Title:
Superheterodyne radio altimeter or locator
United States Patent 2424796


Abstract:
My invention relates to radio systems of the type utilizing reflected radio waves for distance measuring, detection of reflecting objects, or the like. It relates particularly to altimeters or other distance measuring systems which are cyclically frequency modulated and to systems that utilize...



Inventors:
Carlson, Wendell L.
Application Number:
US50803143A
Publication Date:
07/29/1947
Filing Date:
10/28/1943
Assignee:
RCA CORP
Primary Class:
Other Classes:
455/85
International Classes:
G01S13/34
View Patent Images:



Other References:
None
Description:

My invention relates to radio systems of the type utilizing reflected radio waves for distance measuring, detection of reflecting objects, or the like. It relates particularly to altimeters or other distance measuring systems which are cyclically frequency modulated and to systems that utilize the Doppler effect.

In systems of the above-mentioned types it has been the usual practice to apply the reflected signal to a mixer or detector where it is mixed with signal supplied directly from the transmitter to obtain an audio signal which is then amplified by an audio amplifier. It has been found that the amount of audio amplification that can be employed is severely limited because of microphonics in the audio amplifier.

The difficulty caused by microphonics can be eliminated by obtaining the desired amplification in an intermediate-frequency or T.-F. amplifier where the signal being amplified has a frequency above that of the microphonics. The use of the superheterodyne principle in a conventional manner for this purpose would not be entirely satisfactory both because of oscillator frequency drift and, in the case of the FM altimeter, also because of the fact that the I.-P. pass band would have to be wide enough to include the frequency deviation of the carrier wave.

An object of the present invention is to provide an improved method of and means for obtaining a large amount of amplification in FM altimeter or radar systems and in systems utilizing the Doppler effect while, at the same time, avoiding difficulties due to microphonics.

A further object of the invention is to provide an improved radio distance measuring or object locating system of the type utilizing reflected radio waves in which the received signals are amplified in an intermediate-frequency amplifier.

A further object of the invention is to provide an improved frequency-modulated altimeter or object locator of the superheterodyne type which is substantially free from the effects of undesired amplitude modulation.

A further object of the invention is to provide an improved frequency-modulated altimeter or object locator of the superheterodyne type wherein frequency shift of the I.-F. signal due to frequency drift of the transmitter and heterodyne oscillators is minimized.

In one embodiment of the invention, a strong heterodyning signal is supplied to the mixer or detector of the receiver where it heterodynes with the weak reflected signal and with a weak signal obtained directly from the transmitter. The strong heterodyne signal beats with each of the weak signals to produce two intermediate-frequency signals that appear in the mixer output, the two signals having the same I.-F. carrier frequency. These I.-F. signals are amplified in an I.-F. amplifier and supplied to a second detector where they mix to produce a beat-frequency signal. The beat-frequency signal may then be supplied to a suitable indicator such as a frequency counter or frequency indicator.

According to one feature of the invention the output of the I.-F. amplifier is also supplied through a frequency discriminator to an automatic-frequency-control circuit for the heterodyne oscillator whereby frequency drift of either the transmitter oscillator or heterodyne oscillator is compensated and whereby the band width occupied by frequency modulation is reduced.

According to another feature of the invention, the effect of undesired amplitude modulation is avoided by employing a local heterodyne oscillator for producing two I.-F. signals which are amplified in separate I.-F. amplifiers and supplied to the second detector. One of the I.-F. signals is obtained by supplying to a converter or mixer a weak signal from the local heterodyne oscillator and a strong signal directly from the transmitter. Any amplitude modulation on the strong signal will be of negligible value on the I.-F. signal. The other I.-F. signal is obtained by supplying a strong local oscillator signal to the first detector or mixer of the receiver where it mixes with the weak reflected signal. At the second detector the two I.-F. signals beat to produce the beat-frequency signal which may be supplied to a frequency counter.

In a preferred embodiment of the invention the two features of the invention are combined whereby there is obtained a superheterodyne type system that is substantially free from the effects of oscillator drift and of amplitude modulation, and which does not require in an altimeter or radar system an I.-F. amplifier having a pass band as wide as the frequency deviation produced by the frequency modulation. In practice, a comparatively narrow pass band for the I.-F. amplifier may be employed, and in addition, any difficulties that might be encountered due to unavoidable amplitude modulation are avoided. The invention will be better understood from the following description taken in connection with the accompanying drawing in which Figure 1 is a block and circuit diagram of a radio altimeter or locator system embodying one feature of the invention, Figure 2 is a block diagram of a radio altimeter or locator system embodying another feature of the invention, Figure 3 is a block and circuit diagram of a preferred embodiment of the invention, and Figure 4 is a block diagram of another embodiment of the invention. In the several figures, similar parts are indicated by similar reference characters.

Figure 1 shows an embodiment of the invention applied to a frequency-modulated altimeter or radar system comprising a high frequency transmitter oscillator 10 which is cyclically frequency modulated in the usual manner for supplying a frequency-modulated carrier wave to a transmitter antenna 11. The desired frequency modulation may be produced by means of a suitable modulating unit 12, such as a variable capacity unit, that is driven by a cyclically recurring wave supplied by a wave generator 13. The output of the generator 13 may be a wave of any suitable wave form such as a triangular wave, a sawtooth or a sine wave. Merely by way of example, it is assumed that the carrier wave frequency is 500 megacycles per second, that the frequency swing or deviation produced by modulator 12 is 2 megacycles, and that the modulator 12 is driven by a triangular wave recurring at a rate of 120 per second.

The system also includes a first detector or mixer 16 which comprises a detector tube 17. An antenna 15 is coupled through a transformer 18 to the tube 17 for applying thereto the frequencymodulated signal after it has been reflected from the earth's surface or from an object or objects to be located. Signal is also supplied to the mixer tube 17 directly from the transmitter 10 through a transformer 19, this being made a comparatively weak signal.

In order to obtain an I.-F. carrier wave, a local heterodyne oscillator 2 supplies a sine wave signal to the detector tube 17 through a transformer 22. In the example assumed, the I.-F. is 30 megacycles per second and the oscillator 21 operates at 470 megacycles per second.

The strong heterodyning signal from the oscillator 21 beats with each of the weak signals applied to the mixer tube 17 whereby two I.-F. signals appear in the mixer output, both I.-F. signals in this example comprising a 30 megacycle carrier frequency signal that, like the transmitted signal, is frequency modulated but with less frequency deviation than the transmitted signal because of the automatic-frequency-control action described below. However, there is an instantaneous frequency difference between the two I.-F. signals which corresponds to the propagation time of a pulse travelling from the transmitter to the reflecting object and return.

The I.-F. signal output of the mixer tube 17 is amplified by an I.-F. amplifier 26 comprising an amplifier tube 27 and tuned I.-F. transformers 28 and 29. The two amplified I.-F. signals are applied from the amplifier 26 to a second detector 31 where they beat with each other to produce a beat-frequency signal that is representative of the distance to the reflecting object. The beatfrequency signal may then be supplied to a frequency counter 32 or to any desired control or utilization circuit.

In order to make it possible to employ an I.-F. amplifier having a reasonably narrow pass band and good selectivity, I utilize an automatic-frequency-control or A. F. C. circuit for the heterodyne oscillator 21. This comprises a frequency discriminator 33 to which some signal from the I.-F. amplifier 26 is applied. While this signal actually comprises two I.-F. signals, they have such a small instantaneous frequency difference that they are identical so far as the discriminator 33 is concerned. The discriminator 33 may be of any suitable type such as the Seeley discriminator which is illustrated in the embodiment of the invention shown in Fig. 3 and described hereinafter.

The discriminator output is applied to an automatic-frequency-control device 34, such as a reactance tube, which is connected to the oscillator 21 for controlling its frequency.

In operation, any frequency drift of either the oscillator 21 or of the transmitter oscillator 10 will cause a change in the I.-F. frequency and a corresponding change in the amplitude of the discriminator output whereby the frequency of the oscillator 21 will be changed in the direction required to bring the I.-F. signal back to its original frequency. As to the frequency deviation introduced by the frequency modulator 12, the amount of this deviation at the mixer output circuit will be reduced to a fraction of the original deviation, reduced to 10 percent of the original deviation, for example. The time constant of the input circuit feeding the device 34 is such as to allow the A. F. C. to follow slow changes in the discriminator output voltage of up to the 120 cycle frequency modulating rate. Higher rates of change corresponding to the useful beat notes are not efficiently passed on to operate the A. F. C. device 34.

Fig. 2 illustrates the feature of the invention for avoiding the effects of any unavoidable amplitude modulation that may be present on the signal obtained directly from the transmitter.

The circuit includes a converter or mixer 36 to which is applied a strong frequency-modulated signal directly from the transmitter oscillator 10 and to which is applied a weak signal from the heterodyne oscillator 21. The resulting I.-F. signal is substantially free from any amplitude modulation even though there may be undesired amplitude modulation on the strong signal. This I.-F. signal is amplified in an I.-P. amplifier 37 and supplied to the second detector 31 where it beats with the I.-F. signal supplied from the I.-F. amplifier 26. The resulting beat-frequency signal, which is free from amplitude modulation effects, is applied to a suitable indicator such as the frequency counter 32.

Fig. 3 shows one preferred embodiment of the invention that combines the features of Figs. 1 and 2. The converter 36 and the I.-F. amplifier 37 may be the same as the mixer 16 and the I.-F. amplifier 26, respectively, except that the amplifier 37 is provided with two output tubes 38 and 39 which supply I.-F. signal to the discriminator 33 and to the second detector 31, respectively. In this embodiment of the invention, the A. F. C. circuit 33, 34 is controlled only by the I.-F. signal supplied from the amplifier 37, but its action in shifting the frequency of the heterodyne oscillator 21 in the proper direction in response to frequency drift and/or frequency modulation deviation is the same as described in connection with Fig. 1. Any shift in the frequency of oscillator 21 affects in an identical manner the two I.-F. signals being supplied to amplifiers 26 and 27 whereby the audio beat is not affected by said frequency shift.

For purpose of illustration, the well known Seeley frequency discriminator has been shown at 33. This type of discriminator is described in Seeley Patent 2,121,103, issued June 21, 1938. The discriminator 33 comprises a tuned circuit 41 and a center tapped tuned circuit 42, each of which is tuned to the I.-F. carrier frequency, i. e., to the 30 megacycle center frequency. The tuned circuits 41 and 42 are magnetically coupled. If the signal applied to the discriminator is at the resonant frequency of 30 megacycles, the directcurrent output of the diodes 43 and 44 will be zero. If the applied signal frequency is on either side of the resonant frequency, the D.-C. output of the diodes will have either a positive or negative polarity depending upon whether the frequency is on one side or the other of resonance and will have a magnitude depending upon the amount of the departure from resonance. Thus the reactance tube or other A. F. C. circuit 34 is made to shift the local oscillator frequency in the direction to bring the center frequency of the I.-F. signal back to the resonant frequency of the tuned circuits 41 and 42.

From the foregoing, it will be apparent that the circuit of Fig. 3 has the desirable features of both the circuit of Fig. 1 and the circuit of Fig. 2; namely, the I.-F. amplifiers have a comparatively narrow pass band, and any difficulties due to amplitude modulation are avoided.

Fig. 4 shows another embodiment of the invention that employs an I.-F. amplifier having a comparatively narrow pass band. In this system the local oscillator 21 is frequency modulated synchronously with the transmitter oscillator 10 and the resulting modulated signal is applied to the mixer 16 where it beats with the reflected signal. The signal from oscillator 21 also beats with signal supplied directly from the transmitter over a pair of conductors 35, the latter signal being a weak 500 megacycle signal having the same frequency deviation as the transmitted signal. Assuming frequencies of 470 megacycles and 500 megacycles for the oscillators 21 and 10, respectively, there are obtained two 30 megacycle I.-F. signals in the output of mixer 16 which are amplified by the I.-F. amplifier 26 and supplied to the second detector 31. The two I.-F. signals beat in the detector 31 to produce the audio frequency beats.

The amplified I.-F. signal is also applied to an automatic-frequency-control circuit comprising the frequency discriminator 33 and the reactance tube or other A. F. C. device 34 whereby the local oscillator 21 has its frequency controlled in the manner previously described. Thus the frequency shift of the I.-F. signal due to oscillator frequency drift and due to the frequency modulation is greatly reduced.

In the embodiment of Fig. 4, the main reduction in the band width occupied by the frequency deviation of the I.-P. signals may be accomplished by the frequency modulation of the local oscillator 21 by the unit 12. In that case, the A. F. C. circuit 34 need not be designed to control the local oscillator 21 as much as would otherwise be required. If desired, the discriminator 33 and the A. F. C. unit 34 may be omitted, in which case the system depends entirely upon the frequency modulator 12 for reducing the band width occupied by the I.-F. signals.

It will be understood that in all of the foregoing systems employing A. P. C., the local or heterodyne oscillator actually has its frequency changed or "wobbled" synchronously with the transmitter frequency modulation, this being the action that makes the frequency modulation deviation less at the input of the I.-F. amplifier 26 than at the input of the first detector or mixer 16.

I claim as my invention: 1. In a radio locator system of the type utilizing reflected waves, transmitting means for transmitting a radio signal to a wave reflecting surface, means for receiving said signal after it has been reflected from said wave reflecting surface, means including a local heterodyne oscillator for heterodyning said reflected signal to an intermediate-frequency signal and for heterodyning a.signal transmitted directly from the transmitter to an intermediate-frequency signal, amplifying means for amplifying said intermediate-frequency signals, a second detector to which the amplified intermediate-frequency signals are applied for producing a beat-frequency signal, a frequency discriminator to which at least one of said intermediate-frequency signals is applied, and an automatic-frequency-control means for said heterodyne oscillator to which means the output of said discriminator is applied for reducing the width of the frequency band occupied by said intermediate-frequency signals.

2. In a radio locator system of the type utilizing reflected waves, transmitting means for transmitting a cyclically frequency-modulated radio signal to a wave reflecting surface, means for receiving said signal after it has been reflected from said wave reflecting surface, means including a local heterodyne oscillator for heterodyning said reflected signal to an intermediatefrequency signal and for heterodyning a signal transmitted directly from the transmitter to an intermediate-frequency signal, amplifying means for amplifying said intermediate-frequency signals, a second detector to which the amplified intermediate-frequency signals are applied for producing a beat-frequency signal, and means for frequency modulating said heterodyne oscillator synchronously with the frequency modulation of said transmitter for reducing the width of the frequency band occupied by said intermediate-frequency signals.

3. In a radio locator system of the type utilizing reflected waves, transmitting means for transmitting a radio signal to a wave reflecting surface, means for receiving said signal after it has been reflected from said wave reflecting surface, a mixer and a converter, means including a local heterodyne oscillator and said mixer for heterodyning said reflected signal to an Intermediate-frequency signal, means including said heterodyne oscillator and said converter for heterodyning a signal transmitted directly from the transmitter to an intermediate-frequency signal, amplifying means for amplifying said intermediate-frequency signals, a second detector to which the intermediate-frequency signals are applied for producing a beat-frequency signal, a frequency discriminator to which at least one of said intermediate-frequency signals is applied, and an automatic-frequency-control means for said heterodyne oscillator to which means the output of said discriminator is applied for reducing the width of the frequency band occupied by said intermediate-frequency signals.

4. In a radio locator system of the type utilizing reflected waves, transmitting means for transmitting a radio wave to a wave reflecting surface, means for receiving said wave after it has been reflected from said wave reflecting surface, a converter and a mixer, means for supplying said reflected wave to said mixer, means for also supplying said radio wave directly from the transmitter to said converter with comparatively ··~·3bl~solaslWI~·W~m~ _I --~...... I-~ I~,......_ -~, ~ strong amplitude, a heterodyne oscillator for supplying a heterodyning signal to said mixer with comparatively strong amplitude to produce an intermediate-frequency signal and for supplying the same heterodyning signal to said converter with comparatively weak amplitude to produce a second intermediate-frequency signal, amplifying means for amplifying said intermediate-frequency signals, a second detector to which the amplified intermediate-frequency signals are applied for producing a low frequency beat-frequency signal, a frequency discriminator to which at least one of said intermediate-frequency signals is applied, and an automaticfrequency-control means for said heterodyne oscillator to which means the output of said discriminator is applied for reducing the width of the frequency band occupied by said intermediate-frequency signals.

5. In a radio locator system of the type utilizing reflected waves, transmitting means for transmitting a radio wave to a wave reflecting surface, means for receiving said wave after it has been reflected from said wave reflecting surface, a converter and a mixer, means for supplying said reflected wave to said mixer, means for also supplying said radio wave directly from the transmitter to said converter with comparatively strong amplitude, a heterodyne oscillator for supplying a heterodyning signal to said mixer with comparatively strong amplitude to produce an intermediate-frequency signal and for supplying the same heterodyning signal to said converter with comparatively weak amplitude to produce a second intermediate-frequency signal, two intermediate-frequency amplifiers connected to amplify said intermediate-frequency signals, a second detector to which the amplified intermediate-frequency signals are applied for producing a low frequency beat-frequency signal, a frequency discriminator to which at least one of said intermediate-frequency signals is applied, and an automatic-frequency-control means for said heterodyne oscillator to which means the output of said discriminator is applied for reducing the width of the frequency band occupied by said intermediate-frequency signals.

6. In a radio locator system of the type utilizing reflected waves, transmitting means for transmitting a radio wave to a wave reflecting surface, means for receiving said wave after it has been reflected from said wave reflecting surface, said reflected wave being weak in amplitude, a mixer, means for supplying said reflected wave to said mixer, means for also supplying said radio wave directly from said transmitting means to said mixer with comparatively weak amplitude, a heterodyne oscillator for supplying a heterodyning signal of comparatively strong amplitude to said mixer whereby said weak signals beat therewith to produce two intermediate-frequency signals, an intermediate-frequency amplifier connected to amplify said intermediate-frequency signals, a second detector to which the amplified intermediate-frequency signals are applied for producing a low frequency beat-frequency signal, a frequency discriminator to which the intermediate-frequency signals are applied, and an automaticfrequency-control means for said heterodyne oscillator to which means the output of said discriminator is applied for reducing the width of the frequency band occupied by said intermediate-frequency signals.

7. In a radio locator system of the type utilizing reflected waves, transmitting means for transmitting a radio wave to a wave reflecting surface, means for receiving said wave after it has been reflected from said wave reflecting surface, a converter and a mixer, means for supplying said reflected wave to said mixer, means for also supplying said radio wave directly from the transmitter to said converter with' comparatively strong amplitude, a heterodyne oscillator for supplying a heterodyning signal to said mixer with comparatively strong amplitude to produce an intermediate-frequency signal and for supplying the same heterodyning signal to said converter with comparatively weak amplitude to produce a second intermediate-frequency signal, amplifying means for amplifying said intermediate-frequency signals, and a second detector to which 4 the amplified intermediate-frequency signals are applied for producing a low frequency beat-frequency signal.

WENDELL L. CARLSON.