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AWR Technology: FAX +44 (0)1304 369737 alan@buckman-hardy.co.uk

Getting Stable Oscillator Circuits

For accurate timekeeping purposes it is necessary to use Quartz crystal oscillators but everything around them needs to be constant to give a stable frequency. By taking some precautions it is possible to improve on the normal stabilities achievable. The things that can vary are the oscillator circuit voltage; circuit loading; power dissipation, temperature effects and the type of construction of the crystal can. The more parameters you can keep constant the better will be the oscillator. AWR Technology manufacture and sell Sidereal and Universal Time Clocks using the techniques outlined here (see clocks price list).

SOME BASIC REQUIREMENTS
An error of 1 part per million (1 ppm) will give a running error of 1 second in 12 days when used in a clock. Five times better than this would give an acceptable error rate! To achieve this needs special care in circuit design, the adjustment resolution and the drift need to be very small. The discrimination in the frequency setting component (usually a variable capacitor) needs to be about 0.05ppm. For a 180 degree mechanical rotation, about 100 times the discrimination would give an acceptable adjustment range. From experience with the CMOS 4060 oscillator circuit we have found that a 5ppm adjustment equates to about 0.9pF change in the loading capacitance. Special care is needed to design a series / parallel combination of loading capacitors with a variable element to give the desired adjustment range.

CRYSTAL CAPACITORS
The capacitors used around the crystal oscillator need to have zero temperature coefficient. Ceramic capacitors of dielectric type NP0 need to be used. Surface mount types enable a small footprint to be achieved, so reducing the strays and the susceptability from external interference. We have found some variable capacitors wandering on short timescales giving an instability.

VOLTAGE COEFFICIENT
The supply to the HC4060 was found to produce a voltage coefficient on the running frequency of about 1ppm per volt. By adding a series resistor (R2) with the crystal of about 27k (for the crystal under test) we reduced this voltage coefficient to a minimum, about 0.1ppm per volt. This value will be dependant on the crystal frequency, type of oscillator circuit and is tied to the operating power. Taking care of this parameter does improve the performance with the temperature effect on the regulator usually used. Thus the regulator performance also needs to be watched over the working ambient temperature range.

CIRCUIT LOADING
Adding any circuits to the HC4060 first output (pin 9) changes the operating frequency. Beware if you use this pin for calibrating. It is far safer to use an output right at the other end of the divider chain.

TEMPERATURE EFFECTS
Crystals can be 'tuned' during manufacture to provide different variations with temperature. By reducing the specified temperature range and closing up the tolerance it can be possible to achieve turning points within 0 to 25 deg C. The tight tolerance crystals we use have a temperature coefficient of less than 0.2ppm per degree C (typical). It is worth giving the circuit with a blast with the hair drier and monitoring the frequency variations with temperature when the circuit cools down. Freezer spray can also be used. The only way to remove temperature effects is to put the crystal in a small oven, monitored and kept at a constant temperature. AWR Technology do a crystal oven version, running at about 35 degrees C (but settable) which gives 0.2ppm change over an ambient temperature change of 40 degrees C ie 0.005ppm per degree C. Selecting the oven temperature at the turning point improves this immensely but you still have long term drift.

CRYSTAL CONSTRUCTION
Quartz crystals can come in various cans: soldered, cold welded etc and each version will give a particular ageing rate. This is a long term drift in frequency which makes the whole circuit need re-calibration every so often to maintain accuracy. The best types are those which have the lowest initlal drift rates, but even these can be 1ppm per year.

OVENING
The crystal can be kept at a constant temperature (higher than the expected normal ambient temperature) by means of a small heater, diode for temperature measurement and an error correcting amplifier as we do in the OVEN option for the SIDEREAL CLOCKS. Insulation is required and it needs carefull construction to reduce temperature gradients within the working cavity. Although the crystal may have 0.2 ppm per deg C by all other methods taken care of, if you keep the temperature constant to 1 degree C over an ambient of 0 to 35 degreees C then you get a 35 times better error rate ie 0.006ppm / deg C. We use this technique in our Intelligent Handset and oven option for the Sidereal Clock.


ASTROPIP METRONOMES

THE ASTRONOMERS' STOPWATCH
These small instruments are small portable sources of accurate time pips (beeps) for use in the dark. When you have your hands full in guiding on an object through a telescope, these allow you to hear the time and so make accurate exposures. They have another use in the darkroom during process development or when doing test strips. Time you can hear. Both units have an accuracy better than 0.1s over one hour and come in a belt-clip style case.

ASTROPIP MODEL AP200
This unit has ON, MODE and SET buttons. It can be started at the start of an event, the audible pulses recorded on a tape recorder and then record a 'click' at the end of the event. Then you can work out the duration of the event. The mode button allows PIPS every second with a longer PEEP at the minute or changes to being silent with a few PIPS leading up to a PEEP on the minute.

We have not made these for some time. If there is a demand we can make a batch.
List price 40

ASTROPIP MODEL AP2000
This unit has a DISPLAY and ON, MODE, SET and LIGHT buttons. It has all the functions of the AP200 plus a display of the elapsed time. The MODE button works between SECONDS, MINUTES and HOURS; the display shows the indication and two digits of that part of the time. This means that you can synchronise the ASTROPIP with the telephone speaking clock and carry accurate time with you to your observing site, hence the ability to record the absolute time of an event. It is also a lot easier to keep track of the timing of a 10 minute photographic exposure. The LIGHT turns on a low level backlight (red) suitable for use in the dark.
FULL SIZE PICTURE.

We have not made these for some time. If there is a demand we can make a batch.
List price 85


HINTS & TIPS

There are some application notes published in 1986 by Hewlett-Packard that are extremely useful in understanding the the nature of time, timekeeping and frequency calibration. They may still be available - contact Hewlett-Packard. We got our copies in 1993.

1. Application note 52-1 "FUNDAMENTALS OF TIME AND FREQUENCY STANDARDS" ref 5952-7870.
Provides information on accuracy of various time and frequency standards. A useful appendix discussing Solar Time, Universal Time, Coordinated Universal Time, Apparent and Mean Sidereal Time, Ephemeris Time and International Atomic Time. There is also a table of TIME TRANSMISSIONS by radio eg MSF, WWV with frequencies and format of the time signals.

2. Application note 52-2 "TIMEKEEPING AND FREQUENCY CALIBRATION" ref 5952-7874.
Discusses timekeeping and calibration, methods of comparing clocks and working out error equations. This is quite heavy stuff and perhaps less useful to astronomers.

FREQUENTLY ASKED QUESTIONS

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AWR Technology: FAX +44 (0)1304 369737 alan@buckman-hardy.co.uk