Refugee wrote:I have for some time been on the lookout for an off air standard.
Refugee wrote:I have for some time been on the lookout for an off air standard. As far as I can see these things receive the same signal the BBC use for the pips before the news on radio 4. [...] A 10Mhz clock is divided down and phase locked to the pips themselves.
Refugee wrote:The frequency standard I used when I worked in a calibration shop locked on each pip and when the internal oscillator was slightly out the trace on the scope used to drift and then pull back with each pip.
Cathovisor wrote: In my standard, there is an oven-controlled crystal that is compared to the carrier of Radio 4 LW. Radio 4 is received and then passed through a crystal filter to remove the modulation, the recovered carrier then being compared to the output of the crystal.
Cathovisor wrote:You'll find pretty much anything off-air uses one of three sources:
Anthorn carrier (60KHz)
Droitwich carrier (198KHz)
... If making a home-brew, I'd be inclined to go for Anthorn as a reference as you an easily derive a 10KHz square-wave from its carrier to pull say, a 10MHz VCXO with. And a suitable TRF receiver would be a trivial matter to make....
Terrykc wrote:Cathovisor wrote: In my standard, there is an oven-controlled crystal that is compared to the carrier of Radio 4 LW. Radio 4 is received and then passed through a crystal filter to remove the modulation, the recovered carrier then being compared to the output of the crystal.
Is that really necessary? I know that modulation will introduce phase changes in the carrier and there is (or used to be) a low level FSK data stream on the Droitwich carrier - although I can't remember what it was used for - but surely a PLL with a fairly long time loop time constant will average out all of these unwanted perturbations without the need for a crystal filter and ensure that the crystal oscillator remains locked to the mean carrier frequency which is, after all, what is wanted.
Cathovisor wrote:<Goes off, looks at MSF carrier format>
ppppenguin wrote:Some mdoern counters use reciprocal counting (swap the uknown and the gate) to give good resolution of low frequencies without excessive gate times.
Refugee wrote:Perhaps if a keen moderator wants to tidy up we could bump the original thread for the counter with all the extra replies about the counter here.
ppppenguin wrote:How on earth do you do calculate a reciprocal without a CPU or a decent size PROM to make a look-up table.[...]
It may also simplify things if you have a very restricted range of frequencies to cover, as in a mains frequency meter. That may take it into diode array territory.
That Dawe counter must have been expensive.
Refugee wrote:Nixie tube kit is always nice and also well built.
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