[Fwd: LF: Coherent working]
André Kesteloot
akestelo@bellatlantic.net
Fri, 21 Aug 1998 22:11:03 -0400
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Peter Martinez wrote:
> >From Peter Martinez G3PLX Kendal Cumbria.
>
> I have been experimenting with coherent low-level reception for
> a while. I used it to receive all the Omega transmissions on
> 10.2 kHz before they closed down, except Hawaii which is right over
> the North Pole. This was done using a lock-in technique to get my
> receive local oscillator locked in phase to the Norwegian transmitter
> which was the only one strong enough to hear normally, and also
> used the Norwegian transmission to synchronise with the 10 sec
> timing cycle. To receive the others, I then just integrated the noise
> for as long as it takes to build up a good SNR. The limit was that if
> the diurnal phase shift is more than about 90 degrees, then I cannot
> integrate for more than a day!
>
> I used exactly the same idea to measure the LORAN spurious,
> receiving coherently with the 5399th upper sideband of the 6.667Hz
> pulse frequency (the spectral line on 135998.1331 Hz),
> synchronising to the strongest pulse group.
>
> By the way, there IS a common misunderstanding about the
> difference between the terms "coherent" and "synchronous". This
> misunderstanding was made worse by the so-called "Coherent CW"
> system which was only a synchronous technique! Synchronous
> means having my receiver decoder running in step with your
> transmitter encoder, which only helps because I know when you are
> sending and doesn't do anything for signal-to-noise ratio, while
> coherent means having my receiver RF cycle running in step with
> your transmitter RF cycle, which means that I really CAN get 3dB
> more SNR if you send twice, no matter how weak you are.
>
> We could use both synchronous and coherent techniques for
> amateur contacts. For the synchronisation, there are plenty of time
> signal broadcasts sending 1 sec marks, or GPS, or even
> free-running crystals will be good enough: 10ppm is less than 1 sec
> per day, so we could use synchronous CW with 5 sec dots all day.
>
> On the frequency-locking side, external references or Caesiums
> would be nice, but not neccesary. Once enough signal has been
> collected to tell that there is a signal there, using the narrow-band
> FFT techniques we already have, we can lock to it. In a two-way
> contact this lock-in trick is only needed in one direction, as the
> "slave" station can transmit locked to the received signal from the
> "master". The station with the biggest erp would be chosen as
> master of course.
>
> Here's an idea for a simple protocol to allow two stations to work a
> kid of coherent telegraph link which should work right down to the
> diurnal phase-shift limit:
>
> Station A transmits in the first 5 seconds and B in the second 5
> secs of every 10 sec interval, locked to utc, but the phase of each
> transmission stays coherent through the receive periods and
> reception stays coherent through the transmit period. This
> 10-second cycle repeats for the whole contact but is only to allow
> full-duplex and isn't part of the protocol.
>
> The transmitter is retarded in phase to send a zero, and advanced in
> phase to send a one. No phase-change means "I don't have enough
> signal from you yet to tell which of the three possible phase-jumps
> you are sending". We would probably choose 120 degree jumps.
>
> With this simple idea, two stations can exchange data at a
> maximum of one bit in each direction every 10 secs, and if the
> signal in either direction is not strong enough to give a clear
> indication within that time, the protocol just keeps collecting signal
> until it does. At the start both stations send steady bursts until they
> have locked to each other.
>
> 73
> Peter
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From: Peter Martinez <Peter.Martinez@btinternet.com>
Subject: LF: Coherent working
Date: Thu, 20 Aug 98 10:38:50 +0100 ( + )
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>From Peter Martinez G3PLX Kendal Cumbria.
I have been experimenting with coherent low-level reception for
a while. I used it to receive all the Omega transmissions on
10.2 kHz before they closed down, except Hawaii which is right over
the North Pole. This was done using a lock-in technique to get my
receive local oscillator locked in phase to the Norwegian transmitter
which was the only one strong enough to hear normally, and also
used the Norwegian transmission to synchronise with the 10 sec
timing cycle. To receive the others, I then just integrated the noise
for as long as it takes to build up a good SNR. The limit was that if
the diurnal phase shift is more than about 90 degrees, then I cannot
integrate for more than a day!
I used exactly the same idea to measure the LORAN spurious,
receiving coherently with the 5399th upper sideband of the 6.667Hz
pulse frequency (the spectral line on 135998.1331 Hz),
synchronising to the strongest pulse group.
By the way, there IS a common misunderstanding about the
difference between the terms "coherent" and "synchronous". This
misunderstanding was made worse by the so-called "Coherent CW"
system which was only a synchronous technique! Synchronous
means having my receiver decoder running in step with your
transmitter encoder, which only helps because I know when you are
sending and doesn't do anything for signal-to-noise ratio, while
coherent means having my receiver RF cycle running in step with
your transmitter RF cycle, which means that I really CAN get 3dB
more SNR if you send twice, no matter how weak you are.
We could use both synchronous and coherent techniques for
amateur contacts. For the synchronisation, there are plenty of time
signal broadcasts sending 1 sec marks, or GPS, or even
free-running crystals will be good enough: 10ppm is less than 1 sec
per day, so we could use synchronous CW with 5 sec dots all day.
On the frequency-locking side, external references or Caesiums
would be nice, but not neccesary. Once enough signal has been
collected to tell that there is a signal there, using the narrow-band
FFT techniques we already have, we can lock to it. In a two-way
contact this lock-in trick is only needed in one direction, as the
"slave" station can transmit locked to the received signal from the
"master". The station with the biggest erp would be chosen as
master of course.
Here's an idea for a simple protocol to allow two stations to work a
kid of coherent telegraph link which should work right down to the
diurnal phase-shift limit:
Station A transmits in the first 5 seconds and B in the second 5
secs of every 10 sec interval, locked to utc, but the phase of each
transmission stays coherent through the receive periods and
reception stays coherent through the transmit period. This
10-second cycle repeats for the whole contact but is only to allow
full-duplex and isn't part of the protocol.
The transmitter is retarded in phase to send a zero, and advanced in
phase to send a one. No phase-change means "I don't have enough
signal from you yet to tell which of the three possible phase-jumps
you are sending". We would probably choose 120 degree jumps.
With this simple idea, two stations can exchange data at a
maximum of one bit in each direction every 10 secs, and if the
signal in either direction is not strong enough to give a clear
indication within that time, the protocol just keeps collecting signal
until it does. At the start both stations send steady bursts until they
have locked to each other.
73
Peter
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