455KHz IF oscillator with only three parts

Ok Ivan, then I will see if I can find the one I use for training here at work.

You may remember that a couple of years ago we had a discussion about the NRO and I bought a heap of Russian tunnel diodes. I used one of them on a bit of veroboard with two crocodile clips that I could clank across any resonant circuit to make it oscillate.

This I used in classroom demos to measure the lengths of drums/coils of co-axial cable, with surprising accuracy. I have not delivered that course for a couple of years, but if I can find the prototype I will photo and post it here.

BR Harry - sm0vpo

sm0vpo wrote:I wonder if I posted my NRO cable testing oscillator?

Hi Harry,
I think you did not.

VBR from Ivan

This is an interesting circuit. I just love the NROs. Simple and cheap.

Replace the resonator for a capacitor and it will "hoot" nicely at audio frequencies. I have built this type of circuit many times and always impressed by the simplicity.

Yes, Ivan, there are loads of harmonics, mainly even harmonics. With a capacitor the output waveform is a saw-tooth, but if using crystals or ceramic resonators the harmonic content is greatly reduced. This is also the same circuit I use with a tunnel diode instead of the transistor, which gives a much cleaner signal.

I use it to measure cable lengths. Instead of the resonator, connect it (through a capacitor) to one end of a roll of coax, with the far end shorted, then measure the frequency on the oscilloscope or a counter. 100/freqMHz = length in metres, assuming cable VF = 66%. With other cables then (150 * VF) / MHz = length in metres.

I wonder if I posted my NRO cable testing oscillator?

BR Harry - SM0VPO sm0vpo

Hi,
I expect it should work with a two-pin resonator as well. The transistor seems to work as an avalanche generator, so it provides more spikes than a sinewave.

VBR from Ivan