Receiver characterization and the effect of the thermal noise!!

Hi,

Our expected threshold of the receiver is -110 dBm. The following are the observation at the time of characterization of the receiver.

1). We are not able to demodulate signal beyond -80 dbm.( Currently we are using quadrature type demodulator)

2).We have observed C/N ( Carrier to noise ratio) of 10 dB upto -97dBm , also there is a proper FM modulation exist upto -97dBm.

3). We have also observed carrier off noise( ,ie, no Rf input--- means thermal noise) at the demodulator output is very high.( our spec is 150 mv rms and presently we are getting some around ~ 500mv rms).

The demodulator circuit consist of a passive mixer( double balanced mixer) and a phase shifter network(90 deg phase shift), whether mixing of the incoming signal with its own 90 deg phase shifted signal will reduce the threshold?

a.) Here in the demodulator can we use an active mixer instead of this passive mixer ? and this can increase the threshold?

b). I am also thinking to use a phase frequency detector instead of mixer? will this help me in extending the threshold upto -110 dBm?


Waiting for a great help!

if this is a FM receiver, when you put on an unmodulated carrier, the receiver should quieten down. If it does not, try to "kill" each stage in turn (or decoupling their outputs) starting from the Rf working towards the detector until the noise decreases dramatically . Is your bandwidth matched to the bandwidth of the modulation?.
Frank

Sir,

if this is a FM receiver, when you put on an unmodulated carrier, the receiver should quieten down----- Plz explain the statement.Yes the receiver bandwidth matched to modulation bandwidth.

Regards

Avijit

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Our is a FM receiver>

In the absence of a carrier, the noise from the front end will be amplified and demodulated, resulting in your "noisy" output. If an unmodulated carrier is applied, the limiting action in the IF will remove the noise, giving only the demodulated noise which is modulating the carrier (not much I hope!).
Frank

I guess something oscillates on the receiving chain..