19 February 2012

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!"

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