[time-nuts] Pitfalls of Digital 'Scopes

J. Forster jfor at quik.com
Mon Jun 6 18:17:12 UTC 2011

> Aliasing (if that is indeed what's going on) can be a problem with any
> digital sampling device, including your 'scope.

Not quite aliasing, I think, but close.

> 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.

I can only conveniently trigger off the LORAN pulse which is gated RF with
no set or stable relation between the RF carrier phase and the gating



> 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.
>> FWIW,
>> -John
>> ===================
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> .73,
> Brent, KD0GLS, Minneapolis

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