Jerry,
Thanks for the info.
I made final adjustments this afternoon by fishing out an old GC
receiver and using it to receive WWV on 10MHz. I held a piece of wire
onto the Omni TCXO divide-by-two chip output, and dangled the other end
near the GC receiver. Using AM detection I was able to adjust the TCXO
to within about 1/2Hz by listening to the "flutter".
Then I listened to the 40m broadcast band music station and adjusted the
BFOs.
I spotted a simple construction project which uses a ferrite rod antenna
and a high gain TRF chip to produce a 198KHz output from the BBC
Droitwich transmitter. That would form a crude, but useful, off-air
standard. Droitwich used to be 200KHz, which made it simple to produce
other reference outputs, but they changed to 198KHz some years ago when
AM stations moved to 9KHz channel spacing.
Thanks again,
Steve G3TXQ
Dr. Gerald N. Johnson wrote:
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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