US2477434A - Radio direction finding - Google Patents

Description

July 26, 1949.

Filed DSG. 9, 1947 W. H. WIRKLER RADIOJSIRECTION FINDING,

2 Sheets-Sheet 1 WAM TEP H WMD/LEP July 26, 1949. w. H. WIRKLER RADIO DIRECTION FINDING 2 Sheets-Sheet 2 Filed Dec. 9, 194'? anu/vof@ Napa/.AMP

MP W m Patented July 1949 aanronmac'rron FINDING application December 9, 1941, serial 10.790.502

This invention relates to radio direction finders of the type producing an indicating voltage dependent on the deviation of the direction-of signal arrival from the null or reference direction of a directional antenna, and the principles of this invention may be applied to direction lindemploying manual or automatic orientation of the directional antenna as well as to those deriving the indicated bearing by comparison of the indicating voltages derived from a plurality of fixed directional antennas.

The object of this invention is to reduce or substantially eliminate the production of directional indicating voltage from atmospheric or other noise of random character which normally causes considerable error in the indicated bearing.

Another principal object of the invention relates to a direction iinder system of the type employing.A a loop antenna and a sense-of-direction antenna and an associated balanced modulator, which are connected to a superheterodyne radio receiver without requiring the usual phasing circuit between the said modulator and the input tothe receiver.

Another object relates to a radio direction finder system employing a loop antenna and a senseof-direction antenna with a balanced modulator between the loop antenna and the radio receiver 14 Claims. (Cl. 343-121) A 2 j time delay before effecting said modulation and the resultant modulated signal is passed through the usual radio receiver circuits to produce the bearing indication voltage under control only of the components in the receiver output which are of the same frequency as that of said l al beat frequency oscillator. l

Another feature relates to a receiving rrangement for a direction nder of the type employing a directional antenna and a non-directional antenna. and comprising a superheterodyne radio receiving arrangement for producing an intermediate frequency signal which is the resultant of modulation of the carrier signal from the non-directional antenna by the output of a balanced modulator which modulates the carrier from the directional antenna with a locally-generated beat frequency. Said resultant intermediate frequency is of the same frequency as said local beating frequency so that it can =be applied together with the local beat frequency to a detecting circuit of the product detector type to produce a bearing indication signal which is stantaneously proportional to the product of e two applied voltages, and the product detecto is connected to a. lter arrangement for by-p g all except the uni-directional components of the output.

A further feature relatesrto a radio direction and whereby the natural quadrature phase relander of the type employing a loop antenna, and

tion between the voltages from the two antennae is utilized so that the sidebands from the modulator are inphase with respect to the carrier from the sense-of-direction antenna. The sum ofthe carrier and sideband signals in the inciated with amplitude modulation. In the conventi-'anal direction finder arrangement, a special phasing circuit. which is dimcult to design and set up, is required between the usual balanced` `modulator andthe superheterodyne radio rather than phase modulation of the sense-ofdirection signal.

A feature of the invention relates to a method of combining the signals from a directional antenna and a non-directional antenna by heterodyning the signal from one antenna with a frequency derived from a local beat frequency oseillator to produce a beat note which is used to modulate the signal from the other antenna.

' receiver so as to constitute amplitude modulation 45 a sense-of-direction antenna, both of which feed a superheterodyne radio receiver, the sense-ofdirection signal being applied directly to the receiver and the loop signal being modulated in a balanced modulator with a local oscillation, characterized in the novel features that the said local oscillation is derived from a beating oscillator whose output is mixed with a selected intermediate frequency sideband from the receiver to produce said local oscillation. A product detector is also provided and is arranged to be fed with said beating frequency and with a sideband intermediate frequency from the receiver which sideband is equal in frequency to said beating frequency. In order to facilitate the amplification of the resultant bearing indication signal vfrom the product detector, the phase of said local oscillation is continuously reversed at a. predetermined rate by means of a switch, and the output of the product detector is also subjected to mechanical rectification in synchronism with the operation of said switch.

A still further feature relates to the novel ar- This beat note is subjected to a. predetermined rangement and interconnection of parts which cooperate toA provide a more reliable radio direction-finding arrangement.

Other features and advantages not speciiically enumerated, will be apparent after a consideration of the following detailed descriptions and the appended claims.

In the drawing,

Fig. 1 is a schematic illustration, in block diagram form, of a conventiona1 radio direction ilnder arrangement which is used-in explaining the present invention.

Fig. 2 is a schematic illustration, also in block diagram form, of one embodiment of the inventlon.

Fig. 3 is a schematic wiring diagram of a modiiication of Fig. 2.

Fig. 4 is a schematic diagram of a typical pwdduct detector that is used in the embodiment of Figs. 2 and 3.

The usual direction nder circuit, such as is commonly used'in airborne automatic direction finders, is shown in Fig. 1. The directional signal fr from loop I is modulated in balanced modulator 2 by voltage from a low frequency oscillator 8 of frequency fr. The output of modulator 2 is passed through radio-frequency phasing circuit 3 and applied to the input of a superheterodyne radio receiver 4 together with reference signal from sense antenna 5. The intermediate frequency output of the receiver is rectified in second detector 6. 'I'he output of detector 6 contains, in adition to the modulation components of the original signal fs, components of frequency fr. produced in detector 6 by demodul-ating against carrier signal j., from antenna 5, the double sideband derived by modulating in balanced modulator 2 the signal fi from antenna I with the local oscillations fr.. Phasing circuit 3 is required so that the modulation products from modulator 2' will be phased properly with respect to the reference signal from antenna so as to constitute amplitude modulation rather than phase modulation of the reference signal as required in detector 6, since the signal from antenna I isl in phase quadrature with that from antenna 5. The components of frequency f from detector 6 are selected and amplified in audio frequency amplifier 'Ia and thence in the audio frequency amplifier I which selectively amplies frequency fa, this selectively lamplified frequency is then combined in the device 9 with voltage from oscillator 8 to produce the directional indication. Device 9 may, for example, consist of a twocoil electrodyna-mometer instrument which produces the directional indication directly whenever it receives in oney coil components from amplifier 1 of the same frequency as the frequency components from oscillator 8 in theother coil. Usually device 9 is some circuit commonly called a phase detector or product detector which produces an output voltage approximately proportional to the product of the two A. C. input voltages and the cosine of the phase angle between them. Thus, in Fig. 1, the output of device 9 is unidirectional (D. C.) and is shown connected to terminals II through lter I0. The output at terminals II will be zero, of course, when loop I is oriented for zero signal pickup, and the polarity of the output will depend on the angular deviation of loop I from this null orientation, since opposite deviations pick up signals in opposite sense. Thus, during one instant in the modulation`cycle when the signal from the loop for a positive deviation would add to the signal from the sense antenna in detector 6, re-

4. versing the sense of the deviation would cause it to subtract instead, thus resulting in a phase reversal in the j. components from detector 6 as referred to the phase of the oscillations from oscillator i. and consequent reversal o f the polarity at II. Hence terminals II can be connected to a zero-center D. C. instrument for indicating the sense of the .bearing deviation, or to a servomotor control circuit for automatic orientation of the loop to the null position.

Even through the loop be oriented for zero signal pickup. there may be noise arriving from some direction other than that of the signal and which is picked up by loop I as well as by sense antenna l. During moments when this noise voltage in detector 6 is stronger than the signal voltage, the D. C. output of detector I will consist mostly of rectified noise voltage. During one part oi' the modulation cycle in modulator 2, the noise voltage from loop I will add to that from antenna 5, and during another part it will subtract, so that the output of detector 8 will contain components of frequency l.. derived from the noise, resulting in a spurious D. C. indicating voltage at terminals II of polarity determined by the direction of arrival 0f the noise. This in turn will aiect the servo system and cause misorientation of the loop, or "pulling of the indicated bearlng by the noise.

If the noise were stronger than the signal only occasionally, its effect could be reduced materially by a limiting circuit which effectively stops the production of indicating voltage whenever the total, I. F. signal exceeds a threshold value set equal to that of the signal. Unfortunately, however, severe atmospheric noise, after passing the selective circuits of the receiver, is not usually of peaked or intermittent character and might be stronger than the signal practically all of the time. Limiting action would then disable the circuit entirely and prevent it from taking a bearing. Hence in accordance with one phase of the invention the receiver circuits are designed specifically to avoid severe limiting action and overloading, so that indicating voltage will be produced at least part of the time even under comparatively severe noise conditions, and additional means are provided for reducing the production of spurious indicating voltage from the directional noise intercepted, such an arrange'- ment is illustrated in Fig. 2, wherein the parts which are the same as those of Fig. 1 are designated alike.

In Fig. 2, loop modulator 2 is actuated by a beat note of frequency fb derived in mixer I2 by heterodyning the I. F. signal of frequency fr from receiver 4, against the output of I. F. beat-frequency oscillator I3. 'Ihe beat-not output of mixer I2 may be selected and amplified by selective amplifier Il, the output of which is applied as modulating voltage to balanced or carrier suppressing modulator 2. Selectivity in amplifier I4 is not essential to the anti-noise feature of this invention, however. If the frequency of oscillator I3 is lower than that of the I. F. signal, for example, (fb=f-fo). modulation of the loop signal by jb produces two sidebands spaced in frequency from that of the signal by the frequency interval fb. 'I'hese sidebands appear in the I. F. circuits of the receiver I as components of frequency fri-fb and fi-fb, respectively. Since fifb=fo, one of these is of exactly the same frequency as that of oscillator I3, is independent of receiver tuning, and can produce the indicating voltage by various selecting means, one of which is indicated as .y :terasse device I5. Device I5 here takes the form of a product detector, having a pair of input circuits and a single output circuit with I. F. output from the receiver 4 applied to one input circuit, and beat frequency oscillator voltage from oscillator I3 applied to the other input circuit. The D. C. output of device I5 is then applied to terminals II through filter III. Various forms of product detector suitable for use in Fig, 2 are disclosed in my copending patent application Serial No. 790,501, filed on even date herewith, anda typical one of such detectors is illustrated in Fig. 4 hereof. Thus as shown in Fig. 4, the product detector comprises a pair of diodes Di, Dz, connected as a balanced rectifier to the two input circuits and to a common output. Thus there will -be no direct current across the output when input signal is applied to only one of the inputs. The expression "product detector or coincidence detector" will lbe used herein to designate any arrangement similar to that of Fig. 4 which does not produce a characteristic output except when a signal is applied to each of two inputs simultaneously.

In Fig. 1, components derived from loop antenna I and sense antenna 5 `pass through the early circuits of the receiver 4, together with essentially the same time del-ay and do not interact with each other to produce a new component from which the indicating voltage is derived until they reach detector 6. In Fig. 2, however, reference voltage from antenna 5 passes rst through the receiver 4, is then converted to a low frequency in mixer I2, amplified in amplifier I4 and applied to modulator 2 where it interacts with signal from loop I to produce a new component (sideband) which passes through the receiver 4, emerging as a component of frequency fo from which is derived the indicating voltage in device I5. Hence modulator 2 itself serves the function of a product detector to which signal is applied direc-tly from antenna I and indirectly from antenna 5 through elements 4, I2 and I4. Because of the time delay in the receiver circuits and in the beat-note circuits at I4, it is obvious that the major portion of a noise impulse intercepted at antennae I and 5 does not reach modulator 2 from antenna I and from amplifier I4 at the same time so as to cause output in receiver 2.

As explained in my copending application Serial No. 790,501, led on even date herewith, the interaction of an impulse from antenna I with an earlier impulse from antenna 5 produces a component of frequency fo in the receiver outputthe phase relation of which, with respect to that of oscillator I3, is determined by the delay in receiver 4 and amplifier I4 and by the spacing of the impulses which is, for random noise, likewise random. Hence the polarity of the output of device I5 will be random also, consisting of a large amount of alternating components which are rejected by filter I0, and no unidirectional component which could be passed to terminals I I. As explained in said copending application also, the actual value of the D. C. component produced at terminals I I by the eiect of directional noise is ead 'times as strong as it wouldv have been without the action of the delay effect, where e is the base of the natural system of logarithms,

a is the decrement of the tuned loop circuit, and d is the delay, in seconds, involved in traversing elements 4 and I4. Since i a relatively large product ad is obtained even for LlO relatively small values of d, and a relatively large reduction in the effect of noise results. This results from the relatively large value of the decrement a inthe usual loop circuit. and shows the advantage of combining the loop signal with the reference signal at the input of the receiver where the loop noise is still highly damped and decays to a negligible value before that from the sense antenna arrives, even after a relatively short delay. In fact, I have found that with the -arrangements as disclosed in Fig. 2, the incidental delay of the receiver circuits is sufficient to reduce the noise effect as much as is normally required without the deliberate introduction of delay circuits at amplifier I4.

The phasing circuit 3, used in the conventional circuit of Fig. l to assure that the sidebands from modulator 2 can be detected as amplitude modulation in detector 5, is not necessary in the arrangement of Fig. 2 because the desired phase relation between the sideband component of frequency fb and the beat frequency oscillator voltage can be obtained either by the introduction of phase shift in audio circuit I4 or by adjustmentof the phase of the voltage from oscillator I3 applied to device I5. 'Ihis has two advantages. One is the elimination of the design problem involved in applying a 90 phase shift in the lowlevel variable signal frequency circuit independent of tuning adjustment and without loss of signal or introduction of tube and circuit noise. The other advantage is that the natural quadrature phase relation between loop and sense antenna voltages existing Without the use of circuit 3 is such that the sidebands from balanced modulator 2 are so phased with respect to the carrier from sense antenna 5 that the sum of the carrier and sideband signals in the receiver appears as a phase-modulated wave, the modulation sidebands of which are not so readily destroyed by limiting action due to overloading by noise as are those associated with amplitude modulation.

Some of the carrier signal from sense antenna 5, however, may be lost by overloading in the receiver. Fortunately, however, the signal from antenna 5 is large relative to that from loop I when the loop is near its null orientation and the action of the indicator or loop Servo is most critical. As is well known, mixer I2 acts as a frequency translating device under the control of oscillator I3 and can be designed to minimize the effect of overloading so that it produces a component of frequency fb from the signal even if the signal is weaker than the noise, whereas the effect of the weaker signal on the output of second detector 6, Fig. 1, is mostly lost in the nonheterodyne detector, especially if detector 6 is a high-level diode rectifier as is usually the case. One of the major functions of the selective circuit of amplier I4, which is broadly tuned to the low frequency fb of the beat note from I2, is to reject some of the noise from mixer I2 which might otherwise overload balanced modulator 2 and result in loss of directional sensitivity.

To ald in clarifying the operation of the circuit, the phase relations are traced as follows: The signal from the sense antenna may be expressed as Es=Vs cos wa=Vs cos 4a where Ei is the instantaneous strength of the sense antenna signal, V

is its peak strength, we is 21r times the signal frequency.

and t is the time, in seconds, elapsed AWM 2 cos a cos b=cos (a+b) +cos (a-b) that the I. F. output of the first detector can be written.

where K is a detection coemclent. since it is only the lower sideband of the modulation process which is accepted by the I. P. circuits. The I. l". signal output of the receiver can then be written Eu=Vu cos (im-www) =Vn cos ou Ec=Ve cos wot=Vo C08 4m and if the beat frequency oscillator frequency is lower than that of the signal, the beat note output of mixer I2 is Em=Vn cos (oa-oo) The output of amplifier I4 is then given by where is the audio phase shift in circuit I4. The output of loop I when-misoriented from the null direction in a given sense may be written EL= VL cos (weg) y since the loop voltage is in phase quadrature with the sense antenna voltage. The lower sideband output of the loop modulator will then be VL., 00S (+a+o+h+wnda)= VL. cos n. At the I. F. output of the receiver, this becomes Eam=V1Lm` cos (orm-on-wodc) l where wn is 21|- times the beat frequency oscillator frequency, and do is the delay of the receiver circuits at this frequency. 'I'his reduces to Emu: Vis 00B ['#o-I-'n'i-wi-dia-wdiol'] Now ou-wo.=\wb, where us is 2r times the fre- 'quency of the beat note. If the delay of the receiver circuits is substantially the same at the I. F, signal frequency as at the beat frequency 'oscillator frequency, then du=do=dn and we Since =dawa where d. is the delay of the audio beat-note circuit, this can be written 8 Thisisseentobeacomponentofthe samefrsquency as the beat frequency -oscillator voltage applied to mixer I2 but differing therefrom in haso by n(d+df) radians due to delay, in addiradiansduetothequadraturephaseoftheloop voltlse.

A similar analysis shows that when the beat frequency oscillator frequency is higher than that of the signal, the component of the beat frequency oscillator frequency in the receiver output circulti! 'I'hat is, the phase shift due to delay reverses .Isn on opposite sides of zero beat.

Now the product detector at Il requires the two components applied to its input to be in phase to produce an indicating voltage of the correct sense. If they are out of phase the sense will be reversed, and if in quadrature, the sensitivity will be zero. Since this phase relation depends on the beat note and therefore on the receiver tuning, there is provided a tuning indicator Il actuated by the beat-note output of selective amplifier I4. When the receiver is tuned for a maximum indication on tuning indicator Il, the beat note will be of the correct frequency as determined by the peak frequency of amplifier I4. This frequency can be chosen so that (da-l-dr) is an odd multiple of so as to compensate for the quadrature phase relation of the loop voltage. However, the sense of the indication will then be opposite for the is a multiple of r, and the sense will be the same on opposite sides of zero beat.

Referring to Fig. 3, there is shown a modication of. the system of Fig. 2, the parts of Fig. 3 which are the same as those of Fig. 2 being designated alike, In Fig. 3, beat note fb in the output of mixer I2, resulting from the mixing of the intermediate frequency signal l. and the beat frequency fo from oscillator I3, is amplified in the amplifier I4 which is tuned by the network I8, Il. to the frequency fb. 'I'he beat note h in the output of amplifier I4 is periodically reversed in phase by a switching device comprising the movable contact arm 20 and the associated spaced contacts 2I, 22, the arm 20 being oscillated by a cam 23 by means of a suitable motor 24. Consequently, the component in the output of receiver 4 of frequency fn which reacts in the product detector I5 with the beating frequency from oscillator I3, is also reversed in phase periodically. Thus, the output of detector I5 due to this component, is an alternating current of the frequency determined by the frequency of operation of switch arm 2li rather than a direct current increment. It will be observed that the product 9 detector I5 comprises a. conventional plural grid tube 25 which may be of the so-called pentagrid converter type comprising the cathode 26, first control grid 21, shield grids 28, 29, second control grid 30, suppressor grid 3|, an output plate or anode 32. The component fo in the output of receiver 4 is applied to control grid 21, while the beating frequency fo from the oscillator I3 is applied to the control grid 30. 'Ihe alternating current component' in the output of tube 25 is applied to the phase inverter tube 33 and thence to the push-pull amplifier tubes 34, 35. The pushpull output of tubes 34 and 35 is applied to the movable switch arms 36, 31, which are operated in unison with the arm 20 above described. Each of the switch arms 3B and 31 has an associated set of spaced contacts which are connected to the filter circuit I as shown. 'Ihe switch arms 3S and 31 with their associated fixed contacts constitute a mechanical rectifier which is driven in synchronism with the switch arm 20 to produce a direct current signal that is applied to the filter l0. This direct current component, after being filtered in device III, is amplified by the direct current amplifier, comprising amplifier tubes 38, 39, whose output is connected to the output terminals II. Terminals II can be connected to any suitable bearing indicator or servocontrol device in the event that the direction finder is of the automatic type.

One of the advantages of the embodiment of Fig. 3 over that of Fig. 2, is that the components of switch frequency controlled by cam 23, are readily selected from the output of device I and are readily amplified, whereas the selection of a small direct current increment and its amplification as in the system of Fig. 2, involves considerable difiiculty in balancing-out small direct current changes caused by changes in tubes, voltage, temperature and the like. r

While certain particular embodiments have been described herein, it will be understood that various changes and modifications may be made therein without departing from the spirit and scope of the invention.

What is claimed is: v

l. A direction finder arrangement of the type having a loop antenna and a sense of direction antenna the signals from which are to be combined to produce a direction indicating voltage, a radio receiver to whose input the signal from said sense of direction antenna is applied, a source of local beating frequency, means to heterodyne the signal from the output of said receiver with said beating frequency to produce a beat note, a balanced modulator which is fed by the signal from said directional antenna and by said beat note, a time delay circuit through which said beat note passes before reaching said modulator, means to apply the modulated signal from said modulator to the input of said receiver, and circuit arrangements for producing said indicating voltage from only those components in the receiver output which are of the same frequency as said beating frequency.

2. A direction finder arrangement according to claim 1 in which the last-mentioned circuit arrangements include a product detector having a pair of input circuits, means to feed one input circuit from the output of said receiver, and means to feed the other input circuit with said beating frequency.

3. A direction finder arrangement according to claim l in which said time delay circuit includes an audio frequencyamplifler tuned to said beat note.

4. A direction finder arrangement according to claim 1 in which said radio receiver is of the superheterodyne type and the said signal which is heterodyned with said beating frequency is an intermediate frequency carrier from said receiver.

5. In an arrangement of the type described, means to derive two carrier signals of the same frequency fs one of which is a directional signal and the other a sense of direction signal, a superheterodyrie radio receiver, means to apply the sense of direction signal to the input of said receiver, a balanced modulator whose output is also applied to-the input of said receiver, an oscillator for generating a local beating frequency fo, a mixer device, means to apply a selected intermediate frequency from said receiver and also said beating frequency to said mixer to produce a beat note of frequency fb=fs-fo, means to apply said beat note and said directional signal to said balanced modulator to produce in the output of said receiver a frequencyvfs-fb, a product detector having a pair of input circuits, means to apply to one input circuit said frequency fix-fb. means to apply to the other input circuit said beating frequency fo, a pair of output terminals to which a direction signal responsive device is to be connected, and a filter circuit connected to the output of said product detector for passing only the uni-directional components thereof to said output terminals.

6. An arrangement according to claim 5 in which a phasing control circuit is connected between said beating oscillator and said product detector to maintain the frequency fo from the beating oscillator and the frequency (fs-fb) from said receiver at phase quadrature with respect to each other.

7. An arrangement according to claim 5 in which means are provided for regularly and recurrently reversing the phase of the said beat note, and means are provided for mechanically rectifying the signal from said product detector .in synchronism with said recurrent phase reversal.

8. An arrangement according to claim 5 in which the output of said product detector is connected to an alternating current amplifier and thence to a mechanical rectier, said rectier being connected to said output terminals through said filter.

9. An arrangement according to claim 5 in which said beat note is applied to the switch arm of a reversing switch for reversing the phase of the beat note regularly and recurrently at a predetermined rate, said product detector is connected to an alternating current amplifier Whose output is applied to the switch arm of a mechanical rectier, and motor driven cam means are provided for operating both said switch arms in synchronism.

10. The method of producing a direction finding indication voltage by combining two received voltages each of a carrier frequency fs, one of which is a direction signal and the other is a sense of direction signal, which comprises mixing the sense of direction signal with a local generated beating frequency fo to produce a beat note frequency fb, modulating the direction signal with said beat note frequency to suppress the frequency fs while generating a side band frequency (fs-fb=;fo), and applying the said beating frequency and the said side band frequency (fs,fb=fo) to a circuit which produces an outl1 put only in response to the timed coincidence of two applied input voltages.

11. The method according to claim 10 in which the said beat note is subjected to a time delay before modulating the direction signal.

12. The method according to claim 10 in which said output is subjected to a, iilter action to filter out all except the uni-directional components.

13. A direction nder arrangement of the kind employing a directional antenna and a sense of direction antenna which latter antenna feeds a radio receiver and the signal from the directional antenna is subjected to modulation by a separate low frequency, a local source of beating frequency, means to mix the beatingffrequency with the signal from the receiver to produce said low frequency, means to subject said low frequency` to a time delay, means to modulate the signal from said directional antenna by said time delayed low frequency, means to apply the said modulated signal to the input of said receiver, means to select from the output of the receiver a frequency equal to said beating frequency, and a product detector which is fed with said beating frequency and said selected frequency to produce a bearing indication signal voltage.

14. A direction nnder arrangement according to claim 13, in which the means for subjecting said low frequency to a time delay comprises a selective low frequency amplier having substantial time delay betweenits input and output signals.

WALTER H. WIRKLER.

REFERENCES CITED 'I'hetollowing referemces are of record in the 111e of thispatent:

UNITED STATES PA'I'ENTS 2,314,029 Bond. et al. Mar. 16, 1943

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