From G3PLX:
Just a couple of words about the PhaseScope program that Scott VE7TIL has
written, and the ClickLock technique at the heart of it. Some of this
repeats earlier postings, but I thought it might be worth summarising it
again.
The Clicklock technique stems from my observation that if I connect the 1Hz
pulse output of my GPS to my LF receiver, I can hear a repeating pair of
clicks at 1 sec intervals which are the high-order harmonics of the fast
edges of this pulse. Not only does this give me a precise time reference
which I can use if I want to do precision off-air timing measurements, but
if I feed the audio clicks to a phase-comparator, I can see the phase
rotating slowly with time. The rate of rotation is the 'beat-note' between
my receiver frequency and the nearest whole Hz. This 1Hz, coming from the
GPS module, is effectively derived from the most accurate source on the
planet (or at least orbitting the planet). Not only is it stable in
frequency but it can be used as a universal reference for phase measurement
on any frequency. If I use that to lock the software, I can then demodulate
incoming signals in such a way that ALL the residual frequency and phase
drift of the receiver is cancelled completely. If I tune-in a signal which
is also locked to GPS, and the propagation is stable, there will be no
frequency error and no phase drift. At all. Ever.
Scott has implemented this so that the received signal is displayed on what
we have christened an integrating vectorscope. This displays the signal
phase and amplitude simultaneously on a circular display. With no signal, a
dot appears in the centre of the display. If there is some noise present,
the dot moves randomly. With the program set to receive on a specific
GPS-locked frequency (which you can enter to as many decimal places of a Hz
as you like), if there is a signal on that frequency, the dot will start
moving off centre in a particular direction, this direction depending on the
RF phase of the signal relative to the GPS reference. The weaker is the
signal, the slower will the dot move, but, so long as the signal doesn't
change it's phase, there is NO LIMIT to how low in signal level you could
go. This is what we loosely refer to as 'coherent reception'.
For example, I could set the program to the frequency of a LORAN line from a
local transmitter (<1000km), and the dot will move off towards the edge of
the scope. The program is locked to GPS, the LORAN transmitter is stable in
phase, so the direction of movement of the dot (the RF phase of the received
signal) always stays the same. If I repeated the experiment on another day I
would get exactly the same phase reading. I could have left the system on
all day and detected a signal 24 times weaker than if I had left it running
for an hour. If I move 1/4 wavelength closer to the transmitter I would see
the phase change by 90 degrees, even if I switched-off the receiver and the
computer completely during the move.
This opens up an awesome set of possibilities for really weak-signal
reception. Many transmitting stations are already able to transmit
phase-locked to GPS. This technique means we can explore all the
possibilities for coherent transmission and reception, with just an LF
receiver which is already stable to 1Hz, and a GPS module with a 1Hz output.
Scott's program is just an appetizer. Both he and I will make this technique
available to anyone wishing to develope it for amateur use.
73
Peter
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