FX489 Datasheet, PDF (7/12 Pages) List of Unclassifed Manufacturers – GMSK Modem

Application Information ......
PLLacq Rx Hold
PLL Action
“1”
X Acquire: Sets the PLL bandwidth wide enough to allow a lock to the received signal in
less than 8 zero crossings. The Acquire mode will operate as long as PLLacq is a logic
“1”.
“1” to “0” “1”
Medium Bandwidth: The correction applied to the extracted clock is limited to a
maximum of ±1/16th bit-period for every two received zero-crossings. The PLL operates
in this mode for a period of about 30 bits immediately following a “1” to “0” transition of the
PLLacq input, provided that the Rx Hold input is a logic “1”.
“0”
“1”
Narrow Bandwidth: The correction applied to the extracted clock is limited to a maximum
of ±1/64th bit-period for every two received zero-crossings. The PLL operates in this
mode whenever the Rx Hold Input is a logic “1” and PLLacq has been a logic “0” for at
least 30 bit periods (after Medium Bandwidth operation for instance).
“0”
“0”
Hold: The PLL feedback loop is broken, allowing the Rx Clock to freewheel during signal
fade periods.
RxDCacq Rx Hold
Rx Level Measure Action
“0” to “1”
X
Clamp: Operates for one bit-time after a “0” to “1” transition of the RxDCacq input. The
external capacitors are rapidly charged towards a voltage mid-way between the received
signal input level and V_BIAS, with the charge time-constant being of the order of 0.5bit-time.
“1”
X
Fast Peak Detect: The voltage detectors act as peak-detectors, one capacitor is used to
capture the ‘positive’-going signal peaks of the Rx Filter output signal and the other
capturing the ‘negative’-going peaks. The detectors operate in this mode whenever the
RxDCacq input is at a logic “1,” except for the initial 1-bit Clamp-mode time.
“0”
“1”
Averaging Peak Detect: Provides a slower but more accurate measurement of the signal
peak amplitudes.
“0”
“0”
Hold: The capacitor charging circuits are disabled so that the outputs of the voltage
detectors remain substantially at the last readings (discharging very slowly [time-constant
approx. 2,000bits] towards V ).
Table 2 PLL and Rx Level Measurement Operationa^BIAl ^SModes
Rx Clock Extraction
Synchronized by a phased locked loop (PLL)
circuit to zero-crossings of the incoming data, the ‘Rx
Clock Extraction’ circuitry controls the ‘Rx Clock’
output. The Rx Clock is also used internally by the
Data Extraction circuitry. The PLL parameters can be
varied by the ‘Rx Circuit Control’ inputs PLLacq and Rx
Hold to operate in one of four PLL modes as described
in Table 2.
Rx Data Extraction
The ‘Rx Data Extraction’ circuit decides whether
each received bit is a “1” or “0” by sampling the output
of the Rx Filter in the middle of each bit-period, and
comparing the sampled voltage against a threshold
derived from the ‘Level Measuring’ circuit. This
threshold is varied on a bit-by-bit basis to compensate
for intersymbol interference depending on the chosen
BT. The extracted data is output from the ‘Rx Data’
pin, and should be sampled externally on the rising
edge of the ‘Rx Clock.’
Rx S/N Detection
The ‘Rx S/N Detector’ system classifies the
incoming zero-crossings as GOOD or BAD depending
upon the time when each crossing actually occurs with
respect to its expected time as determined by the
Clock Extraction PLL. This information is then
processed to provide a logic level output at the ‘Rx S/
N’ pin.
By monitoring, and averaging, this output it is
possible to derive a measure of the Signal-to-Noise-
Ratio and hence the Bit-Error-Rate of the received
signal.
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