[time-nuts] Input filter for data logger

Attila Kinali attila at kinali.ch
Sun Nov 19 13:19:12 EST 2017

On Sun, 19 Nov 2017 11:40:27 -0500
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 ?

The MV89 is a beast of an OCXO and uses more power at warm-up than any other
I know of. But it is spec'ed ~1W for steady state. Which means the
outside of the OCXO should be about hand-warm. If it's too hot, then
the heater circuit is probably broken and running at full-throttle.

> I woul ask an advise regarding following issue. I have signal generator 
> which produce the square wave. The freq. is 192Hz.
> http://www.patoka.org/OCXO/LOGGER/IMG_20171119_104659813.jpg
> However, the logger measure it TWICE ! I think its because of that 
> signal form. Here is the output (in microseconds):
> http://www.patoka.org/OCXO/LOGGER/IMG_20171119_104838190.jpg
> 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:
> http://www.patoka.org/OCXO/LOGGER/IMG_20171110_123201987.jpg

Can you show us what _your_ circuit looks like instead of some bad
(literal) screenshot?

But guessing from what you showed, I would say that your amplifier
circuit isn't stable and has some gain peaking at around 10MHz.
There are two ways to proceed: Either optimize your circuit or
simplify it using modern components to the input signal you expect.

I personally, would go for simplification, unless you want something

If you only want to time-stamp input pulses and you can ensure
that those pulses are between 0 and 5V, then I'd use a 74LVC1G34
or 74LVC1G04, depending on whether you want inversion or not
(do not use the 74LVC1G14 as the hysteresis will increase the noise level)

If you want to measure sine input as well, then you have to AC
couple and add some bias voltage when using an LVC gate.

It is upon moral qualities that a society is ultimately founded. All 
the prosperity and technological sophistication in the world is of no 
use without that foundation.
                 -- Miss Matheson, The Diamond Age, Neil Stephenson

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