In planning any ‘bandspread’ improvement, it is useful to understand exactly how varactor tuning works. The control level varies the amount of DC voltage applied to the diode. The highest produces the lowest capacitance, while the lowest voltage yields the highest capacitance. The spread is typically about 30 to 40pF. from minimum to maximum.The goal in achieving bandspread or ‘finetuning’ is to achieve a smaller variation in capacitance with the comfortable tuning of a knob.
Any resistor in a range from 500 ohms to 3000 ohms, connected in series with the ground lead of R2, will set a limiting effect on varactor capacitance range and therefore create a “bandspread “ effect. The higher the resistance value, the shorter the available tuning range.
If you wish to reduce the tuning range of R2 down to 40 or 30KHz or even less for a favorite band segment or calling frequency, reducing the value of C4 will decrease the capacitance effect of the varactor diode, D1. Values from 2 to 5pF may give you exactly the tuning range you are looking for.
If you are using your own enclosure and have room for a separate fine tuning control, one useful choice is a 250 ohm speaker attenuation rheostat (Radio Shack model no.
If you want the opposite of bandspread (that is as MUCH frequency range as possible over full rotation of the tuning knob ) increase the value of C4. This will cause the varactor diode to have greater effect on the tuning. Try values of 33pF up to 100pF.
FREQUENCY STABILITY:
A kilohertz of drift is not a big deal in FM circuits or a shortwave broadcast configuration, but ANY frequency instability is annoying when trying to receive CW or SSB. The varactor - controlled Local Oscillator is indeed a VFO, and therefore needs all the consideration given to any VFO circuit. Even though this tunable oscillator design takes good advantage of the SA602’s capabilities and offers nice tuning range from a simple varactor circuit, we do not represent it as suitable for transmitter frequency control or for demanding receiver applications. In any ham VFO design, ANY unshielded or uncompensated oscillator components are susceptible to the influences of temperature change and of nearby moving objects. The use of a suitable enclosure and secure mounting of the PC board within that enclosure will maximize the stability of the oscillator. One to two KHz of slow drift may be expected as components warm to operating temperature; after that the oscillator is reasonably stable
HR40 • 21
