Scope Sunday 45

My friend Eugene pointed me to a Tektronix 661 on eBay (it was the first one I'd seen in quite a while). I've wanted this scope for a long time. I even mentioned back in Scope Sunday 2.

Jim mentioned his Tek 661 quite a few times: He used it in App Note 74, and tt is pictured on the back cover. He used it in Figure 77 of App Note 72 and also in App Note 61. I'm excited to have my own, and if it works, I'll have to get to work on building some more high-speed pulse generators.

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 74 part 2

This app note contains eight appendices (more than half of all the pages), and, as always, the appendices are great stuff.

Appendix A talks a little bit about the history of digital-to-analog conversion and includes a glamour shot of some items from his collection. The Kelvin-Varley divider is very nice. "What Lord Kelvin would have given for a credit card and LTC's phone number."

Appendix B is an updated discussion of oscilloscope-overdrive performance (see App Note 72 Figures 32 to 37 and App Note 47 Figures 45 to 50). Here he has added Figure B1, which compares the topology of various oscilloscopes, and he sings the praises of vintage analog instruments. "Unfortunately, classical sampling oscilloscopes are no longer manufactured, so if you have one, take care of it!" Indeed.

Several of the appendices follow-up on and expand on some issues from the main text. Appendix C discusses calibration of the amplifier delay in the settling-time measurement circuit. Appendix D discusses amplifier compensation (in his usual, seat-of-the-pants way), and the moral of the story seems to be "build it and see." Appendix F shows the circuitry necessary to interface DACs with serial-data interfaces to the settling-time measurement circuit.

Appendix E discusses the special case of using a chopper-stabilized amplifiers and the possible dangers involved therein. The scope trace in Figure E3 is especially frightening, although he admits,

This is admittedly worst case. It can only happen if the DAC slewing interval coincides with the amplifier's internal clock cycle, but it can happen. (Footnote: Readers are invited to speculate on the instrumentation requirements for obtaining Figure E3's photo.)

Such a tease! It'd be nice if he occasionally exposed the trickery behind some of these displays.

Appendix G discusses breadboard construction, and in particular talks about proper steering of the ground currents. Wise advice! As the footnote says, "I do not wax pendantic here. My abuse of this postulate runs deep." This appendix also includes a very nice photo essay on the construction of the settling-time measurement circuit (Figures G1 to G10). I'd love to see some high-resolution color versions of these pictures.

Finally, Appendix H shows some power-gain stages, some of them borrowed from App Note 18 and App Note 47 Appendix C.

The app note concludes with a picture of his work bench instead of a hand-drawn cartoon.

On the left is his Tek 661 sampling scope, with a Tek 454 on top. Next to that is his Tek 547, and in front of the 547 is the General Radio 1422-CL variable capacitor, propped up by a data book. Classic stuff.

(Yes, I really want a 661.)

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Related:

• App Note 74 part 1

App Note 74 part 1

"Component and measurement advances ensure 16-Bit DAC settling time: The art of timely accuracy." 48 pages.

This app note discusses the settling time of digital-to-analog converters. The measurement of settling time is a topic that Jim has discussed before (see Appendix B in App Note 47), but this app note is the most exhaustive treatment so far. This app note is also very, very dense.

There is a plethora of good advice here, included here for several reasons. First of all, the problem is hard. As Jim explains on the first page

In particular, the settling time of the DAC and its output amplifier is extraordinarily difficult to determine to 16-bit resolution... Measuring anything at any speed to 16 bits (0.0015%) is hard.

Secondly, and most importantly, he solves the problem FOUR times. Not content to simply update the settling-time test circuit from App Note 47 (compare App Note 47 Figure B2 with App Note 74 Figure 6), but he also verifies the measurement three more times. One, a bootstrapped clamp in shown in Figure 19. Two, a direct interface to a classical sampling oscilloscope is shown in Figure 26. And three, a unique differential amplifier is employed in Figure 28. These four methods are summarized on page 15.

These results are the fruits of a monumental effort. These four similar scope traces probably represent months of work, and it is an amazing testament to his laboratory skill that they all match so well.

Out of these four measurements, I think the two most interesting ones are the settling-time test circuit and the direct interface to a sampling oscilloscope. The settling-time test circuit in Figure 6 is an update to the circuit discussed in App Note 47. He has made a few improvements to the circuits and replaced a few of the amplifiers with better components. The big improvement is in the choice of the diode bridge; instead of four Schottky diodes, he uses a monolithic array of vanilla diodes with a temperature-control loop. (The temperature control is similar in concept to his temperature-stabilized transistor array in National App Note 299.)

The other interesting measurement uses his Tektronix 661 sampling scope in Figure 26. As he says on page 3, "The only oscilloscope technology that offers inherent overdrive immunity is the classical sampling 'scope." In the footnote, he comments about Appendix B and some of the references and, in particular, "Reference 15 is noteworthy; it is the most clearly written, concise explanation of classical sampling instruments the author is aware of. A 12 page jewel." This reference can still be found on the Tektronix website.

Two more great quotes are worthy of mention. On page 7, he discusses clamp diodes that protect the diode array from damage. In the footnote, he confesses,

This can and did happen. The author was unfit for human companionship upon discovering this mishap. Replacing the sampling bridge was a lengthy and highly emotionally charged task.

On page 17, he discusses the General Radio model 1422-CL precision variable air capacitor. Again, the great quote is the description in the footnote,

A thing of transcendent beauty. It is worth owning this instrument just to look at it. It is difficult to believe humanity could fashion anything so perfectly gorgeous.

There is a similar model capacitor currently listed on eBay for $3,000!

I'll cover the appendices next time.

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Related:

• App Note 74 part 2

App Note 72 part 2

The applications section beings on page 21.

Again, as in App Note 13, he shows a wide variety of applications for this high-speed comparator. He starts with a crystal oscillator (Figure 47), also shown with switchable outputs (Figure 48), temperature compensation (varactor-biasing in Figure 49), and voltage control (more varactor-biasing in Figure 51). Figure 53 shows a voltage-control clock-skew generator with plus-and-minus 10-nanosecond skew. Voltage-to-frequency converters make an appearance in Figure 55 and 57. Figure 55 is a simple V-to-F, while Figure 57 is a precision topology, offering seven decades of frequency output (1 Hz to 10 MHz).

Several instrumentation applications are also included. Some of these circuits are related to the instrumentation circuits that he has used in recent (and will use in future) app notes. Trigger circuits with variable threshold (Figure 60) and adaptive threshold (Figure 63) are shown (we saw the adaptive-trigger circuit in Appendix C of App Note 70). A technique for increasing the comparator gain (using the venerable 733 amplifier, achieving 500-microvolt sensitivity) is shown in Figure 65.

A variable-controlled delay, up to 300 ns, is shown in Figure 69. Two circuits that use this delay are shown next: A high-speed sample-and-hold for repetitive signals (like Figure 29 in App Note 13) is shown in Figure 73. Figure 69's delay is also used in Figure 75 to add a programmable-delay trigger to his favorite pulse-generator topology (I think this circuit is the best circuit in the app note; I really need to build one for myself). Figure 77 shows these fast pulses using a 3.9-GHz sampling scope (a Tektronix 661 with a 4S2 plug-in).

Figure 78 shows a high-speed pulse stretcher that can trigger on pulses as small as 2-ns width input. The final circuit is a overload-protection circuit breaker, shown in Figure 83.

The app note ends with two appendices. The first one, "About level shifts", is from App Note 13. The second one, "Measuring probe-oscilloscope response", is a slightly modified reprint of Appendix D from App Note 47. This version includes Figure B2, taken with the 12GHz sampling oscilloscope that was alluded to in the footnote on page 93 of App Note 47 (seven years after writing that footnote, he finally gets to print the scope trace!).

The app note concludes with a cartoon that compares the LT1394 with other comparators, such as the LM311, the LM360/361, and the AD790.

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Related:

• App Note 72 part 1

App Note 61 part 2

Continuing on, Figure 26 shows an improved high-speed pulse generator. Compared to Appendix D in App Note 47, this pulse generator has a distinct advantage in that it is triggered. "This feature permits synchronization to a clock or other event." The 2N2369 is biased just below avalanche breakdown, and the incoming trigger input causes breakdown (due to the difference between Vceo and Vces). Note the comment about the Tektronix 661 with the 4S2 sampling plug-in ("I'm sorry, but 3.9GHz is the fastest 'scope in my house"). Nice upgrade from the 1-GHz 1S2 sampler that he has used previously. "Ground plane type construction with high speed layout, connection and termination techniques are essential for good results from this circuit." Yep. (I think this circuit is the best circuit of the app note.)

Figure 29 shows a special voltage regulator for flash-memory programming. Unfortunately, he does not compare and contrast this design with the designs in App Note 31. Is this circuit just as good? (It probably is, but why?). What are the features of the LT1109 that make it a good fit for this (touchy and sensitive) application?

Figure 31 shows a low-voltage voltage-to-frequency converter (operating from a 3.3-volt supply). The next two circuits are broadband noise sources. Figure 33 uses a Noisecom NC201 noise diode with a selectable filter to produce the noise, and a RMS AGC loop to set the amplitude. Figure 37 uses a standard zener diode as the noise source, but requires a trim to set the initial noise level. Figure 38 is a switchable-output crystal oscillator, which uses diodes to select which crystal is active in the feedback path of the comparator. Cute.

Appendix A includes significant reprints from the manual of an HP3400A True RMS Voltmeter. Jim compares the approach used here to his approach in Figure 22. The instrument he discusses here is a classic, with an impressively creative solutions to several design challenges. The input buffer in Figure A1 uses Nuvistors (because JFETs weren't good enough in 1965). Figure A2 shows a photograph of the input-buffer circuit board. The "video amplifier" in Figures A3 and A4 is an impressive design with DC and AC feedback loops and clever bootstrapping. The chopper amplifier in Figures A5 and A6 uses neon bulbs and photocells for the chopping action! (As Jim says, "Hewlett-Packard has a long and successful history of using lamps for unintended purposes.") Figure A7 shows the circuit board for Jim's RMS-to-DC converter from Figure 22.

The topic of RMS-to-DC conversion was near and dear to Jim's heart. He covered these circuits in detail in App Note 22, and he used them in his CCFL explorations. He helped to design the LT1088 IC and was its main evangelist. In the footnote on page 28, Jim tries to explain the context: "We are all constantly harangued about the advances made in computers since the days of the IBM360. This section gives analog aficionados a stage for their own bragging rights. Of course, an HP3400A was much more interesting than an IBM360 in 1965. Similarly, Figure 22's capabilities are more impressive than any contemporary computer I'm aware of."

Do I detect some frustration in Jim's voice here? Compare all of the circuitry of the HP3400A with the circuit in Figure 22. In effect, I think Jim is saying, "Look how easy I'm making it for you! Why aren't you buying more LT1088 chips?"

Best quote (page AN61-38): "Incidentally, what were you doing in 1965?"

The app note concludes with a great cartoon (perhaps the best cartoon so far).

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Related:

• App Note 61 part 1

Scope Sunday 2

It is not my intention to purchase an oscilloscope every week while I am writing this blog, but when this 547 came up on Craigslist, I couldn't help myself. The 547 is Jim's favorite scope! You can see it on his lab bench on the cover of his first book, and it's the scope used in many of the oscilloscope photos in the app notes.

It's a little dirty (OK, it's a lot dirty), but it looks like it's in pretty good shape. No physical damage and no missing tubes. It came with a Type M and a 1A4, but the seller said that one channel didn't work (probably in the Type M, since that was the one installed).

According to the seller, this one used to belong to Amar Bose, former MIT professor and founder of the Bose speaker company. (Actually, it just has a Bose property tag on it, so I think it's just Bose company surplus, and I doubt that it belonged to the big man himself... so much for a rare provenance.)

I haven't plugged it in yet. I want to clean it up a little bit first, and I'm concerned about the high-voltage transformer. The HV transformer in this scope is known to go bad. The transformer is potted in an epoxy that absorbs water and eventually fails (or causes the circuits around it to fail). Given that this scope was likely stored in a damp basement for who-knows-how long, is there a way to test the HV transformer for deterioration before just plugging it in and powering it on?

That's it. No more scopes (unless it's a 556; I still want one of those, and a 661, and a 576, and maybe another 7104... I also need a working 109 and a 7B87).