Author Topic: Fundamental Friday - reciprocal frequency counter  (Read 2185 times)

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Offline ChipguyTopic starter

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Fundamental Friday - reciprocal frequency counter
« on: May 09, 2015, 11:07:46 pm »
Principle of reciprocal frequency counting.
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Offline EEVblog

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Re: Fundamental Friday - reciprocal frequency counter
« Reply #1 on: May 09, 2015, 11:22:25 pm »
I have a vague recollection I've done this somewhere?
 

Offline ChipguyTopic starter

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Re: Fundamental Friday - reciprocal frequency counter
« Reply #2 on: May 10, 2015, 07:43:11 am »
Maybe because I suggested that a while ago in a tweet, right after you made the first video about gravity influencing crystal oscillators.
It was not the april fool one, but I think in a video from 2014.

And yes, you mentioned in a sentence or two that those frequency counters use the input signal as the gate to count their own reference frequency and calculate. But not in detail. That could even be in the april fool video 2015, I'd have to re-watch it.
Or maybe I missed that video? Opps....bummer...


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Offline Rerouter

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Re: Fundamental Friday - reciprocal frequency counter
« Reply #3 on: May 10, 2015, 09:32:48 am »
principle of operation, count the reference oscillator cycles, and gate on external input, this gives you the number of cycles that equate to delay between a cycle on the input, you take that count and recipricate it to get the input frequency,

E.g. using a 16mhz reference, and feeding in an input of 48Khz, it would gate on average every 333.3 cycles of the reference, for a purely logic based approach, this may be done using a parallel eeprom to keep the circuit compact,

Or in a micro-controller, you can run a timer all the way up to the oscillator frequency on most, but with reciprocal counters, you really want to either use averaging, or keep the reference frequency a fair amount higher than the input if you want the count to remain accurate,

e.g. my frequency counter project jumps into reciprocal counting below 200Hz for a 250Khz reference, which gets about 0.2Hz resolution per sample (1 count difference is 0.16hz) but if you play it smart, you can increase your number of averages as the frequency increases, further increasing the resolution for a repetitive signal, allowing all the way down to parts per million from a decent reference frequency, e.g. 1ppm at 0.25Hz, or 1000ppm at 250Hz, etc in this particular case.

My own project also had an AD9850 DDS module from ebay, with a minimum frequeny step of 0.0291Hz, and was using the reciprocal counting to measure the modules relative drift over time, with orientation, temperature and impact, and beyond all belief i ended up with 2 crystals that had a drift less than 2ppm from one to another, and a hystereses of 1.5ppm with impacts, and an offset of -7ppm, which remained true over the course of several days (it was new to me then and i wanted to see how well my DIY project fared)

In the application is was intended for i never needed to count less than 0.5Hz or better than 0.5%, but it was nice to know that i had the capacity to measure very slow events very accurately, or by using many samples, had a way to measure fast signals to a very reasonable resolution.
« Last Edit: May 10, 2015, 09:39:32 am by Rerouter »
 


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