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Old 28th Nov 2020, 1:03 pm   #1
mickm3for
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Default Looking for infomation on frequency counter

Hi info wanted / ideas i have this home built counter (not built by me) it reads 9 khz low at 144.500 mhz it has a 100khz timebase xtal to set gate time this has no trimmer cap tried a 150 pf trimmer across the 22pf xtal loading cap it made no diffrence to the readings all i know about the counter is it has a 4049 chip as the xtal osc pin 2/3 going to the xtal a 1 meg across the xtal and a 22pf from each pin of the xtal to ground signal taken from pin 3 to pin 5, on pin 4 etched on the print says 1mhz and a test point. also on the print says G8CKT/G8FTB VHF DFM MK3 couldnot find any info on net? any help Mick
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Old 28th Nov 2020, 1:28 pm   #2
trh01uk
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Default Re: looking for infomation on frequency counter

Mick

hmm......9kHz error at 144.500MHz. I make that 62ppm error - sounds excessive for any normal timebase, which could reasonably be within 5ppm error (about 700Hz) at room temperature.

How do you know that the 144.500MHz signal you are feeding into the counter is accurate?

Is this the first time you have used this counter? Or do you know that it was fine, say 6 months ago?

G8CKT has a website. You could try contacting him and asking for a schematic at least. Can't find G8FTB though.

Richard
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Old 28th Nov 2020, 1:44 pm   #3
mickm3for
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Default Re: looking for infomation on frequency counter

Hi update ist time using counter i am compairing this with a vhf tx but i dont know if this is on frequuency as its old xtal controled and has trimmers in series with xtals so may have drifted its an ic 215 fm rig . update using gb3vhf 144.430 mhz beacon to zero beet a xtal osc to this i know the osc is on 144.430 mhz counter reads 144.421mhz ?mick
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Old 28th Nov 2020, 4:01 pm   #4
PETERg0rsq
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Default Re: Looking for infomation on frequency counter

A 4049 used as a crystal oscillator, in the way you describe, will oscillate at the crystal frequency.

So your description of a 100kHz crystal, but the board marked 1mHz (should it be 1 MHz) doesnt make sense.

Plenty of information on these oscillators on the internet.

Pulling the crystal is possible, but will depend on existing capacitors and resistor loading.

Also an error of 63ppm at 144MHz would equate to an error of less than 0.1ppm on the 100kHz oscilator if the gate is running from it, which might be as good as you can get in such a simple circuit.


But i agree you need a schematic. If it is not to comlicated, then draw out what you have.
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Old 28th Nov 2020, 4:23 pm   #5
mickm3for
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Default Re: Looking for infomation on frequency counter

Hi i have put a 150 pf trimmer instead of the 22pf from pin 3 to ground this now lets me ajust the frequency of the osc counter now reads 144.430/1 the value of cap is 121pf but it works ?? seams right at 10mhz as well as 1mhz from my quartzlock will contact g8ckt as it has his call etched on to the pcb thanks to all Mick
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Old 28th Nov 2020, 9:40 pm   #6
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Default Re: Looking for infomation on frequency counter

Quote:
Originally Posted by PETERg0rsq View Post
...Also an error of 63ppm at 144MHz would equate to an error of less than 0.1ppm on the 100kHz oscillator ...
Sorry but no, 63ppm is still 63ppm, this is a way expressing the error as a ratio of offset to frequency and remains the same whatever frequency.

e.g. The frequency offset at 100kHz will be 1/1440 of that at 144MHz, but then 100kHz is 1/1440 of 144MHz so the ratio remains the same.
(63 ppm is 9kHz at 144MHz or 6.3Hz at 100kHz)

Jim
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Old 28th Nov 2020, 10:20 pm   #7
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Default Re: Looking for infomation on frequency counter

63ppm is pretty normal for a plain vanilla grade crystal in a logic gate oscillator.

A high spec crystal at 10MHz (a bit of a sweet spot) in a cold welded case can be spec'd at 10ppm, worsened a little by osc component tolerances. Temperature compensated oscillators are in the 1ppm to 0,5ppm bracket. Precision ovened ones go down to 0.01ppm.

Sounds like you need a better reference.

David
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Old 28th Nov 2020, 10:48 pm   #8
PETERg0rsq
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Default Re: Looking for infomation on frequency counter

Quote:
Originally Posted by jimmc101 View Post
Quote:
Originally Posted by PETERg0rsq View Post
...Also an error of 63ppm at 144MHz would equate to an error of less than 0.1ppm on the 100kHz oscillator ...
Sorry but no, 63ppm is still 63ppm, this is a way expressing the error as a ratio of offset to frequency and remains the same whatever frequency.

e.g. The frequency offset at 100kHz will be 1/1440 of that at 144MHz, but then 100kHz is 1/1440 of 144MHz so the ratio remains the same.
(63 ppm is 9kHz at 144MHz or 6.3Hz at 100kHz)

Jim
Sorry, yes I stand corrected.

Shouldnt have expressed the measurement as PPM.

Just (badly ) making the point that a small frequency error on a clock running at low frequency will have dramatic effect when clocking a VHF frequency and that the results seen were probably as good as could be expected.
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Old 28th Nov 2020, 11:02 pm   #9
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Default Re: Looking for infomation on frequency counter

The two callsigns are Michael Tooley G8CKT & David Whitfield G8FTB.

They published articles in Practical Wireless for many years, so you might find the design there.
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