US2413939A - Ultra high frequency discriminator - Google Patents

Description

Jan. 7, 1947. n.1'. BENWARE ULTRA HIGH FREQUENCY DISCRIMINATOR Filed March 2l. 1944 Patented Jan. 7, 1947 NITED STATES PATENT OFFICE ULTRA HIGH FREQUENCY DISCRIMNATOR Application March 21, 1944i, Serial No. 527,442

(Cl. Z50-27) 3 Claims.

This invention relates t frequency variation response circuits, and particularly to those circuits which are responsive to frequency variation in the range of frequencies generally referred to as microwaves l While no definite frequency limits are associated with the term "microwaves, this term is sometimes used to identify radio waves whose length may be from less than l centimeter to 30 centi* meters, or whose frequencies may be from about 1000 megacycles to more than 30,000 megacycles.

At these very high frequencies, often referred to as hyper-frequencies, energy maybe transmitted, radiated or received by means of hollow pipe conductors, or wave guides, instead of by the conventional transmission lines and antennae. The principles of operation and the structure of wave guides and resonant chambers are now generally understood by those skilled in the art, and considerable information has been published in technical periodicals and in books, such as Hyper and Ultra-High Frequency Engineering, by Sarbacher and Edson, published in 1943 by John Wiley and Sons. Therefore, it is unnecessary to discuss here the theory and constructional details of such devices. s

It is an object of the present invention to provide, by means of a novel system of hollow wave guides, or resonant cavities, a high frequency circuit that is responsive to the frequency deviations of frequency-modulated radio waves.

Another object 0f the present invention is to provide a frequency discriminator or frequency detector circuit for use at hyper frequencies.

Othei` objects and advantages of thepresent invention will become apparent during the course of the following description with reference tothe associated drawing, in which:

Fig. l is an exploded isometric illustration of one embodiment of the wave guide assembly which forms la part of the circuit of this invention;

Fig. 2 illustrates schematically a frequency discriminator circuit constructed in accordance with the present invention; and

Fig. 3 is a typical'response curve for the circuit of this invention showing output voltage plotted against applied frequency.

Referring to Fig. 1, the hollow rectangular wave guide, or cavity, II, which may be considered as the primary wave guide or primary circuit of the system, is arranged to receive electromagnetic energy at its open end I2. The opposite end of this wave guide is provided with an adjustable tuning means which may comprise a. movable baille or piston I3 of conducting material. For

lll

clarity, this baille is shown exterior `to the wave guide. Attached to each face or broad side of the primary wave guide ll is another section of wave guide, as indicated at il and l5. These two latter wave guides, constituting secondary circuits of the system, are also tted with adjustable tuning means, such as the baffles lli and il. For the transfer of electromagnetic wave energy from the primary wave guide ll into the secondary guides lll and l5 a suitable opening may be located in each face of guide l l, as indicated in the drawing at lli and lll.

Fig. 2 is a cross-sectional view of the Wave guide, or cavity, assembly taken in a, plane containing the longitudinal axes of the primary and secondary wave guides. In this figure the relative positions of the movable pistons in the wave guides are shown, and in addition the location of the coupling probes or loops and the detectors is indicated. The corresponding parts of Fig. 1 bear the same identification in Fig. 2. The primary wave guide, its open receiving end and its movn able piston in an arbitrary position are indicated at I I, l2 and I3 respectively. The secondary wave guides with their baffles are shown at I4, I5, I6 and I'I. The energy coupling holes or irises are shown at I8 and I9. The secondary Wave guides I4 and l5 are each equipped with a conventional probe and detector assembly 20 and 2I respectively. Each detector assembly may comprise a pick-up loop, one end of which is fastened to the wave guide and the other end of which is connected to one terminal of a detecting device which, in the preferred embodiment, is a cartridge type crystal detector. The pick-up loops are illustrated at 22 and 23, while 24 and 25 represent the crystal detectors. The detector assemblies 20 and 2| are equipped with some conventional form of by-pass circuits to permit the maximum available high frequency voltage to be applied across detectors 24 and 25. In the embodiment shown in Fig. 2 the elements 35 and 36, known in the art as quarter wave chokes, form with their respective walls 31 and 38 of the detector assemblies, high frequency by-pass circuits. Elements 35 and 36 may be hollow structures of conducting material, closed at the end adjacent to the crystal detectors 24 and 25. Structures 35 and 36 are mounted within and insulated from the walls 31 and 38 so as to form with these walls open circuited quarter wave sections of transmission lines. The free terminals of detectors 24 and 25 are connected tothe closed end of elements 35 and 36. Since the structures 35 and 36 form, with walls 31 and 38, open circuited quarter wave sections of a transmission this point is, in general, not critical, since the conductor 30 carries low frequency currents only. The other end 3l o f resistor 26 is connected to the free terminal of crystal detector 24 by conductor 32, while the other end 33 of resistor 21 is connected by conductor 34 to the free terminal of crystal detector 25.

The operation of this invention as a frequency discriminator or detector of frequency-modulated radio waves will now be described with reference to Fig. 2 and for the 'IEn mode of propagation, in which the electric field vvector is peran output voltage across this resistor.

the voltage induced in loop 23, due to the vary.

pendicular to the broad sides of the wave guides.

'Ihe electromagnetic energy is received by and transmitted through wave guide Il. vSince this primary wave guide is terminated in a conducting baille I3 the incoming electromagnetic wave is reflected by this plate. The reflected wave combines with the incoming wave to form in guide II a standing wave in which the electric and magnetic components are in phase quadrature. 'I'he electric field is short circuited by the bafiie I3 and-accordingly at regions an integral number of 'half wave lengths from the baille the electric field intensity is minimum and the magnetic field is at maximum intensity. The secondary wave guides I4 and I5 and coupling irises I8 and. I5 are located at a distance which is an integral number ofahalf wave lengths from the baille I3, that is, at `regions of high magnetic field intensity. The secondary guides I4 and I5 may then be considered as inductively coupled to the primary guide Il.`

The Q, or sharpness of resonance, of the wave guide II 'is affected by the amount `of energy coupled yinto the secondary guides I4 and I5 from the primary circuit Il. This is analogous to the conventional tuned circuit of lumped inductance and capacity in which the Q or resonant property of the tuned circuit is lowered by the coupling of a secondary circuit to the tuned circuit. v

In the circuit of this invention the primary wave guide II is tuned by means of sliding piston I3 to the mean or carrier frequency of the frequency modulated radio wave. The magnitude of coupling between the primary circuit Il and the two secondary circuits 'I4 and I5 is preferably such as to cause the primary wave guide II to be resonant with a fairly low Q.

The secondary wave guides I4 and I5 are also terminated by short circuiting pistons I6 and I1 to form resonant chambers. The movable pis` tons I6 and I1 are likewise located at distances an integral number of half lwave lengths' from the coupling holes I8 and I9, thereby forming standing waves of high magnetic eld intensity in the vicinity` of the pistons I6 and I1. The pick-up wires or coupling probes 22 and 23 are accordingly in regions of high magnetic field intensity. The high frequency voltage induced in loop 22 ,is impressed directly across crystal detector 24, the capacitive reactance between the 1rwave length choke and the cylindrical con- The rectified output current of the crystal flows through the detector load resistor 26, producing Likewise,

ing magnetic field in chamber I5, is impressed across'crystal detector 25 to ldevelop across re-l sistor 21 a rectified output voltage.

The secondarywave guides I4 and I5 are so tuned by their respective movable pistons I6 and I1 that one secondary circuit i's resonant to a frequency on one side of the carrier frequency while the other secondary is tuned to a frequency on the other side`of the carrier.

To insure good linearity in the overall response of the discriminator system, it is preferred that the effective Qs of the two secondary circuits be relatively high compared to the Q of the primary circuit. Where such relatively high secondary Qs are desired it is best not to couple the detector circuits too tightly to the secondary wave guides. Iny Fig. 2 the small coupling loops or probes 22 and 23 'provide only moderate coupling, and were found tovbe quite satisfactory.

'I'he frequency discriminating action of the circuit of this invention as shown in Fig. 2 may be described as follows.

Assume that the received carrier wave of normal freque'ncy fo is frequency modulated so as to vary it from a frequency of fr below fo to a frequency f2 above fo, such that fo-fi equals fa-fo. The wave guide II is tuned by movable piston I3 to the carrier frequency fo and the secondary wave guide I4 is tuned by piston I6 to frequency f1 while the other secondary wave guide I5 is tuned by piston I1 to frequency f2.

When the incoming radio wave is of the frequency f1 the intensity of the magnetic field in resonant chamber I4 will be greater than that of the magnetic field in resonant chamber I5 since this is the resonant frequency of I4. There will consequently be a greater voltage induced in loop 22 than in loop 23 and a greater rectified voltage developed across output resistor 26 than resistor 21. Under this condition the end 3i of resistor 26 will be at its maximum potential with respect to the junction point 28. As the frequency of the incoming wave increases the magnetic field strength in wave guide I5 will increase, while that in the wave guide I4 will decrease. When the frequency of the wave reaches ,fz the intensity of the magnetic eld in wave guide I5 will be a maximum since wave guide I5 is resonant at this frequency. The detected output voltages will then be such that the voltage across resistor 21 is a maximum and that across resistor 26 a minimum. If the crystal element 24 has been so connected'that the end 3| of resistor 26 is positive with respect to the junction point 28, then the crystal element 25 should be so connected that the end 33 of resistor 21 is also positive with respect to 'junction 28. Thus, with respect to an output circuit connected to the terminal points 3l and 3 3, the load resistors 26 and 21 are in differential relation, i. e., in opposing relation. 'I'he two'secondary Wave guide circuits are preferably designed to have similar response characteristics, so that when the incoming wave is of frequency lo, equal voltages are developed across resistors 26 and 21. Since the voltages across these two resistors are in oDPOsition the total output voltage developed will then be zero. A typical response curve I0 of the circuit of this invention is shown in Fig. 3.

The output voltage or voltages of the circuit of ductor being negligible at the .carrier frequency. Fig. 2 may be applied by any of the vconventional methods to a signal utilization means. For example, the voltage between points 3| and 33 may be applied to a conventional single sided audio amplier, or with junction point 28 grounded the potentials developed at 3| and 33 may be applied to a push-pull audio amplifier stage.

In one physical embodiment of the invention, the diameter of the coupling holes I8 and I9 was made approximately equal to one-fourth of the width of the broad side of the wave guide.

While the invention has been described with reference to a particular embodiment, it will be apparent to those familiar with the art that various changes and modications may be made in the circuit as described and illustrated Without deviating from the spirit and scope of the invention as dened by the appended claims.

I claim:

1. A frequency discriminator for detecting frequency deviations in hyper frequency carrier waves, comprising a primary cavity resonant at the center frequency of said carrier wave, a first secondary cavity resonant at a predetermined frequency above said center frequency, coupling means between said primary cavity and said secondary cavity, a second secondary cavity resonant at a predetermined frequency below said center frequency. coupling means between said primary cavity and said second secondary cavity, a rst non-linear circuit element responsive to the wave intensity in said rst secondary cavity, a. second non-linear circuit element responsive to the wave intensity in said second secondary cavity, and an output circuit connecting said non-linear circuit elements in differential relation for establishing a detected signal whose magnitude is proportional to the deviation of said carrier wave from said center frequency.

2. A frequency discriminator as claimed in claim 1, wherein said primary cavity comprises a wave guide closed at one end, said rst secondary cavity comprises a wave guide whose length is substantially an integral number of half wave lengths at said first-named predetermined frequency, and said second secondary cavity comprises a wave guide whose length is substantially an integral number of half wave lengths at said second-narned predetermined frequency.

3. A frequency discriminator as claimed in claim 1, wherein all of said cavities are constructed in the form of hollow wave guides, and wherein the coupling between said guides is inductive, and is provided by means of iris openings between said primary and secondary cavities.

ROBERT T. BENWARE.

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