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.
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.
It is also very common for a small trimmer capacitor to be
placed across the crystal. In this way the frequency of the crystal oscillator
can be finely trimmed to the exact frequency required.
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 |
330 |
220 |
33 |
33 |
6.8 |
| 3 6 |
220 |
150 |
33 |
33 |
6.8 |
| 6 10 |
220 |
150 |
33 |
33 |
4.7 |
| 10 20 |
150 |
100 |
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.
Summary
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.
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