US2334473A

US2334473A - Frequency modulation tuning indicator

Info

Publication number: US2334473A
Application number: US405742A
Authority: US
Inventors: Wendell L Carlson
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1941-08-07
Filing date: 1941-08-07
Publication date: 1943-11-16
Anticipated expiration: 1960-11-16

Description

Nov. 16, 1943. w. l.. CARLSON FREQUENCY MODULATION TUNING INDICATOR Filed Aug. '7, 1941 Patented Nov. 16, 1943 FREQUENCY MODULATION TUNING INDICATOR Wendell L. Carlson, Haddonfield, N. J., assignor to i Radio Corporation of America, a corporation of Delaware Application August 7, 1941, Serial No. 405,742 A (ci. '25o-4o) J 4 Claims.

'This invention relates to a frequency modulationtuning indicator for signal receiving systems, of the frequency'modulation type.

It is an object of this invention to provide an improved tuning indicator for frequency modulation signal receiving systems whichv is responsiveto resonance and olf-resonance-tuning and which at the same time indicates the presence or"ab sence of a carrier wave or mean frequency signal, whereby the tuning indicatormay Vdistinguish between signal in tune resonance, and zero `signal and off-resonance tuning.

As is well known, in frequency modulation receiving systems, the discriminatornetwork, and detector provide audio frequency signal output from the frequency modulated signal and at the same time a direct current biasing potential or D. C. component which varies in amplitude and polarity with variations in tuning above` and below a predetermined resonance frequency.

This D. C. potential is utilized ordinarily to operate a polarized indicating device such as a zero-center meter which, however, is subject vto the disadvantage that the meter reads zero or center for a condition of exact tuning to resonance and also in the absence of a received signal.l K It is therefore a further object of this invention to provide an improved tuning indicator for frequency modulation signals for affecting two different kinds of visual'indication in one unit comn prising a response to variations in the AStrength of a mean frequency or carrier signal and a response to the presence of a received signal or the frequency of the received signal. Y

`The invention will, however, be further understood from the following description, when con- .sidered in connection with the accompanying drawing, and its scope is pointed out in the .appended claims.

In the drawing, Figure 1 is a schematic circuit diagram of a portion of a frequency modulation signal receiving system including a frequency ldiscriminator network provided with` atuning :indicator embodying the invention, and

Figure 2 is a similar schematic circuitdiagram :showing a modification of the circuit of Fig. 1 .and also a modified form of tuning indicator embodying the invention.

- Referring to Fig. 1, a source of frequency modu-A lation signals such as an intermediate frequency signal circuit 5, is coupled to a second detector 'comprising rectiers 6 and 'I through a frequency discriminator network 8 comprising a secondary circuit 9 connected in push-pull to the detectors 6 and 'I and a primary circuit I0 coupled to said rectiiiers in parallel.

The discriminator and detector output circuit for audio frequency signals and the D. C. signal component comprises a resistor Il connected between the rectifier cathodes, that is, between the cathode of the tube 6 and ground I 2 and an audio frequencyv output circuit I3. This circuit arrangement represents any suitable second detector for a frequency modulation signal receiving system.

Audio frequency signals from the circuit I3 are applied to an audio frequency amplifier Ill and to the usual loudspeaker I5 which terminates the signal channel of the receiving system.

The D. C. component of the detected signal which varies in amplitude and polarity with tuning is utilized to actuate a portion of the tuning indicating means, which in the present'example comprises apolarized milliammeter element I6 having a light pointerV I'I which moves over a zerocenter graduated scale I8 formed on kan anode or positive plate electrode I9 of an indicator tube 20. This is included in a D. C. path in shunt with the output impedance II from the ground terminal of the discriminator output impedance Il through a lead 2l and with the high potential side of the output impedance II at the circuit I3 through a lead 22 and a decoupling resistor 23.

In the absence of signals, the meter I6 reads zero, or center scale. Likewise at resonance withA a received signal it reads zero or center scaie, for the reason that the D. C. potentials at the terminals of the output impedance I l are equal and opposite. For conditions of olf-,resonance tuning the meter will read to the right or left of zero depending upon whether the tuning is above or below resonance and will therefore indicatethe direction for tuning to correct the off-resonance condition.

The` presence or absence of a received mean frequency or carrier signal is indicated by the same visual indicating element comprising the pointer Il and the plate I9. VFor this purpose the latter is coated with a material such as willemite which becomes fluorescent when bombarded by electrons. The electron source in the present example is the cathode 25 of the tube 28. i

The plate I9 is 4connected through a lead 26 with a positive terminal 21 on a potential supply l source such as a potentiometer 28 to which the cathode is connected at a more negative point 29. The electron iiow from the cathode to the plate or anode I9 is controlled by a control grid 30 interposed between the cathode and the plate as indicated. The grid is operated at a negative potential supplied through a grid resistor 3| from a negative terminal 32 on the supply source 28. The grid is actuated or placed under the control of the incoming signal through a connection 33 with the input circuit I0. A coupling capacitor 34 in theconnection 33 servesto isolate thegrid 30 from D. C. potentials appearing in'the circuit which may be connected to an intermediate fre-- quency amplifier or the like (not shown).

The bias potential between the cathode connection 29 and the grid connection 32 on the supply source 28 is such that the electron flow or anode current of the tube 20 is substantially-Zemin the absence of a received mean frequency signal.

that it may be applied to frequency modulationl circuits vwhich provide a voltage which varies in magnitude and polarity in yresponse ftovariation from resonance of a received frequency modula- Ation signal, that is, proportional to the frequency of an applied signal, and a signal voltage as it 7 varies in intensity or is absent, that is, the strength Therefore the scale I8 is not fluorescent in the l absence of signals. When the signalis'tuned in,

the pointer I1 gradually approaches zero and the scale becomes increasingly fluorescent or illuminated as the strength of the received signal Aincreases in the circuit l0 and on the control grid 30. With one visual indicating device, having two different indicating elements which maybe viewed/at the same time when operating the receiving system, the presence or kabsence of a received signal is indicated, and the `resonance indication in the presence of a received signal differs from the indication provided in the absence of a received signal. c

Referring now to Fig. 2 wherein the samereference numerals refer to like parts as in Fig. l, the D. C. control voltage from the discriminator output circuit Il is applied Yto the control grid 40 of a D. C. amplifier 4l which is coupled across an output resistor 42 to an electron beam deiiector plate 43 in a tuning indicator tube 44. A second deiiector plate is provided on the opposite side of the tube which is indicated at 45'. rIhe beam 46 is between a cathode 41 and an anode 48 which bears a scale 49 adapted to become fiuorescent in the path of the beam which moves across the scale 49 under control of the

electrodes

43 and 45. A positive potential is placed on the electrode 45 through a connection 50 c with a source of anode potential 5I having a grounded negative terminal 52. The anode 4B is also energized from the same source through va connection lead indicated at 53, the cathode 1l-being connected to ground 54.

The strength of the electron beam is controlled by a control grid 55 interposed between the cathode 4lY and the anode 48 and connected to cathode and ground through a suitable source of `biasing potential 56 and a grid resistor 51. Signals are supplied from the input circuit 5 tothe grid 55 through the lsignal supply circuit 33 and the coupling capacitor 34.

`With this arrangement,-1n the absence of re-` ceivedf signals, .the .grid `55 is biased negativelylby the source 56, preventing the flow of any substantial amount of plate current and resultingA in an electron beam which .is linsuflicient to cause iiuorescense of the scalev49. 'As-asignal is tuned in,vthe beam moves over :the scale, increasing in strength until it becomes a iiuorescent moving spot on the scale which comes -toa central position` upon exacttuningto resonance. Thus,y two control functions for the indication of .resonance are provided in a single,rvisual indicator, whereby for frequency .modulation `reception .fan `-oltxerator may distinguish between a received signal l.tuned to resonance and an off-resonance .indication of a received signal, as well .as a vcondition .provided Y ofthe applied signal, to provide the second controlling function in the indicator.

vI claim as" my invention:

1. A frequency modulation tuning indicator comprising a source of illumination, means for varying the intensity of illumination derived from said source in proportion to the strength of an applied signal, a movable indicator within A the iield of illumination of said source, and means lfor varying the position of said indicator in saidlield of illumination proportional to the frequencyof the applied signal. .l

2. A frequency vmodulation tuning indicator comprisinga signal circuit, a source of illumina-A ticn,. a scale within the field of illumination of said source, means forvarying the intensity of illumination on said scale in proportion to Ythc strength of an applied signal through said circuit, a movable indicator associated with said scale within the field of illuminationof said source, and means connected' with said circuit for varying the position of said indicator onsaid scale vpropor-v tional tothe frequency'variation of the applied signal with respectto a predetermined resonance frequency. c

3. A' 'frequency modulation tuning indicator comprising in combinatioman electric discharge tube having a control grid, a cathode and an .an-l ode, means providing an indicator scale on .said anode which becomes visibly fluorescent in response to Va. predetermined electron ow through said tube underI control of said grid, means for applying a received signalto said grid, a frequency discriminator network responsive to received' signals .and having an output circuit providing a control potential which varies in magnitude and polarity with variation in frequency of an applied signal from resonance, and means responsive to said voltage for indicating thefrequency variation of said applied signal on said indicator scale.

4. In a frequencymodulation signal receiving system, a frequency modulation tuning indicator comprising -a frequency Ymodulation signal circuit,anV electron discharge tube having a control grid coupled to said circuit, Ia cathode and anv an-v ode, means providing an indicator Ascale on said anode which -fluoresces in response to the flow of electrons theretofunder'con-t-rol -of said gridv as an indication ofthe signal strength, a frequency discriminator ynetworkcoupled to said circuit and having an output circuit providing a control potential which varies in -magnitude and polarity proportionalto the frequency variation of an' applied .signal from the resonance frequency ofthe network, and means responsive to-saidpotential for imparting tothe vscale 4a vsecond` indication proportional to the frequency variation of anapplied signal from said resonance frequency.

WENDELL .n CARLSON. f f