[time-nuts] Pitfalls of Digital 'Scopes

KD0GLS kd0gls at mninter.net
Mon Jun 6 14:08:15 EDT 2011

Aliasing (if that is indeed what's going on) can be a problem with any digital sampling device, including your 'scope.  You may find the "peak detect" function in the "acquire" menu useful.  I assume your TDS1002 has one as my TDS2024 does.  Also, perhaps the "holdoff" function might improve your triggering stability.

On Jun 6, 2011, at 12:40, J. Forster wrote:

> As some of you know, I've been working on restoring a WW II LORAN-A
> system. To test it, I've been using a Tektronix TDS1002 Digital 'Scope and
> a Military TS-251 Test Set.
> First a bit about the LORAN-A signal. It is a precisely timed Master pulse
> of roughly 2 MHz RF. The spacing of the Master Pulse is precisely crystal
> controlled, but the RF is just gated...  the carrier is not locked to the
> Master Pulse. It looks sort of like this:
> |||||....................................|||||.................
> ^
> | Master Pulse start.
> Anyway, I was triggering on the Master Pulse and looking at several
> succeeding ones, all generated by a TS-251 Test Set.
> For the longest time, I could not figure out why the successive Master
> Pulses envelopes were bouncing around all over the place by perhaps a
> factor of 5.
> Eventually, I worked out that most of the samples taken by the digital
> scope were being taken during the quiet time, and the actual LORAN pulse
> was being badly undersampled. This resulted in displayed waveform that had
> almost nothing to do with what the system was really doing. Also, because
> of slight trigger uncertainties, the LORAN pulse was being "smeared" in
> time.
> IMO, the lesson is that digital scopes do not always accurately depict
> what a circuit is doing. Even a $50 analog 'scope would never have this
> issue.
> -John
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Brent, KD0GLS, Minneapolis

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