[time-nuts] Pitfalls of Digital 'Scopes

[Chuck Forsberg WA7KGX N2469R]

Offhand, I would suspect undersampling or mistriggering....
I picked up a Hameg scope with 200 megasamples/sec a few
years ago.  It saved quite a bit of time when I was putting
together a parallel interface between Linux and a Tek storage tube
terminal.  Push a button and a hard copy pops out of my
Laserjet - definitely beats a Polaroid for what I was doing.

On 06/06/2011 10:40 AM, 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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-- 
Chuck Forsberg WA7KGX N2469R     caf at omen.com   www.omen.com
Developer of Industrial ZMODEM(Tm) for Embedded Applications
   Omen Technology Inc      "The High Reliability Software"
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