Off Air Frequency Standards....
 

Over the years there have been various postings on the subject of Off-Air Fequency Standards on this Forum but those that I have read seem to be unresolved. Many years ago I invested a lot of time and money building a 198kHz off-air frequency standard from the RSGB publication "Test Equipment for the Radio Amateur" but neither me nor a very knowledgeable work colleague could get it to work.

Has anyone on this Forum has built this equipment and if so, did they get it to work? The concensus was that its very simple unfiltered FET receiver was perhaps not up to the job as far North as my home near York at that time .

Alan - G3WXI

Re: Off Air Frequency Standards....
 

I'm looking at this now. I just got my hands on a counter with an OCXO in it which needs calibration. Looks like GPS is the way to go now. Some of the modules have a 10KHz or 1 PPS output which can lock a PLL at your chosen standard frequency.

There are a number of solutions though for 198KHz LW broadcast. This one looks sensible as it has some awareness of the phase modulation: http://www.qsl.net/pa2ohh/07freqstd.htm

The implementation of the frequency standard in the book you mentioned looked hooky to me (I have a copy of that)

Re: Off Air Frequency Standards....
 

Spectrum Communications Off Air Frequency Standard £61.

http://www.spectrumcomms.co.uk/Test.htm

Just remember that the 198 kHz signal won't be on the air indefinitely...

Re: Off Air Frequency Standards....
 

If you don't want to go down the GPS route what about buying one of the used professional quality OCXO online for around £15-20 like the Trimble and build it in a box with a psu and calibrate it against a GPS or Rubidium standard I think quite a few people on the forum have these now.
Chris

Re: Off Air Frequency Standards....
 

Probably best to post up the circuit of the RSGB version.

Back in about 1991/2 I designed my own offair standard for 198kHz for use at home. Back in those days I still did the odd repair for CB/ham operators and the accuracy of my old Marconi TF2430 counter was often questioned even though I used to check it at work now and again.

My target for accuracy at the time was to get within +/- 1Hz of 10MHz using the homemade standard but I later found out it was much better than that. I'm lucky in that I'm quite near Droitwich and on a good day it will stay stable within about +/- 0.03Hz on a fast counter allowing for the odd rogue count.

Evening time stability is not so good and varies a lot. Some nights it won't lock at all although this is rare. My design was quick and dirty using junk box parts, the only exotic item was the programmable divider. I used a 4059 programmable divider. This can be set to divide by 99 by setting logic levels using resistors on the 4059.

I used a (balanced) ferrite rod antenna, then a JFET RF amp followed by a narrow LC BPF then a BJT amp followed by a NE604 limiter and finally a comparator. I used a classic 4046 phase detector and 74HC390 chips to divide the 10MHz oscillator. I designed the 10MHz VCXO using discrete parts and the whole thing was built in an evening and it worked!

It takes maybe 40 seconds to lock up but this depends on conditions. If I was in York I'm not so sure it would work this well. It might not even work at all. I kept meaning to make a tidy version of it and it lived in an old shoebox for about 20 years and was a fragile mess in terms of construction as it was just stored in the box rather than fitted to a box. However, I upgraded it to a plastic ice cream carton a few years ago and tidied up the dead bug construction and fitted the PCBs and the antenna securely and even added an S meter.

Best used during the day and in my opinion a 198kHz offair std should be good enough for nearly all of us. The advantage it has over GPS is that it can be switched on and locked within 30-40seconds. It's not going to match GPS for ultimate accuracy by a couple of orders of magnitude but GPS can take ages to lock and some people resort to running the GPS system 24/7.

See below for an old plot of the stability of the 10MHz output taken over several minutes on a typical day.

Re: Off Air Frequency Standards....
 

This is what made me look at GPS. There are also rubidium standards available for not terrible amounts of money if you can afford to wait a few months and keep your eyes peeled on eBay. I saw one go last year for only £65!

Re: Off Air Frequency Standards....
 

They have limited life expectancy, so with a second-hand rubidium source you may want to check the running hours data.

David

Re: Off Air Frequency Standards....
 

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!

Next came various Mks of Droitwich standards, the most successful of which worked by receiving 198 Khz with a ferrite rod antenna+ preamp, into further amplification via a narrow LC filter block and a two-stage limiter/clipper, which produced a nice 198 KHz square wave which went into a f/99 divider producing 2Khz for the phase detector.
The other input to the PD came from the 10 MHz xtal osc divided by 5000 (result 2KHz). Output from the PD tweaked a varicap in series with the xtal.
A nice long time constant was necessary to iron out the phase mod on the R4 signal. It took about half a minute to lock and a further half a minute to fully settle.
It worked very well, and even listening on a harmonic around 1 GHz it had only a very slight wobble. Details of this can be found on my website under 'Droitwich Mk2 and Mk3'. The site is slowly falling apart as I haven't paid my subs for the last year and I can no longer edit it! So sorry if pics, diagrams etc are missing. http://www.freewebs.com/g1hbe/

These days I just use an old ovened 10 MHz standard whipped from a scrap HP sig gen. According to my Macaroni 20 GHz counter it stays about +300 Hz at its 100th harmonic, or 0.3 Hz at 1MHz. Good enough for me!

Re: Off Air Frequency Standards....
 

Good point. I think the ones I saw were new old stock. Do they have a shelf life as well or is it just operational life?

Re: Off Air Frequency Standards....
 

Someone reckons a lot of Rb sources fail not from Rb exhaustion, but often circuit board components dying from heat, and Rb collecting in the wrong place:

http://www.n4iqt.com/efratom8084008/...bly_repair.pdf

Worth a try if the worst happens.

David

Re: Off Air Frequency Standards....
 

Interesting. Will keep an eye out for one of them on that basis.

Re: Off Air Frequency Standards....
 

A few years ago I bought an Efratom LPRO-101 Rubidium frequency standard module for about £40. There were loads available at the time, I believe from decommissioned mobile phone or data communication networks. They may still be around. It works fine, though the 'lamp voltage' output, which apparently indicates the health of the Rubidium magic, is towards the lower end of the acceptable range. That doesn't matter for me because I only switch it on for a few hours every now and again.

I have never tried comparing it with a GPS-disciplined oscillator or any other reference source, and I'm not honestly sure how accurate it is. Apparently Rubidium sources are good for low drift once set, but I don't know whether that requires them to be kept running 24/7.

My other frequency 'standards' are the chunky-looking OCXO in my HP 5245L counter (nixie tubes, yay), which has a very impressive-looking crystal inside, and the HP 10544A OCXO in my 5335A counter. I generally leave the counters plugged in 24/7 so the ovens are always up to temperature. Very occasionally I compare the Rubidium source and the two HPs against each other and they're never more than 1Hz out at 10MHz which is good enough for what I do.

Chris

Re: Off Air Frequency Standards....
 

Would be nice if we had a 10MHz WWV equivalent in the UK. That would make life much easier. No PLL required - just a simple receiver required to extract the carrier so you can compare it to the reference with a scope.

Re: Off Air Frequency Standards....
 

With a reasonably modern reciprocal counter you could make a simple receiver for 198kHz, pass it into a limiter and then measure the frequency with the counter. Then adjust the counter internal reference to get dead on 198kHz on the display on a slow measurement time. This would give good enough accuracy for most people and you don't need a PLL for this either :)

It would be interesting to see the circuit for the RSGB "Test Equipment for the Radio Amateur" offair standard to see if there are any issues with it that might affect its performance.

Re: Off Air Frequency Standards....
 

I have seen a radio spectrum listing but have forgotten where.
It showed all the wavebands with in the gaps between them timekeeping transmissions.
One of these will almost certainly be the one used by self adjusting clocks.
If we can find one of those it will be good for locking a PLL to make an off air standard.

Re: Off Air Frequency Standards....
 

Quick "phone scan": http://**********/a/UnXJN

I've used imgur instead of the forum as the images are rather large and not possible to view at the reduced size the forum requires.

Edit: first image appears to be unrelated. Not sure what I was doing there! :D

Re: Off Air Frequency Standards....
 

Thanks! I had a quick look and it's an old school/interesting design approach, but it looks to be designed for the old Droitwich frequency of 200kHz. So at best any analysis of this circuit is going to be academic today unless you could find a 990kHz crystal and were happy to create a 990kHz locked reference (198kHz x5 =990kHz) as opposed to 1MHz (200kHz x5 =1MHz).

Even then, it might struggle to work (without some circuit mods) because of the subtle close to carrier phase modulated signals that are on the modern Droitwich signal. I think this is some form of Teleswitch service but I know very little about what subtle close to carrier signals are hidden on the 198kHz transmission today.

Sounds like this was a different circuit to the circuit linked to by MrBungle... Did the RSGB release an updated version for 198kHz?

Re: Off Air Frequency Standards....
 

Mine is a relatively old copy circa 1978. There may be a different version in a later book.

I think the phase modulation was used for economy 7. Not sure.

Re: Off Air Frequency Standards....
 

Not found a updated version for 198kHz, but I have found a Radcom article for modifying a 200kHz receiver to 198kHz. I found it entirely by accident searching to see if anyone had found a manual for the HP 5090B receiver that I have (I did find a HP 5090A manual which will help me with the modification), the article starts on page 74 of the Radcom magazine November 1989 edition;

https://archive.org/details/RadCom_Magazine_1989-11

David

Re: Off Air Frequency Standards....
 

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.

I have sometimes thought about making a 198kHz receiver which will mix a 2kHz signal to get 200kHz then a PLL locks to this to reject the 196kHz sideband. Divide the 200kHz by 100 to get the 2kHz. Minor bootstrap problem, as you need 2kHz to get 200kHz to get 2kHz but all oscillators have a bootstrap problem!