I am using a pair of USRP2s, each equipped with a XCVR2450, for
transmission over-the-air of an RRC-filtered BPSK signal. The Tx antenna
has 3dBi gain, and the Rx antenna has 18 dBi gain. The transmitted
signal is at maximum amplitude, with gain set to 30 dB. The clocks on
each end of the link are running from the internal oscillators - i.e.
the clocks are not locked.
At the receive side, using an MM synchroniser and Costas loop, I am able
to see a BPSK constellation at the receiver when the Rx Gain setting is
The amplitude of the constellation points is around 0.15 in this
instance. However, when I increase the Rx Gain beyond 33 dB (in which
case the constellation is centered around +/- 0.2 on the scope sink),
there seems to be a large IQ amplitude balance, whereby the I signal is
much stronger. Indeed, the Q signal disappears entirely when the Rx Gain
is above around 36 dB.
Is this expected behaviour, and if so, can anyone please explain
why this is expected to occur?