[time-nuts] Pitfalls of Digital 'Scopes

Chuck Forsberg WA7KGX N2469R caf at omen.com
Mon Jun 6 17:57:52 UTC 2011

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.
> -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"
10255 NW Old Cornelius Pass Portland OR 97231   503-614-0430

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