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Re: PSK-31 Satellite Donwlink Opportunity?

Yes, by making the UPLINK FM receiver have a widebandwidth of say 40 KHz,
then even +/- 10 KHz of normal UHF doppler stays in the passband.  THus
the recovered audio is unaffected by uplink doppler.  THis audio then
simply goes into a normal 3 KHz SSB downlink.

Again, this is only a quick-demo way of getting a low doppler multi-stream
PSK-31 downlink to "test the waters".  THe ultimate objective for the
"next" satellite would be the INBAND-INVERTING transponder so that then
you can have 20 independent uplink QSO's... with only 45 HZ total

I have posted the design of the INBAND-INVERTING transponder on

Problem is, I havent figured out how much isolation I can get in such a
tiny space... yet...


 On Sun, 25 Feb 2001 ka1lm@amsat.org wrote:

> On Sun, 25 Feb 2001, MCGWIER ROBERT wrote:
> > You have forgotten the contribution to Doppler caused by the uplink.  
> > Even with an inverting transponder, and practically no frequency
> > tracking in most of the PSK-31 implementations <<that I know about>>,
> > this will be a difficulty.
> Bob:  
> Sticking my neck out, as I have *NO* experience designing receivers or
> other flight hardware... 
> WB4APR said:
> > > On this short fuse experiment, we would simply put 20 PSK-31 audio
> > > streams into a single UHF FM uplink having an onboard wideband (no
> > > doppler) receiver.  THus, the only doppler is the small 600 Hz
> > > experienced on the 10m SSB downlink.  The PSK-31 software on the ground
> > > can EASILY track this, since ALL QSO's move together!
> I think WB4APR Bob's statement about doppler shift is correct *if* the UHF
> uplink receiver is designed as follows:
> The receiver is FM.  Use the discriminator output to drive an AFC circuit.  
> Assume the UHF receiver and AFC circuit has sufficient bandwidth to
> accomodate the audio stream and +/- 9 KHz of doppler shift.  Assume the
> uplink has sufficient ERP to overcome the greater noise in an uplink wide
> enough to meet this requirement (say 20-30 KHz total).  Convert the signal
> to baseband audio before being fed back into the transmitter.
> I think this gives us a no-tune uplink.  The uplink receiver is handling
> doppler tuning via AFC. 
> I recall that the AO-27 uplink uses this approach.  Doppler shift on 145
> MHz is +/-3 KHz, so the total receiver bandwidth can be smaller and
> therefore provide a better SNR.  I **think** (8+ years is a long time) the
> Microsat uplinks use the same approach, albeit again on 145 MHz instead of
> 435 MHz.
> Bob (WB4APR), how do you intend to "process" the uplink - audio bent pipe?
> It seems to me that other than a wider receiver bandwidth due to doppler
> and therefore greater noise in the uplink, the approach is sound.  What do
> you expect the bandwidth to be of the combined uplink signal? (no
> experience with PSK31 here.)
> 73 Steve KA1LM
> -------------------------------------------------------------------------
> Stephan A. Greene                            sgreene@patriot.net
> Amateur  KA1LM@amsat.org  Grid FM18hx        38 59'83.33"N 77 23'6.15"W
> -------------------------------------------------------------------------

de WB4APR@amsat.org, Bob

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