[Fwd: LF: Coherent signal reception]
Tue, 18 Aug 1998 17:07:32 -0400
Andy Talbot wrote:
> >From G4JNT
> >ON7YD Wrote:
> >Another possible way to achieve coherent reception is to lock the TX and RX
> >oscillators to DCF77. Of course some things have to be sorted out first :
> >- is the phase stabilily at both RX at TX good enough (instability could be
> > caused by propagation)
> >- is DCF77 (at 77kHz) reception possible while transmitting at 137kHz
> >If on both sides (TX and RX) a signal with the exact same frequency and a
> >constant phase difference is available very weak signals can be picked up
> >using for example 'lock-in' techniques (used in scientific experiments for
> >years - signal many dB below noise can be detected if the integration period
> >is adequate). Detection can be done directly at 137kHz, so receivers could be
> >relative simple.
> There is no need for both stations to lock to DCF77. Here, the MSF signal
> on 60kHz is available and strong in most parts of the country. I published
> a design for a standard locked to this in the RSGB Magazine RadCom in April
> / May 1994 - built by quite a few people. I now have a much better - and
> simpler - MK2 design available that gives a 5 or 10 MHz output instead of
> the 12 MHz of the original. The original unit with a not too stable VCXO
> gave better than a few parts in 10^-8 short term stability over a few
> minutes (a slight audible 'wobble' to a CW tone at 10 GHz) and of course
> NPL standard long term accuracy. The new one, with an oven controlled
> oscillator in the VCXO, easily achieves 10^-9 and probably better - I
> haven't measured it properly.
> If we integrate a 137kHz signal over, say, a 100 second period the phase
> change for 10^-9 accuracy is 46 degrees which allows plenty of
> discrimination for Binary PSK. A few months ago I started to monitor
> transmissions of MSF and DCF compared to the Caesium standard but it is too
> time consuming and means leaving the computer dedicated to this for several
> days to get a scientifically reliable result.
> The measurements I did manage over one day each in May this year showed the
> 170km path from MSF to give about 30 degrees phase change diurnally, and
> over the day better than 10 degrees. DCF at something like 1000km,
> measured over a 6 hour period during the night gave 50 - 60 degrees shift -
> at roughly the same night and morning points as MSF. Interestingly, the
> power dropped by 5dB quite abruptly at 0400z accompanied by a brief 40
> degree wobble in phase lasting about 20 minutes.
> I'll try to do some more of these LF propagation tests in the near future
> but need to rebuild the PLL for locking my 56002 DSP clock to the frequency
> standard - the last one took an hour to lock up during which the DSP system
> had to be already up and running to enable the CODEC oscillator. Coupled
> with three hours warm up time for the Caesium standard which isn't left on
> continuously as its reached the end of its (official) life, these
> measurements are very time consuming.
> Plenty of opportunity for the coming autumn and winter months!
> Andy G4JNT / G8IMR