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Building stable variable freqency oscillators- Some considerations.

May 14th, 2008
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A stable variable frequency oscillator is the heart of any home brewed ham radio gear. This article will describe various aspects of choosing circuits , components and construction

Choosing a VFO circuit.
Variable Frequency Oscillators (VFO) are used is almost all ham gear.A stable and reliable VFO is the dream of any homebrewer Any circuit that provides stable oscillations can be used as a VFO.There are several circuits available on the internet. Some of the popular ones are Heartly , Colpits Vacker etc. Each amateur has his on preference regarding the circuit combination.

Choosing components.
VFOs can be built around transistors or FETs. I use BFW10 fet or 2n2222/BC107 depending on availability in my junk box.Any other equivalent transistor can be used. Make sure that Ft(unity gain bandwidth) of the transistor is at least five times your operating frequency. The transistor must have sufficient gain(hfe). The other important components are resistors and capacitors. You can use quarter watt carbon film resistors(with 5% tolerance)). As to capacitors, styroflex capacitors are preferred. However the availability of styroflex capacitors may be a problem for most of the homebrewers. NP0 ceramic disk capacitors can be used instead. You can rescue NP0 capacitors form old cordless phones or TV sets. The tank circuit consists of an inductor and an variable capacitor. You can wind your own air core inductor.This can be done on any former. I have wound coils on tap washers , pill bottles ,PVC pipes etc. Use your ingenuity in finding a suitable former. The gauge of the wire is not much important. I generally use what is available in my junk box. Air dielectric metal gang condensers are preferred for variable capacitors. Availability may be a problem in this case also.(Look out of old valve radios). Polyvaricon capacitors used in cheap radios can be substituted instead.
Building VFO
You can use a PCB if it is readily available. Otherwise ugly or Manhattan style construction can be utilized. Double side pcbs are not preferred and it can provide unwanted stray capacitance. If you are designing your own PCB avoid using long PCB traces as this can add unwanted inductance. It is suggested that the VFO supply voltage is kept low from the supply voltage of succeeding stages. This will provide stability. Use a Zener diode to regulate power to the oscillator stage. Always use decoupling capacitors.near the Zener.Finally , keep the VFO in a suitable metallic enclosure. This will prevent unwanted RF leakage.
VFO drift.
VFO drift means small change frequency from the present value. Two kinds of drift is noticed in VFOs. a) Short term drift and b) long term drift. Short term drift is heating up of semiconductor junctions and other components due to RF currents.Long term drift is caused by a variety of reasons such as humidity,ambient temperature etc.
Short term drift is predominant in the first few minutes of VFO operation. It is suggested that if possible keep the VFO powered even standby periods.Chose high quality components such as NP0 capacitors. Avoid magnetic material as core of the tank coil. Trimmers should me air core if possible. Cheap mica trimmers should be avoided.

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