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Design >> RF Design >> Analog or digital RSSI?
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Message started by Visjnoe on May 15^th, 2009, 1:44am

Title: Analog or digital RSSI?
Post by Visjnoe on May 15^th, 2009, 1:44am

Dear all,

In typical FSK receivers, there are 2 ways in implementing the RSSI function: in the analog domain or in the digital domain (after the ADC). In the analog domain this is mainly done by tapping off some limiter chain. In the digital domain, I^2+Q^2 is typically calculated.

What is the benefit of doing it in the digital domain? I can only think of scalability/robustness. Perhaps also you can still make use of DSP/channel filtering in the digital domain to further suppress interferers, thus avoiding their influence on the RSSI.

Any more reasons?

Regards

Peter

Title: Re: Analog or digital RSSI?
Post by cmos_cowboy on May 15^th, 2009, 3:54pm

Hi Peter,

One of the big factors is area. This obviously depends upon your IF frequency, as essentially what you are doing is rectifying and then lowpass filtering. In the analog domain the lowpass can be quite large, and on some chips this was/is done with an external cap (hence costing a pin).

You must also consider what the RSSI signal is used for.
If it is used to control a analogue AGC, to switch in/out an amount of gain or to just to record the signal strength.

What accuracy is required? absolute and relative? will it need calibrating?

I hope this helps,

Regards,

Matt.

Title: Re: Analog or digital RSSI?
Post by loose-electron on May 16^th, 2009, 4:41pm

Analog RSSI (using a log amp) is generally done when you have a wide dynamic range of the signal that is beyond the resolution of the ADC.

For example - a receiver with 96db of dynamic range, has got a lot of freedom in the switching of analog gain stages to get the ADC properly fed.

Title: Re: Analog or digital RSSI?
Post by ndnger on May 19^th, 2009, 1:01am

An RF RSSI is required to set the LNA gain without which the mixer can get saturated.