Circuit Arrangement for Compensating Temperature-Dependent Frequency Variations of a Crystal-Controlled Oscillator

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.

For the compensation of carrier-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-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.

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 either of two frequency bands.

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-controlled oscillator whose temperature-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-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 subcarrier; in one of these sidebands the carrier-frequency 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 subcarrier 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 percentual 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 high-frequency 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 corresponds 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 terms 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.

These and other features of our invention will now be described in detail with reference to the accompanying drawing in which:

FIG. 1 is a block diagram of a message-transmitting system equipped with our frequency-stabilizing circuit arrangement;

FIGS. 2A and 2B are graphs showing sets of curves relating to the temperature dependence of oscillator frequencies generated by an AT-cut crystal and a C-cut crystal, respectively; and

FIG. 3 shows details of a function generator included in the block diagram of FIG. 1.

In FIG. 1 we have shown a conventional crystal-controlled high-frequency oscillator OE emitting a carrier of frequency f.sub.ET subject to fluctuations due to changes in ambient temperature. Close by, within the same thermal environment as indicated by an outline TH, we provide a programmable function generator GF emitting a corrective signal y which is summed with a message signal s, e.g. a voice frequency, in an adder CS.sub.1. The combined signal s+y is fed to a frequency modulator MD also receiving a subcarrier of stabilized intermediate frequency f.sub.O from a nonillustrated source. The frequency-modulated subcarrier f.sub.IF is supplied, together with the carrier f.sub.ET from oscillator OE, to a mixer CC for heterodyning to produce a radio-frequency oscillation f.sub.RF consisting of two sidebands f.sub.ET +f.sub.IF and f.sub.ET -f.sub.IF. This oscillation is passed through a filter FT which suppresses one or the other of the two sidebands.

If the oscillator OE is controlled by an AT-cut crystal, the deviations .DELTA.f from its rated frequency will vary in the manner illustrated in FIG. 2A as a function of temperature departures .DELTA.T from a reference level. A representative curve c, lying between two range limits a and b, is a polynomial function of the form .DELTA.f=-p.DELTA.T+q(.DELTA.T).sup.3. If the corrective signal y of FIG. 1 has the shape of curve c, substantial compensation of thermal frequency fluctuations will occur in the lower sideband f.sub.ET -f.sub.IF so that filter FT can be of the low-pass type. If, however, it is desired to use the upper sideband f.sub.ET +f.sub.IF with filter FT designed as a high pass, signal y should follow a curve d which is the image of curve c with inverted sign. All four curves a-d of FIG. 2A intersect at an operating point O representing normal temperature and frequency.

In the case of a C-cut crystal the function .DELTA.f/.DELTA.T follows a square law as represented in FIG. 2B by a median curve in the form of a parabola c.sub.1 lying between limiting parabolas a.sub.1 and b.sub.1. If a low-pass filter FT is used in FIG. 1, corrective signal y should conform to curve c.sub.1 ; with a high-pass filter it should follow a curve d.sub.1 symmetrical to parabola c.sub.1. All four parabolas of FIG. 2B have a common vertex at an operating point O.sub.1.

In FIG. 3 we have illustrated details of the function generator GF which can be programmed to emit a corrective signal y conforming to any one of curves c, d, c.sub.1 and d.sub.1 discussed above, depending on the type of crystal used and the sideband selected. Function generator GF comprises a resistance bridge VL having an input diagonal connected across a supply of constant reference voltage with terminals +V.sub.c and -V.sub.c, the bridge including two opposite arms formed by resistors R.sub.1, R.sub.2 and two other opposite arms formed by thermistors R.sub.3 and R.sub.4 ; the latter are here assumed to be of the directly heated type with positive resistance/temperature characteristic. When the bridge is in balance (with .DELTA.T=0), no voltage difference exists across its output diagonal. In the event of an unbalance, a basic signal appears with opposite polarities at the ends of this output diagonal as indicated at -x and +x; these signals are respectively sent to a pair of cascaded voltage-multiplier stages MT.sub.1 and MT.sub.2. A selector switch SW.sub.1 feeds either one of these voltages to another input of first-stage multiplier MT.sub.1 ; the latter thus emits a quadratic term .-+.x.sup.2 which is fed on the one hand to multiplier MT.sub.2 and on the other hand to an adjustable attenuator RL.sub.2. Multiplier MT.sub.2 generates a cubic term .-+.x.sup.3 delivered to one input of an adder CS.sub.2 which receives on its other input a fractional value .+-.kx of the basic signal derived from a potentiometer RL.sub.1 inserted between ground and switch SW.sub.1. The output of adder CS.sub.2 thus represents the algebraic sum of the linear term .+-.kx and cubic term .-+.x.sup.3 which is weighted by an adjustable attenuator RL.sub.3 to provide an output signal of a form c' or d', similar to curve c or curve d in FIG. 1, depending on the position of switch SW.sub.1.

The weighted term .-+.x.sup.2 is available from attenuator RL.sub.2 as an alternate output signal d.sub.1 ' or c.sub.1 ' respectively similar to curve d.sub.1 or curve c.sub.1 of FIG. 2B. A switch SW.sub.2 selects either of these output signals as the corrective signal y.