KD2BD Pacsat Modem

Posted on Feb 6, 2014

The KD2BD Pacsat Modem is a low cost, high performance 1200 bps BPSK modem designed to interface between a packet radio terminal node controller (TNC) and an amateur satellite ground station, and allows full-duplex access to the 1200 baud `Pacsat` constellation of amateur satellites. The KD2BD Pacsat Modem consists of a 1200 bit per second BPSK de

KD2BD Pacsat Modem
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modulator for receiving Pacsat transmissions, and a Bi-Phase Manchester encoder for generating Pacsat uplink signals. The demodulator uses coherent phase detection and correlation decoding techniques that are capable of demodulating BPSK signals well into the noise level. It also includes an Automatic Gain Control (AGC) system to compensate for signal strength variations, and an Automatic Frequency Control (AFC) for tuning the downlink receiver in response to Doppler shift during a satellite pass. The demodulator is very sensitive, and is capable of locking into BPSK signals so weak they are barely audible above the ambient receiver noise. The modulator produces low-distortion 1200 bit per second Bi-Phase Manchester code suitable for generating Manchester encoded FSK when used with a standard 2-meter FM voice transmitter, or BPSK when used with an SSB transmitter. The KD2BD Pacsat Modem was originally designed to interface with an MFJ 1270B terminal node controller (TNC-2 clone), and a Yaesu FT-726R multi-mode VHF/UHF communications transceiver. Suitable interfacing modifications should allow proper modem operation with other TNCs and ground station radio equipment. The specifics for doing so are left to the expertise of the reader. Orbital Satellites Carrying Amateur Radio (OSCARs) have been a part of amateur radio for over 30 years. With the growing popularity of terrestrial packet radio communications on the amateur...

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