COM-1002 BPSK/QPSK/OQPSK modulator 
  -> COM-4004 70 MHz IF modulator
    -> variable attenuator 
      -> COM-3004-A 20-90 MHz IF receiver, narrow-band applications
        -> COM-1001 BPSK/QPSK/OQPSK demodulator
          -> COM-1005 BER measurement


Purpose:
Demonstrate the transmission of a 293 Kbit/s BPSK modulated data stream at 75.3 MHz 
intermediate frequency. Test conditions: noiseless channel, internal clock references.
The COM-3004-A narrow-band version is used to avoid noise aliasing. 
The BPSK demodulator uses the AFC to extended frequency acquisition, as the internal clock references within the COM-4004 
and COM-3004 analog modules may create 100ppm of frequency error (100ppm @ 75.3MHz = 7.53 KHz, 
greater than the 2.9 KHz [1% of symbole rate] digital costas loop acquisition range). 


(a) assemble the ComBlock modules listed above. 
Insert a variable RF attenuator (40 dB or about for the initial tests will work) 
between the transmitter assembly and the receiver assembly.
Connect a serial link between a host PC and the COM-1002 module.
Power up the assemblies.
Using the ComBlock control center, enumerate the ComBlocks (2nd button to the left), 
then import (Menu File | Import) the transmitter settings file 1002_4004_3004_1001_1005_tx.stn

(b) move the serial link to the COM-3004 module.
Using the ComBlock control center, enumerate the ComBlocks (2nd button to the left), 
then import (Menu File | Import) the receiver settings file 1002_4004_3004_1001_1005_rx.stn
Both assemblies are now configured. Switch the power supply off then on again.

(c) check the receive baseband signals using an oscilloscope at the COM-3004 receiver. 
  The I/Q baseband signals can be visualized at the COM-3004 receivers at test points
  TP3 (I-channel) and TP4 (Q-channel).
  When the receiver AGC is working properly, the received I and Q baseband signals should be
  1Vpp, filtered (no sharp transition) bit streams. Because of carrier phase offset between 
  transmitter and receiver, the received baseband signals are phase rotated versions of the
  transmitted baseband signals.

(d) Frequency acquisition:
Since the received signal frequency error is larger than 1% of the symbol rate (natural
acquisition range for digital Costas loop), the user must use the AFC to 'zoom in' on the
center frequency. The AFC acquisition range is +/- 10% of the symbol rate, which is 
consistent with the 7.5 KHz possible frequency offset.
Step 1: clear all demodulator loops by writing x84 (instead of the default 0x00) to 
COM-1001 REG7. 
Step 2: enable the AFC by writing x04 to COM-1001 REG7.
Using an oscilloscope, one can visualize the AFC frequency detection at COM-1001 test point
TP3. The square vawe represents the detected frequency error.
One can also visualize the demodulator lock status at TP1.
Step 3: disable the AFC by writing x00 to COM-1001 REG7.
The main reason for disabling the AFC is that it can significantly increase the BER 
for a given Eb/No. 

(e) Verify demodulator lock:
at the COM-1001 demodulator: 
  - TP1 is high when the carrier tracking loop is locked
at the COM-1005 BER measurement: 
  - TP1 is high when the module is synchronized with the PRBS-11
  - TP3 is low (no bit error)
  - TP4 consists of periodic pulses every 2047 bits (52.4 ms).

(f) Compare the transmitted bits and received bits: 
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 BPSK phase ambiguity. 

(g) 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.

(h) receiver sensitivity:
Increase the attenuation between modulator and demodulator while monitoring the bit errors
on COM-1005 TP3 and synchronization on COM-1005 TP1.
The receiver should be able to maintain lock and show no error down to -86 dBm input signal
level. It should also be capable to re-locking after a short signal interruption.
It should also be able to re-acquire using the procedure described in d).