9600 Baud Packet Radio Modem PCB

The conventional packet VHF/UHF radio data rate is 1200 baud because all TNCs provide an internal modem for this speed, and the two-tone AFSK audio spectrum suits unmodified voiceband radios comfortably. However all TNCs can generate much higher data rates, and most FM radios have an unrealised audio bandwidth of some 7-8 kHz or more. So in many cases 9600 baud radio transmission is entirely practical with them.

This design is a high performance full duplex baseband modem designed for terrestrial packet and satellite packet use with typical voiceband NBFM radios, assuming only minor modifications.

A key feature of this modem is its digital generation of the transmit audio waveform. Precise shaping compensates exactly for the amplitude and phase response of the receiver. This results in a matched filter system, which means that the received audio offered to the data detector has the optimum characteristic (eye) for minimum errors. It also allows very tight control of the transmit audio bandwidth.

See: ARRL 7th Computer Networking Conference Papers 1988 pages 135 - 140. (197 kB)


A summary of the modem features:

MODULATION: FM. Audio applied direct to TX varactor. +/-3 kHz deviation gives RF spectrum 20 kHz wide (-60db). Fits standard channel easily.

TX MODULATOR: 8 bit long digital F.I.R. transversal filter in Eprom for transmit waveform generation (12 bit optional) designed using DSP techniques. Gives absolutely "brick-wall" audio spectrum. Typically -6 db at 4800 Hz, -50 db at 7500 Hz. Allows compensation for receiver (the channel) to achieve perfect RX "eye". Up to 16 TX waveforms (32 optional), jumper selectable. Output adjustable 0-8v pk-pk.

SCRAMBLER: (Randomiser): 17 bit maximal length LFSR scrambler. Compatible with satellites UO­22, Kitsat KO-23 etc as well as with manufacturers PacComm NB- 96, Sprint-2, Kantronics DE-9600 , TASCO TMB-965, MFJ-9600, Symek TNC2H, BayCom PCM-4, K9NG, TAPR and many others.

RX DEMODULATOR: Audio from receiver discriminator, 5mv-10v pk-pk. Gentle 3rd order Butterworth noise filter, 6 kHz. Data Detect circuit for use on simplex (CSMA) links. Independent un-scrambler.

CLOCK RECOVERY: Unique digital phase-lock loop (DPLL) clock recovery circuit with 1/256th bit resolution, yet using only 16x clock. Average lock-in time 50 bits, depending on SNR.

CONNECTS to AX.25 TNC Modem Disconnect jack. Suitable for TNC-2 and any other provided the internal modem can be bypassed. Standard TNC digital connections needed: TXData, TXClock (16x bit rate), RXData, Data Detect ("DCD", high or low), GND. RXClock output available. TTL levels. RADIO connections: TXAudio, RXAudio, GND. All connections via 0.1" pitch pads for SIL connectors or direct soldering. Unwired DIN 41612 96-way connector (use optional).

POWER CONSUMPTION: 10 to 15v DC at 40ma (CMOS Roms), 170ma (NMOS Roms). Total 19 ICs (13 CMOS, 2 DACs, 2 Eproms). 5 volt regulator and heatsink.

OTHER FEATURES: The only set-up is TXAudio level. Will run speeds up to 64,000 baud if some filter components changed using notes supplied. Channel calibration facility. Audio loopback. Jumper selectable Data or BERT (bit error rate test) mode.

PCB: 160x100mm single Eurocard format. Top professional quality, double sided, max copper ground plane, plated through, solder resist, yellow silk screen legend. Four 3.3 mm mounting holes. Comprehensive documentation in 16 page booklet.

9600 baud Packet Radio Modem Prices:

I regret that after an amazing 15 years (1988-2003), in which you bought some 4000 PCBs from me - at a price held constant throughout - and vastly more units from licencees such PacCom, Kantronics, Tasco, Symek and many others, I am presently no longer able to supply PCBs.

Thank you for your fantastic support of this project!

Back to Product List


Last updated: 2020 Jul 05