Half Duplex GMSK Modem

FX579

© 1996 Consumer Microcircuits Limited

18

D/579/4

b)

Bandwidth and phase response of the Rx IF filters.

c)

Accuracy of the Tx and Rx carrier frequencies.

(Any difference will shift the received signal towards one of the skirts of the IF filter

response.)

AC Coupling of Tx and Rx Signals

Ideally, the transmit path to the modulator should be dc coupled. Similarly, the Rx demodulated signal

after external adjustment should be dc coupled to the FX579 RXIN pin, providing a signal symmetrically

BT values of 0.3 and 0.5 in peak averaging mode. However, practical applications may require ac

coupling from the FX579's transmit output to the frequency modulator and between the receiver's

frequency discriminator and the FX579's receive input. This creates two problems:

a)

AC coupling of the signal will degrade the FX579's bit error rate (BER) performance, as shown

b)

AC coupling at the receive input will transform a step in the voltage at the discriminator output

to a slowly decaying pulse which can confuse the FX579's level measuring circuits. Sufficient

time should therefore be allowed for the signal into the FX579 to settle before the ACQUIRE

line is activated.

It is recommended that a data message does not contain long sequences of consecutive ones or

zeroes and that the Tx and Rx path frequency responses have cut off frequencies of

respectively, for a data rate of 8kbits/sec and BT = 0.3.

Two-Point Modulation

When designing the FX579 into a radio that uses a frequency synthesiser, a two point modulation

technique is recommended.

This is to prevent the radio's PLL circuitry from counteracting the

modulation process, and so provide a clean flat modulation response down to dc.

The transmit output (after the external RC) should be suitably buffered before being applied to the

radio's synthesizer.

The drive should be applied to both the Voltage Controlled Oscillator and

Reference Oscillator Modulation inputs with suitable ac/dc levels and with correct phase to achieve

good cancellation of the loop's feedback and so provide a flat modulation response.