US 4 , 290, 145

CIRCUIT ARRANGEMENT FOR COMPENSATING 

TEMPERATURE - DEPENDENT FREQUENCY VARIATIONS OF A CRYSTAL - CONTROLLED 

OSCILLATOR 

ABSTRACT

A subcarrier frequency - modulated with a message signal is heterodyned with a carrier from a crystal - controlled oscillator in a mixer whose output is fed to a filter eliminating one of the resulting sidebands. In order to compensate temperature -dependent fluctuations of the carrier frequency，a function generator produces a corrective signal which is added to the message signal in the input of the frequency modulator traversed by the subcarrier. The corrective signal varies as a quadratic or a cubic function of temperature，depending upon the type of out used on the quartz crystal controlling the carrier oscillator，with a polarity determined by which of the two sidebands is passed by the filter.

FIELD OF THE INVENTION

Our present invention relates to a circuit arrangement for compensating temperature -dependent frequency variations of an oscillator of the crystal - controlled type which generates a carrier for the transmission of messages by frequency modulation.

BACKGROUND OF THE INVENTION

For the compensation of carrier 一 frequency fluctuations due to changes in ambient temperature it is customary to use a resistance network including one or more thermistors disposed in the vicinity of the oscillator so as to sense the temperature changes affecting the quartz crystal which acts as a frequency 一 controlling element thereof. A corrective voltage emitted by the resistance network is fed to an adjustable impedance, such as a varactor, in circuit with that crystal to counteract the thermal fluctuations in carrier frequency. For a reasonably precise compensation it is necessary to use relatively expensive thermistors with tolerance limits substantially less than those of ordinary thermistors whose resistances at a given temperature may deviate by as much as +5% from the rated value.

OBJECTS OF THE INVENTION

The general object of our present invention is to provide an improved circuit arrangement for the stabilization of a carrier frequency, subject to modulation by a message signal, which can be realized with relatively inexpensive components and can be mass - produced with satisfactory performance despite unavoidable manufacturing tolerances.

Another object is to provide means in such a circuit arrangement enabling its convenient adaptation to different types of cut of a frequency - controlling crystal.

A further object is to facilitate the utilization of a frequency - stabilized carrier for selective message transmission in rather of two frequency hands.

SUMMATY OF THE INVENTION 

In accordance with our present invention we provide a frequency modulator receiving on a first input a subcarrier and on a second input a message signal to be modulated upon that subcarrier. A function generator, disposed in the vicinity of a crystal 一 controlled oscillator whose temperature 一 dependent frequency fluctuations are to be compensated，emits a corrective signal of a magnitude which varies as a function of temperature related to these fluctuations. The message signal is fed to the second modulator input together with the corrective signal with the aid of a summing circuit connected to the function generator and to the message - signal source；thus, the subcarrier is frequency 一 modulated by the sum of the message and corrective signals. A mixer with input connections to the crystal - controlled oscillator and to the modulator heterodynes a carrier, generated by that oscillator, with the frequency - modulated subcarrier to produce a pair of sidebands respectively representing the sum and the difference of the carrier and the suncarrier; in one of these sidebands the carrierfrequency fluctuations are substantially compensated by the corrective modulation of the subcarrier, this sideband being passed by a filter disposed downstream of the mixer.

With the absolute magnitude of the corrective signal varying substantially in proportion to the thermally induced fluctuations of the carrier frequency, the choice of the sideband depends on the polarity of the corrective signal which either increments or decrements the frequency of the subcarrier. In the absence of a message signal, the upper sideband will have a constant frequency if an increase in carrier frequency is compensated by a decrease in subcarrier frequency and vice versa;  to stabilize the frequency of the lower sideband，the frequencies of the carrier and the subcamer must vary in the same sense.

Since the frequency of the carrier will generally be much higher than that of the subcarrier, the latter will have to undergo substantially larger excursions to balance the thermal fluctuations of the carrier frequency. Thus, minor perceptual variations in the subcarrier frequency due to thermistor tolerances, for example, will have only a negligible effect upon the transmitted carrier frequency.

We have found that the frequency/temperature response of quartz crystals conventionally used in highfrequency oscillators follows an exponential law which can be represented, with good approximation, by a quadratic or a cubic function according to the type of cut used on the crystal. More particularly，the response of a C - cut crystal follows a square law whereas that of an AT - cut crystal corre¬sponds to a third - order polynomial with a linear and a cubic term. The function generator provided in accordance with our present invention，therefore，advantageously includes temperature — dependent impedance means such as one or more thermistors，producing a basic signal，and multiplication means in the form of several cascaded stages emitting respective terms of a polynomial function of that basic signal，in combination with circuit means including adjustable attenuators ( or possibly variable - gain amplifiers) for assigning selected coefficients to these tenns and switch means enabling the selective extraction of one or more terms weighted by these coefficients to form the corrective signal. A polarity switch may also be provided to enable a selective changeover between the two sidebands as discussed above.