US2369585A - Frequency modulation receiver - Google Patents

Description

Feb. 13, 1945. H. T. LYMAN 2,369,585

FREQUENCY MODULATION RECEIVER v I W Filed July 25, 1943 I. SOURCE OF FREQUENCY H U LAT ED 5 AL WAVE5 VOLTAGE FREQUENCY Fig.3.

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FREQUENCY Inventor":

HarolclTLyman;

Attorney.

Patented Feb. 13, 1945 omen-mm PATENT OFFlCE, i z FREQUENCY M33321 REoEIvER v Q 7 v Harold T. Lyman, Milford, Conn., assigno'r' to General Electric" Company,

- New York a corporation of I Application July 23, 1943, Serial No. 495,821 'f 4 Claims; (erase-20) My invention relates tofrequency modulation receivers, more p'articularlyto improvements in detectors or demodulators for frequency modulated carrier waves, and has for its principal object the provision of new and improved signal selector circuits for indicating mistuning of the receiver with respect to a predetermined midband frequency.

, ,In the tuning of amplitude modulation radio or television receivers it is a characteristic that, as the channel is tuned through,-for example, in a superheterodyne receiver as the intermediate frequency approaches and-then recedes from the resonant frequency of the intermediate frequency channel, the signal gradually increases in intensity tov a maximum and then gradually decreases in intensity and fades out. This single maximum in the signal response is a highly desirable tun- 1 g characteristic, since it-enables and'unskilled operator readily to select the proper position of the tuning control for the reception of any desired broadcasting station. I

It is equally well' recognized, however, that detector 'or demodulator circuits which are currently in common use for the detection of frequency modulated carrierwaves possess a tuning characteristic exhibiting three definitely distinguishable maxima of signal response; that is,

as the channel is tuned through, a fairly strong signal having a.poor signal-to-noise ratio is first observed, then a period-0f high distortion, then a range of good reception with high signal-tonoise ratio, then a second period of high distcrtion, and then a second period of fairly good reception with low sigrial-to-noise ratioas in the first condition, followed by a final fading out of the signal. While it is'well known to those skilled in the art-that the optimum tuning position is at the central, or mid-band, maximum of signal response, it i also well recognized that this triple response characteristic is confusing to unskilled; operators who'are; not familiar with the cause of the three-maxima and are therefore unable to select the proper tuning position.- This tuning characteristic, whereby three maxima in signal response are-presented; is afunction of the relative values of the resonant frequency of the signal, channel-and the-mean frequency of the frequency modulated carrier wave --beingtrans:

omine-p m n aveisu h as tuned rad 1 quency receivers, as in superheterodyne receivers in which the meanintermediate frequency is ad-L justable to the rfixed resonant frequency of the intermediate frequency channel.

Accordingly, it is a general object of my in vention toprovidenewand improved selector circuits for indicating the proper-tuning position of a frequency modulation carrier wave receiver.

' It is a vide a frequency discriminator or demodulator for frequency modulation receivers 'which shall be readily tunable to. the, desiredcoincidence of. carrier mean frequency and channel resonant frequency without evidencing undesired tuning indications upon either side of the desired position. i .-.1 It is still another objectof. my invention to pro-;" vide .new and improved means for positively, in-' dicating mistuning of a frequency modulation receiver by attenuation and distortion of unde'e sired side band frequencies. H i It is a specific object of myinvention to provide means for controlling a signal frequency amplifierin response to mistuning of a balanced discriminator circuit. to eliminate confusion on the part of an unskilled operatorin selecting the optimum tuning position of the receiver.

In accordancewith thepresent invention, a unidirectional bias potential of positiveor neg ative polarity, derived from the output of a bale anced discriminator circuit as a result of mis tuning in one or the other direction, is utilized directly to control thegrid of the ,signalmamplifier in such away that mistuning of thereceiver to either side of the mid-band frequency is re n dered highly undesirable. It is well known that a balanced discriminator as for example a dis; crirninator of the type disclosed and claimed if; U. S. Patent 2,121,103,1i'ssued to S. W. Seeley on June 2 1, 1938, includes in its output a unidirectional component of potential proportional in. polarity andintensity to the direction and magnitude, respectively, of any deviation between the carrier mean frequency and the mid-band frequencyiiof the idiscriminator. Inl accordange with the present invention the positive or nega; tive unidirectional. component-{of discriminator output voltage so derived upon ,inistuning is utiliz'ed tobias -the signalarnplifier'either substan: tially to cutoff or into a region of very poor gain, depending upon whether the-bias voltage is negative or positive, respectively. In this way the signal response uponeithersid'e of the de;

sired'midband frequency. is so reducedflby .atf tenusnon and" distortion that the mistuning is further object of myinvention to, pro j forcibly brought to the attention of even the most unskilled operator.

My invention itself will be more fully understood and its objects and advantages further appreciated by referring now to the following detailed specification, taken in conjunction with the accompanying drawing in which Fig. 1 is a schematic circuit diagram of a frequency modulation discriminator embodying my invention, and Figs. 2, 3, and 4 aregraphical representations of certain frequency modulation receiver tuning characteristics illustrative of the invention.

Referring now to the drawing and particularly to Fig. 1, I have shown a frequency modu lation receiver including an antenna II) for supplying frequency modulated signal waves at radio frequency to a signal channel II. The signal frequency by a shunt capacitor 25. The high potential terminal of the primary winding I1 is coupled for intermediate frequency potentials to the midpoint of the secondary winding 24 through a coupling capacitor 26. The adjacent halves of the transformer secondary winding 24 on opposite sides of the midpoint are connected to supply signal modulated intermediate frequency waves for demodulation to a pair of rectifier circuits connected in balanced opposition.-

The rectifier circuits include a common intermediate frequency choke coil 21 connected bechannel ll serves to supply to an electric fdise.

charge device l2 a carrier wave modulated in" frequency about a mean frequency in accordance with desired signals. representing voice, music, or the like. The signal channel I I is provided with a suitable manually controllable tuning de.

vice I3. While it will be evident to those skilled in the art that the invention is equally applicable to a frequencymodulation receiver in which the signal channel I l comprises a plurality of stages of radio frequency amplification, it will also be understood that, in the typical case of a superhetero'dyne receiver, the channel II will ordinarily comprise a suitable number of stages of radio frequency amplification, an adjustable local oscillator, a mixer or converter for reducing the carrier frequency to a desired intermediate frequency by heterodyning the signal wave with the locally generated oscillations, and one or more stages. of intermediate frequency amplification fixedly tuned to the desired intermediate frequency. In such a superheterodyne receiver, the tuning means I 3 is ordinarily arranged to control the frequency determining circuits of the local 5 oscillator. Tuning is then accomplished by adjustment of the mean frequency of the intermediate frequency carrier wave to equal the fixed resonant frequency of the intermediate frequency channel. Assuming for the purpose of illustration that the receiver is of the superheterodyne type, the discharge device l2 may be a limiter of the well known saturation type. The limiter [2 includes a cathode l4 and a control electrode or grid l5 connected to the intermediate frequency channel H and includes also an anode l6 connected to the cathode I 4' through an output circuit including a tuned primary winding ll of a discriminator input transformer l8. The primary winding i1 is tuned to resonance at the desired intermediate frequency by means of a shunt capacitor Fla. The output circuit of the discharge device l2 may be traced from the anode it through the primary winding I1 and a" resistor l9 to the positive terminal of the battery 20, the negative terminal of which is connected through ground to the cathode l4. As shown, the limiting discharge device l2 also includes a screen grid 2i connected through a resistor 22 to the positive terminal of the battery '28 and tween the midpoint of the transformer secondary winding 24 and the midpoint of a load resistor '28, Opposite terminals of the load resistor 28 are connected through a pair of unidirectional conducting, devices 29 and 30 to opposite terminals of the transformer secondary winding 24. As shown, the unidirectional conducting devices 29 and 30 are two-element electron discharge devices having their cathodes connected to opposite terminals of the load'res'is'tor 28 and'their anodes connected to opposite terminals of the transformer winding 24. The lower terminal of the load resistor 28 is shown grounded at 35.1

The discriminator circuit described in the foregoing paragraph is of a well known type and its operation is clearly explained in the Seeley pat ent referred to hereinbefore. Briefly,- however, it may be noted that the tuned discriminator input transformer l8 supplies to the balanced rectifier circuits high frequency potentials which vary oppositely in intensity with frequency modulation of the received signal wave. The high frequency potential applied to each rectifier circuit is the vector sum ofthe induced'voltage in one-half of the transformer secondary winding and the transformer primary winding voltage impressed upon the transformer secondary winding through the coupling capacitor. The induced voltages in opposite halves of the secondary winding are equal and opposite at all times and, by reason of the tuning of the transformer windings, they'are displaced by from the primary winding voltage when the instantaneous frequency'of the car'- rier wave is equal to the resonant frequency of the transformer. The induced'voltag'es are shifted oppositely in phase with respect to the'primary winding voltage upon any deviation of the carrier frequency from the transformer resonant fre quency, so that the net voltages applied to the rectifier circuits vary oppositely inintensity with frequency deviations. In this way the net voltages across the two .halves of the transformer secondary winding 24 are modulated oppositely in amplitude in accordance with frequency modulation of the signal wave applied to the primary winding l1. Thus, the rectified currents, though flowing in opposite directions through the load resistor 28 on opposite sides 'of the midpoint, have the same average unidirectional component when the carrier mean frequency coincides with the resonant frequency of the tran's'form'e'r l8 and vary oppositely in' instantaneous intensity with instantaneous frequency deviations "ofth'e carrier wave. At Fig. 2 I have shown a curve A which repr'esents the sum of the opposing instantaneous voltage drops between the terminals of the load resistor 28 as the frequency of the signal wave deviates to one side or the other of the trans former resonant frequency; The total signalyolt age appearing across theterminals of the"load resistor 28 is seen to comprise an'alternating' component at signal frequency and a un'idire'c 'n'atorload "resistor 28 in response to miss-- sonar compone t. having a "polarity depending upon the-direction of departure of the" carrier mean fr quency from-the transformer resonant frequency and a 'inagnit'ude' proportional; to the magnitudeol the departure. 1 n view of the foregoing -'explanation,"' itywill be clear that when the carrier' 'frequency' is equal tothe resonantf'requency of v the transformer and no signal modulation" is present, no signal voltage appears stress the terminals of the load resistor 28. In the presence of signal jmodulw tion,'a;signal frequency voltage with a zero unidirectional component appears across the resistor 28. If, howeven thgmean frequency ofthe car rier wave difiers from the resonant'frequencyg of the transformer [6, the average-voltage drop across one-half of the resistor 28 exceeds that across the other'half so that-the upper terrriinal ofthe resistor assumes a net positive or negative unidirectionalpotential with respect to ground independently ofsi'gnal modulation, the-polarity of the unidirectional'potential'depending upon the direction of departure of the carrier'mean frequency from the transformer resonant -fre quenc'y. Any alternating component of signal voltage developed from frequency modulation is superposed upon the unidirectional potential."

' 'The signal voltage developed as above across the terminals of the load-re'sistorZB- is placed across the volume control resistor 36 through an intermediate frequency rejection-'and f'high frequency de-emphasis' filter comprising airesistor 3! and a pair of capacitors 38 and- 38a connected between opposite ends of the resistor ,3-Ifand ground. By means m suitable slider 40 on'the volume control resistor; 36, the'alternating component of'signal voltage drop appear ing'across any desired 'portion ofthe volume control resistor isimpressed through a blocking capacitor-4| and a demodulating resistor42 upon s grid 43 of a signal, frequency amplifier comprising -'a'njelectric discharge device 44 having an anode 45 and a cathode. 'The cathode'4B is connected to ground through a cathodebias resistor 41 which is by-passed for signal tree I quency currents b condenser 48. "The" con trol grid 43 of thedischarg'e device 44 is connected to ground through a grid bias resistor 49.-- The of illustration. I have showna loud speaker ing a voice coil a. Y I

A discriminator, amplifier and reproducer of the type thus far described has a tuning characteristic such as represented in the curve B of Fig. 3. This curve shows signal response at the voice coil in terms of frequency departure of the carrier mean frequency from the transformer resonant frequency. As illustrated at Fig. 2, the signal response is a maximum and fairly constant in the region of the mid-band frequency, decreases to a minimum .on each side where the signal is distorted bya zero rate of change of the curve A of Fig. 2, and then increases to an undesired maximum on either side before the signal fades out. I

' Forfthe purpose of controlling the gain and fidelity of the signal amplifier 44 in accordance gain or to cutofi, respectively; To provide such resistors to ground, Y

' In operation, it will be understood that if the mean frequency of the' carrier wave coincides with the resonant frequency of the discriminator transformer l8, no unidirectional component of potential appears across the volume control resistor 36 so that no bias is impressed on the grid 43 through the signal rejectionfilter' 5|, 52, 53. If now the receiver is mistuned in such a direction that the upper terminal of the discriminator load resistor 28 assumes a negative unidirectional component of potential with respect to ground, such negative potential will be impressed as a negative bias upon'the signal amplifier grid 43 through the signal rejection filter, 51,52, 53. Such negative grid potential willbias the signal amplifier -44 toward-cutofi thereby sharply to reduce the amplification and the signal response. In the typical case of a radio receiving apparatus, it will become immediately obvious to the listener that a maximum of audio response has not been reached and that the receiver is mistuned. Stated-in another way, the cutofi bias of the tube suppresses the signal and eliminates the undesired maximum in audio response appearing at one side of the mid-band tuning position. o

If, however. the receiver-is mistuned in such direction that the upper terminalof the volumecontrol resistor 36 assumes a positive unidirectional potential with respect to ground, such positive potential, when impressedupon the grid v43 of the signal amplifier 44 throughth'e signal rejection filter 5l', 52', and 53, drivesthe amplifier into a region of poor gain. In this region the amplification and fidelity of reproductionof the amplifier are both-reduced so that'the signal response is low. The amplification of the signal amplifier 44 is not reduced to the same extent by the positive bias as by the negative cutoff bias. However, in'thepositive bias region, the amplifier 44, while still conducting, delivers such a severely distorted signal that the positive bias is equally as efiective in reducing the signal response and apprising the operator of mistuning as is the negative cutoff bias. In the typical case of a radio receiver substantially no "signal is heard when the receiver is mistuned to' impress I upon the audio amplifier a negative cutoff bias, while when the receiver is mistuned to impress upon the audio amplifier a positive bias, the output is so severely distorted that it is immediately obvious to the operator that the correct tuning position has not been reached.

At Fig. 4 I have shown a signal response curve of the type shown at Fig. 3, but for a receiver embodying my invention. It will be observed that due to attenuation of the side band signal response the curve C of Fig. 4 evidences a sharper maximum of response at the mid-band frequency and shows very appreciable reduction of the undesired maxima of signal response upon either side of the proper tuning position.

In view of the foreging description, it will now be evident that the whole purpose of my invention may be effectively carried out with very lit tle apparatus additional to that conventionally required in a frequency modulation discriminating apparatus of the type described. It is only necessary to couple the high potential terminal of the discriminatorload resistor to the control grid of the first signal amplifier through a signal frequency rejection filter. This coupling results in suppression of the signal response upon mistuning in one direction and in distortion and partial suppression of the signal response upon mistuning in the other direction. It is not necessary that the signal be made to fade out symmetrically upon both sides of the optimum tuning position, asin an amplitude modulation receiver. According to the present invention, mistuning is indicated by fading upon one side and, at least partially, by distortion upon the other side. The distortion, while unpleasant to the,

manymodifications will doubtless occur to those skilled in the art and I therefore wish to have it understood that I intend in the appended claims to cover all such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In combination, a source of carrier waves signal modulated in frequency about a mean frequency, a load resistor, means including a tuned circuit for deriving from said carrier waves and supplying across said resistor a signal potential having an alternating component at signal frequency and a unidirectional component proportional in polarity and intensity to the direction and magnitude respectively of any difference between said mean frequency and the resonant frequency of said tuned circuit, an electron discharge device having a control electrode, means for supplyinga selectable portion of said alternating component of signal potential to said control electrode for amplification, and means for continuously supplying directly to said control electrode a fixed portion of said unidirectional component of signal potential thereby to reduce the amplification of said discharge device.

2. In combination, a source of carrier waves signal modulated in frequency about a mean frequency, a load resistor, means including a tuned circuit for deriving from said source of waves and supplying across the terminals of said load resistor a signal potential having an alternating aaoacsu component at signalfrequency and a unidirectional component proportional;;in polarity and intensity to the direction and magnitude respectively of variations between said mean frequency and the resonant frequency of said tuned circuit, an electron discharge device having a control electrode, potentiometer means for supplying to said control electrode a selectable portion of said alternating component of signal potential for amplification by said discharge device, and means for continuously supplying directly to said control electrode a fixed portion of said unidirectional component of signal potential independently of the position of said potentiometer means thereby to control ,the amplification of said discharge device.

3. In combination, means including a tunable circuit for supplying a carrier wave signal modulated in frequency about a mean frequency, means including a load resistor for derivingv from said carrier wave an alternating potential at sig: nal frequency, said means including means for providing across the terminals of said load resistor a unidirectional potential of positive or negative polarity depending upon the direction of mistuning of said tunable circuit, an electron discharge device, means for supplying said alternating potential at signal frequency to said discharge device for amplification, and means including a signal frequency rejection filter for sup:- plying said positive or negative unidirectional potential directly to said discharge device to reduce the amplification of said device.

4. In a tuning system for a receiver of the ire quency modulation type the combination comprising, means for supplying a carrier wave signal modulated in'frequency about a mean frequency subject to variation, means including a tuned circuit and a load resistor for deriving from said carrier wave an alternating potential at signal frequency, said means including means for providing across the terminals of said load re sistor a unidirectional potential having a positive or negative polarity in accordance with the direction of a departure of said mean frequency from the resonant frequency of said tuned circuit, an electron discharge device having a control electrode, manually controllable means for supplyinga selectable portion of said alternating potential to said control electrode for amplification, and means including signal frequency rejection means for supplyingvfixed portions of said positive or negative unidirectional potential directly to said control electrode to decrease the amplification of said discharge device upon the occurrence of a departure in either direction of the mean frequency from said resonant frequency.

' HAROLD T. LYMAN.

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