Dear all,

In most papers on FSK receivers, the stages of a chain of limiting amplifiers are tapped and rectified to provide a RSSI function.

However, in most papers it is presented as if the RSSI can be calculated based on 1 signal path (e.g. I or Q). However, I would think that e.g. in a direct-conversion receiver, the TX-RX carrier phase offset might result in no (significant) signal in the I or Q path.

So I would think you need to combine the (rectified) currents from both the I and Q signal paths. Is this correct?

Regards

Peter

Visjnoe wrote on Jul 30^th, 2009, 9:24am:


This is only true if there is zero frequency error between tranmsitter and receiver. If there is frequency error (and in most practical situations there are), then a fixed phase offset between RX & TX cannot persist for long, so one finds that signal amplitudes on I and Q will be equal (once the the relative RX-TX phase offset has fully cycled 360^o) even if its "direct conversion". Since RSSI is an "averaging" function, you'll find that most direct conversion receivers only need do RSSI on I or Q since over time signal amplitudes on I and Q are the same, if there is finite frequency error. If however your RSSI needs to be quick in relation to the frequency error, or there is no frequency error, then you'll have to combine I & Q to get a true instantaneous RSSI value as you say.


cheers
aw