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)
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.
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).