FX579 Datasheet, PDF (14/23 Pages) CML Microcircuits – HALF DUPLEX GMSK MODEM

Half Duplex GMSK Modem
FX579
ACQUIRE Pin
"0" to "1"
"1"
"0"
Rx Level Measurement Mode
Clamp
Lossy Peak Detect
Peak Averaging
PLL Mode
Narrow → Wide Bandwidth
Wide Bandwidth
Narrow Bandwidth
Rx Level Measurement Clamp: Operates for a minimum of two bit times, maximum three bit
times, after a "0" to "1" transition of the ACQUIRE input. The external DOC capacitors are
rapidly charged towards the input signal level, with the charge time constant being of the order
of 0.5 bit time.
Rx Level Measurement Lossy Peak Detect: The detectors rapidly capture the +ve and -ve
going signal peaks of the Rx filter output signal, these peaks being stored on the external DOC
capacitors. The detectors operate in this mode whenever ACQUIRE is at a logic "1", except for
the initial Clamp period.
Rx Level Measurement Peak Averaging: Provides a slower but more accurate measurement
of the signal peak amplitudes. This operating mode depends on the PLL circuitry being in lock.
PLL Wide Bandwidth: Sets the PLL bandwidth wide enough to allow a lock to the received
signal in 8 zero crossings. This mode will operate as long as ACQUIRE is at logic "1".
PLL Narrow Bandwidth: The correction applied to the extracted clock is limited to a maximum
of ± 1/16th bit period for every 4 zero crossings received. The PLL operates in this mode
whenever the ACQUIRE input is set to logic "0".
Table 3 Receive Operating Modes
Rx Data Formats
The receive section of the FX579 works best with data which has a reasonably 'random' structure, i.e.
the data should contain approximately the same number of 'ones' as 'zeroes' with no long sequences
(> 100 bits) of consecutive 'ones' or 'zeroes'. Also, long sequences (>100 bits) of patterns without "11"
and "00" should be avoided.
For this reason, it is recommended that data is scrambled in some manner before transmission, for
example by 'exclusive-ORing' with the output of a binary pseudorandom pattern generator.
Where data is transmitted in bursts, each burst should be preceded by a preamble designed to allow
the receive modem to establish timing and level lock as quickly as possible. This preamble should be
at least 16 bits long, and should preferably consist of alternating pairs of "1"s and "0"s i.e.
"110011001100...." A "10101010...." preamble sequence will yield poor peak levels for the receive
circuits, although performance is better with BT = 0.5 than with BT = 0.3.
© 1996 Consumer Microcircuits Limited
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D/579/4