TABLE OF CONTENTS The first LO in most receivers must be capable of being tuned over a frequency range, offset from the basic receiver RF tuning range. In some cases, this requires tuning over many octaves, while in others, a much more narrow frequency range may suffice. Prior to the advent of the varactor diode and good switching diodes, it was customary to tune an oscillator mechanically by using a variable capacitor with air dielectric or, in some cases, by moving a powdered iron core inside a coil to make a variable inductor. This resulted in tuning over a range of possibly as much as 1 to 1.5 octaves. For greater coverage, mechanical wafer switches were used to switch among different coils and/or fixed capacitors.
At higher frequencies, transmission lines can be used for tuning. Short-circuited lines less than a quarter-wave long present an inductive reactance at the open end and may be tuned by a variable capacitor shunted across this end. Open lines shorter than a half-wave may be tuned to resonance at one end by connecting a variable capacitor across the other.
Whether the oscillator is tuned mechanically or electrically, the capacitor is most often the source of small frequency changes, while the coil generally is switched for band changes. This can be achieved more or less easily in many different oscillator circuit configurations. Figure 7.108 shows a number of circuits used for VFOs. Different configurations are used in different applications, depending on the range of tuning, whether...
