Oven Controlled Crystal Oscillators

Oven Controlled Crystal Oscillators

Introduction:

Oven controlled crystal oscillators use a proportionately controlled oven in which the crystal is placed and this forms the basis for the oscillator stability. The proportionate control varies the amount of power to the oven so that the temperature of the oven is kept constant with the change in ambient temperature of the oven. A thermistor is used sense the oven temperature and is an element of a thermistor resistor bridge, which controls the voltage to the oven. With a good design an OCXO can achieve stabilities as good as ± 1 X 10-9 over 0° C to +60° C.

Ageing

The ageing associated with an oscillator result from the frequency changes in the crystal with time. This frequency change is caused by the stresses within the quartz and other effects. The crystal ages rapidly immediately after manufacture and stabilises after a few weeks and achieves the best ageing after a few months. All the crystals used in our OCXOs are aged in an elevated temperature chamber, which accelerates the ageing process . The ageing data acquired each day and only those crystals that qualify our ageing specifications are used in a OCXO.

Ageing of OCXOs are measured in our ageing racks which have several hundred OCXOs are powered and the frequency of each of the OCXO is measured at periodic intervals, usually once every 24 hours. The data is acquisition starts after the oscillator is stabilised for one day. The data acquired after 30 days is used to project the ageing of the oscillator over a longer period of time using the method of least squares to the function:



Where f (T) is the frequency of the OCXO T days after the start of the data acquisition, A, B and fo are constants from the least square fit and fo is the frequency of the OCXO at the beginning of the ageing cycle.

OCXOs utilising AT cut crystals offer ageing from 1 X 10-8 per day to 1 X 10-9 per day whereas OCXOs with SC cut crystals can achieve ageing rates better than 1 X 10-10e per day.

Warm Up

The crystal in OCXO is designed so that its upper turning temperature is at least 10 to 15° C higher than the specified higher temperature point of the operating temperature range. If an OCXO is specified at -10 to + 60° C, then the crystal upper turning point is designed to be at say + 70 or 75° C. This is achieved by the controlling the angle of the cut of the crystal. As soon as the OCXO is powered on, the oven starts to warm up to the temperature of the upper turning point of the crystal. And after achieving the temperature, starts stabilising at that point with the proportionate oven control circuit. The time taken by the OCXO to achieve the specified frequency temperature stability is the Warm up time.

Phase Noise

The output of a crystal oscillator consists of a complex spectrum consisting of the output frequency and other noises, which may include harmonic distortions and other effects. This noise is called the phase noise. Phase noise is measured in the frequency domain and is expressed as a ratio of signal power to noise measured in 1 Hz bandwidth at a given offset from the desired signal., for example, as -135dBc/Hz at 1 Khz. Simple clock oscillators above 80 MHz which sometime use frequency multiplying techniques have very poor phase noise characteristics and indirect compensation TCXOs and VCXOs have slightly better phase noise characteristics. OCXOs utilising SC cut crystals have the best phase noise characteristics.

Short Term Frequency stability

Since the complex signal of the oscillator output will be made of several components, the signal will not be constant but will change from time to time. Short-term frequency stability is the general descriptive term to indicate the fluctuation of instantaneous frequency from a constant value and implies that other deterministic effects( such as frequency ageing, Frequency vs. temperature, frequency Vs voltage change etc) have not been considered.