RF Amplifier Power Limiter Stage

[alftel]

Hello my brothers in arms!

I do have a specific issue with RF amplifier that I developed for one of my users - it is a VLW (very low frequency) power amp that supposed to take as an input signal not exceeding 0dBm, otherwise output signal is distorted/saturated. User is going to use a variety of SDRs and other gadgets to supply input stimulus and has no control over output power from these devices. My initial idea was to a) tune pre-amp stage to accommodate presumably known signal level which in turn will require a "unique" atteniation value per RF source in question, or b) employ attenuator to drive input signal down. The problem is that input stimulus signal is not exactly known, and user is not knowledgeable enough in order to measure exact power level(s) from his RF gear. The only fact that he knows is that signal can be anywhere between 0dBm and all a way up to 30dBm. I searched carefully all commercially available offerings (like Minicircuits etc.) but couldn't find anything suitable. Another approach taken was to make power limiter out of two pairs of zener and silicon diodes, as per article here: https://www.maximintegrated.com/en/design/technical-documents/app-notes/4/4035.html (picture/figure #8) - it works in terms of protecting my pre-amp stage from over-voltage case(s), but signal is of course distorted in case input stimulus exceeds certain Vpp value. Is there any way to design lumped components circuit that will limit input signal up-to 30dBm to 0dBm without distortion? Any ideas? I am out of options, tried everything. Attenuators will be "straight between the eyes" solution, but then again, not practical in this particular case due to the fact that user will use different SDRs with a different TX power level(s). Any help, advice, reference will be greatly appreciated.

Cheers,
Alex

 

[BigBoss]

You have to use Output Power Observer then a Variable Attenuator at the Input by a Controlling Feedback.

 

[vfone]

The name of the circuit that you have to use is: AGC (Automatic Gain Control).
Searh the net for information, papers, theory and schematics. For low frequency range you will find a lot of information.
The idea is that, is not enaugh to get a variable attenuator (as transistors, PIN diodes, etc.) but also to get the right circuits to drive them, function input levels.

 

[FvM]

Are you dealing with CW carrier, or is the signal modulated and you want to keep the relative signal dynamic? What's the signal frequency range?

A smallband signal can be limited by a diode clipper followed by a low- or bandpass filter.

 

[biff44]

there are probably a number of ways to do it.
1) receive the signal, amplify it, clip it to be a stable power level, then use a fixed attenuator to drop the output level to 0 dBm. if the clipping causes some issues, lowpass filter the harmonics that are generated by it.

2) injection lock an oscillator to the signal. the oscillator is at a constant output power, so it can be attenuated to 0 dBm output. the only drawback, the oscillator runs even if there is not input signal.

3) a variation of 2) is the SuperRegenerative amplifier (a technique used in receiver back in the day). in that case there is not output if there is no input

4) amplify, send thru a variable gain amplifier that is controlled by an envelope detector at the output, then attenuate the whole thing to be 0 dBm output

5) amplify, sample the whole thing with an analog to digital converter that has a LOT of bits (high dynamic range). use digital math to change the digital gain, then drive a digital to analog converter.

6) have a VCO running at approximately the input frequency. Use an analog phase detector to phase lock the VCO to the incoming signal. Since the VCO output is constant in power, just attenuate the output to 0 dBm. use the large dynamic capture range of the PLL to catch anywhere from tiny to large input signals. You might need to limit the tuning range of the VCO to always be near where the input frequency will be.

7) set up an amplifier, and a delay line loop, to make a "frequency memory amplifier". i.e. you couple the input signal into the delay line, the amplifier boosts it, then sends it back to itself. you sample off the frequency circulating in the delay line loop for your output

8) amplify the input, send it AC coupled to a CMOS gate. Have the DC bias on the gate so it is right at the trigger level between a "1" and a "0". when the input signal is present, the gate puts out a square wave of the same exact frequency of the input signal, at a constant output voltage. then attenuate and lowpass filter it, and you have a constant power copy of the varying power input signal.


Come on guys, who can think of some more ways...