[time-nuts] Input filter for data logger
orin.eman at gmail.com
Sun Nov 19 13:17:11 EST 2017
Look at the falling edge of your square wave - I'd guess there is some
undershoot and it's triggering the extra counts. With the back to back
limiting diodes in that circuit, I don't think the ringing shown in your
scope trace will matter, however, ringing around the 0V level will matter.
I've seen similar with HP 5335A and 5370A counters with inputs set to DC
and the level pots set to preset. Change it to AC, or tweak the level pots
and the problem goes away.
You can probably see the effect on the scope by setting the trigger level
somewhere around 0V - sometimes it will trigger on the rising edge and
sometimes on the undershoot on the falling edge.
On Sun, Nov 19, 2017 at 8:40 AM, Vlad <time at patoka.org> wrote:
> I am doing another fun project. It is data logger at this time. The
> "heart" is MV89A OCXO and the "brain" is STM32.
> "The box" has 1PPS input to control DAC which will control OCXO. BTW, this
> M89A is pretty hot to touch. My other OCXO's not that much warmed. Is it
> normal for this Morion model ?
> I woul ask an advise regarding following issue. I have signal generator
> which produce the square wave. The freq. is 192Hz.
> However, the logger measure it TWICE ! I think its because of that signal
> form. Here is the output (in microseconds):
> In average delta is .0026041 s, which is almost equal to 384 Hz (192x2)
> For the input I am using the schema similar as HP was using in one of
> their counters:
> I tried another source (DDS sine generator, referenced from GPSDO) and
> that beast produce exact number of events:
> However, it is some "spikes" in the data flow (see above the number "
> 26.23559295", which suppose to be something as "26.58..."). I can't
> understand the reason for that.
> The "delta" is .0052 s, which is almost equal to 192 Hz. But data flow is
> not that "smooth" though.
> In square wave mode, "school grade" Wavetek generator produced exact 192
> events with no issues. (Except its output frequency is not that stable)
> However, if I switch Wavetek to sine, my logger detected twice number of
> event (384)
> I would assume, some improvement needs to be done for the data logger
> input. I am using 2N5485 and 74AC04 elements there. Any advises will be
> appreciated ! Thanks !
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