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Emady
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Posted: 21 June 2018 at 5:07am | IP Logged Quote Emady

Hi,

I use a test set by N7ZWY for evaluating motional parameters of crystals, combined with an oscilloscope. It works great but ties the scope up, requires several manual measurements and need to write them down somewhere so it is time consuming.

For a while now I have been thinking of ways to automate this and have the possibility of storing the results on an SD card for later use and this is where the MM fits in.

The procedure involves taking few voltage and frequency readings. Now I am OK with measuring voltages but am struggling to understand how to do frequency measurement with the MM. The manual says that COUNT function does that but is limited to 300kHz. The crystals I measure go up-to 27MHz so not sure how to make the MM measure that. I would prefer to avoid using a pre scaled.

Is this feasible and if so any starting help anyone please?

Kind Regards
Elia Mady
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Emady
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Posted: 21 June 2018 at 5:28am | IP Logged Quote Emady

I have been thinking some more and wonder if using two timers would work. One of them using interrupts, although not sure how. Is this better using an fFunction?
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Geoffg
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Posted: 21 June 2018 at 7:29am | IP Logged Quote Geoffg

Measuring 27MHz on the Micromite is not feasible (for many reasons). Off the top of my head the best way that I can think of is to build a frequency counter using discrete chips then read their value.
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matherp
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Posted: 21 June 2018 at 7:57am | IP Logged Quote matherp

Quote:
The crystals I measure go up-to 27MHz so not sure how to make the MM measure that.


I think this is possible by using the frequency as input to one of the PIC's timers and then reading the timer at intervals. I believe it could be done with a CFunction. Of course you will need a trivial circuit to convert the crystal output to a TTL signal as input to the timer
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Azure
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Posted: 21 June 2018 at 9:14am | IP Logged Quote Azure

This might not be exactly what you are after but maybe will spark ideas.

Silicon Chip published a 6GHz+ Touchscreen Frequency Counter which was an add on module for the Micromite Plus Explorer 100 in the Oct/Nov/Dec 2017 issues. Since it is on the MM Plus and the basic programs are supplied you can change it and add other functionality to suit your purpose.
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Emady
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Posted: 22 June 2018 at 3:07am | IP Logged Quote Emady

Geoff, Mather,

Thank you for the replies. As per Geoff's idea I did some searches on Google and found this:
https://www.swharden.com/wp/2016-09-05-vhf-frequency-counter-with-pc-interface/

It looks promising, I will have a go at porting the simple python code to MMBasic.

Azure, thanks, will have a look at the magazine article. Maybe that will be an option too.

Kind Regards,
Elia, M0ZHN
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f1fco
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Posted: 22 June 2018 at 10:39pm | IP Logged Quote f1fco

Hi Elia,
you can try on a test board two (or 3) 74LS90 (divide by 10) with your xtal oscillator
with /100 (or /1000), you got a range of 300 KHz maxi
it is ok to count with a MicroMite

73s from F1FCO
Pierre, from Nimes, south of France

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CaptainBoing
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Posted: 23 June 2018 at 6:39am | IP Logged Quote CaptainBoing

I built a Frequency meter with a USB serial interface on an old euro card.

I wanted it absolutley perfect to whole Hz and used a "gold" TCXO at 12.288MHz 100ppb as the reference XTAL and divided it down to give a very precise 1 second gate into that same 8154.

It was more involved than that one above because I divided down using discrete counter chips (4017s !), I didn't want to rely on a processor clock for the gate, but it is incredibly accurate and good from 0Hz to about 70MHz in 1Hz increments

I have a small vid of it working here. https://www.youtube.com/watch?v=pakCeRi2fJQ&t=97s

If you are interetsed, I can bundle the circuit schematics in Eagle and post them. It was originally powered by a PIC16F877 but was the subject of a brain transplant to a 44pin micromite. It has given sterling service and is a daily driver for me.

I intend to put a frequency generator in the same case... one day... just need to get round to it.

Edited by CaptainBoing on 23 June 2018 at 6:43am
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Warpspeed
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Posted: 23 June 2018 at 1:26pm | IP Logged Quote Warpspeed

You can always divide the frequency right down to something manageable and measure that as already suggested.
If you divide it down far enough, its even possible to measure the period. Accuracy and resolution should not be a problem, but its going to be a slow measurement.

Edited by Warpspeed on 23 June 2018 at 1:30pm


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bigmik
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Posted: 23 June 2018 at 4:55pm | IP Logged Quote bigmik

The only problem with dividing is you lose resolution.

ie. a divide by 10,

You could be as much as 9 counts out, as the source has to change states 10 times before the output changes once.

Regards,

Mick

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Warpspeed
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Posted: 23 June 2018 at 5:07pm | IP Logged Quote Warpspeed

Not if you increase the measurement interval by the same proportion.

If you have 1Mhz and you count over one second, you will have a one million count, with 1Hz resolution, but with a one count uncertainty.

If you have 10Mhz, first divide that by ten down to 1Mhz, and then count over ten seconds and you will have a ten million count and get the same 1Hz resolution with the same one count uncertainty.

The measurement just takes ten times longer to do, but accuracy and resolution remain the same.

If you are stuck with a slow hardware/software and you wish to measure up to very high frequencies with good resolution, you can still do it, but the trade off is time required to make the measurement.

Edited by Warpspeed on 23 June 2018 at 5:31pm


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bigmik
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Posted: 23 June 2018 at 7:50pm | IP Logged Quote bigmik

@Tony,

Yes, you are correct, I didnít consider that option.. but can be excessive if it is divide by 100 or 1000..

Regards,

Mick

Edited by bigmik on 23 June 2018 at 7:51pm


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