[amsat-bb] Re: AO-16 report

[Tom Clark, K3IO]

Mark, N8MH commented

> Actually, this is a great idea, Greg.  It's the basis for AE4JY Moe Wheatley's 
> AO40RCv program--the 400BPSK demodulator.  In fact, his source coded is 
> posted--it should help with the programming I'd think.  It provided button 
> pushes, com port selection, radio choices, etc.
>
> Not sure there is ENOUGH carrier..at least from what I've been hearing.  But I 
> haven't looked at it with a waterfall yet;  Drew KO4MA was doing that during 
> our initial tests--he could see the carrier well as I recall.
>   
The BPSK modulator should have zero carrier in the middle unless it has 
drifted out of balance. I would be surprised if the carrier rejection is 
any greater than ~-20 dB below the sideband level -- this corresponds to 
the supposed 180° phase shift being off by about 0.1 radians (i.e. ~6°).

If you try to inject carrier by simply turning on a BFO and receiving 
the signal as DSB, you will get distortion. This is because you want to 
extract the 2 sidebands with the same phase. You will be better off 
throwing away half the signal and receiving it as a SSB signal.

One way to properly phase the signal is with a Costas loop demodulator. 
Such widgets were the hardware PSK demodulators offered by TAPR 
(designed by K3IO & N7CL) and a different design by G3RUH; the TAPR 
design was sold commercially by Paccomm as the PSK-1 (Google Paccomm 
PSK-1). These designs have been built with both 400 BPS (for AO-40) and 
1200 BPS (for the PACSATs) data filters. These devices have a PLL to 
generate a coherent carrier and do not suffer the 3dB SNR loss; the PLLs 
will also help to track the satellites Doppler.

As Mark pointed out, a similar design was done in software for use with 
a Soundblaster for AO40's 400 BPS BPSK by Moe Wheatley, AE4JY. You can 
fetch a copy at Moe's web site at 
http://www.moetronix.com/ae4jy/ao40rcv.htm. Moe's web site has the 
source code in C++ and you will probably want to modify the data filters 
by a factor of 3-4. Moe has also implemented a DSB demodulator in his 
SpectraVue SDR software that supports N4IP's RF Space SDR-14 and SDR-IQ 
receivers (see http://rfspace.com/Home.html).

For those of you using a Flex  SDR (http://www.flex-radio.com)like the 
SDR-1000 or -5000 (or the SoftRock, or a similar QSD design)as your 
receive IF, you should be able to do a proper demod on the AO-16 signal 
(including a properly phased carrier generated in a software PLL) using 
the [DSB] mode (also please try the Synchronous AM [SAM] detector)in 
PowerSDR.

I haven't tried any of the SDR approaches myself, so I don't know about 
the AFC characteristics in [DSB] and [SAM]. Perhaps AE4JY or N4HY or 
AB2KT can comment.

73, Tom