QPSK modulator COM-1002 ->  [0-80 MHz] modulator COM-4004

This test setup helps assessing the QPSK modulated spectrum (sidelobes, spectrum lines, etc).
The COM-1002 generates a PRBS-11 pseudo random data stream at 200 Kbit/s QPSK = 100 Ksymbols/s. 
The COM-4004 up-converts the signal to 70 MHz IF.

(a) Connect an external 10 MHz frequency reference to the EXT_CLK input of the COM-4004
before powering up the assembly.

(b) import the 1002_4004.stn settings into the assembly.

(c) using a spectrum analyzer, check the COM-4004 output.
The output is a QPSK modulated signal at 70 MHz.

The continuous spectrum in any 3 KHz band outside 
+/- 2*the modulation bandwidth is < - 67 dBc

Two spectrum spurious lines are at -50 dBc at +/- the symbol rate. 
Besides, no other spectral spurious lines are detected above -60 dBc.


(d) Change the COM-1002 symbol rates and observe the resulting spectrum at 70 MHz:
REG0/REG1/REG2 = 5C 8F 02  for a symbol rate of 100 KSymbols/s
REG0/REG1/REG2 = 9A 99 19  for a symbol rate of 1 MSymbols/s
REG0/REG1/REG2 = A4 70 FD  for a symbol rate of 9.9 MSymbols/s

In general, the lower the symbol rate, the cleaner the output spectrum.
At 1 Msymbol/s, a few anti-aliasing sidelobes are seen at -57 dBc in a 3 KHz band.
At 9.9 Msymbols/s, sidelobes are seen at -54 dBc.