COM-1012 DS spread-spectrum modulator 
-> COM-4004 70 MHz IF modulator
-> 40 dB attenuator 
-> COM-3004 20-90 MHz IF receiver
-> COM-1011 DS spread-spectrum demodulator
-> COM-1005 BER measurement


Purpose:
Demonstrate the transmission of a 155.9 Kbit/s data stream modulated with 
a 9.9 Mchips/s direct-sequence spread-spectrum modulation. 
Spreading code period (spreading gain) : 127 chips
Spreading code: maximal length sequence
Modulation: QPSK
Intermediate frequency: 50 MHz
Reference clock for IF frequency synthesizer: internal
Channel: noiseless
The demodulator is configured with the AFC enabled to increase the carrier frequency 
acquisition window to +/-10% of the symbol rate (+/- 7.8 KHz), which is generally 
less than the COM-4004 and COM-3004 combined drift caused by internal crystals.

(a) assemble the ComBlock modules listed above. 
Insert an RF attenuator (40 dB will work) between the transmitter assembly
and the receiver assembly.
Connect a serial link between a host PC and the COM-1012 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 1012_4004_3004_1011_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 1012_4004_3004_1011_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) Verify demodulator lock:
at the COM-1011 demodulator: 
  - TP1 is high when the code tracking loop is locked
  - TP2 is high when the carrier tracking loop is locked
  - TP3 shows a square wave representing the carrier frequency error between transmitter and
    receiver (a few KHz. < 5 KHz typ).
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).

(e) 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-1012 TP8/TP9). The bits should be identical, taking into account processing
delay and inversions due to QPSK phase ambiguity. 

(f) 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.