QPSK modulator COM-1002 -> Multi-path simulator COM-1024 -> 
QPSK demodulator COM-1001 -> BER measurement COM-1005.  
Test configuration example: 19.8 Mbit/s, QPSK, no frequency error, noiseless, no-multi-path.

Back to back digital modem operations can be verified at baseband. 
The QPSK modulator is configured in signal generator test mode whereby a periodic 
2047-bit sequence is being transmitted. The end to end BER is measured using the 
COM-1005 module. 
The COM-1024 simulates multi-path conditions between QPSK modulator and demodulator.


Proper operation can be verified as follows:

(a) using an oscilloscope probe:

COM-1001 TP1 is high, indicating demodulator carrier lock 

COM-1005 TP1 is high, indicating synchronization with the 2047-bit periodic test pattern 

COM-1005 TP3 is low, showing no bit error pulse 

COM-1005 TP4 shows regular periodic start of frame pulses every 2*2047 bits = 206.76 us.
Trigger on COM-1005 TP4, and compare demodulated bits (COM-1005 TP5/TP6) with the 
transmitted bits (COM-1002 TP2/TP3). The bits should be identical, taking into account processing
delay and inversions due to QPSK phase ambiguity. 

(b) from the ComBlock control center check the BER (COM-1005 status).
It will show no bit errors (REG 1 through 4) and the synchronization bit (REG5 bit0) is high.
Depending on the demodulated phase ambiguity, the BER can be either 0 (no inverted bit) 
or 0x0F4240 (all bits inverted)

(c) Switch from a direct path with no frequency offset and no attenuation, to 
a path with 100KHz offset and 6 dB attenuation. Observe BER recovery.
Change the COM-1024 registers as follows:
REG0/1: 00 00
REG2/3: 00 80
Observe the COM-1005 BER measurement synchronization TP1 and bit errors TP3.
The recovery is in the order of 0.3 seconds.
The BER returns to zero (using method in (b)).