[Date Prev][Date Next][Thread Prev][Thread Next] - [Date Index][Thread Index][Author Index]

Re: AO40 telemetry tutorial?



>Center frequency was about 1100 Hz most of the time.

1100 Hz is probably a little low. The BPSK modulation creates two
mirror-image sidebands around the carrier frequency, and the
Manchester encoding puts peak sideband energy at +/- 400Hz from the
carrier, i.e, at 700 and 1500 Hz. Each sideband has a null at the
carrier frequency and another at +/-800 Hz from the carrier, i.e., at
300 and 1900 Hz.  Signal energy beyond these limits can probably be
neglected.

Most SSB radios cut off at about 300 Hz on the low end, so this
doesn't leave much margin for the group delay distortion in the
crystal filter as you get close to the passband edge.

>I did try
>different center frequencies and didn't see any difference in the way
>it decoded.

Did you try higher frequencies? Check the specs for your radio for the
nominal SSB audio passband. Average the upper and lower limits and
make this your carrier frequency. E.g., if your radio goes from
300-2700 Hz, then set the carrier at (300+2700)/2 = 1500 Hz.

--Phil




----
Via the amsat-bb mailing list at AMSAT.ORG courtesy of AMSAT-NA.
To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org



AMSAT Top AMSAT Home