MSK (Digital Demod)
The VSA offers two methods of demodulating signals: type 1 and type 2. Both methods demodulate MSK Minimum Shift Keying modulated waveforms, but they differ in the compensation used and in the way they detect bits, as shown in the following table.
|
Features |
Type 1 |
Type 2 |
|---|---|---|
|
Carrier lock |
|
|
|
- Frequency correction |
Yes |
Yes |
|
- Phase correction |
Yes |
Yes |
|
Symbol lock |
Yes |
Yes |
|
Burst amplitude droop |
No |
Yes |
|
I/Q origin offset |
No* |
Yes |
|
Differential detection |
Yes |
No |
* MSK Type 1 does not compensate for I/Q origin offset (in other words, it doesn't remove I/Q origin offset) but it does report it in the symbol table.
MSK Type 1
demodulation is done with carrier lock and symbol lock and uses differential data decoding.
MSK type 1 does not compensate for I/Q origin offset and burst amplitude-droop. Therefore, I/Q origin offset and burst amplitude-droop affect the IQ Meas trace data.
The data is assumed to have been differentially encoded using:
dOut(i) = not[dIn(i) XOR dIn(i-1)]
where: dOut(i) is the data bit that is output.
dIn(i) is the current data bit in.
dIn(i-1) is the previous data bit in.
XOR is the exclusive-OR operation.
not is the negation operation.
When Constellation Sync Search is OFF, dIn(i-1) is assumed to be one (1).
MSK Type 2
demodulation is done with carrier lock and symbol lock but does not use differential detection. Furthermore, MSK type 2 compensates (removes) any I/Q origin offset or burst amplitude-droop.
With MSK type 2, I/Q origin offset and burst amplitude-droop do not affect the IQ Meas trace data. However, the VSA reports these errors in the symbol table.
Additional Details
For additional details, see Digital Demodulation.
See Also
