GMSK Packet Data Modem and RF Transceiver
CMX990
The CMX990 has a built in AGC algorithm. Programmable gain has 4 steps of 15dB (i.e -5dB, +10dB,
+25dB and +40dB). It is beneficial to allow as large an operating window as possible as the AGC has a
certain amount of hysteresis allowing the signal to rise more than 15dB before the gain is backed off.
The CMX990 measures the signal level in baseband. This information is used to control the AGC but is
also available via the control interface. This information must be combined with the AGC setting to
determine RSSI.
Signal Processing
The CMX990 includes the demodulation functions, these are implemented digitally. Algorithms are
consistent with Mobitex mobile burst timing structure and requirements. The received waveform can be
inverted to allow high side or low side mixing in the receiver (see section 5.1.4.4). It will be noted that
when powered on the CMX990 needs to acquire base channel. The baseband has been optimised to
achieve this while also giving minimum power consumption in normal operation (i.e. maximum sleep
times).
Figure 26 ADC Input Dynamic Range Partition
ã 2004 CML Microsystems Plc
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D/990/1
CMLMICRO [ CML MICROCIRCUITS ]
