[time-nuts] Scopes and 1 PPS signals?

Dave Brown tractorb at ihug.co.nz
Wed Oct 24 00:21:15 EDT 2007

----- Original Message ----- 
From: "Didier Juges" <didier at cox.net>
To: "'Discussion of precise time and frequency measurement'" 
<time-nuts at febo.com>
Sent: Wednesday, October 24, 2007 3:58 PM
Subject: Re: [time-nuts] Scopes and 1 PPS signals?

> ); SAEximRunCond expanded to false
> Errors-To: time-nuts-bounces+tractorb=ihug.co.nz at febo.com RETRY
> I took some pictures of my Thunderbolt's PPS output. While it's 
> pulse is not
> quite as narrow as what you report, it's 10uS, way too narrow for 
> the scope
> to display it in normal ("Sample") mode at 250mS/div (the fastest 
> setting
> that will allow to measure the period of a PPS signal) on my TDS-210 
> digital
> storage scope.
> There are at 
> http://www.ko4bb.com/Test_Equipment/Thunderbolt/PulseStretching
> The names are self explanatory, but just in case :-)
> 1_PPS_10xProbe is the PPS measured at the end of a 3 foot cable with 
> a 10x
> probe
> 2_PPS_Sample Mode is the PPS signal in normal (sample) mode, as you 
> probably
> would see it on any DSO without any special mode. It's fair to say 
> it would
> be hard to make sure what you are seing or measuring. Please note 
> that some
> of the time, the scope triggers, but the trace is flat. I had to 
> wait to
> make this shot, which was the best I saw in about 30 seconds.
> 3_PPS_PeakDetect_Mode is the same signal, with the scope in 
> Peak-Detect
> mode. It is easy to check the period (if not very acurate). In this 
> mode,
> all sweeps look the same.
> 4_PPS_Diode is with a 1N4148 between the PPS output and the scope 
> probe. See
> how the 10uS pulse is now a solid 250uS at 1/3 the height. It would 
> be much
> easier to see on any scope, I need to pull the old trusty Tek 2245A 
> and see
> what it does on it.
> 5_PPS_Diode_5.6nF is the same diode with a 5600pF capacitor across 
> the
> probe. The pulse is now strecthed to ~10mS.
> 6_PPS_Diode_5.6nF is the same signal at the 250mS/div sweep rate.
> That's a heck of a lot cheaper than $45. Does not even need a power 
> supply
> :-)
> Didier KO4BB

I took Brookes original inquiry to be about measuring the width of the 
nominally 10 uS  wide PPS pulse-in which case anything that alters the 
pulse width is a no-no.  Precise and stable triggering delay to place 
one of the pulses in the on-screen area is all  that is required.
DaveB, NZ

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