App Note 98 part 1

Signal sources, conditioners and power circuitry: Circuits of the fall, 2004. 28 pages.

This app note is another circuits collection of more-or-less random projects, as he has previously done in App Note 45, App Note 61, and App Note 75.

The app note starts with a simple implementation of a current source in Figure 1. The real application circuit in Figure 2, produces "alternating, equal amplitude, opposed polarity linear capacitor ramps". An interesting choice for the output waveform: I wonder what this signal was used for?

Figure 4 shows a simple sine-wave oscillator (well, really a heavily filtered triangle-wave oscillator). I think the interesting piece of this circuit is the use of the one-sided output from the single-supply A3 to implement half-wave rectification in the amplitude-control loop. Cheesy, but clever.

Figure 6 shows a wide-band high-voltage level shifter, capable of producing 50-volt pulses without overshoot. The footnote exposes a brave design choice:

Transistor data sheet aficionados may notice that the –50V potential exceeds Q1, Q2, Q3 VCEO specifications. The transistors operate under VCER conditions, where breakdown is considerably higher.

The next few circuits include several different pulse generators. He seems to be exploring a wide array of alternatives to his usual avalanche-based pulsers (such as Appendix B in App Note 79). Figure 8 uses a single-chip oscillator that achieves a 400-ps rise time (as shown in Figure 10) and a 320-ps fall time (Figure 11). Nice and simple. Figure 12, inspired by the calibrator circuit from the Tektronix 485 oscilloscope, produces a 850-ps rise time, with a flat-top pulse. Clean and pretty. Figure 15 uses a unique (and discontinued) HP tunnel diode to produce a 20-ps (twenty! picoseconds!) rise time. Yikes. Figure 19 produces pulses with controlled widths, down to 1 ns.

The next three circuits are instrumentation applications. Figure 25 shows a single-supply amplifier, which uses the chopper-clock output to drive a charge pump to allow true-zero-volt output swing. This circuit is them used in the milliohmmeter in Figure 27 and the instrumentation amplifier in Figure 30.

Figure 32 show a wide-band low-feedthrough switch using transconductance amplifiers, developed for settling time measurement (we'll see this circuit again in App Note 120).

I'll cover the rest of the circuits next time.

Vintage scopes are better part 4

Vintage scopes are better (also see part one, part two, and part three).

Reason number 4: Repairabililty and inspiration.

It is nearly impossible to get a useful service manual for a modern oscilloscope, and even if you could, the parts to repair it are generally unobtainable custom ICs. However, many vintage scopes are repairable, and complete service manuals are readily available for many models. While some vintage parts are getting hard to find (cough, tunnel diodes, cough), many repairs are possible with easily obtained parts. In his second book, Jim explains,

Older equipment offers another subtle economic advantage. It is far easier to repair than modern instruments. Discrete circuitry and standard-product ICs ease servicing and parts replacement problems. Contemporary processor-driven instruments are difficult to fix because their software control is "invisible", often convoluted, and almost impervious to standard troubleshooting techniques. Accurate diagnosis based on symptoms is extremely difficult... Additionally, the widespread usage of custom ICs presents a formidable barrier to home repair. [1]

In addition, vintage scopes are extremely well-designed and well-constructed. Much can be learned from examining and exploring the internals of classic oscilloscope.

The inside of a broken, but well-designed piece of test equipment is an extraordinarily effective classroom... The clever, elegant, and often interdisciplinary approaches found in many instruments are eye-opening, and frequently directly applicable to your own design work. More importantly, they force self-examination, hopefully preventing rote approaches to problem solving... The specific circuit tricks you see are certainly adaptable and useful, but not nearly as valuable as studying the thought process that produced them. [2]

His love of vintage scopes was a continual source of inspiration. He studied, referred to, and borrowed extensively from classic instruments and their service manuals. For example,

• The high-speed ECL logic in the "King Kong V/F" in App Note 14 was inspired by the trigger circuitry of the Tektronix 2235.
• The adaptive threshold trigger circuit in Figure 130 of App Note 47 was also inspired by the Tektronix 2235.
• In his discussion of CCFL power supplies in [3], Jim explored the high-voltage resonant power supplies in the Tektronix 547 and the Tektronix 453.
• In his discussion of low-noise power conversion (App Note 70, Appendix A), he discusses circuits from the Tektronix 454 and 7904 (Figures A1 and A2).
• Figure 12 in App Note 98 was derived from the calibrator circuit in a Tektronix 485.

References:

[1] Jim Williams, "There's no place like home," in The Art and Science of Analog Circuit Design, Jim Williams, Ed. Boston: Butterworth-Heinemann, 1995, ch. 17, pp. 269–277.

[2] Jim Williams, "The importance of fixing," in The Art and Science of Analog Circuit Design, Jim Williams, Ed. Boston: Butterworth-Heinemann, 1995, ch. 1, pp. 3–7.

[3] Jim Williams, "Tripping the light fantastic," in The Art and Science of Analog Circuit Design, Jim Williams, Ed. Boston: Butterworth-Heinemann, 1995, ch. 11, pp. 139–193.

App Note 95

Simple circuitry for cellular telephone/camera flash illumination: A practical guide for successfully implementing flashlamps. 12 pages.

This short little app note with a long title discusses the LT3468, which is a monolithic power-converter chip for driving camera flashtubes. As Figure 1 testifies, flashlamps are much brighter with better color temperature than LEDs, and so are the preferred illuminator for digital cameras. After a brief discussion of the physics, Jim discusses the support circuitry and construction requirements. "Capacitor, wiring and lamp impedances typically total a few ohms, resulting in transient current flow in the 100A range."

This app note contains a single application schematic, shown in Figure 6. Most of the text is dedicated to measurements of the circuit performance (Figures 7 to 10). Figures 9 and 10 are particularly interesting, with vertical scales of "relative light per division". Surprisingly, Jim does not reveal how he made the "relative light" measurements.

A carefully worded caution appears near the end of the text:

In cases where the lamp is triggered with a user-selected transformer and drive scheme, it is essential to obtain lamp vendor approval before going to production.

That quote sounds like the voice of unfortunate experience.

The app note ends with a cartoon. "Nice seeing you again. Yeah, nice seeing you again, too."

Vintage scopes are better part 3

Vintage scopes are better. (See part one and part two.)

Reason 3: Overdrive resilience.

Several of Jim's App Notes were concerned with the measurement of settling time (for examples, see App Notes 120, 86, 79, 74, 47, and 10). In App Note 79, he measures the 0.1%-settling time of a high-speed op amp (5-millivolt settling on a 5-volt step). In App Note 120, he attempts to measure the LSB-settling time of a 20-bit DAC (10-microvolt settling on a 10-volt step). In these kinds of measurements, the primary concern is avoiding overdrive in the oscilloscope. An input-gain setting that allows the user to see 5-mV-settling details also allows the 5-V waveform to travel off the screen (and overload the vertical amplifier).

In Appendix B of App Note 120, he explains the problem,

Oscilloscope recovery from overdrive is a murky area and almost never specified. How long must one wait after an overdrive before the display can be taken seriously? (App Note 120, page 14)

The horrors of overdrive are readily apparent on page 16.

The waveform looks fine in Figures B2 to B4, but when part of the transient is off the screen, as in Figures B5 to B7, the oscilloscope is clearly having trouble. "It is obvious that for this particular waveform, accurate results cannot be obtained at this gain."

However, in this application, vintage scopes are better. Specifically, the classical sampling oscilloscope is immune to overdrive. To explain, Jim compares the architecture of and analog scope, a digital scope, and a classical sampling scope, all shown in Figure B1.

Describing the advantages of the classical sampling scope, he explains,

The classical sampling oscilloscope is unique. Its nature of operation makes it inherently immune to overload. Figure B1C shows why. The sampling occurs before any gain is taken in the system. Unlike Figure B1B’s digitally sampled ‘scope, the input is fully passive to the sampling point. Additionally, the output is fed back to the sampling bridge, maintaining its operating point over a very wide range of inputs. The dynamic swing available to maintain the bridge output is large and easily accommodates a wide range of oscilloscope inputs. Because of all this, the amplifiers in this instrument do not see overload, even at 1000x overdrives, and there is no recovery problem. Additional immunity derives from the instrument’s relatively slow sample rate—even if the amplifiers were overloaded, they would have plenty of time to recover between samples. (App Note 120, page 14)

A final word from Jim, "Unfortunately, classical sampling oscilloscopes are no longer manufactured, so if you have one, take care of it!"

App Note 94

Slew rate verification for wideband amplifiers: The taming of the slew. 12 pages.

This app note discusses slew-rate measurement, which is another great application for Jim's high-speed pulse generators. The text is only 12 pages long, but it is not a "brief" project. Previously, he has used a variety of pulse generators for measuring oscilloscope bandwidth and amplifier settling times. For examples of the circuits used in some of these other applications, see

• App Note 79, Appendix B
• App Note 72, Appendix B
• App Note 61, Figure 26
• App Note 47, Appendix D
• App Note 45, Figure 27

Here he is directly measuring slew rate, which requires even faster pulse rise times.

The app note begins with a quick history lesson in footnote 2,

The term "slew rate" has a clouded origin. Although used for many years in amplifier literature, there is no mention of it on the Philbrick K2-W (the first standard product op amp, introduced in January 1953) data sheet, dated 1964. Rather, the somewhat more dignified "maximum rate of output swing" is specified.

He quickly gets to the heart of the matter, the discussion of the necessary subnanosecond-rise-time pulse generator. The schematic in Figure 8 shows his 360-ps design, which is a great piece of work. This design is an improved version of Figure B1 in App Note 79.

Although the complete schematic is shown, a picture of the actual construction is missing. Much of the "secret sauce" of high-speed pulse generation is in the construction technique (as alluded to in footnote 11), so it is unfortunate that a photoessay isn't included. (I've seen one of these boxes, and it makes Figure B3 in App Note 72 look like child's play.)

Figures 11 to 17 show the calibration of pulse damping, the 360-ps rise time, and the measurement of the LT1818 slew rate. Several of these figures (such as 11, 12, 13, 16, and 18) were made with his Tektronix 7104 with 7A29 and 7B15 plug-ins that he discusses on page 4. These figures are the first appearance of his 7104 (note the legend in Figure 2). I discussed this marvelous scope in a previous post.

Appendix A discusses measurement integrity, using a time-mark generator (Figure A1) and a 20-ps rise-time pulse generator (see the table in Figure A3). Yes, Figure A4 is produced by a pulse with a twenty-picosecond rise time! Appendix B discusses level-shifting (no active circuits here; your only hope is to use excellent transformers).

Appendix C discusses the horrors of connectors, cables, adapters, attenuators, and probes at these high speeds. The best quote is on page 11:

Skepticism, tempered by enlightenment, is a useful tool when constructing a signal path and no amount of hope is as effective as preparation and directed experimentation.

The cartoon shows off his 7104 scope, with a 7M13 alphanumeric readout plug-in (which produced his "signature" WILLIAMS03 in the corner). "I'm going as fast as I can."

Vintage scopes are better part 2

Vintage scopes are better. (See the introduction in part one.)

Reason number 2: Sensitivity and bandwidth. With the appropriate plug-ins, analog oscilloscopes provide superior sensitivity compared to digital scopes. In discussing low-level noise measurements in App Note 70, Jim describes the oscilloscope requirements and laments,

Current generation oscilloscopes rarely have greater than 2mV/DIV sensitivity, although older instruments offer more capability. Figure B11 lists representative preamplifiers and oscilloscope plug-ins suitable for noise measurement. These units feature wideband, low noise performance. It is particularly significant that the majority of these instruments are no longer produced. This is in keeping with current instrumentation trends, which emphasize digital signal acquisition as opposed to analog measurement capability. (App Note 70, page 29)

While 2 millivolts-per-division is commonplace in digital oscilloscopes, plug-ins are available for 500-series and 7000-series scopes with sensitivity down to 10 microvolts-per-division. Yes, microvolts. In Appendix D of App Note 124, Jim lists the high-sensitivity, low-noise amplifiers of choice.

Of course, sensitivity and bandwidth are related (the wider the bandwidth, the higher the expected noise floor). However, in conjunction with superior noise floor, some vintage analog scopes also provide very large bandwidths. Some of Jim's favorites were

• Tektronix 556 with a 1S1 sampling plug-in, 1-GHz bandwidth (App Note 72, page 9, Figures 16 and 17)
• Tektronix 547 with a 1S2 sampling plug-in, 3.9-GHz bandwidth (App Note 79, page 19, Figure B4)
• Tektronix 661 with a 4S2 sampling plug-in, 3.9-GHz bandwidth (App Note 72, pages 34 and 35, Figure 77 to 82)
• Tektronix 7104 with 7A29 and 7B15 plug-ins, 1-GHz real-time bandwidth (App Note 94, page 4, particularly Figures 2, 11, 12, 13, 16, and 18)

There are modern digital scopes available today with wider bandwidths, but "vintage" and "analog" does not mean "slow." Of course, Jim would want me to point out that bandwidth isn't everything. In App Note 47, he explained,

Intimate familiarity with your oscilloscope is invaluable in getting the best possible results with it. In fact, it is possible to use seemingly inadequate equipment to get good results if the equipment’s limitations are well known and respected. All of the circuits in the Applications section involve rise times and delays well above the 100MHz-200MHz region, but 90% of the development work was done with a 50MHz oscilloscope. Familiarity with equipment and thoughtful measurement technique permit useful measurements seemingly beyond instrument specifications. A 50MHz oscilloscope cannot track a 5ns rise time pulse, but it can measure a 2ns delay between two such events. Using such techniques, it is often possible to deduce the desired information. (App Note 47, page 20)

To be honest, the first sentence of that quote applies no matter what oscilloscope you have.

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Footnote: One last comment while we're discussing plug-in oscilloscopes. The Tektronix 556 dual-beam instrument provides flexibility that is not found in modern instruments.

The Tektronix 556 offers an extraordinary array of features valuables in converter work. This dual beam instrument is essentially two full independent oscilloscopes sharing a single CRT. Independent vertical, horizontal and triggering permit detailed display of almost any converters operation. Equipped with two type 1A4 plug-ins, the 556 will display eight real time inputs. The independent triggering and time bases allow stable display of asynchronous events. Cross beam triggering is also available, and the CRT has exceptional trace clarity. (App Note 29, pages 43-44)

In App Note 65, he exploited these dual-beam advantages in a number of measurement. Figure 36 shows six waveforms, with independent triggering of the top two versus the bottom four traces. Figure 42 shows the ringing bursts at the resonant frequency of the Royer converter, with the explanatory footnote

The discontinuous energy delivery to the loop causes substantial jitter in the burst repetition rate, although the high voltage section maintains resonance. Unfortunately, circuit operation is in the "chop" mode region of most oscilloscopes, precluding a detailed display. "Alternate" mode operation causes waveform phasing errors, producing an inaccurate display. As such, waveform observation requires special techniques. Figure 42 was taken with a dual-beam instrument (Tektronix 556) with both beams slaved to one time base. Single sweep triggering eliminated jitter artifacts. Most oscilloscopes, whether analog or digital, will have trouble reproducing this display. (App Note 65, page 38)

Finally, the flexibility of the Tektronix 556 allows for some great measurement displays. In Figure 34 of App Note 35, he showed a 115-volt sine wave, its distortion products, and its frequency spectrum all in one shot.

In the accompanying footnote, Jim teased,

Test equipment aficionados may wish to consider how this picture was taken. Hint: Double exposure techniques were not used. This photograph is a real time, simultaneous display of frequency and time domain information. (App Note 35, page 16)

This picture was (most probably) produced with his trusty Tektronix 556 with a vertical-amplifier plug-in in one bay (perhaps a 1A2 or 1A4), and a spectrum-analyzer plug-in in the other bay (perhaps the 1L5 50Hz-to-1MHz spectrum analyzer).

App Note 93

Instrumentation applications for a monolithic oscillator: A clock for all reasons. 20 pages.

This app note discusses applications of the LTC1799 oscillator chip. The approach here is classic Jim: Linear Technology produces a simple oscillator chip, and Jim's first thought is "temperature measurement!" The frequency of the LTC1799 is set with a single resistor, so Jim immediately replaces it with a platinum RTD (Figure 5) or a thermistor (Figures 7 and 9), in a variation of his normal voltage-to-frequency-converter theme. Figure 9 also includes galvanic isolation. Other unique sensors also appear. Figures 11, 13, and 15 use a fragile capacitive humidity sensor (which we've seen before), using the LTC1799 as either a reference oscillator or a clock source.

Figures 17 and 19 continue the instrumentation theme with a chopper-stabilized amplifiers. Figure 17 uses a bipolar-op-amp input, while Figure 19 uses JFETs for lower bias current, but slightly higher noise. He also includes two sine-wave generators, Figure 21 uses a resonator loop and Figure 23 is a memory-based DDS (see also Appendix E in App Note 35).

A few almost-digital circuits wrap up the note. A clock-tunable notch filter is shown in Figure 28 and a clock-tunable interval generator in shown in Figure 30. The last circuit (Figure 32) is a single-slope analog-to-digital converter that shares some topology with the interval generator.

The two appendices discuss the LTC1799 in a little more detail. Appendix A discusses the internal topology and the master oscillator, and Appendix B discusses the care and feeding of the RSET pin (and potential bootstrapping).

The best quote is the pedantry in the first footnote,

Strictly speaking, an oscillator (from the Latin verb, "oscillo," to swing) produces sinusoids; a clock has rectangular or square wave output. The terms have come to be used interchangably and this publication bends to that convention.

The cartoon extols the simplicity of the LTC1799, "Everything should be as simple as possible, but not too simple."

App Note 92

Bias voltage and current sense circuits for avalanche photodiodes: Feeding and reading the APD. 32 pages.

More fiber-optic-laser support circuits (see App Notes 89 and 90) are discussed here, in particular, the design of voltage-biasing supplies and current-monitoring circuits for avalanche photodiodes (APD). These circuits are used on the receiving end of an optical communications system.

Monitoring supply current is hard, especially in a high-voltage, single-supply application. Jim works through several approaches, starting with a direct instrumentation amplifier in Figures 2 and 3, both of which are impractical. His first real solution is the AC-coupled lock-in amplifier in Figure 4, using an LTC1043 and a charge pump to create a negative power-supply voltage (remember Figure 4 in App Note 3?). A DC-coupled amplifier is shown in Figure 5, with an amplifier floating up at the high-voltage rail.

Several high-voltage power supplies are presented, with output-noise measurements. A programmable flyback (with voltage-tripler) APD voltage supply is shown in Figure 6. A careful measurement of the 200-microvolt output noise in shown in Figure 7. Figure 8 combines the features of Figures 5 and 6. Figure 9 implements the features of Figure 8 using a transformer instead of a voltage-tripler circuit for the high-voltage output.

Figure 11 is a simple inductor-based design, but it requires a cascode on the LT1946, which gives Jim an excuse to reference the 1939 paper by Hickman and Hunt again (see reference 11). Figure 13 and 15 produce the low-noise supply voltage using the LT1533 controller (which was discussed, at length, in App Note 70). Again, the pedantry in footnote 11 makes me very happy,

Noise contains no regularly occurring or coherent components. As such, switching regulator output "noise" is a misnomer. Unfortunately, undesired switching related components in the regulated output are almost always referred to as "noise." Accordingly, although technically incorrect, this publication treats all undesired output signals as "noise."

Figures 16 and 18 use a crazy flying-capacitor scheme, reminiscent of Figure 23 in App Note 3, this time with optically driven switches. Figures 19 and 20 discuss digital schemes for output-current monitoring.

The various circuits are summarized in the table in Figure 21, along with Jim's usual love for summary tables,

Figure 21's chart is an attempt to summarize the circuits presented, although such brevity breeds oversimplification. As such, although the chart reviews salient features, there is no substitute for a thorough investigation of any particular application’s requirements.

Appendix A discusses two schemes for deriving the feedback voltage on the high-voltage supply, while minimizing the induced error in the current measurement. Appendix B discusses a few reasons why vintage oscilloscopes are so prized in his laboratory.

Appendix C reprints Appendix C from App Note 70. (He is getting quote a bit of mileage out of this Appendix, but it makes good sense: once you've written the definitive treatise on noise measurement, you might as well use it!)

Appendix D discusses the LT1150 chopper-stabilized amplifier, which has a clock-output pin that can be used to charge-pump a negative supply voltage in single-supply applications. "The circuit provides a simple way to obtain output swing to zero volts, permitting a true "live at zero" output."

Appendix E lists a few protection circuits for the expensive APD module, including a voltage clamp, a current limiter, and a bias-voltage crowbar.

The app note ends with a cartoon, betraying some obvious frustration with coworkers, perhaps? "Someday, when I don't have to make anything work, I'm gonna wear a tie too."

Vintage scopes are better part 1

I was discussing these application notes with a colleague, and he commented, "Jim sure did love his vintage oscilloscopes. I wonder, is there anything that you can do with a vintage scope that you can't do with a modern digital one?"

"Yes!" I cried, and I listed off several things, but I don't think I convinced him. Over the next four Sundays, I'm going to enumerate and explain the list of reasons why vintage scopes are better than modern digital abominations, including

1. Trace clarity (resolution and spot size)
2. Sensitivity and bandwidth (and noise floor)
3. Overdrive resilience (of sampling plug-ins)
4. Repairability and inspiration

Mostly, I'll just be quoting the relevant passages in Jim's app notes. Truly, they don't make them like they used to...

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Reason number 1: Trace clarity. The low-level-measurement resolution of a oscilloscope is limited, in part, by the minimum size of the trace on the screen. A well-designed (and well-calibrated) vintage scope can have a vanishingly small spot size on the CRT. With a digital scope, the resolution of the input analog-to-digital converter often becomes apparent as you increase the vertical gain on a digital scope (or you are limited by the size of the LCD pixels).

Discussing oscilloscope selection for low-level noise measurements in Appendix B of App Note 70, Jim commented,

The monitoring oscilloscope should have adequate bandwidth and exceptional trace clarity. In the latter regard high quality analog oscilloscopes are unmatched. The exceptionally small spot size of these instruments is well-suited to low level noise measurement. (Footnote: In our work we have found Tektronix types 454, 454A, 547 and 556 excellent choices. Their pristine trace presentation is ideal for discerning small signals of interest against a noise floor limited background.) (App Note 70, page 29)

He continues this train of thought to say,

The digitizing uncertainties and raster scan limitations of DSOs impose display resolution penalties. Many DSO displays will not even register the small levels of switching-based noise. (App Note 70, page 29)

Again discussing noise measurements in Appendix C of App Note 90, Jim says,

Diehard curmudgeons still using high quality analog oscillscopes routinely discern noise presence due to trace thickening. Those stuck with modern instruments routinely view thick, noisy traces. (App Note 90, page 22)

The detail that is visible with well-focused oscilloscope trace is evident on Jim's Tektronix 556 in Figures B9 and B10 in App Note 70.

I'd like to see these plots replicated on a modern all-digital scope.

App Note 90

Current sources for fiber optic lasers: A compendium of pleasant current events. 24 pages.

This app note continues the fiber-optic-laser-diode theme from App Note 89. Now, attention is paid to the power supply of the laser diode itself, a precision current source. The app note begins with a discussion of performance requirements and protection issues for the laser. "The delicate, expensive load, combined with the uncertainties noted, should promote an aura of thoughtful caution."

Figure 2 shows a simple current-source topology, which is refined in various ways throughout the app note. Figure 3 uses a switching regulator for improved efficiency. I like his intuitive explanation that "It is useful to liken the switching regulator’s input (VCC), feedback (FB) and output (VSW) to the transistor’s collector, base and emitter."

Figure 4 uses an LT1970 "power op amp" to allow a grounded cathode on the laser diode. I have to laugh whenever I see a "power op amp" that comes in a TSSOP body. A real power op amp, like Bob Widlar's LM12, comes in a TO-3 package! (It also drinks scotch.)

The best circuit is Figure 6, which includes a full array of protection circuits: low-voltage drop out, a self-enable regulator, an input current clamp, and an open-circuit lock out. The oscilloscope traces in Figures 7 to 11 demonstrate the performance and operation of these features.

The rest of the circuits in the app note implement low-"noise" switching regulators to provide higher-power output (I heartily endorse footnote 2 on page 8). Figure 15 uses the LT1683, while Figures 17 and 19 use the LT1533 (discussed in App Note 70).

Appendix A discusses using regular diodes and a simple circuit (Figure A2) for simulating a laser diode. Appendix B discusses switching regulator "noise" measurement, using excerpts from App Note 70. Appendix C discusses current transformers for low-level measurements. The Tektronix CT-1 is very nice. The best quote is the footnote in Appendix C,

Diehard curmudgeons still using high quality analog oscillscopes routinely discern noise presence due to trace thickening. Those stuck with modern instruments routinely view thick, noisy traces.

The app note ends with a cartoon, ghost-written by Jim's wife.

App Note 89

A thermoelectric cooler temperature controller for fiber optic lasers: Climatic pampering for temperamental lasers. 12 pages.

This app note discusses thermoelectric coolers (TECs) for laser-diode temperature control. The Figure 3 shows the main circuit, built around the LT1923 TEC-controller chip. Most of the app note (pages 3 to 7) discusses compensating the temperature-control feedback loop. As Jim says, "The unfortunate relationship between servo systems and oscillators is very apparent in thermal control systems." Figures 6 through 9 show a trial-and-error approach to setting the loop gain.

Best quote (with footnote):

Figure 6 shows large-signal oscillation due to thermal lag dominating the loop. A great deal of valuable information is contained in this presentation. (When a circuit "doesn’t work" because "it oscillates," whether at millihertz or gigahertz, four burning questions should immediately dominate the pending investigation. What frequency does it oscillate at, what is the amplitude, duty cycle and waveshape? The solution invariably resides in the answers to these queries. Just stare thoughtfully at the waveform and the truth will bloom.)

Wise words.

Once the loop is "optimized", as in Figure 9, the temperature "stability" is measured (Figures 10 through 12). The short-term stability is good (Figure 10), but disturbance rejection (the response to changes in ambient temperature) in Figure 11 could be better. In this application, proportional control is good, but proportional-plus-integral control would be better. Jim likes to mock "control aficionados" (see footnote 7), but really, we can help!

The last section of the app note discusses switching-regulator "noise" (ripples and spikes) in the voltage to the laser diode. Like the LT1533 discussed in App Note 70, this controller also uses slew-rate control on the switching edges to minimize the high-frequency harmonic content. Figures 13 to 15 and Figure 17 to 19 show the advantage.

The app note ends with a cartoon. "That kind of talk makes me feel so coherent."

App Note 86

A standards lab grade 20-bit DAC with 0.1ppm/°C drift: The dedicated art of digitizing one part per million. 52(?) pages.

The main text of this app note is only four pages long, and there are 48 glorious pages of appendices.

Well, "glorious" is the wrong word. Unfortunately, one of the appendices contains TWENTY-TWO pages of source code (Appendix D). Really? Couldn't Linear Tech have used a smaller font? Printed the code in two landscape columns? Or simply not printed it at all? (Do you think anyone, even one single person, typed in this code and used it? Really?) If we include the source code, this app note is the fourth longest one. However, if we ignore the code, then this app note is only 30 pages long, a little longer than average. It still results in a large appendix-to-main-text ratio, but it's not the less-than-ten-percent ratio we initially suspected...

This app note discusses a 20-bit digital-to-analog converter. The DAC circuit includes an interesting "table-turning" topology: DACs are often used in the feedback path to create a good ADC (for example, as in the successive-approximator topology); in this circuit, a high-quality analog-to-digital converter (the LTC2400) is used in the feedback path to implement a high-quality DAC.

The construction of the 20-bit "slave" DAC is interesting. Jim says, "The sole DAC requirement is that it be monotonic. No other components in the loop need to be stable." The circuit is shown in Figure 2, which using two 16-bit DACs, with eight bits of overlap, and four bits of sub-LSB twiddle.

The results are (briefly) shown on pages 3 and 4, which are heavily footnoted (referring the reader to the numerous appendices). For example, the following quote appears on page 3, with footnote,

Figure 3 is a plot of linearity vs output code. The data shows linearity is within 1ppm over all codes (Establishing and maintaining confidence in a 1ppm linearity measurement is uncomfortably close to the state of the art.).

Appendix A talks a little bit about the history of digital-to-analog conversion and includes a glamor shot of some items from his collection (we saw this photo before in App Note 74). Appendix B lists some of the specifications of the LTC2400 ADC.

Appendix C discusses the operation and use of a Kelvin-Varley divider to verify the linearity of the 20-bit DAC. "The actual construction of a 0.1ppm KVD is more artistry and witchcraft than science." The individual components must be selected (see the table in Figure C4 for the LTC1152 chopper-stabilized op amp) to obtain the necessary performance. Figure C7 shows the complete schematic for the voltage source. The construction of this circuit is a work of art (read the text carefully). "Adjust for 5.000000V at A." That's a lot of zeros! The best quote appears in the footnote on page 11,

The author, wholly unenthralled by web surfing, has spent many delightful hours "surfing the Kelvin." This activity consists of dialing various Kelvin-Varley divider settings and noting monitoring A-to-D agreement within 1ppm. This is astonishingly nerdy behavior, but thrills certain types.

Appendix D contains the source code for the digital comparator. Jim once commented to me, "The summing junction of this circuit is in software. It took a while for me to get my head around it, but it really does work."

Appendix E discusses linearity correction for the LTC2400, and Appendix F discusses improved output-buffer stages. Appendix G shows a gain-of-2000 settling-time measurement circuit, modified from App Note 74.

Appendix H discusses microvolt-level noise measurement. A high-gain preamplifier is necessary, as well as an oscilloscope with a plug-in capable of low-level measurements (2mV/division is not good enough). Figure H3 shows the instrumentation setup, using the same cookie-tin as the noise measurement in App Note 83 Figure 6 (he really did like those cookies!).

Appendix I discusses voltage references for this application, with the LM199A and the LTZ1000A receiving highest marks. Both of these parts use a temperature-control loop to maintain constant temperature on the Zener diode, thus improving the temperature drift to sub-ppm. Figure I2 shows an example circuit for the roll-your-own LTZ1000A. (Bob Dobkin designed these parts. The LM199A is good, but (I suspect) that the loop gain isn't high enough to reject ambient changes in temperature. He got it right in the LTZ1000A with the external loop.)

Appendix J discusses parasitic thermocouples and other construction pitfalls, a topic that he previously discussed in App Note 9 and App Note 28.

Readers finding [Figure J4's] information seemingly academic should be awakened by Figure J5. This chart lists thermoelectric potentials for commonly employed laboratory connectors. Thermocouple activity of some types is more than 20 times greater than others. Be careful!

This app note does end in a cartoon, but I can't reproduce it here. The cartoon demonstrates "one part per million" by printing one million dots. A JPEG on your computer screen doesn't do it justice. Go download the PDF file, print out the last page (on a good printer), grab a magnifying glass, and ponder the real-world difficulty of 20 bits.

Scope Sunday 25

When I visited the Computer History Museum three weeks ago, I also stopped at two electronic surplus shops, WeirdStuff and HSC (I also visited these stores back in October). Unfortunately, the pickings were slim.

At WeirdStuff, there was a Tektronix 502 oscilloscope. Unfortunately, it was priced at $160.

The Tektronix 7904 that I saw last time was still on the back wall, now with a sticker that says "no power", but still with a price tag of $250. (This scope has been there for at least six months.)

I think both of these scopes are priced five times too high. Fifty dollars is a fair price for a broken 7904.

I saw similar overpricing at HSC. They seem to have a lot of untested gear in the $45 to $75 range. I buy a lot of electronic surplus junk, but my what-the-hell price (as in, "what the hell, I'll just buy it and see") for a random untested, unidentified box is a lot less than $45.

However, I have bought things at HSC in the past, and I will buy things at HSC in the future. For one thing, I appreciate and support any business that does this:

This picture shows part of the databook library at HSC. This collection of databooks puts the collection of databooks in the M.I.T. Library to shame. I have referenced these books in the past, and I hope to be able to reference them in the future. Not everything is available on the web (as my commentary in the "Bibliography of Jim Williams" attests). Having real, printed databooks and handbooks available for perusal is a valuable resource and a public service. Thank you, HSC.

App Note 85

"Low noise varactor biasing with switching regulators: Vanquishing villainous vitiators vis-à-vis vital varactors." 24 pages.

This app note discusses low-noise bias-voltage generators for varactor diodes. The main application, as shown in Figure 1, is VCO tuning for phase-locked loops. This application is extremely sensitive to power-supply ripple, as any corruption of the bias voltage will cause spurs in the VCO output. In the circuit shown in Figure 6, careful power-supply design and appropriate filtering produce the varactor-bias voltage shown in Figure 11, which exhibits only 20 microvolts of ripple and noise.

However, the major emphasis of this app note isn't the circuitry, but the instrumentation (this topic is well-worn territory for Jim, of course). Measuring 20 microvolts of ripple and noise is HARD, and Figures 12 through 17 show several ways that improper measurement technique ruin the measurement. Figures 19 through 24 show frequency-domain measurements of the VCO output using a spectrum analyzer. Again, it is shown that improper measurement technique, or careless construction and layout, degrade performance significantly.

Appendix A is a primer on varactor diodes written by Neil Chadderton of Zetex.

Appendix B discusses amplifier and oscilloscope selection in order to facilitate the 20-microvolt sensitivity needed for Figure 11. This appendix is borrowed from Appendix B in App Note 70. I can't resist quoting the footnote again:

In our work we have found Tektronix types 454, 454A, 547 and 556 excellent choices. Their pristine trace presentation is ideal for discerning small signals of interest against a noise floor limited background.

Appendix C is copied from App Note 70 Appendix C (as he explains in the footnote).

The app note ends with a varactor cartoon. Probably the only varactor cartoon in the world. "I never had it so good."

Scope Sunday 24

In response to my post last week about the lack of analog instrumentation the Computer History Museum, one person asked, "Haven't they ever heard of ANALOG computers?" The answer is "sort of"...

The museum does have a small number of analog computers in the main exhibit hall, including one of my personal favorites, an EAI PACE TR-48.

There is also a small display case that discusses the history of operational amplifiers, including a copy of Henry Paynter's "Palimpsest on the Electronic Analog Art", a Philbrick K2-W op amp, a Fairchild uA709 op amp, and a National LM10 op amp.

Over in the "Digital Logic" section of the museum, there are a few analog integrated circuits in the corner, along with a picture of Bob Widlar.

But really, that's about the extent of the analog content (being something of an analog bigot, I'd prefer to see much more).

One thing that I note with disappointment is an item that was formerly on display. When the museum had its "Visible Storage" exhibit hall, they had a beautiful analog computer by the GPS Instrument Company on display. This computer was a lovely, monstrous beast that showed the scale, and the artistry, of "big iron" analog machines. The museum does have a large picture of the machine hanging in the lobby (because, really, it is a work of art), but they no longer have the machine itself on display.

This machine is also close to my heart for two other reasons: 1. It was built in Massachusetts, not far from my house, and 2. Before the museum knew what they had, I found a reference that described the machine and the history of the GPS Instrument Company (the paper was Per A. Holst, "Sam Giser and the GPS Instrument Company: Pioneering compressed-time scale (high-speed) analog computing", in John McLeod, editor, Pioneers and Peers, San Diego: The Society for Computer Simulation International, 1988.) Finding this paper is why my name appears on this page of credits.

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UPDATE: In response to this post, I received a very nice email from Alex Bochannek, who is a Curator at the Computer History Museum. He gave me permission to share his comments here.

In developing the exhibit, I made the conscious decision to focus on general purpose, indirect analogs, i.e., differential analyzers and electronic analog computers. The goal is to show a historical arc which begins with the work done under Vannevar Bush at MIT and ends with hybrid digital/analog systems and DDAs. The continuity in this changing technology story lies with the users’ needs for rapid, interactive calculation for (primarily) engineering problems.

The operational amplifier is displayed as an example of changing implementation technology and as a contrast to the integrator in the DA story. The latter also is a nod to mathematical instruments; slide rules and sectors you will find in the Calculators gallery. The op-amp text is slightly more technical in nature as indicated by the words Tech Talk on the panel. As you noted, you will find Bob Widlar repeated in the Digital Logic gallery.

Analog computing techniques in special purpose applications are represented by many of the devices in the Real-Time Computing gallery, for which I also was responsible. Direct analogs (e.g., network analyzers, electrolytic tanks, etc.) were excluded from the exhibit for lack of artifacts and available exhibit space.

I intentionally limited the content of the Analog Computing gallery to general purpose systems to highlight the broad applicability of analog computers. As you can imagine, most people in the computing field are not only unfamiliar with analog computers but tend to write them off as a pre-digital curiosity which surely must have disappeared many decades ago. By displaying unexpected and beautiful artifacts that represent a vibrant and innovative computing community, I hope to raise our visitors’ awareness of this important story in computing history.

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Here is my response:

I truly enjoyed the exhibits. As I commented in "Scope Sunday 23" (the one about the oscilloscopes), I am impressed with the stunning collection of artifacts that the Museum has on display. Many computing professionals don't appreciate the importance of analog computers. I am reminded (and constantly humbled by) a quote from Ted Nelson:

There are two kinds of computers: analog and digital. Analog computers are so unimportant compared to digital computers that we can polish them off in a couple of paragraphs. (Nelson 1974)

Of course, Ted Nelson is a smart guy (coined the word "hypertext", inspired the creation of the Web) and many people seem to agree with him, but he doesn't (or didn't) know his history. I appreciate that the Museum is working to correct this view (and I hope Ted Nelson no longer feels this way!)

I also agree with your exclusion of direct analogs from the museum exhibits. Some people think that direct analogs are a "under-appreciated story" in the history of computing, but I do not. For example, William Aspray wrote a paper on Edwin Harder and the Westinghouse Anacom, which is little known compared to Bush's Analyzers. In his paper, Aspray laments

What accounts for the difference in recognition accorded these machines? One may be that Bush’s machines were built earlier that the Anacom… Perhaps more important was that Bush’s machines had a high profile at MIT, where they were used by many industries, as well as by many students who went on to important computing careers… It may also be an artifact of historical scholarship. Bush’s prominent role in organizing science for the Second World War and his plan for postwar government support of science has attracted scholars to examine other aspects of his career, including his calculating machinery. MIT as an institution had received considerable attention from historians of science and technology, whereas Westinghouse has been surprisingly neglected given its long history of contributions to electrical technology. (Aspray 1993)

I disagree with Aspray, here. The key issue is that the Differential Analyzers were true general-purpose computers, while things like the Anacom, network analyzers, and electrolytic tanks were really just scale models. While the work performed on these machines was of profound importance to the research teams using them, they were not analog computers. Furthermore, these "non-computing analogies" did not contribute significantly to the "culture of computing" that arose in the middle of the 20th century.

As you say, it is this culture of computing, grown on analog computers, that the Museum celebrates.

App Note 83

"Performance verification of low noise, low dropout regulators: Silence of the amps." 20 pages.

This app note discusses testing LDO regulators, primarily the measurement of output noise. In effect, this app note is the linear-regulator version of App Note 70 (which discussed the measurement of output noise for low-noise switching regulators).

The schematic of the measurement chain is shown in Figure 2. Note that single-pole highpass filters on use on the input and output to remove any DC components from the signals. This arrangement leads to a non-optimal low-frequency cutoff. (See the frequency response in Figure 3, which shows a nice high-frequency roll-off, but a considerably "rounder" low-frequency roll-off... Butterworth is better. He could have done better by moving the second-order Butterworth poles to the third-order locations, since he already had a pole (or two) on the axis.)

Nevertheless, the plots on pages 8 and 9 are very nice, and are clearly the product of painstaking work.

The best quote (from page 3, with footnote): "The metal can encloses the regulator under test and its internal battery power supply (the cookies were excellent, particularly the thin ones with sugar on top)."

Appendix A discusses the internal architecture of low-dropout regulators. This verbiage is an updated and expanded version of Appendix A from App Note 32. Note that Figures A3 and A4 show the transient response for a load step (from 10mA to 100mA); he will return to this topic in a later app note.

Appendix B discusses capacitor selection, and warns of the voltage-dependent and temperature-dependent horrors of some ceramic types.

Appendix C is an in-depth discussion of RMS voltmeters. He has discusses RMS voltmeters before (most notably in Appendix C of App Note 65), but this discussion is the most detailed, and includes comparisons of voltmeter types (Figures C1, C2, and C3), as well as comparisons of specific models (Figure C5). Of course, the HP3400A and HP3403C work very well, as does his special baby, the LT1088-based circuit in Figure C6. Thermal great, logarithmic good, rectify and average bad.

The app note ends with a cartoon. "Speak softly and carry a big PNP."

App Note 81

"Ultracompact LCD backlight inverters: A svelte beast cuts high voltage down to size" 24 pages.

This short app note (seven pages of main text) is the sad conclusion to the grand saga of cold-cathode fluorescent lamps (CCFLs). Just as a reminder:

• Part 1 is Figure 36 in App Note 45
• Part 2 is App Note 49
• Part 3a is App Note 55 (part 1 and part 2)
• Part 3b is Chapter 11 in his second book
• Part 4 is App Note 65 (part 1, part 2, part 3, part 4, and part 5)
• Part 5 is App Note 81

This conclusion is sad, because this app note contains a great idea (using piezoelectric transformers) that utterly failed to achieve traction in the marketplace. Jim gave a talk on this subject at M.I.T., and he seemed genuinely disappointed that this technology wasn't more widely accepted. (If I recall correctly, the issue was in assembly. Mounting the piezoelectric transformer on a circuit board requires specialized assembly equipment, and none of the usual-suspect laptop vendors wanted to invest in the technology.)

Piezoelectric transformers are not new to Jim. In fact, he discussed them in National Application Note 285, "An acoustic transformer powered super-high isolation amplifier", back in 1981. He also used one in Figure 51 of LTC App Note 29.

The enabling technology here is shown in Figure 7: the transformer terminal labeled "resonance feedback" allows for simplified drive circuitry, ensuring start-up and oscillation at resonance. The complete circuit is shown in Figure 9.

Appendices A and B were written by his coauthor at CTS Wireless Components. Appendix A is a short introduction to piezoelectric transformers, and Appendix B is a longer theoretical treatise.

Appendix C provides the best quote

Veterans of feedback loop compensation battles will exercise immediate caution when confronted with a pure and lengthy delay in a loop. Neophyte designers will gain a lesson they will not easily forget.

Again, I think he's mocking me in footnote 2. I must admit that "glop comp" is not a terminology that I am familiar with (although I do use "dominant-pole compensation" in everyday conversation).

The app note ends with a cartoon. "I can't believe I've been replaced by that skinny nothing."

Scope Sunday 23

Last week, I spent a few hours at the Computer History Museum in Mountain View, California. I had previously toured the "Visual Storage" exhibit hall several years ago, but they have a new, beautiful display hall called "Revolution" that I hadn't seen yet. I spent the afternoon wandering around the exhibits and taking pictures. The Museum has a stunning collection of computer-related artifacts. Simply breathtaking.

However, (since it is "Scope Sunday") I have to report that they only have two pieces of Tektronix gear on display in their collection (other than the equipment currently on Jim Williams's desk). In the "PDP-1 Restoration" exhibit hall, there is an original PDP-1 that has been restored and is occasionally demonstrated to the public. In the back of the exhibit hall is a Tektronix 535A, which is probably the "historically accurate" instrumentation for working on a PDP-1!

In the "Revolution" exhibit hall, in the "Minicomputers" section, there is a PDP-8/E that is part of some custom hospital equipment that mapped the brain's response to stimuli during surgery. This "brain surgery station" includes several pieces of Tektronix gear, including a RM561 oscilloscope (with 2A60 and 72 plug-ins), a 161 pulse generator, and a 162 waveform generator.

However, if you look closely at the other exhibits and photographs, there is much evidence that Tektronix oscilloscopes are important to the development of computers. In many of the historic photographs, there is Tektronix equipment plainly visible in the background (and foreground). It's like a subliminal Tektronix exhibit. Here's a list of the Tektronix gear that I found:

1. MADDIDA Customer Demonstration

   What do your customers want to see? A Tek 511A front and center.

2. UNIVAC System brochure

   This sales brochure has several pictures of a UNIVAC installation that include a Tektronix oscilloscope. Check out pages 2, 11, 13, and 15. I especially like how the drawing on page 2 includes an oscilloscope (looks like a Tek 535 or 545). Of course you would want one!

3. IBM 701 assembly floor

   Two oscilloscopes are seen on the assembly floor (looks like a Tek 511 in front and a Tek 512 in back).

4. NTDS Combat Information Center training

   Two oscilloscopes are seen in the training center (one front and one back). The one facing forwards looks like a Tek 535/545.

5. Assembly of IBM 1401 computers

   Look very closely at this photo. I count a dozen Tektronix scopes! The one in front looks like a Tek 535A or 545A.


6. Building the ORACLE computer CPU

   Is that a Tek scope behind the 19" rack?

7. RAMAC Assembly

   I see three Tektronix scopes in this picture, but there must be more.

8. Los Alamos MANIAC computer

   One scope in the very back of the lab, looks like a Tek 511.

9. Operators at the ILLIAC IV at NASA Ames Research Center

   The lone 400-series scope in these pictures. Perhaps a Tek 465?

10. Max Palevsky and Robert Beck, in the lab

    Another laboratory picture with a Tek 535A/545A scope front and center.

11. PDP-1 computer

    A Tek scope on a cart, with a rack-mount scope above it.

12. Fairchild co-founders in discussion

    Tek 575 curve tracers, as far as the eye can see! There are six on the nearest bench (three facing forwards and three facing backwards).

13. The Beast at Johns Hopkins Applied Physics Lab

    Robot development at APL, with a Tek 551 dual-beam scope.

14. Sven Wahlstrom and Nils Nilsson with Shakey

    In the very back of the lab, there appears to be the back of a Tek scope on a cart.

15. Spiral scanner at Lawrence Berkeley National Laboratory

    In the back, a Tek 535A/545A scope on a cart.

With that many pictures of oscilloscopes in the museum, shouldn't Tektronix scopes have an exhibit of their own?

(Sorry for the giant list of links. My cell-phone photographs of the photographs didn't turn out very well, so I thought that it was better to link directly to the source.)

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Postscript: I am aware of the Vintage TEK Museum in Portland, Oregon (I haven't had a chance to visit yet, but it's on my 2012 list). However, I think that classic Tektronix oscilloscopes belong in all museums, including computer history museums, science museums, fine art museums, etc... Don't you?

App Note 79

"30 nanosecond settling time measurement for a precision wideband amplifier: Quantifying prompt certainty" 32 pages.

While technically the long-awaited update to App Note 10, to be honest, this app note is an abridged and modified version of App Note 74. Here the application is the measurement of amplifier settling time instead of DAC settling time, but the techniques (and the text) are significantly similar. The measurement problem tackled here is actually easier; amplifier settling is determined at the 0.1% point, while 16-bit DAC settling requires measurement of the 0.0015% point. (The amount of laboratory work involved was no less, however; Appendix E is proof of that fact.)

The settling-time measurement circuit is shown in Figure 6 (compare to App Note 74 Figure 6). The modifications for this application include a change to the input drive (to drive a voltage step to the op amp, instead of a digital command to the DAC), removal of the output amplifier, and removal of the temperature-control loop on the diodes (both because 0.0015% accuracy is no longer needed). Also, the sample delay and window generator is now implemented with LT1720 comparators instead of 74HC123 TTL logic.

Also, instead of three alternative measurement methods for comparison (as in App Note 74), he provides just one: the classical sampling oscilloscope. The "essentially identical" results of these two measurement are shown in Figures 18 and 19. Perhaps he felt he has less to prove this time?

The best quote is the app note's conclusion: "Examination of the photographs shows nearly identical settling times and settling waveform signatures. The shape of the settling waveform is essentially identical in both photos. This kind of agreement provides a high degree of credibility to the measured results."

Many of the appendices previously appeared in App Note 74. Appendix A is the same as App Note 74 Appendix B. Appendices C and D are similar to App Note 74 Appendices C and D (with light modifications for the new topology of the settling-time measurement circuit, and for op-amp settling instead of DAC-output-amp settling, respectively).

Appendix B is new, discussing one of his favorite topics, subnanosecond pulse generators. He complains about the prices of current production units ($10,000 to $30,000), discusses his favorite vintage units (HP-8082A, HP-215A, Tek 109, and Tek 111), and then shows his own design in Figure B1. This circuit, as he would likely say, "is the beneficiary of considerable attention over a protracted period of time." It is now loaded with features, including a fully adjustable pulse amplitude, an external input to determine repetition rate, and an output trigger pulse that is settable from before-to-after the main pulse. Figure B4 shows the high-speed pulse in all its glory, measured with a Tek 547 with 1S2 sampling plug-in.

Appendix E discusses breadboard construction (like App Note 74 Appendix G) and includes another photo essay on the construction of the settling-time measurement circuit (Figures E1 to E6), proving that the lab work was exhaustive.

The app note ends with a cartoon, of course. Thirty nanoseconds is hard!

App Note 75 part 2

The waveform sampler in Figure 19 is another great circuit. This "track-and-not-hold" topology is basically the core of his settling-time measurement circuit from App Note 74.

Several random instrumentation circuits are included in this app note. Figure 24 is a extremely low-power chopped amplifier. Figure 25 is a thermocouple amplifier for detecting the pilot-light flame in a furnace or hot-water heater (some home improvement during his sabbatical, perhaps?). Figure 26 is a tip detector for shipping containers.

Several oscillator circuits are also included. Figure 27 is the simple 32,768-hertz crystal-oscillator circuit that is used in Figure 10. Figures 29 and 31 are 10-MHz complementary-output oscillators, the former with 50% duty cycle and the latter with nonoverlapping outputs.

A CCFL power-supply circuit returns in Figure 33. I can't believe he isn't completely burned-out on CCFL circuits. Interestingly, this circuit uses the Royer topology, but does not use one of Linear Technology's specialty Royer control chips. Perhaps this circuit is for a low-cost application? A single footnote refers the reader to App Note 65.

The applications section of the app note concludes with more three power supplies. Figures 36 to 39 were contributed by Jeff Witt. Figure 40 is a low-noise off-line power supply using the LT1533 (see App Note 70). The drive on the cascode transistors and the power-limit/current-limit circuits are very interesting.

Best quote (discussing Figure 44): "This data was taken with no input filtering LC components and a nominally nonoptimal layout."

Second-best quote (footnote 12):

Veterans of LTC Application Notes, a weary brigade, may recognize this reference as the object of Application Note 70's (Footnote 14) champagne prize offer. The mystery solved, the messenger was compensated as specified (Veuve Clicquot Ponsardin).

This paper (Reference 24) is the earliest use of the word "cascode" in the literature.

The appendices are updated versions of "Box Sections" A and C from App Note 23. Appendix A discusses low-power techniques and the design evolution of the zoo circuit. Figures A1, A2, and A3 are from App Note 23. Figure A4 is the original Zoo Circuit, and Figure A5 is from App Note 45 Figure 21. Figures 1 and 4, as discussed last time, are the latest circuits in this evolution ("utilizing contemporary components").

Appendix B discusses the effects of test equipment on micropower circuits (in short, avoid loading, or worse, powering, your micropower circuits with your test equipment). Figure B3 is a new addition, borrowed from App Note 47 Appendix E.

The app note concludes with a cartoon that expresses his dog's point of view.

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Related:

• App Note 75 part 1