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Old 17th Jul 2017, 11:25 pm   #41
G0HZU_JMR
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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?
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Old 18th Jul 2017, 9:24 am   #42
G8HQP Dave
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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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Default Re: Off Air Frequency Standards....

Craig
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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Old 23rd Jul 2017, 1:04 pm   #45
G0HZU_JMR
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Default Re: Off Air Frequency Standards....

Alan kindly sent me the schematic for the Practical Wireless version he is working on and I've attached it below in a word doc. Hopefully it will appear less fuzzy on the forum thread when uploaded in a word doc format compared to a jpg or gif.

I spotted a couple of connections that were missing and I've added these in red and also a couple of earth symbols were missing but these are just cosmetic. I've added a few bits of info on there and Alan advised me that R24 is 10nF and R41 is 40Meg (4 x 10Meg).

It looks like it mixes in a 200kHz signal at the MC1496 chip and this will generate a 2kHz heterodyne that goes through the op amp amplifier/limiter. It looks like the opamp section is a tuned 2kHz BPF with the CF set by R19.

The S meter circuit looks a bit odd because R21 looks to be a very large value and it's tapped into the early part of the 2kHz IF strip. I would have expected it to be connected further down where the signal is bigger?

The 74HC390 divider circuits look OK for the 200kHz and 2kHz taps and the MC1496 connections look 'reasonable' but I don't have any experience in using it in a circuit like this. I suppose you could test the lock performance by disconnecting the receiver section and forcing an accurate 2kHz into the opamp stage and then watch and wait to see if it eventually starts to lock in. The lock process is probably going to be quite slow even with the speed up diodes D6 and D7? But you will need to have a very accurate 2.00000kHz test source, ideally synthesised from a stable reference accurate to a few ppm. Otherwise this basic test will fail as the lock frequency will fall outside the very narrow tuning range of the 10MHz VCXO circuit.
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File Type: docx PW_Offair.docx (329.1 KB, 44 views)
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Old 23rd Jul 2017, 2:49 pm   #46
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Default Re: Off Air Frequency Standards....

Quote:
Originally Posted by G8HQP Dave View Post
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.
Yes, I think it's best to use these things during the daytime when the propagation effects are not so significant. I'm only 25 miles from Droitwich and I get good results during the day. However, to reliably get beyond about 2e-9 performance it's best to average over time and maybe recheck this several times a day.

To get the very best performance I think you are meant to average over several hours during daytime because the propagation effects can't cause a long term error. So it should be possible to get reasonably close to the performance of the BBC standard if averaged over several hours. But I'm not interested in getting this level of accuracy. I just want something that keeps my various 10MHz OCXOs in my RF test gear within about 0.1Hz of 10MHz. eg I have various items here that use a classic Agilent 1E5 10MHz OCXO.

I did check mine against a Quartzlock E8-Y GPS 10MHz reference a few years ago and it did seem to reliably deliver <0.01Hz error (at 10MHz) when averaged over about 10 seconds. This was done numerous times during the day. I did try averaging over longer periods but I can't remember how good the results were.

For me, the appeal of the 198kHz offair standard is that you can switch it on (with a small integral antenna) and get +/- 0.02Hz typical accuracy (at 10MHz) within a couple of minutes after switching it on. When I had the E8-Y GPS standard here I tried various tricks to upset it and it was easy to get it to give significant errors even when it still reported itself to be in lock. It also took ages to get lock and required an external antenna and monitoring via RS-232 to get a reliable signal that didn't cause it to misbehave. Too much hassle!

If the 198kHz service goes away one day then I guess I'll have to consider GPS. But for me the extra hassle involved (i.e. long and unreliable lock time and crap portability) make it a poor alternative because I don't 'need' better than 2e-9 performance.
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Old 24th Jul 2017, 8:28 am   #47
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Default Re: Off Air Frequency Standards....

By way of an update - I have added the missing pieces of track which Jeremy brought that to my attention and I am now reversing component changes introduced by well intentioned others in an attempt to get the thing working.

This is a slow and laborious process due to age related deterioration of my eyesight and dexterity so in the first instance I have to be guided by spotting resistors of a different body colour to those that I originally used, then by checking one by one visually and with a digital resistance meter.

Thank you to everyone who has contributed to this thread which seems to have generated a lot of interest.

Regards - Alan
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