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From: "Dick Rollema" <d.w.rollema@gironet.nl>
To: "LF-Group" <rsgb_lf_group@blacksheep.org>
Subject: LF: Fw: [psk31] Lowfer/Medfer PSK31
Date: Fri, 15 Oct 1999 14:28:26 +0200
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-----Oorspronkelijk bericht-----
Van: G.J.Huijsman <gjhuijsm@gironet.nl>
Aan: Dick Rollema <d.w.rollema@gironet.nl>
Datum: donderdag 14 oktober 1999 11:46
Onderwerp: Fw: [psk31] Lowfer/Medfer PSK31 


>
>-----Oorspronkelijk bericht-----
>Van: Peter Martinez <Peter.Martinez@btinternet.com>
>Aan: psk31@aintel.bi.ehu.es <psk31@aintel.bi.ehu.es>
>Datum: woensdag 13 oktober 1999 21:06
>Onderwerp: [psk31] Lowfer/Medfer PSK31 
>
>
>>>From Peter Martinez G3PLX
>>
>>Congratulations to Clint KA7OEI on the hardware implementations of
>>PSK31 for low-frequency work. Personally I think the hardware
>>implementations of PSK31 have been rather neglected. Perhaps this is
>>a side-effect of the software version being cost-free!  Some
>>interesting applications of PSK31 have probably not yet seen the
>>light of day as a result. Clint's work is valuable in this respect.
>>
>>Here's a few of my own ideas for exploring the PSK31 scene with
>>hardware for VLF and other areas:-
>>
>>1. Transmitting "squarewave" PSK31, although it would be considered
>>very antisocial on HF radio, is a valid way of doing it when we are
>>not concerned with adjacent-channel QRM. I am thinking of underground
>>induction field communication at VLF, and other non-radio
>>applications where we are fighting the noise level and not the QRM. A
>>Class C power amplifier, designed more like a switchmode PSU than a
>>transmitter, fed with PSK31 generated with an XOR gate, will be ideal
>>for this task. Be careful that if this kind of transmitter is
>>connected to a very high Q VLF antenna, there can be disastrous
>>effects when the transmitter reverses phase and the high level of
>>stored energy in the antenna circuit is dumped back into the
>>transmitter! Check the reverse ENERGY capability of the transmitter
>>rather than the reverse POWER rating. This probably doesn't apply to
>>USA lowfer activity, but could apply to the European 136kHz
>>allocation where you might have a 1 kW transmitter to radiate the
>>permitted 1 watt e.r.p.
>>
>>2. Note that if the transmitter is hard-limited in this way, the
>>receive filter in the current software PSK31 receive-side is not
>>quite optimum, since this is matched to the cosine envelope of the
>>software PSK31 transmit-side. Slightly better results (by which I
>>mean performance in Gaussian noise and not adjacent-channel
>>rejection) can be achieved with a filter matched to the square-wave
>>transmit envelope. The classic "Integrate and dump" filter (or the
>>equivalent using analogue delay lines) is right for this task, and is
>>quite easy to build in hardware. More good reasons to experiment with
>>simple hardware PSK31 receivers.
>>
>>3. On HF the PSK31 receive signal is demodulated incoherently, by
>>comparing the phase of one symbol with the previous one. The
>>transmission is encoded differentially to match this, but we can gain
>>an extra 3dB of weak-signal performance if the path is stable enough
>>to let us demodulate coherently. We don't even need to reconfigure
>>the transmission coding to do this, since we can get the same answer
>>as differential incoherent demodulation by using coherent
>>demodulation followed by differential decoding. VLF applications,
>>whether radio or induction-field, are certainly stable enough. The
>>only penalty will be that it will take a little longer to lock in at
>>the start of a transmission, but we could easily switch between
>>incoherent and coherent if that's a problem. This is another area for
>>exploration with simple hardware demodulators.
>>
>>4. Although QPSK is noticably better that BPSK on HF where there is
>>QSB and burst noise, when the limiting factor is Gaussian noise, BPSK
>>is better. This could mean that simple BPSK may be the best choice
>>for some non-HF applications of PSK31. The Lowfer/Medfer area might
>>suffer from atmospheric static which could mean QPSK had the
>>advantage there, but for underground reception of VLF, or laser
>>working, or perhaps even underwater acoustics, hard-limited BPSK
>>transmit and coherent integrate-and-dump would be my suggestion for
>>the best way to go.
>>
>>73
>>Peter
>>
>>
>>+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
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>>
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>>+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
>>
>