[amsat-bb] Operating PSK-31 through PCSAT2

["Robert Bruninga" <bruninga@xxxxxxxx>]

Here is the scoop from the inventor of PSK-31 on
how best to use the PCSAT2 PSK-31 transponder.

It gets word-wrapped here, but I posted a clean
TXT copy on:  
http://www.ew.usna.edu/~bruninga/pec/PC2PSKnotes.txt

Bob
>>> "Peter Martinez" <peter.martinez@btinternet.com> 08/17/05 6:12 AM >>>
>From Peter G3PLX:

Taking up Bob's point about the fastest routes to getting active via the 
PSK31 transponder, here are a few things that I think will help the 
first-time user.

1) The transponder downlink audio is not pre-emphasised. That is, the higher 
audio frequencies are not boosted like they are in conventional voice FM 
transmitters. Most FM receivers have de-emphasis in the receive audio, and 
this means that the end-to-end frequency response of the system - from the 
transponder receiver output to your soundcard input - will droop at the HF 
end if your FM receiver is of the conventional type. If your FM transceiver 
has a direct output from the discriminator which bypasses the de-emphasis 
(sometimes labelled as being suitable for 9600 baud packet reception), then 
use this one to feed the PC soundcard, and that will give a flat response 
overall. If you can't do this, then bear in mind that downlink signals at 
the top end of the audio band will be lower in audio level than those at the 
bottom, but not necessarily further down in the background noise.

2) The PCSAT2 transponder receiver 'dial frequency' is 29401.1 kHz. By this 
I mean that if the ISS was stationary and you transmitted to it on that 
frequency, you would (hypothetically of course) hear a zero-beat in the 
output.

3) The transponder receiver audio frequency response is centred on 1650Hz, 
and extends down to 300Hz and up to 3000Hz. The receiver RF passband is 
therefore 29401.4 to 29404.1 kHz.  It's not too steep at the edges, so these 
figures are really just a guide.

4) If you wanted to sit right in the centre of the passband, you would set 
your transmitter dial on 29401.1 kHz and set the PSK31 transmit audio tone 
at 1.650 kHz, or any combination of the two that results in their sum being 
29402.75 kHz.  If the ISS was stationary, this would result in your uplink 
signal on 29402.75 kHz appearing right in the middle of the downlink audio 
band at 1650Hz.

5) Because of Doppler shift, if you do transmit as in (4) above, your 
downlink signal will start the pass higher than 1650 Hz, by as much as 
700Hz, and end up low by the same amount.  From this you can see that 
there's room to spare. You could transmit as high as 29403.4 or as low as 
29402.1 and your downlink signal would always be within the 300-3000 Hz 
range during a worst-case (overhead) pass.

It's much better to calculate where you propose to transmit and stay there, 
than to discover, halfway through the pass, that your signal is drifting off 
the bottom of the downlink and you have to QSY up. It's also much easier for 
other stations to receive you!

6) The transponder receiver has AGC, and if you transmit too much uplink 
power, it will reduce it's gain and keep the downlink audio level constant. 
This will prevent lower power signals getting through. It's therefore very 
important that you know how to recognise the signs of overdrive, but you 
will not be able to do so by ear. Set up some way to display the downlink 
audio visually. Even a simple audio voltmeter, or recording level meter, or 
a scope will be enough, but some people may want to use a soundcard spectrum 
analyser program, perhaps on a separate computer screen, so that they can 
pick out individual stations in the downlink. The aim is to learn how to 
recognise when YOUR signal is dominating the downlink, and to then reduce 
power. The transponder downlink deviation is about 800Hz when the AGC is 
operating, so it's possible to calibrate the downlink level display so you 
can know precisely how hard you are driving the transponder. Later on, we 
need to decide the best way to share between multiple users, but for the 
moment it's enough to know that you only need to have a few hundred Hz 
downlink deviation to be 100% readable on PSK31. As a guide, on the first 
tests I found that I only needed 2 watts into a 5/8 wave vertical to give 
500Hz downlink deviation (and no AGC) when the satellite is at a range of 
500km at 45 degrees elevation.

While activity is still low and while we are still learning, I think the 
best strategy for a first-time user is to set up the transmitter frequency 
and power before the pass, leave the transmitter on continuously with a 
repeating short identification message, and work full duplex. Check that you 
can copy your own signal early in the pass before the Doppler drift gets too 
fast. Don't waggle your transmitter frequency around, leave it steady and 
let it drift. Spend your time finding and following other signals, 
concentrating on those where the Doppler shift is getting less and less, 
which you stand a better chance of copying just before they vanish. Don't 
try too hard to get a two-way QSO at the start. This really isn't easy 
without Doppler correction both ways.

Later on, uplink-corrected PSK31 software will be available and things will 
get much easier when we can all adopt this technique. It will be possible to 
treat the downlink as if we were all working full duplex on a dedicated 
'0.3-3.0 kHz' band, with no Doppler shift apparent to anyone after the 
initial signal aquisition. It is potentially even better than an HF band 
because we have the remarkable ability to set all the downlink signals to 
the same level, and this means we could reliably work at much smaller 
channel-spacing than we do on HF.  This linear/FM transponder technique is 
going to be very interesting indeed.

73
Peter


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