Bandwidth vs rise time of digital 'scopes: Behavior of the TDS3054


Inspired by recent postings in this group, I've decided to describe my experience with a TDS3054 and a TDS3054B.

These 'scopes have a published bandwidth of 500 MHz on all channels.
The TDS3000-series Service Manual (071-0382-01) on page 1-3 mentions a *calculated* rise time of 700 ps.
This complies with BW = 350/tr (BW in MHz, tr in ns).
350 is the accepted number for analog 'scopes (Gaussian fall-off). Apparently, Tek didn't want to use the larger (400 - 450) value that makes sense for digital 'scopes with their much steeper fall-off.
I measured both TDS 'scopes to indeed have about 700 ps rise time on all four channels, with only a few percent overshoot, in line with the rise time as per the specifications (although mentioned there as calculated).

Two interesting things:

- Both TDS 'scopes have their -3 dB points clearly above 600 MHz on all four channels! If this applies to "all" samples of this model, it indicates that 400 - 420 would be more applicable than 350 but Tek decided in their sales documentation to do as if the number 350 that everybody knows from the analog age would apply, calculating BW from tr. The assumption "Nobody is interested in BW, rise time is what counts" wouldn't be surprising in these digital times.

- Tek sold a TDS3064 having a BW spec. of *600 MHz* with V/div. settings that nicely match my checking results for the two TDS3054's!

AFAIK, checking and (automatically) calibrating frequency behavior in these 'scopes is done by adjusting the step behavior.

Any comments?


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