Home Brew Permeability Tuned VFO

For something like twelve years, I've been trying to build a good VFO. Unfortunately, such is the nature of Genus Hammus that after I built each one my requirements became stricter. A friend once told me that what I wanted was a crystal—with a knob on it. Actually, I've tried that, but the range is too limited. I replied, wittily, that all that I needed to build what I wanted was: a case built like a battleship, a device with power gain that produced no heat, and a variable reactance that wasn't subject to vibration.

To some extent the features that I wanted are incorporated in the present VFO, and the result is pretty good. I built it out of scrap material using hand tools and a quarter inch power drill. The VFO drifts only a few cycles from turn on. has plenty of bandspread and is practically insensitive to vibration.

The oscillator uses basically the Clapp circuit. The eapacitive swamping is not as heavy as usually used, but this seems to have no bad effects in this case.

Circuit

The circuit as shown has been tested with various inductances and capacitances and oscillates up to 25 me. It would probably work above that frequency.

The transistors are 2N70S's. These are NFN silicon high frequency transistors operated in this circuit at very small fractions of their ratings. At least one manufacturer sells these at less than $1.50 each. Other similar transis-

W4VRV is an Asst. Professor of EE at Clem-son on leave to Ohio Slate while he's working on his Ph.D. and their radio observatory. He likes to design and build VHF gear.

tors will undoubtedly work in the circuit, but the 2N70S's are about as inexpensive as any readily available which have good high frequency characteristics.

The oscillator is followed by an emitter follower. This is followed by a class A amplifier which is followed by another emitter follower for low impedance output to a cable. The VFO shown will produce a useful voltage across a 50 ohm load. A 75 ohm load gives a little more voltage still.

It is not, of course, necessary to use the permeability tuning shown with this circuit, but the series capacitance should be kept fairly high for oscillation to start readily if a variable capacitor and fixed inductor are used.

Mechanical details

The circuit is built on Vector board using Hea clips.. Standard components were employed throughout, and no difficulty was encountered due to any unwanted couplings. The transistors are soldered in (afte.r mounting all the other components to the flea clips). The silicon -transistors are very tolerant of heat, but the iron should not be applied to the leads for more than about five seconds. That will be more than enough. No beat sinks were used.

The box used in this case to house the unit was made from !i" aluminum, top, bottom, front and back, With !s" aluminum sides. This is undoubtedly overdoing the rigidity bit, but it was available. A sufficiently rigid enclosure can be made from ?»" aluminum (rack-panel type) with angle stock at the corners (do it yourself stock available in hardware store or from Sears, Roebuck). A Minibox might do, but I've never found them rigid enough for VFO's. Look at the construction of a BC221, for example.

The heart of the VFO, LI, a permeability tuned coil wound on thick plastic stock.

Permeability Tuned Oscillators

The heart of the VFO, LI, a permeability tuned coil wound on thick plastic stock.

Use batteries for the power supply. The VFO frequency varies with voltage and even with a well regulated laboratory type AC supply some FM could be detected as a rough note at the 5th harmonic. Batteries smoothed it right out. Three "D" size flashlight batteries should last quite a while since the drain at 4.5 volts is 3 ma. This figures out to a total power input to the VFO of 13.5 milliwatts, which is one of the main reasons for the stability, There's practically no internal heating,

The inductor

To keep changes in the box from stretching the coil from the inductor is constructed to be supported by the box at only one point.

The slug, which gave the inspiration for this mode of construction, is from an old ferri-Ioopstick broadcast coil 3i" in diameter, with a 4-40 screw on one end, and a hole the right size to take a 4-40 screw in the other.

The bass and uprights were constructed from rectangular plastic stock /b" x Ja". The coil form itself is fs" round plastic. A hole )i" in diameter was drilled through the center of the coil form and then smoothed slightly by wrapping fine sandpaper around a smaller drill and working it back and forth until the slug slid easily through the form. When plastic is drilled with a high speed drill it tends to grab and melt and otherwise behave badly, so the drilling should not be rushed. The centering of the hole exactly is not extremely important, but it should be straight and as parallel to the form as possible. One end support was then glued on with polystyrene cement and the ii" hole continued through the coil form through the support. (Let the cement harden thoroughly first.) Both supports are then glued to the base and the coil form glued to the other support. After drying, the hole is drilled back through the first support and coil form and through the remaining Support. This

View of the transistors in the VFO. Note the solid construction.

Permeability Tuning

View of the transistors in the VFO. Note the solid construction.

way, all the holes line up exactly.

A 4-40 nut is then put on the screw in the slug and the slug is slid into the form, The nut is then heated with a soldering iron until it melts securely into the support. Another 4-40 screw, at least two inches long is then threaded an inch or so into a nut, and %" or so of its threaded end is liberally smeared with epoxy cement. The hole in the slug is also smeared 011 the inside with epoxy cement for about Ji". (A toothpick serves well.) The slug, in the coil form, should then be positioned so that when the nut on the two inch screw is melted into the remaining end the screw will go into the hole for about the 11" that has been glued.

Once assembled in this fashion, the nuts, screws and form will be in alignment, and the slug should revolve freely for S" or more of travel inside the form. Unfortunately, once assembled in this fashion, il is impossible to take the assembly apart without breaking something, so be careful!

This may sound involved, but once the drilling of the form and supports is completed and the plastic cement has hardened, the remaining steps take about as long as it takes to tell about them. The important thing is the order of assembly—which should be fairly obvious.

The coupling from the dial to the inductor is a piece of tubing with an inside diameter that the 4-40 screw on the slug will slide inside. The tubing is slotted (with a coping saw) on one end, and built up with wire on the other to fit the U" hole in the dial. A short strap is soldered into the slot in the screw on the slug. This strap rides in the slot as the slug rides in and out and serves to transmit the circular motion from the dial to the slug. This method of construction also^ avoids transmitting any lateral motion to the slug due to the expansion of the case from heat.

The winding was put on the form and cemented with Q-dope. Since I was only interested in proving a point, rather than absolute linearity, only an approximation was made to a winding that would give the ultimate in linearity. Even so, one revolution of the screw

Fig. 1, A Home Brew Permeability Tuned VFO. See text for LI. Atl transistors are 2N708. Battery voltage is not critical; a 4.2 volt mercury ■05 out battery is recommended for best per-pHQr^ formance. rb gives 35 kc at one end of the range (5 mc) and 50 kc at the other end (5.8 mc) with the winding as shown. Further adjustment of the spacing of the winding before gluing would have improved that. This particular frequency range mixes with my 9 mc filter output for eventual use at 50 mc.

The winding in use is twenty-seven turns of #22 enamelled wire. The first ten turns are close-wound, and the next seventeen with gradually increasing spacing. The spacing between the last few turns is about vro".

Results

The resulting tuned circuit is temperature sensitive because the dimensions of the ferrite slug are temperature sensitive. However, the input to all four transistors is around a tenth af a watt, and the oscillator shows no discernable drift caused by internal heating during operation.

Uncompensated for temperature, the VFO was taken from a cold car' into a heated room, hooked to a counter and turned on. The drift was down in frequency ancT~steady, about ten cycles per minute. The VFO was turned off and left in the room for a couple of hours. Turned on again, after reaching room temperature, the total drift was 150 cycles in 24 hours from a cold start. If the temperature where the VFO is to be used is not steady, some form of temperature"' compensation should be employed. Negative temperature coefficient capacitors should do nicely here.

As currently constructed, the VFO has several drawbacks. There wasn't enough heavy aluminum available to build the box big enough to put a good turn-count dial on it. A ten turn Revod.ex dial has been tried, but is not entirely satisfactory. The output voltage is only a volt 01' so mis. For use with a mixer, this is enough voltage, but if the VFO is to drive a crystal oscillator stage, another stage of amplification will be needed. The input resistor in the grid circuit of such an amplifier stage could be 100 ohms or so, thereby removing the necessity for neutralization.

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