[time-nuts] Scopes and 1 PPS signals?

Didier Juges didier at cox.net
Tue Oct 23 19:37:48 EDT 2007

Some DSOs (my lowly Tek 210 does) have a peak detect acquisition mode where
it will analyze the signal at the highest speed of the A/D converter
regardless of sweep speed and report the min and max for each displayed
pixel line. Main advantages are 1) to make aliasing obvious and 2) to be
able to see very narrow pulses at low sweep speed. I can see 20nS wide
pulses at 1 PPS easily. Of course, the precision of the measurement (if you
use the period function) is limited to the display resolution, but at least
you can get in the ballpark. My scope is in that mode all the time, little
reason not to.

If you do not have such mode, put a diode (i.e. 1N4148) in series with the
probe. The diode will charge the input capacitance of the probe, which will
discharge exponentially in the 10 Mohm resistance and give you something you
can see. Can't beat that for cost. Add some capacitance in parallel with the
probe if needed to widen the pulse.

Didier KO4BB

> -----Original Message-----
> From: time-nuts-bounces at febo.com 
> [mailto:time-nuts-bounces at febo.com] On Behalf Of Brooke Clarke
> Sent: Tuesday, October 23, 2007 6:24 PM
> To: Discussion of precise time and frequency measurement
> Subject: Re: [time-nuts] Scopes and 1 PPS signals?
> Hi Bill:
> I'm sure that would work, but it's something else that needs 
> to be done sometime in the future.
> I checked the pulse coming from the M12+T and it's more like 
> 100 ms wide, probably for this reason.
> Have Fun,
> Brooke Clarke
> http://www.PRC68.com
> http://www.precisionclock.com
> http://www.prc68.com/I/WebCam2.shtml 24/7 Sky-Weather-Astronomy Cam
> WB6BNQ wrote:
> > Hi Brook,
> > 
> > You could try using a 74123 (or newer variants) 
> retriggerable one shot 
> > to extend the pulse width to a level that would allow you 
> to see it on the scope.
> > 
> > Bill....WB6BNQ
> > 
> > Brooke Clarke wrote:
> > 
> > 
> >>Hi:
> >>
> >>While trying to figure out the O-1814 Rb source I found a 1 
> PPS signal 
> >>that's less than 10 us wide.  But using an HP 54501A DSO 
> it's almost 
> >>impossible to determine the period.  
> >>http://www.prc68.com/I/O1814.shtml
> >>
> >>If the display is 1,000 dots wide:
> >>at 2 us/sq the trace takes up about 4.5 squares or 450 dots, at 200 
> >>us/sq the trace takes up about 0.045 squares or 4 dots but 
> the second 
> >>pulse is way off screen.  At slower sweep speeds you can't 
> see the pulse.
> >>
> >>Are there scopes that allow the period of a narrow pulse to be 
> >>measured that don't cost an arm and leg?
> >>
> >>--
> >>Have Fun,
> >>
> >>Brooke Clarke

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