US2420199A

US2420199A - Frequency modulated altimeter or distance indicator

Info

Publication number: US2420199A
Application number: US445720A
Authority: US
Inventors: Jr Royden C Sanders
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1942-06-04
Filing date: 1942-06-04
Publication date: 1947-05-06
Anticipated expiration: 1964-05-06
Also published as: GB594766A

Description

May 6, 1947 R. c. SANDERS, JR 2,420,199

FREQUENCY MODULATED ALTIMETER OR DISTANCE INDICATOR Filed June 4, 1942 Fr-@amar Qs@ 155W @dcnfandcra Jr.'

Gttorneg Patented May 6, 1947 ZAZQJQQ FREQUENCY MODULATED ALTEVIETER OR DISTANCE INDICATOR Royden C. Sanders, Jr., Haddonfield, N. J., as

signor to Radio Corporation of America, a corporation of Delaware Application June 4, 1942, Serial No. 445,720

9 Claims.

My invention relates to radio systems such as radio altimeters for indicating distance or height and Will be described particularly with respect to systems wherein the radiated wave is frequency modulated.

Systems of this character are described in Bentley Patent No. 2,011,392 and Espensch-ied Patent 2,645,071. In these systems, the radiated frequency modulated wave is reflected from the earths surface or other object and the refiected wave is received in a heterodyne receiver located in the vicinity of the transmitter. The heterodyning or mixing signal for the receiver is obtained directly from the transmitter whereby the receiver output includes a signal of the beat or difference frequency which frequency is determined by the time required for .the radiated signal to reach the reflecting object and return to the receiver.

It has been found that one of the things that limits the full use of the sensitivity of the receiver is the interfering signal resulting from the amplitude modulation normally prese-nt on the mixing or heterodyning signal applied to the heterodyne detector. When the optimum mixing Voltage, i. e., optimum with respect to noise ratio, is introduced on the heterodyne detector, the amplitude modulated` signal in the output of the receiver is stillten to fifty times the noise level when the transmitter signal is swept through a` twenty megacycle bandwidth.

'it might be expected that there would be no amplitude modulation on the mixing signal. This would be true if it were not for the diiiiculty of transmitting a wide frequency band signal over a line with uniform attenuation at all frequencies. In actual practice there are reflections on the line to the transmitter antenna `and on the line between the transmitter and the receiverl because of practically unavoidable imperfect line terminations, for example.

While amplitude modulation on the mixing signal causes the greater part of the above mentioned interference, there also may be some interference, usually almost negligible, caused by amplitude modulation on the received reflected signal. The latter modulation is introduced because of such factors as imperfect termination of the line from the transmitter to the transmitter antenna and possibly because of frequency selective renection. of the signal.

An object of my invention is to provide improved means for reducing the interfering amplitude modulation signal appearing in the output of a receiver in a system of the above described type, A further object of the invention is to provide an improved receiver for a system of the above described type.

In a preferred embodiment of the invention, a balanced diode detector is employed in which each detector tube has the reflected signal and (Cl. Z50-1.68)

2. the heterodyning signalapplied thereto with the two signals having a phase relation with respect to each other at one tube that is the opposite of their phase relation with respect to each other at the other tube. The reflected signal may be applied to the tubes in parallel relation while the heterodyning signal is applied to them in push-pull relation, for example. By taking the output signal off the anode of one tube and olf the cathode of the other tube and properly combining the two output signals, any amplitude modulation of the applied signals, and particularly of the heterodyning signal, may be balanced out while the heterodyne or difference frequency signal appears in the detector output circuit. Likewise, any interfering or jamming sig-I nal may be balanced out.

The invention will be better understood from the following description taken in connection with the accompanying drawing in which:

Figure l isl a circuit and block diagram of one embodiment of the invention;

Figure 2 is a :circuit diagram of a receiver -designed in accordance with a preferred embodiment of the invention; and

Figure 3 is a view showing the physical ary rangement of a portion of the receiver illustrated in Fig. 2.

In the several figures, like parts are indicated by the same reference characters.

Referring to Fig. 1, the invention is shown applied to a radio altimeter comprising a frequency modulated radio transmitter Ill which radiates a frequency modulated carrier wave from an antenna H. Suitable means for sweeping the carrier wave frequency through a certain frequency range periodically is indicated at I2.

The associated receiver comprises a pair of diodes I3 and I4 which have signal applied thereto from the secondary I6 of a transformer I'I, one end of which is connected directly to the anode of diode I4- and the other end of which is connected through a capacitor I8 to the anode of diode i3. The midpoint of secondary I5 is connected to ground through the secondary I9 of a transformer 2I. The cathode of diode I3 is connected directly to ground While the cathode of diode IB is connected to ground through a blocking capacitor 22.

The reected signal is picked up by a receiver antenna 23 and is supplied through a coaxial line to the primary 24 of transformer 2|. Thus the reflected signal is applied with the same polarity to the anodes of diodes I3 and I4. Stated differently, it is applied in parallel relation to the diodes.

The same frequency modulated signal that is supplied to the transmitter antenna I I is also supplied through a coaxial line 26 to the primary 21 of transformer II as a heterodyning signal whereby a difference or beat frequency signal is produced which is a measure of the time required for the signal radiated from antenna II to reach antenna 23 by reflection and, therefore, a measure of the distance to the reflecting surface. It will be seen that the heterodyning signal is applied with opposite polarity to the diodes I3 and I4, i. e., it is applied in push-pull relation instead of in parallel relation.

The detector push-pull input circuit is tuned to the mid-frequency of the heterodyning signal by a variable capacitor 28. This circuit may be traced from the center point of winding I6 through the upper half of this winding, through capacitor 28 and through the lower half of winding I6 back to the center point. The two parallel input circuits are tuned to the said mid-frequency by selecting proper inductance values, particularly for coil |9. The latter circuits, which are tuned in part by the diode capacities indicated at 29 and 38, may be traced as follows: One circuit from ground through coil I9, the upper part of coil I8 and through the diode I3 to ground, and the other circuit from ground through coil I9, the lower part of coil I6 and through the diode I4 and the blocking capacitor 22 to ground.

The beat or difference frequency output signal is taken off the anode of diode I3 and off the cathode of diode I4 through radio frequency .choke coils 3| and 32, respectively, and applied to opposite ends of a balancing resistor 33. A variable tap 36 is connected to one end of a comparatively high resistance resistor 34 which may be the grid leak of the following audio amplifier tubes. The other end of resistor 34 is grounded.

Any undesired amplitude modulation on the reflected signal or on the heterodyning signal may be balanced out by setting the tap 36 at the proper point since the envelope of any amplitude modulation has the same polarity at the anode of diode I3 as at the anode of diode I4 and since the audio output of diode I4 is reversed with respect to that of diode I3.

The desired beat frequency signal, on the other hand, will not balance out because the beat frequency signal at the anode of diode I3 is 180 degrees out of phase with the beat frequency signal of diode I4. Therefore, when these two signals are reversed in relative polarity by taking one signal off the anode and the other signal o the cathode, the two signals add in the resistor 34,

It will be seen from the foregoing that the invention is based upon the fact that if a heterodyning signal of one polarity is mixed with a received signal to produce a, beat frequency signal and if this same heterodyning signal, but of opposite polarity, is mixed with the received signal to produce another beat frequency signal, then the two beat frequency signals are of opposite polarity, i. e., 180 degrees out of phase. It is also based upon the fact that the detected amplitude modulation signal, i. e., the amplitude modulation envelope, is of the same polarity at the anodes of the two dio-des.

In Fig. 2 there is shown schematically a preferred embodiment of the invention which operates in a manner similar to that of Fig. 1. It comprises a balanced diode detector which includes the

diodes

16 and 11 having their anodes 18 and 19 connected to a tuned line 8|. This line includes two

coaxial conductors

82 and 83, the inner conductors of which are connected to the anodes of the

diodes

16 and 11. The outer conductors of the coaxial conductors are connected at each end to the inner conductor by means of

capacitors

84, 86, 81 and 88 whereby the inner and outer conductors are at the same potential at radio frequencies.

The cathode of diode 16 is conductively connected to ground preferably through a low impedance resistor 88 and a capacitor 9| in parallel. The cathode of diode 11 is connected to ground at radio frequencies through a capacitor 92. The cathode 88 may be connected directly to ground, if desired, but the connection through capacitor 9| is preferred since it makes the circuit more symmetrical and more easily balanced. This is true because capacitor 9| at diode 18 corresponds to capacitor 82 at diode 11, the latter capacitor being required where the beat frequency signal is taken off the cathode, The resistor 89 is required to complete the direct current path for diode 16. It may have a resistance of 108 ohms, for example. Each side of the heater filament for cathode 88 is held at ground potential at radio frequencies by means of

bypass capacitors

98 and 95. Also, one side of the heater filament for cathode 85 is held at ground potential at radio frequencies by means of a bypass capacitor 98, the other side of the filament being connected directly to ground.

Referring again to the tuned line 8|, the ends of the outer conductors of

lines

82 and 83 are connected to ground through

connections

93 and 94. An adjustable shorting bar 96a/-96b is provided for tuning the two circuits formed by each coaxial line and the center conductor 94 to the mid-frequency of the received signal. It will be seen that there is one resonant circuit that may be traced from the anode 18 through the capacitor 84, the conductor 82, the shorting bar section 96a and the conductor 94 to ground; and a second resonant circuit that may be traced from the anode 19 through the capacitor 81, the conductor 83, the shorting bar section 96h and the conductor 94 to ground.

A third resonant path is provided by a capacitor 91 that is in adjustable contact with the outer conductors of

coaxial lines

82 and 83. This path may be traced from the anode 18 through the capacitor 84, the conductor 82, the capacitor 91, the conductor 83 through capacitor 81 to the anode 19 and hence through the

cathodes

88, 85. This third path is made resonant at the midfrequency of the heterodyning or mixing signal, this mid-frequency being the same, of course, as the mid-frequency of the received reflected signal.

Referring now to the detector output circuit, it will be seen that the beat frequency signal is taken off the anode 18 through the inner conductor of the coaxial line 82 and supplied through a conductor I8I, a resistor |82 and a portion of resistor |83 to a variable tap |84 and the output resistor 34. The beat frequency signal also is taken oif the cathode 85 through a low impedance filter resistor |88 (having a resistance of 100 ohms, for example) and supplied through a resistor |81 and the other portion of resistor |03 to the variable tap |84 and the output resistor 34. A lter capacitor |88 is connected between ground and the end of filter resistor |86 remote from the cathode. The resistor |85 and the capacitor |88 are provided to keep the radio frequency out of the output circuit. The beat or audio frequency circuit for diode 11 is completed from the anode 19, through the inner conductor of coaxial line 83 and through a connection |89 to the grounded conductor 94.

As Ain the circuit of Fig. 1, the mixing signal from the transmitter is applied to the tuned circuit 8| in push-pull relation while the 'reflected signal is applied thereto in parallel relation. This is accomplished as follows: The mixing `signal is supplied to a loop II I which, `as shown in Fig. 3, is positioned vcentrally with respect to the tuned circuit BI and with its plane parallel `to the plane of the coaxial conductors 872 `and 83. The lines -of force from` loop II I will cut the

lconductors

82 and 83 as indicated in Fig. 3 whereby the flow of transmitter-or lmixing `signal in tuned circuit 3i will be as indicated by the arrows fr in Fig. 2. Thus, the vmixing sig- 'nal at the anode 18 is 180 degrees out of phase with the mixing signal at the anode '59.

The Yreflected signal is supplied through a co axial 4conductor H2 to a loop H3 which may be terminated by a tuning capacitor I I4. The loop I I3 may be untuned like loop HI, if preferred. This loop is positioned centrally with 'respect Vto tuned circuit BI and is located with its plane at right angles to the plane of

conductors

82 and 83. The lines Yof force from loop `I I3 cut the conductors At2 and 83 as shown in Fig. 8 whereby the flow of reected signal will beas shown by the arrows :in in Fig. 2. It will be seen Athat the reflected signal on the anode i8 is in 'phase with the reflected signal on the anode T9. The loops il! and AH3 are adjustable paralle] to the plane of the lines 82 and 33 to rpermit balancing adjustments to compensate for any 'difference in diode characteristics or in the physical construction of the tuned circuit -82-83 or lother parts of Ithe receiver.

From the foregoing it will be evident that 4the operation of the circuit -of Fig. 2 is similar to that of Fig, l previously described, that is, any amplitude modulation may be balanced -out by adjusting the tap i055 since the modulation at one anode `is in phase with that at the other anode. The beat frequency signal, on the `other hand, is 180 degrees out of phase at one anode with respect to the beat frequency signal at the other anode whereby these signals add due to phase reversal obtained by the output connections.

`In Fig. 3 electrostatic sheldslare shown at H5 and II'l `positioned `between the tuned line 32-53 andthe loops I I-I and H3, respectively. Actually,

these shields are not required if there is exact symmetry between thc two loops and the line 82-8- The shields H6 and il? are indicated schematically in Fig. 2. Also, in Fig. 2 a shieldingbox of aluminumor the like for-the receiver is indicated at H8.

nIt may be noted, merely by way of example, Vthat all the capacitors shown in Fig 2, except the tuning capacitor iid, have a capacity of `55 micro-microfarads. Also, resistors -IilZ and |07 may be 5000 ohms each, resistor |03 may be 25,000 ohms and resistor 34' may be 47,000 ohms. These Values are for a receiver operating at a 440 lmegacycle carrier frequency that is being swept through a range of 40 megacycles.

It should be understood that the mixing signal and the yreflected signal may be applied to the detector in parallel relation and push-pull relation, respectively, if desired instead of in the opposite way described above.

It maybe noted that the invention may be utilized to advantage to measure the Doppler effect Yfor the purpose of determining the ground speed of an-airplane for example. When the apparatus is so used, the frequency modulator I2 is `disconnected from the transmitter so that a constant frequency carrier wave is transmitted. In this case there will be no amplitude modulation 4on the mixing signal but there may be undesired amplitude modulation on the reflected signal.

I claim as my invention:

11. A radio receiver comprising a pair of rectiers connected in balanced relation, each rectier having an anode and a cathode, means 'for so applying two carrier waves to said recntii-"lers that they have one phase relation with respect to each other at the anode of one rectiner andthe opposite phase relation with respect to Leach vother at the anode of the other rectifier, 4a frequency responsive device, an output impedance unit `connected to supply signal to said frequency responsive device, means for taking the output signal of one rectifier off its anode and means for vtaking the output signal of the other rectifier off its cathode, and means for supplying the signals from said anode and from said cathode to said output impedance unit whereby any amplitude modulation on said carrier waves may be balanced out while supplying said beat frequency signal to said frequency responsive device.

2. A radio system comprising means for transmitting a frequency modulated carrier wave to a reflecting surface, a receiver system for receiving said wave after reflection from said surface, said receiver comprising a pair of diodes connected in balanced relation, each diode having an anode and a cathode, means for applying said modulated wave directly from said transmitter to said diodes as a mixing signal whereby a difference or beat frequency signal is produced, said lreflected wave and said mixing signal having one phase relation with respect to each other at one diode and the opposite yphase relation with respect to each other at the other diode, a frequency responsive device, an output'I impedance unit connected to supply signal to said frequency responsive device, means for taking Vthe output signal of one diode off its anode and means for taking the output signal of the other diode off its cathode, and means for supplying the signals from said anode and from said cathode to said output impedance unit whereby any amplitude modulation signal may be balanced out while supplying said beat frequency signalto said frequency responsive device.

3. A radio distance determining system comprising means for transmitting a frequency modulated carrier wave to a reflecting surface, a receiver for receiving said wave after reflection from said surface, said receiver comprising a pair of diodes connected in balanced relation, each diode having an anode and a cathode, Vmeans for applying said reflected wave to said diodes and means for applying said modulated wave directly from said transmitter to said diodes whereby a difference or beat frequency signal is produced, one or" said waves being applied to said diodes in push-pull relation and the 4other wave being applied to said diodes in parallel relation, and means for taking the resulting output signal of one diode off its anode and for taking the output signal of the other diode off its cathode, and means for adding said output signals with such relative amplitudes that any amplitude modulations of said output signals are substantially balanced out whilethe beat frequency signal is passed to a utilization circuit.

'4. A radio distance determining system comprising means for transmitting a frequency modulation carrier Wave to a reflecting surface, a receiver system for receiving said wave after reflection from said surface, saidreceiver comprising a pair of diode detector tubes connected in balanced relation, means for applying said modulated wave directly from said transmitter to said detector tubes as a mixing signal whereby a difference or beat frequency signal is produced, said reflected wave and said mixing signal having one phase relation with respect to each other at one detector tube and the opposite phase relation at the other detector` tube, a frequency responsive device, and means for balancing out any amplitude modulation signal While supplying said beat frequency signal to said frequency device, said means including an output impedance unit, a connection from said impedance unit to the anode of one of said diodes and a connection from said impedance unit to the cathode of the other of said diodes.

5. A radio receiver comprising a tuned line and a pair of heterodyne detector tubes connected in balanced relation thereto, means for applying a first carrier Wave to said detector tubes and means for applying a second carrier wave to said detector tubes whereby a diierence or beat frequency signal is produced, one of said waves being applied to said detector tubes in push-pull relation through a coupling loop positioned with its plane parallel to the plane of said tuned line, and the other Wave being applied to said detector tubes in parallel relation through a coupling loop positioned with its plane at right angles to the plane of said tuned line, and means for adding the resulting output signals of said detector tubes in such phase relation and with such relative amplitudes that any amplitude modulations of said output signals are substantially balanced out while supplying said beat frequency signal to a utilization circuit.

6. A radio system comprising means for transmitting a carrier wave to a reflecting surface, a receiver for receiving said Wave after reflection from said surface, said receiver comprising a tuned line and a pair of heterodyne detector tubes connected in balanced relation thereto, means for applying said reected wave to said detector tubes and means for applying said carrier wave directly from said transmitter to said detector tubes whereby a difference or beat frequency signal is produced, one of said waves being applied to said detector tubes in push-pull relation through a coupling loop positioned With its plane parallel to the plane of said tuned line, and the other wave being applied to said detector tubes in parallel relation through a coupling loop positioned with its plane at right angles to the plane of said tuned line, and means for adding the resulting output signals of said detector tubes in such phase relation and with such relative amplitudes that any amplitude modulations of said output signals are substantially balanced out while said beat frequency sig nal is passed to a utilization circuit.

7. A radio receiver comprising a tuned line and a pair of diodes connected in balanced relation thereto, each diode having a cathode and an anode, means for applying a rst carrier wave to said diodes and means for applying a second carrier wave to said diodes whereby a diiierence or beat frequency signal is produced, one of said waves being applied to said diodes in push-pull relation through a coupling loop positioned With its plane parallel to the plane of said tuned line,

and the other Wave being applied to said detec tor tubes in parallel relation through a coupling loop positioned with its plane at right angles to the plane of said tuned line, means for connecting the cathode of one diode through a conductive connection to the low potential end of said tuned line and for connecting the cathode of the other diode through a capacity connection to the low potential end of said tuned line, and means for adding the resulting output signals of said detector tubes in such phase relation and with such relative amplitudes that the amplitude modulations of said output signals are substantally balanced out, said last means comprising an output impedance unit, a conductive connection from the anode of said one diode to said impedance unit and a conductive connection from the cathode of said other diode to said impedance unit.

8. A radio receiver comprising a tuned line and a pair of diodes connected in balanced relation thereto, each diode having a cathode and an anode, means for applying a rst carrier Wave to said diodes and means for applying a second carrier Wave to said diodes whereby a dilerence or beat frequency signal is produced, one of said Waves being applied to said diodes in push-pull relation through a coupling loop positioned with its plane parallel to the plane of said tuned line, and the other wave being applied to said detector tubes in parallel relation through a coupling loop positioned with its plane at right angles to the plane of said tuned line, said tuned circuit having an adjustable shorting bar for tuning it to the frequency of the carrier wave that is applied through the right angle loop and having a capacitor connected thereacross and adjustable along the line for tuning it to the frequency of the carrier Wave that is applied through the parallel loop, means for connecting the cathode of one diode through a conductive connection to the low potential end of said tuned line and for connecting the cathode of the other diode through a capacity connection to the low potential end of said tuned line, and means for adding the resulting Output signals of said detector tubes in such phase relation and With such relative amplitudes that the amplitude modulations of said output signals are substantially balanced out, said last means comprising an output impedance unit, a conductive connection from the anode of said one diode to said impedance unit and a conductive connection from the cathode of said other diode to said impedance unit.

9. The invention according to claim 8 wherein each side of said tuned line comprises a c0- axial line having an inner conductor and an outer conductor, said inner conductors being connected to the anodes of the diodes, said outer conductors having said shorting bar and said tuning capacitor in adjustable contact therewith, and capacitors connecting each end of said outer conductors to the corresponding end of its inner conductor.

ROYDEN C. SANDERS, JR.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,268,587 Guanella Jan. 6, 1942 2,268,643 Crosby Jan. 6, 1942