[time-nuts] Scopes and 1 PPS signals?
didier at cox.net
Tue Oct 23 22:58:34 EDT 2007
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
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
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
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