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Design >> RF Design >> RSSI for FSK receiver
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Message started by Visjnoe on Jul 30^th, 2009, 9:24am

Title: RSSI for FSK receiver
Post by Visjnoe on Jul 30^th, 2009, 9:24am

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

Title: Re: RSSI for FSK receiver
Post by aaron_do on Jul 30^th, 2009, 10:21pm

Hi Peter,

for a low-IF architecture you can do image rejection and then just use one side, but for a direct conversion receiver I believe you are right. Have a look at this paper where they did exactly what you described.

A. Porret, et. al, "An Ultralow-Power UHF Transceiver Integrated in a Standard Digital CMOS Process: Architecture and Receiver" IEEE JSSC, VOL. 36, NO. 3, MARCH 2001

cheers,
Aaron

Title: Re: RSSI for FSK receiver
Post by ACWWong on Aug 1^st, 2009, 3:43pm

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

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.

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

Title: Re: RSSI for FSK receiver
Post by Visjnoe on Aug 2^nd, 2009, 7:41am

Hello,

Aaron, thanks for your comments and references to papers.
Aw, thanks for your remark with regard to the zero frequency error.

Regards

Peter