Transistor crystal oscillator circuit

- circuit design information and data for the circuit of a transistor crystal oscillator

Crystal oscillators are used in a variety of applications. In some instances crystal oscillators may be used to provide a cheap clock signal for use in a digital or logic circuit. In other instances they may used to provide an RF signal source. In view of the fact that quartz crystals offer a very high level of Q and they are stable, crystal oscillators are often used in oscillator circuits to provide stable, accurate radio frequency signals.


Colpitts crystal oscillator

There is a great number of different types of circuit that can be used for crystal oscillators, each one having its own advantages and disadvantages. One of the most common circuits used for crystal oscillators is the Colpitts configuration as shown below.

The circuit uses a capacitor divider network comprising C1 and C2 to provide the feedback and the output is taken either from the emitter as shown. Alternatively it is possible to place a resistor or choke in the collector circuit and take the output from there. In either case it is wise to employ a buffer after the crystal oscillator circuit to ensure the minimum load is applied.

In this configuration, the crystal operates in a parallel mode. When running in this mode, the crystal should be presented with a load capacitance to operate on its correct frequency. This load capacitance is specified with the crystal and is typically 20 or 30 pF. The crystal oscillator circuit will be designed to present this capacitance to the crystal. Most of this will be made up by the two capacitors C1 and C2, although the remaining elements of the circuit will provide some capacitance.

Transistor crystal oscillator circuit

Typical Colpitts transistor crystal oscillator circuit

The disadvantage with this circuit is that the resistor bias chain shunts the series combination of C1 and C2 as well as the crystal. This means that additional gain and current are required in the crystal oscillator circuit to overcome this, and also the stability may be affected to some degree. The other effect the bias resistors have is to reduce the Q of the crystal. The problem can be overcome to some degree by using a field effect transistor for the active device, but these devices are generally not as stable as bipolar devices and they often need a higher operating current.

Often a small trimmer capacitor can be used to finely adjust the frequency of the crystal oscillator to compensate for any inaccuracies, the circuit conditions and any ageing. If a parallel capacitor is used, this should enable the crystal, operating in its parallel mode to have the correct load capacitance, but care should be taken not to excessively load the crystal as this could affect the Q of the tuned circuit and reduce the performance for some applications in terms of phase noise or stability. If these parameters are critical, it may be encessary to opt for a series capacitor, although care has to be taken to ensure the value does not go too low where the oscillation will cease. Often a fixed value capacitor, sufficient to maintain reliable oscillation, will be added in parallel with the series variable capacitor in these instances.


Crystal oscillator gain and drive level

In order to obtain the best performance from a crystal oscillator, it is necessary to ensure the crystal is driven at the correct level. If the drive level for the crystal is too high then the parasitic resonances of the crystal may be excited. Alternatively the crystal oscillator may even run on the incorrect frequency. Additionally if the drive level is too high then the phase noise performance of the crystal oscillator will be degraded

Additionally the crystal can be damaged if the drive level is too high. In particular the miniature types are susceptible to damage. Even if permanent damage is not caused, the high level of drive within the crystal oscillator increases the rate of ageing and can cause a frequency shift. It is therefore important to ensure the level of drive within the crystal oscillator circuit is approximately correct.

In view of the need to ensure the correct operating conditions for the crystal oscillator itself, it is necessary to optimise the circuit for stability, gain and drive level. This may lead to a lower output level, but this can be overcome in the following stages.


Crystal oscillator component value optimisation

The circuit conditions are fundamentally governed by capacitors C1, and C2 along with the bias resistors R1 and R2, and the emitter resistor R3. As the circuit is frequency dependent the values will change according to the frequency of operation. Typical values are given below.


Frequency
range
C1
pF
C2
pF
R1
kohms
R2
kohms
R3
kohms
1 - 3 220 330 33 33 6.8
3 - 6 150 220 33 33 6.8
6 - 10 150 220 33 33 4.7
10 - 20 100 150 33 33 2.2

These values will give provide a good solution for many circumstances. The transistor can be a BC109 or similar general purpose transistor.

The transistor crystal oscillator circuit described provides a good stable reference signal that will be satisfactory for many applications. In some circumstances highly stable oscillators will be needed and it may be necessary to use a purpose designed and made oven controlled crystal oscillator (OCXO). These are considerably more expensive, but offer very high levels of performance in terms of stability, frequency accuracy and phase noise. If these are needed then the additional cost may be justified.

By Ian Poole


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