Half Duplex GMSK Modem
FX579
Receive Signal Path
The function of the Rx circuitry is to:
a)
Accept an incoming signal from the radio's frequency discriminator at a defined level via
suitable external signal and dc level adjustment
b)
c)
d)
e)
Clean the signal by filtering
Provide dc level thresholds for clock and data extraction
Provide clock timing information for data extraction and external circuits
Provide Rx data as an output in binary form
The output of the radio receiver's frequency discriminator after suitable external signal and level
adjustment is applied to the RXIN pin. With V = 5V, nominal input level when receiving a continuous
DD
"1111000011110000..." data pattern should be 1V pk-pk (level is proportional to V ) centred around
DD
½ V . Positive going signal excursions about V
at RXIN will produce a logic "1" on the DATAIO
DD
BIAS
pin, negative going signal excursions will produce a logic "0".
The signal is then applied to the low pass Rx filter, which has a -3dB corner frequency of 0.56 times the
data bit rate, before being applied to the Level Measuring and Clock and Data Extraction blocks.
Level Measuring Circuit
The 'Level Measuring' block consists of two voltage detectors one of which measures the amplitude of
the 'positive' peaks of the received signal, while the other measures the 'negative' peaks. These
detectors use the external capacitors connected to the DOC1 and DOC2 pins to form voltage hold or
integrator circuits.
Results of the two measurements are then processed within the modem to establish the optimum dc
level decision thresholds for the Clock and Data Extraction circuits, depending on the received signal
amplitude, BT and any dc offset present.
The receive circuits operate in several control modes as defined by the logic level applied to the
acquire pin. These are explained later - See 'Acquire Sequence'.
Rx Clock Extraction Block
The 'Rx Clock Extraction' circuit is based on a zero crossing tracking loop which uses a multi resolution
digital phase locked loop (PLL). The wide bandwidth mode allows for fast initial phase acquisition.
Eight good zero crossings are required for correct operation.
The highest timing resolution is obtained when the PLL is in its narrow bandwidth mode. This mode of
operation yields the least amount of phase jitter, which is responsible for the associated bit error rate
(BER) performance degradation.
The PLL operating mode is defined by the logic level applied to the ACQUIRE pin - See 'Acquire
Sequence'.
Rx Data Extraction Block
The 'Rx Data Extraction' circuit decides whether each received bit is a "1" or "0" by sampling the
received signal, after filtering, in the middle of each bit period and comparing the sampled voltage
against a threshold derived from the 'Level Measuring' circuit. This threshold is adapted from bit to bit
to compensate for intersymbol interference depending on the chosen BT. The extracted data is output
from the DATAIO pin and should be sampled externally on the rising edge of the received data clock.
ã 1996 Consumer Microcircuits Limited
11
D/579/4
CMLMICRO [ CML MICROCIRCUITS ]
