On Fri, 2008-09-05 at 11:58 +0100, Steve Hunt wrote:
> Thanks Lee and Rick for your suggestions.
>
> I solved the problem this morning - I hardly dare admit that it was
> mostly "technician error"!
>
> I decided to check the obvious, and measured the final 20m LO injection
> frequency. It was in error by 130Hz - which handily ruled out the BFO
> frequencies. I checked the LO injection frequency on 2MHz, 14MHz and
> 30MHz and discovered that the offset was not fixed - it varied in
> proportion to the LO frequency. So my earlier posting was rubbish - I
> hadn't checked carefully enough!
>
> This pointed to the TCXO being slightly off; so rather than trust my
> (now suspect) frequency counter I tuned in a 40m broadcast band AM
> station which was playing music. Assuming the station would be on a
> "1KHz multiple", I tuned the Omni so that the last two digits were
> zeroes and then adjusted the TCXO until the music sounded OK. I find
> this a much more accurate way of detecting "zero beat", because the ear
> is very sensitive to any frequency error in musical harmonics.
>
> So everything is now fine. But I'm left with the problem that, without a
> really accurate frequency reference, I could still be slightly out on
> the TCXO and be compensating with a slight shift on the BFO. Which leads
> me to ask if anyone knows of a reasonably priced off-air frequency
> standard which would give me something against which to calibrate my
> test gear for the future?
>
> 73,
> Steve
>
CHU on 7335 has pretty good accuracy. There may still be a British,
continental or Russian version of WWV or WWVH on 5, 10, and 15 MHz with
super transmitted accuracy but on HF it takes a month of averaging to
get down to 0.1 ppm. There's still Rugby England on 198 KHz with very
good accuracy, I think and on packet radio there's a regular bulletin
about creating your own local frequency standard from receiving them.
Sometimes the sweep frequency of analog TV is controlled to parts in the
10^-9 accuracy, unfortunately in the US the standards are at the local
stations now, not just at network HQ, so there are phase jumps between
local sources and the network standard. Shortwave BC stations may have
pretty good accuracy, or be off from an agreed to frequency by several
KHz. Some short wave stations in less regulated countries may be rogues
or pirates.
One of the best standard signals in the US is WWVB at 60 KHz, but that
takes a good antenna and a TRF receiver to make use of its precision. I
use it with a good oscilloscope to set my local standards to parts in
10^-9 which is their stability limit. I trigger the scope sweep with 100
KHz or 1 MHz from my house crystal standard and put the received signal
on the vertical. That gives multiple traces, like the three phases of
three phase AC power. I speed up the sweep to maybe a microsecond per CM
and the vertical gain so I can pick out a crossing of a couple of those
sweeps and monitor its motion in time, ignoring noisy periods and times
of reduced power from WWVB (10 db amplitude keying once per second).
When I have the local oscillator down to moving that intersection less
than 0.1 microsecond in 15 minutes I've achieved 10^-9 adjustment. It
takes a good oscillator to achieve that stability and at that point I'm
suspicious of short term propagation changes because the speed of light
is about 1 nanosecond per foot and holding to within 100 ns is holding
to 100 feet of propagation distance as well as the suspended vertical
wire at the transmitter.
My VLF receiver is a modified one made by Florida Scientific or some
such company. I was able to modify its tuning range by adding small
toroids to one tuned position to raise its maximum frequency from 25 to
60 and 100 KHz.
Then there is Loran C at 100 KHz which I've never succeeded in using for
frequency standard though its oscillators have very good precision also.
73, Jerry, K0CQ
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