FX579 Datasheet, PDF (18/23 Pages) CML Microcircuits – HALF DUPLEX GMSK MODEM

Half Duplex GMSK Modem
FX579
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
centred about V_BIAS. Figure 9 shows the bit error rate (BER) performance under these conditions for
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
in Figure 10 for a data rate of 8kbits/sec, with V_DD = 5V and BT = 0.3.
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 ≤5Hz and ≤20Hz
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.
© 1996 Consumer Microcircuits Limited
18
D/579/4