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90 MHz VFO for Frequency Modulation

A controllable source of RF for experiments on the FM Broadcast frequencies

Notice the abundance of 2N4401, 1 k resistors and 330 Ohm resistors? That's because I have a lot of each. 2N4401's weren't intended to be used as varactors, but they work just fine. Don't be afraid to experiment with the parts you have on hand.

Build this on a ground plane board. Layout is important. If you place the parts relative to one-another in correspondence with their placement on the schematic, it should work pretty well. For making one or two of something, I don't like to bother with a printed circuit boards. I just cut off a 2.5 x4 cm piece of copper clad board and soldered grounded parts onto the copper cladding fiberglass, and the ungrounded parts directly onto the leads of the grounded parts. This "dead bug" construction is a very speedy way to put high frequency circuits together and it works pretty well at these frequencies. One thing to be careful of is to make sure all the components are packed tightly so as to minimize parasitic resulting from long leads. In the ones I built, the components are leaded, though in one of them I used chip capacitors for the bypass capacitors, though I couldn't see any difference in performance.

It was necessary to shield the circuit so that it would not be affected by other circuits nearby, particularly additional RF stages. The loose coupling for the RF, provided by the 330 build-out resistor on the emitter follower further serves to isolate the oscillator from its load. These result in greater stability. The shield was made by forming a five-sided box (one side missing) from tooling copper (a thin copper sheet) and soldering it onto the groundplane of the copper clad board. Once, for something similar, I made a shield box by soldering pieces of copper clad board together. Nearly anything will do as long as its reasonably low resistance material (brass shim stock should work if you have a piece laying around).

The inductor is made by winding 8 turns of #24 insulated solid copper wire on a 5 mm screwdriver. I used a conductor from a piece of category 5 quad twisted pair, left over from wiring the house with Ethernet and this seems to work well and to be easy to handle. Stripping the vinyl insulation is much easier than scarping or burning off enamel.

This was easily tuned up and down the bottom of the FM band using a 10 turn panel mounted potentiometer. The upper limit to the frequency modulation is set by the 39 k input resistor and the shunt capacitance (I would guess around 20 pf max), so that shouldn't be a concern until the modulation frequency reaches many MHz.

The 2N4401's give a span of 8 MHz with 6 volt change in control voltage.

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