US2679585A

US2679585A - Frequency discriminator

Info

Publication number: US2679585A
Application number: US123323A
Authority: US
Inventors: Elbert J Drazy
Original assignee: Bell Telephone Laboratories Inc
Current assignee: AT&T Corp
Priority date: 1949-10-25
Filing date: 1949-10-25
Publication date: 1954-05-25
Anticipated expiration: 1971-05-25

Description

May 25, 1954 E. J. DRAZY FREQUENCY DISCRIMINATOR Filed Oct. 25, 1.949

FIG. 5

.02 '04 EERCENTAGE DEVIAT/ON 0F INPUT FROM REFERENCE mzousn/cr /N|/EN TOR E J DRAZY Arrom/Ev Patented May 25, 1954 UN ITE D PATENT FREQUENCY DISCRHVIINATOR: r

Elbert J. Drazy, East Orange, N. .L, assignor to, Bell Telephone Laboratories, Incorporated, New'York', N. Y.', a corporation of New York Application'0ctober25, 1949, Serial No. 123,323

13 Claims.

This invention relatesto: circuitswhich are responsive to frequency variation, and more particularly to a frequency discriminator adapted for use in the microwave range.

An object of the invention-is to -produce an output voltage which is proportional to, and the sign of which is determinedbmthe deviation of the input signal from a reference frequency.

A further object is to increase the sensitivity of a frequency discriminator.

Wave transmission systems often require a devicewhich will develop across its output terminals a voltage the magnitude of which is substantially proportional-to the deviation from the nominal or reference frequency of the inputsignal, and the sign of which depends upon whether the frequency is higher or lower than. the reference. Such a device, called afrequency discriminator, finds application, for example, as a component in circuits for oscillator frequency stabilization or as a demodulator for frequencymodulated signals.

Thefrequency discriminator of the present invention makes use of hollow wave-guide structures and is particularly adapted for operation in the microwave frequency range. Inthe embodiments shown, by way of example only, the discriminator comprises a main wave-guide, a cavity resonator coupled to one'endthereof, two auxiliary wave guides directionally coupled to opposite sidesof the main guide, andmeans at one end of each auxiliaryguide for deriving a unidirectional potential proportional to the microwave energy therein. The auxiliaryguides are short-circuited at their other ends at points so chosen that the differencebetween the length of the electrical path from the input end of the main guide to the shorted end of one of the auxiliary guides and-the length of the path from the same point to the shorted end of the other auxiliary guide is approximately equal to an odd integral number of quarterwavelengths, and that each of these. lengths differs from the length of themain guide by approximately an odd integral number of eighth wavelengths, at the resonant frequency of the resonator. These two potentials are combined in the series-opposing relationship to provide a voltage which is proportional to the deviation 'fromfthe resonant frequency ofthe resonator, and the sign'of which is positive foran increase in'frequency and negative for a decrease.- The-resonator, which is resonant at the reference frequency, contributes materiallyto the sensitivity of the discriminator, since its. 'reactance' changes sign at its-resonant frequency and has asteep-slope on either side thereof. The microwave energy detectors may, for example, be crystals, thermistors, .or.thermionic diodes.

The nature of the invention will be more fully understood from the following detaileddescription and by reference to theaccompanying drawings, of which Fig. l is a longitudinal sectional View of a frequencydiscrimina-tor in accordance with. the invention;

Fig. 2 is an end view'of-the discriminator of Fig. 1 as seen from the right Fig. 3 shows the portion of Fig.- 1 to the left of the line XX and illustrates a modification in which the crystal detectors are-replaced by thermistors; I

Fig. 4 is similar to Fig. Band showsanother modification in which the crystals are replaced by thermionic diodes; and-- Fig. 5 is a typical discrimination character istic of the discriminator showing the ratio of output to maximum voltage, plotted-against the deviation of the input frequency from the-:reference frequency.

The frequency discriminator shown in: Figs.-

1 and 2 comprises a hollow main-waveguide ID of rectangular cross-sectiom a cavity res onator ll, two aperiodic rectangular auxiliary wave guides l2 and I3, anddetecting means 14 and I5 for deriving unidirectional potentials proportional to the microwave energywithin the auxiliary guides 12 and I3; Microwave energy of nominal or reference frequency from some suitable source is fed into the left endaof the main guide ID, as indicated by-thearrow; The

unidirectional output voltage appears at the terminals l6 and I1 and is measured-by means of a direct-current voltmeter or other appro-- priate indicating device.

The cavity resonator II is electromagnetically coupled to the right-hand end of the main guide H] by means of the orifice I8 and is adjusted-to resonate at the frequency f by means of the slidable piston |9.- Formaximum discrimination the resonator II should match the guide H) in impedance at the frequency f.'. The auxiliary guides l2 and I3 adjoin-the opposite

wider sides

21 and 22 of the main guide-wand are'eleetromagnetical-ly coupled-thereto by the

coupling orifices

23, 24, 25', and 26, 21-,- 28, respectively. These orifices are so spaced and the areas thereof are so chosen that the couplings between the main guide land the auxiliary guides l2" and l3 are equal and are directionally selective;

that is, left -to right signal propagation in the main guide ID will induce left-to-right signal propagation in the auxiliary guides I2 and I3, but will not produce appreciable right-to-left signal propagation therein.

The right-hand ends of the auxiliary guides I2 and I3 are closed by the slidable short-

circuiting istons

30 and 3|, respectively. These pistons are so positioned that the difference between the length of the electrical path from the input end 32 of the main guide ID to one of the pistons 30 and the length of the electrical path from the input end 32 to the other piston 3I is approximately equal to an odd integral number of quarter wavelengths and, in addition, that each of these lengths differs from the length of the main guide III by approximately an odd integral number of eighth wavelengths, at the frequency ,1. Therefore, the distance S between the planes of the inner faces of the

pistons

30 and 3| is made approximately equal to m/l, where x is the wavelength within the guide and n is any odd integer. In Fig. l, n is unity, and the length of the auxiliary guide I2 is longer than that of the auxiliary guide I3. Also, the distance D1 between the plane of the inner face of the piston 3I and the plane 33 of the orifice I8 is made approximately equal to an odd integral number of eighth wavelengths and the distance D2 between the plane 33 and the inner face of the piston 30 is made approximately equal to an odd number of eighth wavelengths. Therefore, D1 is approximately equal to m1 8 and D2 is approximately equal to max/8, where m1 and me are odd integral numbers and may be either the same or different. In Fig. 1, m1 and ma are each equal to unity.

The left-hand ends of the auxiliary guides I2 and I3 are terminated in devices for detecting and rectifying the microwave energy therein. As shown in Fig. 1, these devices are the crystal detectors I4 and I5, respectively, represented schematically. These crystals are connected in series-opposing relationship between the output terminals I3 and I1, and are paired and individually matched in impedance to the associated auxiliary guide at the operating frequency ,"f.

In order to prevent the escape of microwave energy from the auxiliary guides I2 and I3 a quarter-wave choke is provided between each of the detectors I 4, I5 and the associated output terminal. This comprises a cylindrical conductive sleeve 31 which surrounds the conductor 38. The sleeve 31 has a length approximately equal to M4, is open at the outer end, and at its inner end 39 is short-circuited and conductively connected to the conductor 38. 1

When the frequency discriminator is used in applications such as oscillator stabilization, where only comparatively slow drifts of frequency are to be corrected, the crystal detectors I4 and I5 can be replaced by the pair of thermistors 40 and 4|, the equal resistances 4B and 47, and the battery 48, as shown in Fig. 3. Thermistors have the advantage that their impedance match can be maintained more easily over wider bands than is the case with crystals. Also, in some applications, the crystals I4 and I5 can advantageously be replaced by the pair of

thermionic diodes

42 and 43, as shown in Fig. 4. The sources 44 and 4-5 supply the cathode current for the

diodes

42 and 43, respectively.

An explanation of the operation of the frequency discriminator shown in Figs. 1 and 2 will now be given. When a microwave signal, hereafter called the incident signal, traveling from left to right in the main guide I0, reaches the coupling orifices 23 to 28 it induces in the auxiliary guides I2 and I3 equal signals proportional to, and propagating in the same direction as, the incident signal. When these induced signals in the auxiliary guides I2 and I3 reach the righthand ends thereof they arereflected by the

shortcircuiting pistons

30 and 3|, reverse their direction of propagation, and travel to the left ends of the auxiliary guides where they aid in exciting the crystal detectors I4 and I5.

The portion of the incident signal in the main guide In which passes the orifices 23 to 28 continues traveling to the right until it reaches the orifice I8 of the cavity resonator II, where it is partially reflected. The part which is reflected depends, both in magnitude and in phase, upon the deviation of the incident signal from the reference frequency f, and increases in magnitude as the deviation increases. This reflected signal in the main guide I0 propagates from right to left and, as it passes the orifices 23 to 28, induces in the auxiliary guides I2 and I3 equal signals proportional in magnitude to that of the reflected signal in the main guide II].

There thus exist in each of the auxiliary guides I2 and I3 two components of signal, one induced by the incident signal in the main guide I0 and the other induced by the reflected signal in the main guide. When the incident signal is of frequency ,7 there is a minimum reflection of it at the orifice I8 because the resonator I I is resonant at this frequency and presents a substantially nonreactive impedance which referably matches the characteristic impedance of the guide ID. Due to the positions of the pistons 30 and 3I with respect to each other and with respect to the plane 33 of the orifice I8, the two signal components in each of the auxiliary guides I2 and I3 are in quadrature, and their vector sums are equal. Therefore, the rectified voltages appearing across each of the detectors I4 and I5 are equal, and since these are connected series opposing the output voltage at the terminals I6 and I! will be zero.

The reactive component of the impedance of the resonator II, as viewed at the orifice I8, passes through zero at the frequency ,f and has a negative slope on each side thereof. Therefore, this component increases with the frequency deviation and is capacitive above 1 and inductive below 1. This reactance, in turn, causes a phase shift in the reflected signal in the main guide In which increases with the frequency deviation and is leading below 1 and lagging above 1. When this phase shift is lagging, the angle between the two signal components in the auxiliary guide l2 becomes acute and their vector sum is increased, While the angle between the signal components in the auxiliary guide I3 becomes obtuse and their vector sum is decreased. As a result, the detector I4 is more strongly excited than the detector I5 and, because of the series-opposing connection of the detectors, a net positive potential is obtained between the output terminals I6 and I1. This positive potential increases in magnitude as the frequency of the incident signal increases from I.

On the other hand, at frequencies of the incident signal below 1, the leading phase shift produces an obtuse angle between the signal components in the auxiliary guide I2 and an acute angle between the signal components in the auxiliary guide I3. As a result, the detector I5 is more strongly excited than detector I4 and there appears at the output terminals l6 and H a net negative potential which increases as the signal frequency decreases from f.

Fig. 5 shows a typical discrimination characteristic for the frequency discriminator of Figs. 1 and 2. The ratio of the rectified output voltage to the maximum voltage appearing at the terminals l6 and I1 is plotted against the percentage deviation of the input or incident signal from the nominal or reference frequency ,f. This characteristic applies to a discriminator in which the impedance of the cavity resonator II substantially matches the characteristic impedance of the main wave guide I 0 at the frequency f, and the resonator II has a Q of 5000. The factor Q may be defined as the ratio of the energy stored to the energy dissipated per cycle. It is seen that at the frequency f, where the deviation is zero, the output voltage is also zero but increases negatively for the deviations below I, and positively for frequencies above 1. Over the deviation range from 0.01 to +0.01 per cent the characteristic is quite steep and is substantially linear. The slope in this region, and therefore the sharpness of discrimination, may be increased by increasing the Q of the resonator ll.

Many modifications of the frequency discriminators herein disclosed will readily occur to persons skilled in the art. For example, in the structure shown in Figs. 1 and 2, either the piston 30 or the piston 3| may be moved either to the right or to the left by any small integral number of half wavelengths without substantially affecting the operation of the discriminator. Also, the orifice [8 may be moved either to the right or to the left by any small integral number of half wavelengths without substantially changing the performance. Furthermore, the orifice I8 may be moved either to the right or to the left by any small odd integral number of quarter wavelengths with only the result that the polarity of the output voltage at the terminals l6 and I! will be reversed. Of course, the resonator ll must be retuned to the frequency 1 when the orifice I8 is moved, and neither the orifice IS, the piston 30 nor the piston 3| can be moved so far to the left that it will interfere with the proper functioning of the right-

hand coupling orifices

25 and 28.

What is claimed is:

1. A frequency discriminator comprising a main wave guide adapted at one end for the reception of electromagnetic waves of nominal frequency f, a cavity resonator resonant at approximately the frequency f and approximately matching said main guide in impedance at said frequency, means for electromagnetically coupling said resonator to the other end of said guide, two aperiodic auxiliary wave guides directionally coupled to opposite sides of said main guide, means for short-circuiting the ends of said auxiliary guides towards which said waves are directionally propagated, two detectors located, respectively, at the other ends of said auxiliary guides for deriving a unidirectional potential proportional to the high frequency power therein, each of said detecting means being approximately matched in impedance to the associated auxiliary guide, and means for connecting said detectors in series, the difference between the length of the electrical path from said one end of said main guide to the short-circuited end of one of said auxiliary guides and the length of the electrical path from said one end of said main guide to the short-circuited end of the other of said auxiliary guides being approximately equal to an odd integral number of quarter wavelengths, and each of said lengths differing from the electrical length of said main guide by approximately an odd integral number of eighth wavelengths, at the frequency J.

2. A frequency discriminator in accordance with claim 1 in which said detectors are paired at the frequency f.

3. A frequency discriminator in accordance with claim 1 in which each of said detectors comprises a crystal.

4. A frequency discriminator in accordance with claim 1 in which each of said detectors comprises a thermistor.

5. A frequency discriminator in accordance with claim 1 in which each of said detectors comprises a thermionic diode.

6. A frequency discriminator in accordance with claim 1 which includes means for adjusting the lengths of said auxiliary wave guides.

7. A frequency discriminator in accordance with claim 1 which includes means for adjusting the resonant frequency of said resonator.

8. A frequency discriminator comprising a main wave guide adapted at one end for the reception of electromagnetic waves of a selected nominal frequency, a cavity resonator resonant at approximately said frequency and approximately matching said main guide in impedance at said frequency, an orifice coupling said resonator to the other end of said guide, two aperiodic auxiliary wave guides directionally coupled to opposite sides of said main guide, means for short-circuiting the end of one of said auxiliary guides towards which said waves are directionally propagated at a point A/8 to one side of the plane of said orifice, means for short-circuiting the end of the other of said auxiliary guides towards which said waves are directionally propagated at a point M8 to the other side of said plane, two detectors located, respectively, at the other ends of said auxiliary guides, each of said detectors being approximately matched in impedance to the associated auxiliary guide, and means for combining the output voltage of said detectors in series-opposing relationship, the longitudinal axes of said wave guides being approximately parallel and A being the wavelength within the auxiliary guide under consideration at approximately said frequency.

9. A frequency discriminator in accordance with claim 8 in which each of said detectors comprises a crystal.

10. A frequency discriminator with claim 8 in which each of comprises a thermistor.

11. A frequency discriminator with claim 8 in which each of comprises a thermionic diode.

12. A frequency discriminator in accordance with claim 8 in which said short-circuiting means are adjustable along said auxiliary guides.

13. A frequency discriminator in accordance with claim 8 which includes means for adjustthe resonant frequency of said resonator.

References Cited in the file of this patent UNITED STATES PATENTS in accordance said detectors in accordance said detectors Number Name Date 2,153,728 Southworth Apr. 11, 1939 2,413,939 Benware Jan. 1 47 2,420,892 McClellan May 20, 1947 2,423,390 Korman July 1947 2,562,281 Mumford July 31, 1