I am running QPSK Demodulator (QPSK, ½ rate FEC) at 2.7KSPS( BW=3.5KHz) with AD9364 transceiver (fmcomm-S4 evaluation board)in following configurations.
· QPSK, ½ rate FEC
· 50MSPS sampling rate
· IF offset= 0MHz or 5MHz
· RF frequency = 1GHz
· FDD, CMOS mode
· AGC on
· Signal level = -45dBm to -75dBm
Following are the observations:
1. Demodulator is losing lock intermittently below 7dB SNR.
2. Constellation is observed to be elongated
Though symbol rate more than 5KSPS, this phenomena is not observed & demodulator works absolutely fine.
Digital Demodulator is simulated & tested thoroughly in AWGN environment.
We strongly feel, following could be the reason:
1. The internal AGC of the 9361 does not seem to operate correctly when using these low bandwidths, resulting in an elongated constellation. The incorrectly operating AGC seems to wrongly apply discrete gain jumps which are difficult for the demodulator to track.
2. There also seems to be significantly increased phase noise, which just serves to exacerbate the problem. The magnitude of this noise can exceed the loop bandwidth of the demodulator carrier tracking loop, resulting in the losses you are seeing.
Please let us know the possible solutions for above problems