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Ian Braithwaite, G4COL, believes that, even in an era of direct digital synthesis (IDDS), there is still a place in relatively simple home-brew gear for the variable crystal oscillator. He writes: "The main challenge is obtaining as wide a pulling range as possible [without resorting to ceramic resonators normally available on only a few frequencies-G3VA]. It is often overlooked that the pulling range of a single crystal can be extended by using two basic oscillator circuits, with a given crystal transferred from one to the other.

"Most variable crystal oscillator (VXO) or voltage-controlled crystal oscillator (VCXO) circuits use the crystal as a series device in a single transistor circuit. Good behaviour and pulling range can both be obtained quite easily, though the pulling range depends crucially on the parameters of the crystal itself. Criteria for good behaviourare:

Fig 1 - G4COL's preferred VCXO circuit using the crystal as a series device.

"My favourite circuit is shown in Fig 1. The crystal is a series device with insufficient gain for oscillation without the crystal in circuit. A low-Q collector-tuned circuit prevents oscillation at remote frequencies, while giving sufficient gain at the crystal frequency to keep oscillating over a reasonable pulling range. Using a series inductor, the frequency can be pulled both below and above the series resonance of the crystal.

"However, as the frequency is pulled upwards (towards parallel resonance) by means of a very small capacitance in series with the crystal, the oscillator loop-gain falls and oscillation ceases. This means that there is still 'unexplored territory' around the parallel resonance. Access to this can be gained in a second oscillator using the crystal as a parallel element.

Fig 2 - G4COL's two-device VCXO with the crystal as a parallel resonant device- A combination of the two VCXOs provides him with an extended pulling range of some 112kHz with 10MHz crystals not designed for frequency pulling.

"A suitable circuit is shown in Fig 2. Two transistors are used to give the gain and phase-shift required for oscillation, with very small (2.2pF) coupling capacitors. The aim should be to keep capacitance in parallel with the crystal to a minimum in order to reach as high a maximum frequency as possible. The gain of the circuit, which can be set by the emitter and source feedback resistors, must not be made too large, or the circuit will oscillate without the crystal, risking the loss of crystal control. I modelled the circuit using TINA circuitanalysis software, based on a representative crystal equivalent circuit. When built the arrangement worked perfectly, first time.

"With a 10.106MHz (QRP frequency) crystal, the series oscillator gave a range of 10.09910 to 10.10940 MHz (10.3 kHz range). The parallel oscillator covered 10.10842 to 10.11208 MHz (3.66 kHz range). Both used a tuning voltage control range of  0 to 13 volts. There is no point in using a greater voltagerange since there is an overlap. The total inband coverage from the two oscillators is thus over 12kHz, a very satisfactory pulling range for a crystal not specially selected or designed for pulling.

"In theory, the crystal could alternatively have been pulled above its parallel resonance frequency by using an inductor in parallel with the crystal. However, this runs the risk of oscillation without the crystal in circuit and I have not tried it. Clearly, to make full use in practice of this technique, the crystal must be swapped between the two circuits. This could be done by physically transferring it between sockets, or by using a switch or relay. The choice is left to your ingenuity. Note that the variable capacitance diodes can share a common control voltage." [The exact pulling rangeswill,of course, depend on the precise characteristics of the crystal, but the results achieved by G4COL representing rather more than one part in a thousand without any loss of stability are probably representative of most crystals used as indicated - G3VA]