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My amateur radio interests are mostly in the microwave regime. Recently I snagged some 10GHz Gunn sources from SHF Microwave to try out. Luckily, I have access to a spectrum analyzer at work that measures signals up to 40GHz! So I hooked a horn up to the analyzer for receiving, pointed the Gunn source at the receiving horn, and powered up the Gunn source. The result is a spike at the Gunn source's frequency, which is modifed by a tuning screw on the Gunn source. Changing the position of this tuning screw modifies the resonant frequency of the small chamber that the Gunn diode is in, and thus changes the frequency of the signal that is spewed out of the horn on the end.

Normally one can modulate the Gunn source signal in one of two ways:

• Direct modulation of the Gunn source's power supply. This is basically direct AM modulation, although the modulating signal could itself be FM modulated.
• Indirect frequency modulation of the Gunn source by biasing a varactor diode. Varactor diodes have a capacitance that varies with the bias voltage applied to them; by changing this capacitance you change the resonant frequency just as if you were twiddling the tuning screw.

The "experimenter" sources I bought don't have a varactor in them, but luckily I have acquired some samples normally used for ESD suppression. Plus, the Gunn sources have a screw-on back panel that is easily removed. So nothing prevents me from taking the Gunn source apart and sticking a wired-up varactor in there. Perhaps it will even work.

The next experiment involves actually making a link with the gunn sources. The bandwidth of this link is unfortunately limited by both the gunn source (which can really only be modulated a few Mhz, I am told), and the FM receiver which expects an audio signal and so filters out everything above a few khz.
kg6cga

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