# Why is my experiment to produce 3rd order harmonics by saturating a receiver not working

I've been trying to devise an experiment to produce a third order harmonic in radio receiver that's been saturated, but I'm not seeing the expected results.

My understanding of the theory is that when you drive an amplifier ( refer to the diagram below ), past the point of linear operation whether by an on or off frequency signal, it will produce harmonics of and primary frequencies input into it. ( See diagram below ) I thought I could observe this effect with a handheld two-way radio and 2 signal generators I have on hand.

My experiment set up is thus: A multiple band, 2-way FM transceiver, programmed to to receive on 471MHZ and two signal generators. One signal generator generates an FM 157MHZ signal, modulated with a 1kHz tone, the other signal generator is used to produce an unmodulated CW carrier. Note that 157MHZ x 3 = 471MHZ. I expected that I could use the CW signal to force the receiver into non-linear operation causing the amplifier to produce 3rd order harmonics of the 157MHZ FM signal injected below saturation.

The results were not what I expected. No matter what frequency or power level the CW was set to, it didn't seem to affect when the receiver opened up. If the FM signal was set so that it's 3rd order harmonic was being received, I could overload the receiver by cranking up the CW wave though, so I know it was getting into the receiver. I was expecting that the CW signal would force the receiver into non-linear operation, and that I would be able to receive the 3rd order harmonic of the FM signal at a lower threathold than by itself. Anyone have an idea why I didn't see what I expected?

• Are you sure the modulated 3rd harmonic is coming from the front end of the radio and not the signal generator? May 7 at 17:35
• @Duston I'm pretty sure. Using a IFR1200 as the primary signal generator. -60dBm is where the harmonic is opening the receiver. That's well with in normal operation for this instrument, so I wouldn't expect it to produce harmonics. But I will verify on my bench this afternoon. May 7 at 18:04

Receivers have passive band-pass filters at their input. Except possibly ferrite cores (which are unlikely to be used for a filter above HF), the components used to construct these filters are very linear. For a receiver tuned to 471MHz, the output of your signal generators at 157MHz is surely well within the filter stop-band.

Although the filter's stop-band attenuation isn't infinite, and this means at some power you could still saturate the receiver, it's going to require a very high power for the fundamental, after being attenuated, to have enough power to cause significant nonlinear behavior in the active components behind the filter.

Further consider, the signal generators produce harmonics as well. The receiver's filter attenuates the fundamental from the signal generators, but not the harmonics.