GMSK Modem
CMX589A
As well as using the Rx Hold input to freeze the Level Measuring and Clock Extraction circuits during a signal
fade, it may also be used in systems which use a continuously transmitting control channel to freeze the Rx
circuitry during transmission of a data packet, allowing reception to resume afterwards without losing bit
synchronization. To achieve this, the CMX589A Xtal clock needs to be accurate enough that the derived
RxClock output does not drift by more than about 0.1 bit time from the actual received data-rate during the
time that the RxHold input is ‘Low’.
However; the RXDCacq input may need to be pulsed High for 2 bit durations to re-establish the level
measurements if the RxHold input is Low for more that a few hundred bit-times (exact number depends on
system crystal tolerances).
The voltages on the Doc1 and Doc2 pins reflect the average peak positive and negative excursions of the
(filtered) receive signal, and could therefore be used to derive a measure of the data signal amplitude.
Note: These pins are driven from very high-impedance circuits, so that the DC load presented by any
external circuitry should exceed 10MW to VBIAS
.
5 Application
5.1 Radio Channel Requirements
To achieve legal adjacent channel performance at high bit-rates, a radio with an accurate carrier frequency
and an accurate modulation index is required. For optimum channel utilization, (e.g. low BER and high data-
rates) attention must be paid to the phase and frequency response of both the IF and baseband circuitry.
5.1.1 Bit Rate, BT, and Bandwidth
The maximum data rate that can be transmitted over a radio channel depends on the following:
Channel spacing
Allowable adjacent channel interference
Tx filter bandwidth
Peak carrier deviation (Modulation Index)
Tx and Rx carrier frequency accuracies
Modulator and Demodulator linearity
Rx IF filter frequency and phase characteristics
Use of error correction techniques
Acceptable error-rate
As a guide to MOBITEX operation, a raw data-rate of 8kbps at 12.5kHz channel spacing may be achievable -
depending on local regulatory requirements- using a ±2kHz maximum deviation, a BT of 0.3, and no more
than 1.5kHz discrepancy between Tx and Rx carrier frequencies. Forward error correction (FEC) could then
be used with interleaving to reduce the effect of burst errors.
Reducing the data-rate to 4.8kbps would allow the BT to be increased to 0.5, improving the error-rate
performance.
5.1.2 FM Modulator, Demodulator and IF
For optimum performance, the eye pattern of the received signal (when receiving random data) applied to the
CMX589A should be as close as possible to the Transmit eye pattern examples shown in Figure 10.
Of particular importance are general symmetry, cleanliness of the zero-crossings, and for a BT of 0.3, the
relative amplitude of the inner eye opening.
To achieve this, attention must be paid to:
Linearity and frequency/phase response of the Tx frequency modulator. Unless the transmit data is
especially encoded to remove low frequency components, the modulator frequency response should
extend down to a few hertz. This is because two-point modulation is necessary for synthesized
radios.
Bandwidth and phase response of the Rx IF filters.
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.
ã 2002 CML Microsystems Plc
15
D/589A/4
CMLMICRO [ CML MICROCIRCUITS ]
