LF: Coherent signal reception]

André Kesteloot akestelo@bellatlantic.net
Tue, 18 Aug 1998 17:06:21 -0400

Rik Strobbe wrote:

> > From G4JNT
> >
> > Marco - you mentioned that you could use a Rubidium standard for future
> > tests - I have a Caesium standard and this opens up new possibilities of
> > coherent reception.  If you can achieve 10^-9 (usually the spec limit for Rb
> > sources - usually a lot better in practice) this means that your phase as
> > monitored here will shift at the most 360 degrees in 2 hours.  This makes
> > PSK, even generated by using a changeover relay in the antenna feed, simple
> > to implement for data rates of a few seconds per bit.  I may be able to
> > detect this in real time, but others could save the data to a .WAV file via
> > a Soundblaster card and digitally downconvert to zero IF to receive the
> > signal.
> >
> > Is there anyone else with stabilities of 10^-7 or better who could try
> > coherent tests ?
> >
> > Andy  G4JNT / G8IMR
> A big advantage of our LF band is the low frequency, making receiving
> techniques possible that are much more difficult HF and up.
> One of these techniques is coherent reception, I have been playing with the
> idea for a while. But only very few will have acces to Rubidium or Caesium
> frequency standard (and unfortunately not me).
> 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.
> For the moment I have a lot of other problems to tackle at LF (eg. getting
> heard in the UK) but if someone wants to start some tests in this direction I
> am always willing to participate.
> 73, Rik  ON7YD