MSK (Minimum Shift Keying) CML Microcircuits, MSK (Minimum Shift Keying) Datasheet - Page 22

• Manufacturer Part Number: MSK (Minimum Shift Keying)
• Description: Minimum Shift Keying and its Application to Wireless Data Transmission
• Manufacturer: CML Microcircuits

MSK and its Application to Wireless Data Transmission
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Pseudo-Code for Bad Zero Crossing Detector
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3.2.2.2 Carrier Detect Sliding Window Filter
The sliding window filter maintains a count of the number of bad zero crossings within the last L samples. It can be
modeled as a finite impulse response filter of length L with all coefficients set to 1. See section 3.4 for pseudo-code of a
finite impulse response filter. To mimic response time of MX-COM's serial MSK modems the length of the window should
be eight symbols so L=N x 8 where N is the number of samples per symbol.
3.2.2.3 Carrier Detect Comparator
The carrier detect comparator monitors the output of the sliding window filter to determine if the zero crossing spacings
are consistent enough to look like MSK. It has hysterisis to prevent chattering in noisy conditions. The comparator's
upper threshold is set to five while its lower threshold is set to one. Thus, it trips to the detect state when there are no bad
zero crossings in the last L samples and trips to the non-detect state when there are more than five. Section 3.4 shows
pseudo-code for a comparator with hysterisis. That models output should be inverted to have an active high MSK carrier
detect.
3.2.3 Bit Synchronization
The Bit Synchronization section re-times the data from the demodulation section. There are two blocks in this section, the Phase Lock
Loop block (PLL) and the re-timing block. The PLL creates the clock that is used to re-time the data.
© 1997 MX COM Inc.
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OK = TRUE;
if (input[i] = 1) {
if (counter > max. value in STOP array)
output[i] = logic inverse of OK;
OK = FALSE;
for j=0 to size of STOP and START arrays
reset counter;
}
then
else
if ((counter >= START[i]) & (counter <= STOP[i])) // check if counter is in window
{ OK = FALSE; reset counter; }
increment counter;
then OK = TRUE;
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Figure 28: Bit Synchronization Block Diagram.
// flag defaults TRUE for no zero crossing input
// if zero crossing occurs this sample
// then flag defaults FALSE and window is checked
// counter is reset at every zero crossing
// window expired -> spacing is too large for MSK
// output pulses high for bad zero crossings
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// loop over all valid-windows
// set flag TRUE if in window
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Doc. # 20830084.001
APPLICATION