Please review method of checking RF amplifier stability i use (schottky diode probe)

[Terminator3]

I guess when we using spectometer, we can just check the spectrum when device is powered on and powered off, then find out if oscillation is present.

As i have no decent signal analyzer, i use another approach:

I make a simple probe with schottky diode, one pin grounded and other pin connected to OpAmp. Next, i put this probe near working FET amplifier and move my hand around. If there is some signal from OpAmp, amplifier is oscillating. OpAmp can be connected to small speaker or earphone. Probe is totally stand-alone device with it's own ground and power.

Idea is that diode working as receiving mixer. If FET is oscillating, it provides LO to diode. LO reflecting from moving hand gives it a doppler shift, so diode simultaneous receives two frequencies: FET's oscillating frequency and the same frequency shifted by moving hand or object. As a result of mixing in diode, we obtain some low frequency IF, that indicates existance of RF source.

Is it reliable method to check RF existance? Actually i tried some FET's and here what i get:

- when slowly changing bias on gate toward negative (-2v), there is a point, where oscillation stops. First, signal becomes more noisy and distorted, and at suddenly dissapears.
- it is easy to find oscillation at frequencies higher than 2GHz (the higher frequency is, the higher doppler tone is, so easy to hear it).
- sometimes there is still very weak signal received, even i thought FET is biased and matched almost perfectly. I think that FET is actually stable, but it amplifies some air signals, and output from it's drain becomes enough for the diode to provide an IF.

So i did a conclusion, that most of times (or always?) if something becomed oscillating in amplifier, it oscillates very well, and produced RF power is easily detected by schottky self-mixing probe.

 

[jiripolivka]

I guess when we using spectometer, we can just check the spectrum when device is powered on and powered off, then find out if oscillation is present.

As i have no decent signal analyzer, i use another approach:

I make a simple probe with schottky diode, one pin grounded and other pin connected to OpAmp. Next, i put this probe near working FET amplifier and move my hand around. If there is some signal from OpAmp, amplifier is oscillating. OpAmp can be connected to small speaker or earphone. Probe is totally stand-alone device with it's own ground and power.

Idea is that diode working as receiving mixer. If FET is oscillating, it provides LO to diode. LO reflecting from moving hand gives it a doppler shift, so diode simultaneous receives two frequencies: FET's oscillating frequency and the same frequency shifted by moving hand or object. As a result of mixing in diode, we obtain some low frequency IF, that indicates existance of RF source.

Is it reliable method to check RF existance? Actually i tried some FET's and here what i get:

- when slowly changing bias on gate toward negative (-2v), there is a point, where oscillation stops. First, signal becomes more noisy and distorted, and at suddenly dissapears.
- it is easy to find oscillation at frequencies higher than 2GHz (the higher frequency is, the higher doppler tone is, so easy to hear it).
- sometimes there is still very weak signal received, even i thought FET is biased and matched almost perfectly. I think that FET is actually stable, but it amplifies some air signals, and output from it's drain becomes enough for the diode to provide an IF.

So i did a conclusion, that most of times (or always?) if something becomed oscillating in amplifier, it oscillates very well, and produced RF power is easily detected by schottky self-mixing probe.

In principle your method using a probe with a detector can detect RF oscillation, but:

1. you would have to characterize the probe-detector response over a wide band using a calibrated RF signal generator. Otherwise, your probe can be insensitive over an important RF band.

2. Using a diode as a mixer is possible, but hearable sounds only develop if both signals to beat are stable which a parasitic RF oscillation is not. At 2 GHz I doubt you could hear anything as the frequency changes fast.

3. In a RF amplifier, the easiest way to detect unwanted oscillation is by monitoring the DC current (plate, collector, drain). If it varies smoothly with the smoothly applied input power, there rarely is an oscillation. Abrupt current changes (up or down) indicate a parasitic oscillation.