GMSK Modem
CMX589A
Ideally, the Rx demodulator should be DC coupled to the CMX589A Rx Signal In pin (with a DC bias
added to center the signal at the Rx Feedback pin at VDD/2 [VBIAS]). However, AC coupling can be
used provided that:
The 3dB cut-off frequency is 20Hz or below (i.e. a 0.1mF capacitor in series with 100kW).
The data does not contain long sequences of consecutive ones or zeroes.
Sufficient time is allowed after a step change at the discriminator output (resulting from channel
changing or the appearance of a RF carrier) for the voltage into the CMX589A to settle before the
RXDCacq line is strobed.
5.1.3 Two-Point Modulation
When designing the CMX589A into a radio that uses a frequency synthesizer, a two-point modulation
technique is recommended. This is both to prevent the radio's PLL circuitry from counteracting the
modulation process, and to provide a clean flat modulation response down to DC.
Figure 12 shows a suggested basic configuration to provide a two-point modulation drive from the CMX589A
Tx Output using the FX019 or MX019 Digitally Controlled Quad Amplifier Array. The FX019/MX019 elements
provide individual set-up, calibration and dynamic control of modulation levels. Level setting control of the
amplifiers/attenuators of the FX019/MX019 is via an 8-bit data word. Note that the FX019/MX019 frequency
response only supports data rates as high as 8kbps.
With reference to Figure 12:
The buffer amplifier is required to prevent loading of the CMX589A external RC circuit.
Stage B, with R1/R2, provides suitable signal and DC levels for the VCO varactor; C1 is RF
decoupling. The drive level should be adjusted (digitally) to provide the desired deviation.
Stage C, with R3/R4, provides the Reference Oscillator drive (application dependent). This
parameter is set by adjusting for minimum AC signal on the PLL control voltage with a low-frequency
modulating signal (inside the PLL bandwidth) applied.
Stage D could be used with the components shown if a negative reference drive is required.
Stage A provides buffering and overall level control.
Figure 12: An Example of Two-Point Modulation Drive with Individual Adjustment Using the
FX019/MX019
ã 2002 CML Microsystems Plc
16
D/589A/4
CMLMICRO [ CML MICROCIRCUITS ]
