LF: Fw: [psk31] Lowfer/Medfer PSK31]

[Andre' Kesteloot]

Dick Rollema wrote:

> -----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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> >>
> >