56KB Modem Specifications

Frequency:

Determined by your choice of crystals and linear transverter used. Possible choices include 50 to 1296 MHz.

Bandwidth:

70 KHz

Modulation:

Bandwidth limited MSK generated by digital state machine. Frequency shift is 14 KHz (1/4 baud rate). Phase shift is +/- 90 degrees for each bit. Amplitude variations are 3 db.

Transmitter:

Exciter outputs 1 milliwatt on 29 Mhz. Crystal controlled carrier is modulated by two double balanced modulators (1496 chips). Bandwidth limited modulation waveforms generated by digital state machine and DAC's. Linear transverters are used to convert from the 29 MHz output to the desired band of operation.

Receiver:

Sensitivity 1 microvolt with low error rate. 1st IF 29 MHz. 2nd IF 10.7 MHz. 3rd IF 455 KHz. Uses custom L-C 70 KHz wide 455 KHz IF filter. Demodulation by MC3359 quadrature detector chip.

Data Format:

Synchronous NRZI

Scrambler:

x = x^-17 + x^-5 + 1 (17 bit shift register with taps at 17 and 5)

Computer/TNC Interface:

RTS Request to Send (PTT)

RXD Receive Data

RXC Receive Clock

TXD Transmit Data

TXC Transmit Clock

GND Signal Ground

/CD Carrier Detect