QPSK modulator COM-1002 -> AWGN generator COM-1023 -> Variable decimation COM-1008 -> QPSK demodulator COM-1001 -> BER measurement COM-1005. Test configuration example: - modulation: 400 Kbit/s, QPSK non-differential (200 Ksymbols/s) - no frequency error - 7.5 (approx) Eb/No within the modulation bandwidth - 1:32 decimation prior to QPSK demodulator Back to back digital modem operations in an additive white noise environment can be verified at baseband. The transmission channel is represented by 2 10-bit complex samples at 40 Msamples/s. The QPSK modulator is configured in signal generator test mode whereby a periodic 2047-bit sequence is being transmitted. A precise digital noise generator (COM-1023) is used to add white gaussian noise. The end to end BER is measured using the COM-1005 module. The variable decimation module (COM-1008) prevents noise aliasing when subsampling from at the QPSK demodulator input. The sampling rate is reduced from 40 Msamples/s to 1.25 Msamples/s using successive 1:2, 1:4 and 1:4 filter/decimation stages. Proper operation can be verified as follows: (a) import the 1002_1023_1008_1001_1005.stn settings into the assembly, using the ComBlock Control Center file | import menu. (b) 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 TP4 shows regular periodic start of frame pulses every 2*2047 bits = 10.23 ms. 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 bit errors, processing delay and inversions due to QPSK phase ambiguity. (c) The intermediate signals in this test setup are represented by complex 10-bit I/Q baseband samples: - modulator output - modulated signal + noise - decimated received signal As these signals can be converted to analog and displayed on a oscilloscope by using the COM-2001 dual D/A converter, the interface of which is compatible with the Comblock modules used here. Simply break the chain and insert a COM-2001. NOTE: Because of the large ratio between symbol rate and 40 Msamples/s sampling rate, severe quantization occurs within the COM-1023 module (the signal scaling coefficient is very small (3!)). Therefore, this setup should not be used for doing calibrated Eb/No measurements. In theory (i.e. without quantization within the COM-1023 module), the following Eb/No can be created: COM1023 REG4 = 0x02 -> Eb/No = 4 dB COM1023 REG4 = 0x03 -> Eb/No = 7.5 dB COM1023 REG4 = 0x04 -> Eb/No = 10 dB