Unstable VCO

[fuseless]

Hi Folks,

I am trying to tune a voltage controlled oscillator (Colpitts type) I recently designed from scratch. I will be using it as an RF modulator to inject a 1.050 KHz signal, for alignment purposes, through the front end of a weather band receiver I will be building in the future and that will operate between 162.400MHz - 162.550MHz. I can get a strong and quiet signal on my store bought receiver but only as long as I keep the tuning wand on the frequency control voltage potentiometer. As soon as I remove the tuning wand, the frequency shifts. If I put my hand a few inches from the circuit board, without touching it, the signal reappears on the receiver. The VCO at present is operating on batteries only, including the frequency control voltage. Anyway, I just don't understand how my body, without touching the circuit board, can couple to the VCO enough to cause a shift in frequency.

Does anyone know how to prevent this frequency shift from happening?

 

[Boncuk]

Build your oscillator circuit carefully and use a metal enclosure for shielding.

For extra frequency stability use a controlled heater circuit for constant temperature.

Getting close to the circuit you act as a capacitor shifting frequency.

Boncuk

 

[RadioRon]

Boncuk makes a good point. When you place a part of your body close to the board, you are adding a tiny bit of capacitance in parallel with some of the critical resonant circuitry on the VCO board. Imagine that your VCO is resonating with, say, 33.000 nH inductance and 29.0682 pF of capacitance (just a wild guess for example). Now, how much change in the capacitance would move the frequency by, say, 30 KHz (enough to move it out of the channel of your receiver)? Well, it would take 0.01073 pF. That's a very small capacitance change, and moving your hand nearby easily adds this much.

The way to fix this problem is to shield your circuitry. One of the easiest ways is to solder additional pieces of unetched copper pcb board into the shape of a box around your circuits. For the best performance, all DC going in and out (except GND) should go through a feedthrough capacitor, and RF should be taken out through a coaxial cable connector. Just as easy is to use sheet brass, commonly available at auto parts stores but they call it "shim stock". Buy it in a conveniently workable thickness, like .010 inches. Brass solders very easily and is easily cut with common tin snips. If you get the really thin stuff, like .002 inches, you can cut it with scissors too, but when it is this thin, it gets a bit wobbly and you run into another common VCO problem. Its called microphonics.

Microphonics is where your VCO is unintentionally modulated by nearby sound, or any vibration for that matter. I've made VCOs like this, where you just need to talk loudly at the VCO and you get FM from it! This is caused by very microscopic vibration of the components on the circuit board, or even the individual turns of your tank inductor. The solution is to use surface mount components and inductors that are potted in solid plastic.

Oh, and about that tuning wand. Well,that's a common problem too and not easily fixed. If your tuning wand is not metallic, then the problem is that your capacitor setting shifts very slightly when the pressure of the wand is removed. You could try a better quality variable capacitor, but they all do it to some degree. If your wand is metallic, then you are adding additional capacitance near the variable part, which is bad. Don't use a metallic wand. I've also seen where the part where the metal wand touches the capacitor is conductive and in this case that side of the capacitor should be the grounded side.

 

[TiagoSilva]

Or you can always use an IC with this function, Maxim has some pretty good VCOs, have a try, the Maxim VCOs are very stable, i've seen some RF simple circuits that use them. Elektor has a simple FM microphone that uses a Maxim microphone amplifier and a VCO for 88 - 108Mhz band. (I think) This same VCO can be used for 162Mhz (if not, just find the right version of the same VCO), just change the inductor and then the voltage applied in the VCO tunning pin.

 

[Boncuk]

If you get the really thin stuff, like .002 inches, you can cut it with scissors too, but when it is this thin, it gets a bit wobbly and you run into another common VCO problem. Its called microphonics.

That problem can be overcome by soldering pieces of copper wire on the brass. (inside of course because of the beauty of work ) The wire just adds a bit weight and strenghtens the brass.

 

[Bob Scott]

Shield the circuit completely. Install enclosing shields on both sides of the PCB.
Use NPO (COG) ceramic capacitors. They have the lowest drift with temperature.
Use a non-metallic adjustment tool if required.

The industry used to use crystal ovens in order to keep crystal oscillators at a constant temperature to keep them from from drifting. Nowadays, they use temperature compensating components instead. ie: If the oscillator tends to drifts up in frequency with temperature, change a capacitor to one with a temperature characteristic that tends to reduce the frequency. That way, the errors cancel.

Bob

 

[fuseless]

Thank You

Boncuk, RadioRon, TiagoSilva, and Bob Scott... Thank you all very much for your input. So far I've switched from an external power supply to batteries located inside an aluminum inclosure. Also, I removed the VCO control voltage potentiometer from the printed circuit board and replaced it with a panel mount type so I tune the unit from outside the enclosure shielding. It's amazing what effect shielding has. There is very little coupling between me and the VCO now. O yeah, I reduced the control voltage range because the potentiometer I'm using is only a ¾ turn type. It's allot easier to hit the target frequency without flying past it.

Thanks Again...

Fuseless...