Crystal Oscillator
Theory

Quartz Crystal Oscillators

The comprehensive range of Hy-Q temperature compensated quartz crystal oscillators (TCXO), voltage controlled quartz crystal oscillators (VCXO) and a combination of both (TCVCXO), provide a choice of package size, supply voltage, output waveform and performance where a low power, stable frequency source is required. In addition to the above types of oscillators, Hy-Q also offers a wide range of Clock Oscillators (SPXO) including DIL type packages, for applications where relatively high frequency stability is not required.

VCXO, TCXO & Clock Oscillators

Voltage Compensated Crystal Oscillator (VCXO)

Voltage controlled quartz crystal oscillators (VCXO) use varicap diodes to alter the frequency of oscillation in the same manner as that used in TCXO’s. However, with a VCXO the bias point of the varicap diode is made available to the user in the way of an additional pinon the base of the oscillator. The temperature stability of a VCXO is inherently restricted to the performance of the quartz crystal itself and associated components, therefore even over a relatively narrow temperature range of 0° to +60°C a frequency stability of ±10ppm is all that is practically achievable.

As VCXO’s use the same technique to vary the operating frequency as a TCXO it is also possible to temperature compensate them. However, due to the required increase in pulling sensitivity (crystal temperature compensation + frequency adjustment) it is not always possible to obtain quite the same degree of temperature stability as that of a TCXO when wide frequency adjustment ranges are required.

Hy-Q can produce VCXO’s with frequency adjustment ranges of up to ±200ppm over a sweep voltage range of 0 to +5 volts or even greater if the sweep voltage range is increased. Not only can we provide very wide adjustment ranges but also a very high degree of linearity to better than 1%.

Temperature Compensated Crystal Oscillator (TCXO)

A temperature compensated quartz crystal oscillator (TCXO) is an oscillator which has been specially compensated to achieve a specific frequency stability over a specified temperature range (see Table 1 below). TCXO’s use temperature dependant components (usually a thermistor network) to provide a bias voltage to a varicap diode which makes up part of the quartz crystals’ load capacitance. TCXO’s can be produced to work over any temperature range between -55° to +125°C but of course the narrower the temperature range the higher the frequency stability.

Clock Oscillators
Dual In Line (DIL)
Simple Packaged Quartz Crystal Oscillator (SPXO)

Whatever you choose call them, they are an oscillator which has no means of temperature compensation or control and thus it’s frequency temperature stability is inherently that of the basic quartz crystal.

As SPXO’s are less complex in design and therefore use less components than their counterparts, they are relatively inexpensive and available in almost any package size including the popular dual line type (DIL).

Terms and Definitions

Nominal Frequency: The frequency at which the oscillator will operate under specified conditions.

Temperature Stability: The frequency deviation of the oscillator from nominal frequency due to changes in temperature. In order to give a guide of the tolerances readily obtainable from a TCXO, Table 1 below shows the typical performances over some selected temperature ranges. These are representative only, our sales staff will be able to advise on other tolerances and temperature ranges. Fig. 1 is an example of a high stability TCXO temperature stability graph.

Table 1
Temperature (Deg. C)	Frequency Deviation (ppm)
-40 to +85	±3.0
-30 to +75	±2.5
-20 to +70	±2.0
-10 to +60	±1.0
+10 to +45	±0.5

Temperature Stability Graph

Fig. 1

Ageing: The frequency deviation of the oscillator from nominal frequency over a period of time. Hy-Q quartz crystals age approximately ±2.0ppm in their first year of operation. However due to the logarithmic nature of the aging curve, 1.0ppm is established within the first 40 days of operation.

Voltage Coefficient: The frequency deviation of the oscillator from nominal frequency due to an incremental change in the supply voltage.

Load Coefficient: The frequency deviation of the oscillator from nominal frequency due to an incremental change in load capacitance.

Operating Temperature Range: The temperature range over which the oscillator will function within the specified frequency tolerances.

Operable Temperature Range: The temperature range over which the oscillator will function although not necessarily within the specified frequency tolerances.

Storage Temperature Range: The temperature range over which the oscillator may be stored but not operated.

Supply Voltage: The voltage at which the oscillator must be operated. Hy-Q supplies oscillators at various supply voltages in the range of +5.0 to +24.0 volts D.C.

Power Consumption: The maximum power consumed by the oscillator under reference conditions. Power consumption as low as 15mW can be achieved.

Output Characteristics: Hy-Q produce a range of oscillators with outputs fully compatible with TTL, LSTTL, CMOS and HCMOS logic. In addition sinewave and open collector types are available.

Frequency Adjustment: frequency adjustment of an oscillator is required to offset for effects of ageing, etc. Adjustment can be achieved by various methods as outlined below:-

External potentiometer.
Internal trimming capacitor.
Internal potentiometer.

Frequency adjustment of VCXO’s is achieved via an external voltage source and is required not only to offset for the affects of ageing, etc. but also to allow the oscillator to be modulated or used in phase locked loops. Hy-Q can produce VCXO’s with frequency adjustment ranges of up to ±200ppm over a sweep voltage range of 0 to +5 volts D.C.

Linearity: The ability of an oscillators frequency deviation to match a straight line linear curve over a specified sweep voltage range (see Fig. 2).

Frequency Adjustment Graph

Fig. 2

Nominal Frequency Offset: When very tight frequency temperature characteristics are required from a TCXO, a manufacturer may request that a positive or negative offset frequency be engraved on the oscillator can. The purpose of this is to inform the user that the oscillator is not to be set exactly to nominal frequency at room temperature but to a frequency slightly above or below this.

An example for the requirement of an offset frequency is shown in Fig. 3 below. The dotted curve shows a 6.4MHz oscillator adjusted to nominal frequency inclusive of a -2Hz offset frequency so as to achieve a performance +35/-0.5ppm from nominal frequency over the temperature range. The solid curve shows the oscillator without the offset frequency making its performance +0.66/-0.12ppm from nominal frequency over the temperature range.

Frequency Offset Graph