Initially I was using RFPD-1611to get the IR beat frequency. Its gain was not very high, so I was getting a very low signal of power -37 dBm.
I used ZHL-32A-S with a gain of 25 dBm to amplify it before feeding it into the spectrum analyser.
I connected the ground of the amplifier circuit to the red of the power supply, which blew the amplifier.
I learnt that there is a small tab indicating the ground side of the BNC to banana connectors which I should have noticed.
I learnt to plug in the side with th little tab on it into the ground of the power supply. (Learnt it the hard way I guess!!)