How to obtain received signal strength using log amps or similar chips.

[rwgast_lowlevellogicdesin]

I got my hands on some decent LT power detectors and AD log amps, and I would like to build a circuit that is completely generic and plugs in to the RF front end to obtain received signal strength. This sounds silly but I want to build an antenna diversity controller that is wide band and generic for use with any SDR or Analog radio.

The more I thought about this the more I think I am going about this the wrong way and it may have to be hardware dependent. Would a standard S meter or rssi meter be built by tapping a directional coupler to the IF and then running the signal to a power detector? If so this shoots wide compatibility since most SDRs do the IF internally and would call for specific drivers that receive the signal strength from the PC. Dave just had some meter in a mailbag that could test receive strength of an antenna though, so maybe there is a way to do this on the RF front end before the antenna without IF tapping??

[Kalvin]

Analog Devices has a set of nice RMS-power detectors usable from DC up to a few GHz. If you place a wide-bandwidth MMIC amplifier to feed the RMS-power detector, you are pretty much done.

[rwgast_lowlevellogicdesin]

I guess what I don't understand is how that tells the received strength of the signal your tuned to? I would think you would have to measure the strength of the IF not the front end?

[Kalvin]

I was referring to the first part of your original post of building something using LT's power detectors and ADI's log amps.

[Kalvin]

If you can gain access to the IF section you can measure the received signal strength. Otherwise you need to have another receiver or some other tunable circuit to measure the signal strength at the frequency you are listening, something like an SDR dongle.

[rwgast_lowlevellogicdesin]

Well basically what I'm trying to do is use some omoron RF relays to switch between  4 patch arrays, this way I can use hi gain antennas and still get omni directional coverage. I'm using RTL dongles and airways, the only way I can think of to get signal strength is read it from the PC. The problem is this is driver dependent and I'm not even sure if I can access that info while other programs are using the sdr.

I wanted to build a totally driver/software independent solution that could be used with any sdr to switch any antennas. Anyone have any thoughts on doing this? I thought about maybe piping sound output and measuring that... The problem is if your running a program which doesn't output noise your kind of screwed.

[Kalvin]

Raspberry Pi can be used with some SDR-dongles. However, I do not know how well that will suit your needs. There are some demos in Youtube about using the Raspi with a SDR-dongle.

[rwgast_lowlevellogicdesin]

I already have a pi running with and RTL Dongle and GNU Radio etc. Im acually trying to design an antenna controller for myself and to sell. I figure there are alot of people out there with a few different types of antennas that would want to be able to switch between them which is easy enough, but I want to be able to cater to people like me who want to run arrays with electronic steering and post processing.