COM-1019 DSSS digital modulator 
  -> COM-2001 Dual D/A converters 
    -> COM-4002 L-band quadrature modulator
      -> 20 - 30 dB attenuator 
        -> COM-3002 L-band receiver 
          -> COM-1018 DSSS digital demodulator
            -> COM-1005 BER measurement


Purpose:
Demonstrate the transmission of a 12 Mchip/s QPSK modulated direct-sequence 
spread-spectrum signal at 1.0 GHz. 

Conditions:
12 Mchip/s. QPSK DSSS. Barker code length 11. Data rate 2.18 Mbit/s
Demodulator AFC enabled. 
Internal clocks used for RF frequency synthesizer
No channel noise.


(a) assemble the ComBlock modules listed above. 
Insert an RF attenuator (20 to 30 dB will work) between the transmitter assembly
and the receiver assembly.
Connect a serial link between a host PC and the COM-1019 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 1019_2001_4002_3002_1018_1005_tx.stn

(b) move the serial link to the COM-3002 module.
Using the ComBlock control center, enumerate the ComBlocks (2nd button to the left), 
then import (Menu File | Import) the receiver settings file 1019_2001_4002_3002_1018_1005_rx.stn
Both assemblies are now configured. Switch the power supply off then on again.

(c) verify that the RF frequency synthesizers are locked:
at the COM-4002 modulator: SYNC_LOCK is low, PLL_REF is a 10 MHz clock
at the COM-3002 receiver: PLL_LOC is low, PLL_REF is a 10 MHz clock.

(d) check the baseband signals using an oscilloscope. 
- At the COM-2001 dual D/A converter: probe at the capacitors C3 and C24 near the output SMA connectors.
  The signals should be 1Vpp, filtered (no sharp transition), bit streams. The I-channel is near C3, 
  the Q-channel at C24. 

- At the COM-3002 receiver. 
  The same baseband signals can be visualized at the COM-3002 receivers at test points
  TP1 (I-channel) and TP2 (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.

(e) Verify demodulator lock:
at the COM-1005 BER measurement: 
  - TP1 is high when the module is synchronized with the PRBS-11 pseudo-random bit stream
  - TP3 is low (no bit error)

(f) Place the transmitter in 'unmodulated mode' by changing COM-1019 REG18 from x96 to xA6.
Using an oscilloscope, probe the COM-3002 test points TP1 and TP2. A sinewave is seen, 
the frequency of which is the total frequency error between the COM-4002 and COM-3002.
The demodulator AFC is capable of acquiring a frequency error up to 10% of the 
symbol rate (here 12 Mchips / 11 length of barker code * 10% = +/- 109 KHz).