05 February 2012

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...

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.

1 comment:

Anonymous said...

A very important point seems to have been missed:a digital scope can be subject to aliasing and totally mangled display. You'll find a detailed discussion in the comments to web page

This affects observation of a short segment with the timebase set for a low repetition rate, e.g., the detail of a PAL television signal with the timebase set to display two fields (40ms). Although a DSO can capture samples at a huge rate, it can't store so many samples, so the sampling rate is decreased radically as the time/div increases. The most expensive scopes have resolved this to some extent,

See the discussion (URL above). HTH, pol098