GB2220317A

GB2220317A - Temperature stabilised oscillator

Info

Publication number: GB2220317A
Application number: GB8812773A
Authority: GB
Inventors: Zalman Schwartzman; Yair Shapira; Alex Shani; Shlomo Berliner; Shmuel Barman; Shmuel Riych
Original assignee: Motorola Israel Ltd
Current assignee: Motorola Solutions Israel Ltd
Priority date: 1988-05-28
Filing date: 1988-05-28
Publication date: 1990-01-04
Anticipated expiration: 2008-05-28
Also published as: JPH04504035A; SG22895G; GB2244877B; JP2814638B2; KR900702657A; GB2244877A; HK87595A; GB9018896D0; GB2220317B; SG22795G; WO1989011756A1; HK87695A; GB8812773D0

Abstract

There is provided a system for automatically integrally calibrating an oscillator (10) and/or a phase-locked loop (30, 32, 34) to achieve frequency accuracy and stability in a synthesized radio, particularly a radio where the temperature of an oscillator can be set. Temperature calibration is performed by integrally determining the present optimal operating temperature for the oscillator and integrally resetting the temperature of the oscillator to that optimal temperature. Thus, frequency vs. temperature stability is achieved; frequency calibration may follow.

Description

AUTOMATICALLY SELF-CALIBRATING OSCILLATORS IN HETERODYNED RADIO RECEIVERS Field of Invention This invention relates to automatic calibration of oscillators. More particularly, this invention relates to temperature and frequency calibration of oscillators in High Frequency Single Side Band (HF SSB) heterodyned radio receivers to achieve frequency accuracy and stability.

Background of the Invention Frequency accuracy and stability are major problems in radio communications, particularly in HF SSB. The oscillating frequency of oscillator components (and especially the oscillating crystal) for radios are particularly sensitive to temperature variation and are, therefore, commonly isolated from ambient temperature conditions by being maintained at a constant temperature within oven enclosures.

The dominent frequency contribution of the oscillator comes from the crystal. A typical frequency deviation vs temperature characteristic for a crystal is illustrated in Figure 1. Thus, substantial frequency stability can be achieved by regulating the temperature of the crystal about the point of minimum frequency deviation for given changes in temperature (i.e., zero slope). Also, any method that regulates the operating temperature of the entire oscillator according to the basic characteristics of the crystal itself will achieve substantial frequency stability.

This optimum temperature for maximum frequency stability varies considerably from crystal to crystal and migrates in each crystal over time due to aging. Thus, although the oven temperature is ordinarily individually set within a certain tolerance by the use of precision instrumentation at the factory for each oscillator before being assembled into the radio and then individually frequency adjusted once assembled, each will require adjustment throughout its lifetime in the field and as it ages.

What is needed, then, is a method -- integral to the radio itself -- for individually regulating the temperature of the oscillator according to its prevailing temperature vs frequency characteristics and subsequently adjusting the frequency for maximum accuracy. In modern HF SSB radios, it is now possible to do automatic calibration within the radio itself in the field, without the use of precision test instrumentation but simply using elements of the radio itself, because the frequency error is multiplied in the radio reciever such that it can easily be measured at the audio output with conventional microprocessors using known methods of digital signal processing/frequency sampling.

It is, therefore, one object of the invention to provide a method for automatically and integrally calibrating the temperature and frequency of the oscillator according to the basic prevailing characteristics of the oscillator to achieve appreciable frequency stability, using only an external frequency reference signal.

Summary of the Invention According to the present invention, there is provided a system for automatically and integrally calibrating the temperature of an oscillator to achieve frequency stability in a system where the temperature of an oscillator can be set. The temperature calibration is performed by integrally determining the present optimal operating temperature for the oscillator and integrally resetting the temperature of the oscillator to that optimal temperature. Thus, frequency vs. temperature stability is achieved; frequency calibration may follow.

Brief Description of the Drawings An exemplary system in accordance with the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a graph that illustrates a typical frequency deviation vs temperature characteristic for a crystal.

Figure 2 is a block diagram of a synthesized radio incorporating the present invention.

Figure 3 is a block diagram of the Processing Unit of Figure 2 and constructed according to the present invention.

Detailed Description of the Invention Figure 2 is a block diagram of a synthesized radio incorporating the present invention. The synthesized radio includes a frequency oscillator providing a reference frequency, a synthesizer providing a first and second injection frequency for the first and second Intermediate Frequency mixers and whose programmable frequency dividers are loadable from the processor, and a demodulator, whose ordinary audio output is advantageously used in accordance with the present invention to provide frequency deviation and calibration information to the processor.

The oscillator of this synthesized radio can be accurately calibrated because the frequency error between the external reference frequency recieved by the radio reciever and its internal reference frequency is transformed to the base-band and multiplied by the ratio of the recieved frequency to the reference frequency. Thus, with an internal reference frequency of 11.4 MHz and with the reciever tuned to a 10 MHz and an external frequency of 10.001 MHz, a tone of approximately 1 KHz will be produced at the audio output -- a 0.1 ppm oscillator error is reflected in the audio output as a 1000 ppm deviation. This multiplication of the frequency error is then easily measured in the audio output by modest microprocessors clocked by low cost crystals.

Figure 3 is a block diagram of the Processor of Figure 2 and constructed according to the present invention. The Processor includes a microprocessor coupled to the audio output of the reciever and is used in accordance with the present invention to provide frequency deviation and calibration information to the processor and further includes digital-to-analogue converters to supply temperature and frequency calibration information to the oscillator.

To calibrate the oscillator, the reciever is tuned to a calibration frequency (recieved from a calibrated transmitter, from an external precision test instrument or from a transmitted frequency standard -- time and frequency reference signals are commonly available in the HF band for navigation and time distribution purposes). Next, the voltage generated using the Temperature Digital-to-Anologue converter (D/A-2) is varied to drive the oven temperature of the oscillator across its design range from its minimum temperature to its maximum temperature. After sufficient time has passed for the oven temperature to stablize following each change in voltage, the change in frequency deviation at the audio output is measured and noted (a microprocessor is utilized to note said resulatant frequency deviations using digital frequency sampling techniques).

Once a sufficient number of frequency/temperature points have been noted (for example, a minimum of three is required for the parabolic curve approximation of Figure 1), the oven temperature at which the least frequency deviation exists can be determined. Again, for the basic parabolic characteristic of Figure 1, this optimum temperature (where the slope is zero) is determined from coefficients of the parabola f=At**2+Bt+C where: Tcalibration = B/2A and where A= [ (fl-fO)(t2-tO)-(~2-fO)(tl-tO) ] /(tl-tO)(t2-tO)(tl-t2) and B= [ f2-f0-A(t2**2-t0**2) )/t2-t0 from the temperature/frequency pairs (tO,fO), (tl,fl) and (t2,f2).

Finally, the voltage corresponding to the optimum temperature Tcalibration is generated through the Temperature D/A and the Frequency Tuning D/A is adjusted (varying the voltage on varicap diodes in the oscillator) until the frequency deviation measured at the audio output disappears. The calibration is complete when the values of both the Temperature and the Frequency Tuning D/As are stored in non-volitile memory.

Thus, what has been provided is a system for automatically and integrally calibrating the temperature of an oscillator to achieve frequency accuracy and stability in a synthesized radio where the temperature of an oscillator can be set. The temperature calibration is performed by integrally determining the present optimal operating temperature for the oscillator and integrally resetting the temperature of the oscillator to that optimal temperature.

Thus, frequency vs. temperature stability is achieved; frequency calibration may follow.

This method of calibration is suitable for use with any oscillator where frequency accuracy and stability is a concern, including those that are not oven temperature controlled. It is particularly advantageous for field frequency adjustments to compensate for crystal aging.

While the preferred embodiment of the invention has been described and shown, it will be understood by those skilled in the art that other variations and modifications of this invention may be implemented.

Claims

CLAIMS:

What we claim and desire to secure by Letters Patent is: 1. A method of automatically and integrally calibrating the temperature of an oscillator to achieve frequency stability in a synthesized radio where the temperature of an oscillator can be set, characterized by: integrally determining the present optimal operating temperature for the oscillator and integrally resetting the temperature of the oscillator to that optimal temperature, whereby frequency vs. temperature stability is achieved.
2. As claimed immediately above, further characterized by calibrating the operating frequency of said oscillator following said temperature calibration, whereby frequency accuracy is achieved.
3. As claimed immediately above, wherein said frequency calibration is further characterized by noting the resultant frequency deviation following said temperature calibration and adjusting the frequency of said oscillator to eliminate said frequency deviation.
4. As claimed anywhere above, wherein a microprocessor-driven digital-to-analogue converter is utilized to adjust the frequency of the oscillator by altering the voltage supplied to varicap diodes in the frequency generator of the oscillator.
5. As claimed anywhere above, wherein said integral determination is further characterized by: deriving the specific frequency vs temperature characteristic of said oscillator and calculating the optimal operating temperature where relative temperature differentials result in relatively minimal frequency deviations.
6. As claimed immediately above, wherein said derivation is further characterized by: adjusting the operating temperature of the oscillator across its operating temperature range, noting the resultant frequency deviations and calculating the coefficients of a representative polynomial equation and calculating the relative minima of said polynomial equation, whereby said relative minima represents the optimal operating temperature of said oscillator.
7. As claimed immediately above, wherein three frequency vs temperature pairs are noted to resolve the coefficients and to calculate the relative minima of a parabolic quadratic equation representative of a crystal oscillator.
8. As claimed anywhere above, wherein a microprocessor-driven digital-to-analogue converter is utilized to adjust the operating temperature of the oscillator by altering the voltage supplied to the temperature control of an oscillator.
9. As claimed anywhere above, wherein a microprocessor is utilized to note said resulatant frequency deviations using digital frequency sampling techniques.
10. As claimed above wherein a microprocessor-driven digital-to-analogue converter is utilized to generate a current proportional to the optimal operating temperature and to supply said voltage to the temperature control of an oscillator.
11. A method of automatically and integrally calibrating an oscillator to achieve frequency stability in a synthesized radio where the oven temperature of a crystal oscillator can be set, characterized by: integrally determining the present optimal operating temperature for the oscillator by: deriving the specific frequency vs temperature characteristic of said oscillator and calculating the optimal operating temperature where relative temperature differentials result in relatively minimal frequency deviations by:: adjusting the operating temperature of the oscillator across its operating temperature range, noting the resultant frequency deviations calculating the coefficients of a representative polynomial equation and calculating the relative minima of said polynomial equation (whereby said relative minima represents the optimal operating temperature of said oscillator), by: noting three frequency vs temperature pairs to resolve the coefficients and to calculate the relative minima of a parabolic quadratic equation representative of a crystal oscillator, integrally resetting the temperature of the oscillator to that optimal temperature by: utilizing a microprocessor-driven digital-to-analogue converter to adjust the operating temperature of the oscillator by altering the voltage supplied to the temperature control of an oscillator by:: utilizing a microprocessor-driven digital-to-analogue converter to generate a voltage proportional to the optimal operating temperature and to supply said current to the temperature control of an oscillator.

and calibrating the operating frequency of said oscillator following said temperature calibration by: noting the resultant frequency deviation following said temperature calibration and adjusting the frequency of said oscillator to eliminate said frequency deviation by: utilizing a microprocessor-driven digital-to-analogue converter to adjust the frequency of the oscillator by altering the voltage supplied to varicap diodes in the frequency generator of the oscillator, wherein a microprocessor is utilized to note said resulatant frequency deviations using digital frequency sampling techniques and whereby frequency vs. temperature stability is achieved and whereby frequency accuracy is achieved.