Wide Bandwidth Crystal Frequency Discriminator Circuit

This invention relates generally to the detection of frequency modulated signals, i.e. the conversion of an ac signal into a dc signal having a value indicative of the frequency of the ac signal. More particularly, the invention relates to a crystal frequency discriminator circuit of substantially increased bandwidth.

Frequency discriminator circuits employing piezoelectric crystals as frequency sensitive impedance elements have been useful in many applications on account of their good stability, high sensitivity and low power dissipation. In the past such discriminator circuits were designed with at least one piezoelectric crystal resonator having a series resonant frequency below the center frequency of the discriminator and usually in the vicinity of the discriminator lower cutoff frequency. Since practical crystal resonators have spurious resonances at frequencies higher than the desired resonant frequency, prior art crystal discriminators had to be designed with a sufficiently limited bandwidth so that the discriminator upper cutoff frequency would lie below the lowest spurious resonant frequency. As a result, frequency discriminators employing AT crystal resonators had useable bandwidths restricted to about 100 KHz to 150 KHz, while the useable bandwidth of discriminators employing overtone AT crystal resonators was only about 50 KHz.

It is an object of the present invention to provide a crystal frequency discriminator having a bandwidth substantially wider than crystal frequency discriminators of the prior art, while retaining the good stability, high sensitivity and low power dissipation achievable with crystal discriminators.

It is a further object of the invention to provide a crystal frequency discriminator which may be constructed using fewer crystals than otherwise comparable crystal discriminators of the prior art.

A frequency discriminator according to the invention includes input circuitry for receiving a frequency modulated input signal. A crystal filter coupled to the input circuitry has a predetermined voltage versus frequency characteristic including a portion wherein the voltage varies substantially linearly as a function of frequency. The filter includes a crystal resonator having a series resonant frequency at a frequency higher than the frequencies corresponding to the substantially linear portion of the voltage versus frequency characteristic. Detector circuitry coupled to the crystal filter rectifies the output voltage from the crystal filter. Voltage level shifting circuitry coupled between the input circuitry and the detector algebraically combines the output voltage from the detector with a dc voltage such that the algebraic sum of the dc voltage and the output voltage from the detector is zero when the instantaneous frequency of the input signal is at the discriminator center frequency. Since a frequency discriminator according to the invention is constructed without the use of any crystal resonator having a series resonant frequency within the discriminator passband, spurious series resonances occur outside of the discriminator passband, and hence are not troublesome.

Additional objects, advantages and characteristic features of the invention will become readily apparent from the following detailed description of preferred embodiments of the invention when considered in conjunction with the accompanying drawings wherein:

FIG. 1 is a schematic circuit diagram of a frequency discriminator circuit according to one embodiment of the invention;

FIGS. 2a and 2b are simplified equivalent circuit diagrams illustrating respective lattice arm impedances for the circuit of FIG. 1;

FIG. 3 shows pole-zero plots for the impedances of FIGS. 2a and 2b;

FIG. 4 is a graph illustrating the output voltage as a function of frequency for the circuit of FIG. 1; and

FIG. 5 is a schematic circuit diagram of a frequency discriminator circuit according to another embodiment of the invention.

Referring to FIG. 1 with greater particularity, a frequency discriminator circuit according to the invention may be seen to include a pair of input terminals 10 and 12 between which a frequency modulated rf input signal is applied. The terminal 12 may be connected to a level of reference potential illustrated as ground. A transformer 14 has its primary winding 16 connected between input terminals 10 and 12, while a capacitor 18 is connected across secondary winding 20 of transformer 14.

A crystal resonator 22 is connected between the ungrounded end of secondary winding 20 and one end of a tank circuit 24, while a capacitor 26 is connected between the ungrounded end of winding 20 and the other end of tank circuit 24. As a specific example, crystal resonator 22 may be formed from an AT-cut quartz crystal, although crystals of other materials and other cuts are also suitable and may be employed instead. Tank circuit 24 includes a parallel-connected capacitor 28 and inductor 30, the latter having a grounded center tap. A diode 32 and a capacitor 34 are connected in series between the resonator-connected end of tank circuit 24 and ground, with the cathode of diode 32 connected to capacitor 34. A resistor 36 is connected between the junction between diode 32 and capacitor 34 and a terminal 38 from which the dc output voltage from the discriminator may be obtained.

Connected in series between the ungrounded end of secondary winding 20 and output terminal 38 are a capacitor 40, a diode 42 and a resistor 44, with the cathode of diode 42 connected to capacitor 40. A resistor 46 is connected between ground and the junction between capacitor 40 and diode 42, while a capacitor 48 is connected between ground and the junction between diode 42 and resistor 44.

Transformer 14, capacitors 18 and 26, crystal resonator 22 and tank circuit 24 function as a crystal filter having a predetermined voltage versus frequency characteristic including a portion wherein the voltage varies substantially linearly as a function of frequency. Diode 32, capacitor 34 and resistor 36 function as a detector circuit which eliminates negative voltage excursions from the crystal filter output. Diode 42, capacitors 40 and 48 and resistors 44 and 46 provide a level shifting circuit which derives a dc voltage from the rf input voltage and adds the proper dc level to the detected crystal filter output so that a zero dc output voltage is obtained from the discriminator at the discriminator center frequency.

In order to facilitate understanding of the operation of the discriminator circuit of FIG. 1, simplified equivalent circuit diagrams for respective lattice arm impedances Z.sub.a and Z.sub.b in the crystal filter portion of the discriminator are presented in FIGS. 2a and 2b, respectively. In FIG. 2a, L.sub.1 and C.sub.1 represent the series, or motional, inductance and capacitance, respectively, of the crystal resonator 22. In FIGS. 2a and 2b, L.sub.p represents twice the inductance of secondary winding 20 and twice the inductance of each portion of inductor 30 from the center tap to the respective ends of tank circuit 24, while C.sub.p represents the equivalent parallel combination of the capacitance of capacitors 18, 26 and 28 and the static capacitance of crystal resonator 22.

The impedances Z.sub.a and Z.sub.b may be expressed as a function of the applied frequency .omega. by the following equations: ##EQU1## where .omega..sub.0 is defined by: ##EQU2## and .omega..sub.1 and .omega..sub.2 represent the lower and upper cutoff frequencies, respectively, of the crystal filter portion of the discriminator.

The manner in which the impedances Z.sub.a and Z.sub.b vary as a function of frequency may be better appreciated by making reference to the pole-zero plots shown in FIG. 3 for positive values of the frequency .omega.. From Equation (1) and FIG. 3 it may be observed that the impedance Z.sub.a has poles (i.e., parallel resonances) at the frequencies .omega..sub.1 and .omega..sub.2 and a zero (i.e., series resonance) at .omega..sub.0, while from Equation (2) and FIG. 3 it becomes apparent that the impedance Z.sub.b possesses a zero at .omega..sub.0.

The frequencies .omega..sub.0, .omega..sub.1 and .omega..sub.2 at which the poles and zeros shown in FIG. 3 occur are selected according to the desired voltage versus frequency characteristic of the discriminator. A specific exemplary plot illustrating the output voltage E appearing at terminal 38 versus the frequency .omega. for the circuit of FIG. 1 is shown by curve 50 of FIG. 4. It may be seen from the curve 50 that the crystal filter portion of the discriminator circuit is designed as a bandpass filter having a center frequency at .omega..sub.0 and a bandwidth BW.sub.f ranging from lower cutoff frequency .omega..sub.1 to upper cutoff frequency .omega..sub.2. As a specific example, the frequencies .omega..sub.1 and .omega..sub.2 may be those frequencies at which the output voltage E is 3 db below its value at the center frequency .omega..sub.0.

It may further be seen from FIG. 4 that the voltage versus frequency characteristic 50 includes a portion 52 slightly below the filter lower cutoff frequency .omega..sub.1 along which the voltage E varies substantially linearly as a function of the frequency .omega., and it is this portion 52 of the curve 50 which is used to produce frequency discrimination. The center frequency .omega..sub.c of the discriminator, i.e., the frequency at which the discriminator output voltage E is zero, occurs at a preselected frequency below the filter lower cutoff frequency .omega..sub.1. In the specific exemplary arrangement depicted by curve 50 of FIG. 4, the discriminator center frequency .omega..sub.c corresponds to that point on the curve 50 for which the filter response is 6 db below its value at the filter center frequency .omega..sub.0. The bandwidth of the discriminator BW.sub.d is such that the discriminator upper cutoff frequency occurs in the vicinity of the filter lower cutoff frequency .omega..sub.1. Although in the specific exemplary arrangement illustrated, the discriminator upper cutoff frequency coincides with the filter lower cutoff frequency .omega..sub.1, these two frequencies need not be equal. In fact, the filter lower cutoff frequency .omega..sub.1 will often lie slightly below the discriminator upper cutoff frequency, especially when "intermediate band" filter design techniques are employed.

As an illustrative example, a discriminator according to FIG. 1 and employing a "wide band" filter design may be constructed using the specific parameter values listed below in the Table I. It should be understood, of course, that these parameter values are given solely for purpose of illustration, and a wide range of other parameter values are also suitable.

.omega..sub.c = 30 MHz

Bw.sub.d = 300 KHz

.omega..sub.0 = 30.75 MHz

Bw.sub.f = 1200 KHz

.omega..sub.1 = 30.15 MHz

.omega..sub.2 = 31.35 MHz

Another embodiment of the invention is illustrated in FIG. 5. Components in the embodiment of FIG. 5 which correspond to respective components in the embodiment of FIG. 1 are designated by the same second and third reference numeral digits as their counterpart components in FIG. 1, along with the addition of a prefix numeral "1." The circuit of FIG. 5 differs from that of FIG. 1 in that secondary winding 120 of transformer 114 has a grounded center tap, while the end of tank circuit 124 electrically remote from crystal resonator 122 is connected to the ground level.

Since a frequency discriminator circuit according to the invention is constructed without the use of any crystal resonator having a series resonant frequency within the discriminator passband BW.sub.d, spurious series resonances occur outside of the discriminator passband, and hence are not troublesome. Therefore, a frequency discriminator according to the invention is able to provide a useable bandwidth substantially wider than crystal frequency discriminators of the prior art. In addition, a frequency discriminator circuit according to the invention may be constructed using fewer crystals than otherwise comparable crystal frequency discriminators of the past. In fact, only a single crystal 22 or 122 (as shown in FIGS. 1 and 5, respectively) need be employed, although additional crystals may be added in parallel with the crystals 22 or 122 in order to enhance the linearity of the discriminator voltage versus frequency characteristic.

Thus, while the invention has been shown and described with reference to particular embodiments, various changes and modifications which are obvious to a person skilled in the art to which the invention pertains are deemed to lie within the spirit, scope and contemplation of the invention.