Can an I.F. signal be used like a local oscillator?

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Larry Lanberg:
Been trying to build my own VHF converter, for connection to shortwave radio, but I'm having a difficult time getting the l.o. section to work. In fact I can't get it to oscillate at all. Maybe the (115 MHz) crystal that I have is bad, I don't know. I've tried several different l.o. designs, but none will oscillate for me. However, yesterday I noticed that my Cable TV box puts out a strong 135 MHz signal. I think that's an I.F. signal. Am wondering if I can use that 135 MHz signal as a 'proxy' l.o. to convert off of, by running that into the mixer circuit as though it were my local oscillator.

Does anyone know if the I.F. signal of another device could stand in for a local oscillator? Thank you.

Dieter Kuespert:
It does actually not matter what generates the signal as long as frequency and amplitude are what you need.
From own experience I can tell you that xtal oscillators can be hard to get going at frequencies above about 60 MHz because they are 5th overtone. You might try one of those active xtal oscillator circuits.

Larry Lanberg:
Thank you very much. I'm going to try to complete this, using the 135 MHz signal from my Cable TV box, will see what happens.

Dieter Kuespert:
It took me a while to locate this: http://www.qsl.net/sv1bsx/2m_converter/2m_conv.html
The oscillator of this circuit did not work without the L*. After inserting it, however, it was strong. Good circuit for downconverter.

Larry Lanberg:
Quote from: KA4POL on May 12, 2013, 10:44:27 AM

It took me a while to locate this: http://www.qsl.net/sv1bsx/2m_converter/2m_conv.html
The oscillator of this circuit did not work without the L*. After inserting it, however, it was strong. Good circuit for downconverter.


Thank you, KA4POL. Actually, that's been a large part of my problem so far: getting/making the right value inductor to force the VHF crystal into resonance. Apparently it must be just right, not a little off. I tried to wind my own small value L's -- e.g., 5 turns on a 7 mm form, as per a schematic I found -- but I don't have the proper tools to go that small, physically, and get it done right. So I've ordered some inductors from a parts vendor. Right now I can do most things except wind small form coils.

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