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Old 17th Jul 2017, 11:25 pm   #41
Join Date: Sep 2010
Location: Cheltenham, Gloucestershire, UK.
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Default Re: Off Air Frequency Standards....

I can offer up a few coarse guesstimates as to the probable design approach using the 4046 VCO if that helps?

I think the original design probably had a very clean and well regulated 12V supply and the 10k resistors and the cap used in the VCO section would have been very stable parts. So no drifty ceramic caps allowed here. Probably a very stable poly type is required here and a decent 12V linear regulator was used.

We know that the phase detector frequency is 500Hz so the loop bw will probably be 50Hz or lower because it should be 1/10 or less of the PD frequency.

I'm going to guess that a free running 4046 VCO at 1MHz will have maybe 10Hz of short term wobble on it (as in wobble per second or so) so the loop BW will probably be relatively wide for an offair standard as the loop will need to be fast enough to correct the wobble. Normally one would use a crystal oscillator here and the idea would be to slowly tickle/nudge it with a very, very slow loop until it locks. i.e. something as slow as my design in my earlier youtube video. This means that the phase noise of the 10MHz VCXO will remain quite good once locked.

But if a wobbly VCO like the 4046 is used then I can only assume that G4FZH adopted a different approach. I think his loop needs to be much faster to take control of the wobbly 4046 VCO and clean away the wobble and keep it disciplined to be close to the correct frequency. I had a quick look at the components in the loop filter and made a guess as to the VCO gain in kHz/V and worked out that the loop probably has a natural frequency of about 3Hz. Hopefully it will be fast enough to keep the 4046 VCO tamed within the target accuracy of the standard. I kind of expected to see a higher natural frequency than I calculated but maybe 3Hz is adequate here?

Is there anything in the literature that tells you how stable the standard is in terms of short term stability?
Regards, Jeremy G0HZU
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Old 18th Jul 2017, 9:24 am   #42
G8HQP Dave
Join Date: Sep 2008
Location: Solihull, West Midlands, UK.
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Default Re: Off Air Frequency Standards....

For off-air reception the issue is not how stable is the standard but how stable is the ionosphere. I vaguely recall seeing some discussion of that some years ago, but I can't remember where.
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Old 18th Jul 2017, 9:48 am   #43
Craig Sawyers
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Default Re: Off Air Frequency Standards....

Coming to this thread late. I use a Trimble Thunderbolt GPS unit. That has outputs at 10MHz and 1pps. The 10MHz is the external standard for my HP 5328A counter.

It is fed from a Trimble bullet antenna on a pole on the end of the house with a good view of the sky.

The Trimble has an RS232 output, which is fed via an RS232-USB convertor from Maplin.

Last edited by AC/HL; 18th Jul 2017 at 7:32 pm. Reason: As requested
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Old 18th Jul 2017, 5:28 pm   #44
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Location: Bristol, UK.
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Default Re: Off Air Frequency Standards....

Just be aware you have given your exact address in your screen shot, I blur out the Latitude and Longitude in those shots.
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