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Re: PSK-31 Satellite Donwlink Opportunity?



I get it.  Assuming a large uplink to overcome
the threshold detection problems associated
with FM will do it.  I missed the FM on the uplink.

Bob

----- Original Message -----
From: <ka1lm@amsat.org>
To: "MCGWIER ROBERT" <rwmcgwier@home.com>
Cc: "Bob Bruninga" <bruninga@usna.edu>; <digitalradio@yahoogroups.com>;
<carcomm@egroups.com>; <psk31@bipt106.bi.ehu.es>; <amsat-bb@amsat.org>
Sent: Sunday, February 25, 2001 3:45 PM
Subject: Re: [amsat-bb] PSK-31 Satellite Donwlink Opportunity?


> 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
> -------------------------------------------------------------------------
>
>

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