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Design >> Analog Design >> RSSI
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Message started by aaron_do on May 21^st, 2007, 1:49am

Title: RSSI
Post by aaron_do on May 21^st, 2007, 1:49am

Hi all,

i'm designing a receiver signal strength indicator (RSSI) in analog, and i've got a question. One way to do the RSSI is to have a log amplifier followed by peak detection. The log amp usually consists of a series of limiting amplifiers with each of their outputs added together. So I noticed that for N limiting amplifier stages, most designers used N peak detectors and then added the DC signals together. However, to save power, I want to add the AC signals together and have one peak detector. Does anyone know why this isn't normally done?

thanks,
Aaron

Title: Re: RSSI
Post by didac on May 21^st, 2007, 2:37am

Hi,
I don't know for sure but I suppose that it's caused due to phase effects. If you sum DC signals(all with the same sign) you will have the maximum, if you sum AC signals each one will probably have a slight delay with the previous one(seen in a transient point of view) or a different phase(seen in a frequency point of view) due to the amplifier chain, in this case if you sum AC signals not in phase you won't obtain the maximum.

Title: Re: RSSI
Post by ACWWong on May 21^st, 2007, 6:52am

i agree with didac, also most of the limiting amp structures give you a simple peak recified level (for no or next to no extra power consumption) which when added together gives a good piecewise linear rssi.
out-of-interest, how would you propose to "add the AC signals together" ?

Title: Re: RSSI
Post by loose-electron on May 21^st, 2007, 11:17am

aaron_do wrote on May 21^st, 2007, 1:49am:

for N limiting amplifier stages, most designers used N peak detectors and then added the DC signals together.

Depending on signal input amplitude, some of the limiting amplifiers are slammed on, those are not contributing to usable singnal amplitude level information. Thus grabbing the information off of each stage. Did one of these things about 4 years back, probably still have the design buried someplace.

Jerry

Title: Re: RSSI
Post by aaron_do on May 21^st, 2007, 6:27pm

The way i want to add the AC signals together is quite simple. You have a differential amplifier, and instead of having only one input pair you divide it into several input pairs each one fed by one of the limiting amps. So you add the currents. In the actual design I combined the adder with the peak detector.

Also as for the point about phases i guess you are right. Actually I already took that into account. In my design, the phase shift from beginning to end is only 10 degrees which results in an error somewhere around 1-3%. Now that you mention it though, i had to force the limiter bandwidths to somewhere around 30 times the operating frequency which resulted in higher power consumption. So i guess maybe the standard way is better.

BTW Jerry, I don't understand your point about the limiters in limiting region not contributing to usable signal amplitude information. When you add the limiter outputs up, the limiting outputs contribute the largest part to the overall amplitude...

thanks everybody,
Aaron

Title: Re: RSSI
Post by RobertZ on May 25^th, 2007, 12:10pm

I did one RSSI before, not so great. But I used only one amplifier, one peak detection and some digital stuff.