BRIFE DESCRIPTION OF THE DRAWING
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 一 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 blocking diagram of FIG. 1.
SPECIFIC DESCRIPTION
In FIG. 1 we have shown a conventional crystal - controlled highfrequency oscillator OE emitting a carrier of frequency fET 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 CS1. The combined signal s + y is fed to a frequency modulator MD also receiving a subcarrier of stabilized intermediate frequency fO from a nonillustrated source. The frequency - modulated subcarrier fET is supplied, together with the carrier fET from oscillator OE，to a mixer CC for heterodyning to produce a radio 一 frequency oscillation fET consisting of two sidebands fET + flF and fET-flF. 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 △T - cut crystal, the deviations △f from its rated frequency will vary in the manner illustrated in FIG. 2A as a function of temperature departures AT from a reference level. A representative curve c, lying between two range limits a and b, is a polynomial function of the from △f = -p△T + q ( △T) 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 fET-flF so that filter FT can be of the lowpass type. If, however, it is desired to use the upper sideband fET + flF 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. 2 A intersect at an operating point 0 representing normal temperature and frequency.
In the case of a C - cut crystal the functionAf/AT follows a square law as represented in FIG. 2B by a median curve in the form of a parabola c1 lying between limiting parabolas a1 and b1. If a low - pass filter FT is used in FIG. 1，corrective signal y should conform to curve c1; with a high - pass filter it should follow a curve dl symmetrical to parabola c1. All four parabolas of FIG. 2B have a common vertex at an operating point O1.
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, c1 and d1 discussed above, depending on the type of crystal used and the sideband selected. Function generator GF comprises a resistance brigde VL having an input diagonal connected across a supply of constant reference voltage with tenninals 十 Vc and 一 Vc , the bridge including two opposite arms formed by resistors R1，R2 and two other opposite arms formed by thermistors R3 and R4 ； the latter are here assumed to be of the directly heated type with positive resistance/temperature characteristic. When the bridge is in balance ( with△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 signal are respectively sent to a pair of cascaded voltage -multiplier stages MT1 ; the latter thus emits a quadratic term + x2 which is fed on the one hand lo multiplier MT2 and on other hand to an adjustable attenuator RL2 . Multiplier MT2 generates a cubic term + x2 delivered to one input of an adder CS2 which receives on its other input a fractional value + kx of the basic signal derived from a potentiometer RLJ inserted between ground and switch SW1. the output of adder CS2 thus represents the algebraic sum of the linear term ± kx and cubic term 士 x2 which is weighted by on adjustable attenuator RL3 to provide an output signal of a form c ’ or c1', similar to curve c or curve d in FIG. 1，defending on the position of switch SW1.
The weighted term ±x2 is available from attenuator RL2 as an alternate output signal d1 ’ or c1 ’ respectively similar to curve dlor curve c1 of FIG. 2B. A switch SW2 selects either o fthese output signals as the corrective signal y.
We claim ：
1. In a message - transmitting system using frequency modulation, in combination ：
a crystal - controlled oscillator generating a carrier of a frequency subject to temperature - dependent fluctuations ；
a frequency modulator having a first input for receiving a subcarrier and a second input for receiving a message signal to be modulated upon said subcarrier;
a function generator in the vicinity of said oscillator emitting a corrective signal of a magnitude varying as a function of temperature related to the temperature dependence of said fluctuations ；
summing means connected to said function generator and to a source of said message signal for supplying the latter together with said corrective signal to said second input whereby said subcarrier is frequency - modulated by the sum of said message and corrective signals ；
mixer means with input connections to said oscillator and to said modulator for heterodyning said earner with the frequency — modulated subcarrier to produce a pair of sidebands respectively representing the sum and the difference of said earner and subcamer, the temperature — dependent fluctuations of said carrier being substantially compensated by the corrective modulation of said subcamer in one of said sidebands ； and filter means downstream of said mixer means for suppressing the other of said sidebands.
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