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Old 29th Jun 2017, 11:40 am   #21
cmjones01
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Default Re: Off Air Frequency Standards....

Quote:
Originally Posted by G8HQP Dave View Post
Most radio clocks in the UK will use either MSF on 60kHz from Cumbria or DCF on 77.5kHz from Germany. These use phase modulation for the clock data, which will complicate locking a PLL to them for frequency purposes.
The MSF signal has various components, but from memory I think it's a simple on/off carrier. I designed and built a decoder for it many years ago and there were two codes: the 'fast' code, a bit stream sent once a minute, and the 'slow' code which is encoded at one bit per second, one logic state being a 100ms break in the carrier and the other being a 200ms break in the carrier. I don't remember there being any phase modulation at all. I think DCF is similar but I've never looked in to the details.

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Old 29th Jun 2017, 12:17 pm   #22
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Default Re: Off Air Frequency Standards....

It's Droitwich that uses phase mod. You have to use a very long time constant in the loop to get rid of it. I think France Inter uses the same system.
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Old 29th Jun 2017, 1:12 pm   #23
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Default Re: Off Air Frequency Standards....

Yes, just checked. MSF uses on/off keying, so perhaps less of a problem for PLL. DCF uses both amplitude and phase modulation, although the PM uses a 512-bit PRBS and the average phase is unchanged by the modulation.
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Old 29th Jun 2017, 2:27 pm   #24
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Default Re: Off Air Frequency Standards....

You could divide the 198 by 99 to get 2.
 
Old 29th Jun 2017, 3:31 pm   #25
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Default Re: Off Air Frequency Standards....

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You could divide the 198 by 99 to get 2.
See post 8
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Old 29th Jun 2017, 4:06 pm   #26
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Default Re: Off Air Frequency Standards....

Yes, I realise that 198/99=2. Dividing by 99 just requires a bit more logic, and may result in an output with some 4kHz in it; dividing by 100 can be done in a way which ensures that the first harmonic is the third at 6kHz.

I seem to recall once managing to buy a 198kHz crystal off ebay, but I don't know where it is. I still can't decide whether 198kHz or 60kHz is a better long term bet. Anyway, I am still waiting for a round tuit and there are other jobs ahead of it in the queue.
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Old 29th Jun 2017, 4:33 pm   #27
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Default Re: Off Air Frequency Standards....

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There may be a different version in a later book.
Hi.

There was indeed a later version of this frequency standard in a later book, the author of the book was Clive Smith, G4FZH. I am also contemplating building this frequency standard using the 198kHz carrier from the Droitwich transmitter. I even managed to get hold of a 198kHz crystal. The circuit is attached below and hope it is legible.

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Old 29th Jun 2017, 5:07 pm   #28
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Default Re: Off Air Frequency Standards....

Sorry, forgot to add the block diagram for post #8.
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Old 29th Jun 2017, 5:43 pm   #29
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Default Re: Off Air Frequency Standards....

Excellent thanks for posting that. What year / edition is that? I will update my copy a bit
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Old 29th Jun 2017, 7:07 pm   #30
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Default Re: Off Air Frequency Standards....

Related to that, I seem to remember a project, possibly in Elektor, to produce a 200kHz reference once Drotiwich moved. It took the 198kHz signal and divided it by 99 (giving 2kHz), mixed that with the 198kHz signal and extracted the sum frequency.

May be worth searching for (but I have no idea of the issue...)
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Old 29th Jun 2017, 7:11 pm   #31
Philips210
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Default Re: Off Air Frequency Standards....

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Excellent thanks for posting that. What year / edition is that?
Hi MrBungle.

It is the Third edition dated 1995.

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Old 29th Jun 2017, 7:42 pm   #32
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Default Re: Off Air Frequency Standards....

Thank you
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Old 30th Jun 2017, 12:00 am   #33
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Default Re: Off Air Frequency Standards....

I just grabbed my 'Walls Ice Cream Tub' homebrew offair standard off the storage shelf and tried it and it is working well tonight. It took just over 1 minute to settle to within 50mHz of 10MHz and during the day it is a bit quicker than this.

Tonight, on a 10 second gate time on my counter it is typically staying within 3mHz of 10MHz on each count every 10 seconds. However, I can spoil this by causing vibration to the offair standard or by moving stuff on the bench near it. If I go to a 1 second gate time it effectively does less averaging and the jitter is more like +/- 30mHz. On a bad night it won't be this good but during the day it is usually at least this good in terms of stability.

A few years ago I wrote a Windows app that reads the counter reading the offair standard over GPIB and averages the reading over many minutes or even over an hour. This gives very repeatable results. Much better than I need and I'm more than happy with the 1 second gate time with maybe +/- 30mHz of jitter with the odd rogue blip. I'm too lazy to wait 10 seconds. This is easily good enough to keep my various 10MHz OCXOs in my test gear within their spec limits every year or so. I usually dig out the offair standard once every 6 months to a year and trim up all the OCXOs so they agree with it.
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Old 13th Jul 2017, 5:14 pm   #34
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Default Re: Off Air Frequency Standards....

Many thanks to everyone who has contributed to this thread and apologies for not acknowledging it sooner. Google had thrown a wobbly and was not allowing my email client to connect.

I have looked out the book and attached scans of the relevant pages which I hope will be readable.

The circuit is intended for use on 198kHz and I now need to check if the crystal that I was given ever go put into circuit.
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Old 14th Jul 2017, 9:28 pm   #35
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Default Re: Off Air Frequency Standards....

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Originally Posted by Andrew2 View Post
My first homebrew standard used horiz sync pulses from BBC-1. A crystal osc on 10 MHz was divided by 640 to give 15.625 KHz which was compared to the incoming sync provided by a TV tuner/IF amplifier/demod cct. It worked a treat, but the closing of analogue TV put a stop to it!
A lot of us used the 3rd harmonic of the Holme Moss 405-line sound carrier, a welcome intruder in the 2m amateur band at 144.750MHz

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Old 15th Jul 2017, 11:35 am   #36
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Default Re: Off Air Frequency Standards....

Egg on face time & apologies to to Clive, G4FZH. I can now categorically say that my Off-Air standard was built from an article in Practical Wireless. The reference on the underside of the PCB is "WR343 1995 Practical Wireless Publishing".
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Old 15th Jul 2017, 9:30 pm   #37
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Default Re: Off Air Frequency Standards....

I had a quick look at the G4FZH circuit and I'm impressed with the way he did the logic for the dividers to get the ratios of 99 and 4 to get /396 and 500Hz. I was lazy and used a programmable 4059 /N chip for my offair standard design.

However, I'm a bit concerned that it looks like the design uses the 4046 internal VCO for the 1MHz oscillator. It looks like the 1MHz xtal oscillator is only there as a backup if the 198kHz offair signal is lost.

I'd expect there to be some close in phase noise and stability/spurious issues when using an RC based VCO and I wonder what loop BW that design uses? There must be a compromise here and I wonder if anyone has measured the phase noise close to carrier?

I'd expect the 4046 VCO to be quite noisy and wobbly even when running at 1MHz and I don't know how this will affect the overall performance of that standard. There must be a compromise when defining the loop bandwidth. I would have expected to see a 1MHz VCXO here rather than an RC based VCO. Or have I missed something here?

To see what mine looks like for phase noise I dug out my ice cream tub 198kHz standard and fed the 10MHz VCXO output to my 8GHz Tek 3408 spectrum analyser and looked at the close to carrier noise of the 10MHz VCXO when it is locked. It isn't that bad across 10Hz to 1kHz offset but I can't easily measure below a 10Hz offset with this analyser when looking at phase noise. But it looks like an 'average to mediocre' 10MHz crystal oscillator on the spectrum analyser across 10-1000Hz offset and this is hardly surprising. I designed the VCXO in a hurry using junk box parts and a cheap 10MHz crystal and there will also be added close to carrier noise from the offair standard signal
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Last edited by G0HZU_JMR; 15th Jul 2017 at 9:40 pm.
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Old 16th Jul 2017, 12:59 pm   #38
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Default Re: Off Air Frequency Standards....

To put some numbers to my post above, a typical 'standard' (as in low cost) crystal oscillator at 10MHz can probably achieve:

-95dBc/Hz at 10Hz offset
-120dBc/Hz at 100Hz offset
better than -135dBc/Hz at 1kHz offset

However, these numbers can be expected to degrade a bit if the circuit is designed to be a tunable VCXO using something like a varactor diode.

From memory from last night, I think I measured about -80dBc/Hz at 10Hz and -117dBc/Hz at 100Hz for my 10MHz VCXO when locked. Probably close to average performance for a low cost 10MHz VCXO.

I'm not sure what the close in phase noise will be like on the 4046 VCO at 1MHz but it's going to be fairly noisy...
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Old 16th Jul 2017, 7:53 pm   #39
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Default Re: Off Air Frequency Standards....

I made a quick youtube video showing the lock characteristic of my old homebrew offair standard that I designed and built back in a bout 1991/2. The video also gives an idea how stable it is on a good day because I've used a fairly fast counter.

It isn't this good every day but this is fairly typical. At night the stability can suffer a lot and sometimes it won't lock at all. But this is fairly rare.

I think one of the caps in the loop filter has aged and degraded over the years because it used to only overshoot by about 6Hz. But now it is more like 18Hz and it seems less damped and takes longer to settle than it used to. But I'm still pleased with the performance.

https://www.youtube.com/watch?v=F5gN...ature=youtu.be
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Old 16th Jul 2017, 8:08 pm   #40
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Default Re: Off Air Frequency Standards....

It's difficult to make out the circuit of the G4FZH offair standard but I think it does use/lock the internal VCO as the 1MHz oscillator when the 198kHz Radio 4 signal is detected.

Obviously this must have worked well enough to include in a magazine but I'm surprised that a 4046 VCO would give good stability and noise performance. At the very least it would require very good quality external resistors and a highly stable capacitor to get the best from the RC based VCO. Also the +12V power supply noise and regulation would need to be good at the 4046 in order to prevent VCO stability and noise issues.

I'm also a bit confused by the way it selects between the VCO and the 1MHz xtal oscillator. I can see how the receiver detects the RF and emits a flag to enable the 4046 VCO via the NOR(?) logic gate but I'm not so sure how it manages the 1MHz crystal oscillator during all this. Does it turn it off when in offair mode or does it do something clever with it? I'm not sure the 'selection' logic looks right here but the circuit is very fuzzy. It must have worked for G4FZH so hopefully his original schematic is reproduced error free in the magazine itself.

I did look at the counter/divider connections and the /396 and /2000 circuits look OK to me.
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Last edited by G0HZU_JMR; 16th Jul 2017 at 8:35 pm.
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