GB2101432A - RF signal receiver with phase- locked loop demodulator having variable bandwidth means - Google Patents

Abstract

An FM receiver apparatus includes a frequency tuner and a phase-locked loop demodulator having a loop amplifier filter means for recovering of the modulated signal. The phase-locked loop detector includes a high quality voltage controlled oscillator (9) driving a tuned signal phase comparator (8) connected to a loop filter (10) which includes an active loop amplifier. The output of the filter is connected to drive the voltage control oscillator. The loop filter serves to set the bandwidth of the demodulation in accordance with the amplifier gain, the resistance capacitance filter components and the like. The loop filter includes a fixed capacitance (11) establishing a large signal bandwidth. A second fixed capacitor (21) is connected in series with FET transistor (22) in a series branch connected in parallel with the fixed bandwidth capacitor (11). An RF signal signal amplitude detector (24, 25) actuates the FET transistor to reduce the bandwidth of the phase-locked loop demodulator with the RF signal such that the loop filter pass-band tracks the tuner pass-band characteristic, particularly when a weak signal is received. <IMAGE>

Description

SPECIFICATION RF signal receiver with phase-locked loop demodulator having variable bandwidth means This invention relates to a radio frequency (RF) signal receiver for receiving electromagnetic frequency modulated signals and particularly to such a receiver including a phase-locked loop type demodulator or detector apparatus operable to demodulate the frequency modulated received signal.

Sound and video transmission systems advantageously use frequency modulation (FM) of a radio frequency carrier signal. Frequency modulation of the signal is desirable to provide a high fidelity sound signal. The receiver unit includes a demodulator or detector circuit tuner to select an RF signal and the modulation band and a demodulator or detector circuit which recovers the modulating or information signal from the modulated received signal. Quadrature detection and demodulation circuits have been used in high quality receivers. A quadrature detector has an S-shaped detection characteristic with relative small center portion which can be made substantially linear by creating a wide bandpass characteristic substantially greater than the bandpass of the tuner. Thus, the substantially linear portion extends over the total RF signal to be received.As the bandwidth increases the quality of the sound decreases because the filter characteristic includes more non-linear-portions of the bandpass curve. The extremely wide bandwidth response requirement, thus prevents optimum signal receiving characteristics. An alternative demodulation means which has been suggested is a phase-locked loop type demodulating unit, but such means has not been commercially available because of costs and high fidelity limitations. A phase-locked loop detector unit is advantageous because it operates with an essentially linear detection characteristic and has excellent rejection amplitude modulation of the FM signal. However, the phase-locked loop demodulation unit has generally been used in applications in which lower carrier frequency such as ten (10) megahertz (MHZ), is used and sound fidelity was not of great significance.Thus, teletype transmission system, various military applications and the like have used phase-locked loop demodulation unit. When applied to FM high frequency signal transmission system, the bandwidth of the phase-locked loop type detector unit may be selected to be somewhat wider than the bandpass of the RF signal tuner. The bandpass curve is the linear over practically the complete length of the bandpass limits and thus as long as the limits are at least equal or somewhat greater than the detection limits and located to the opposite sides of the tuner curve or IF skirt, a distortion free RF signal is produced. The linear bandpass of course minimizes, distortion of the received signals. In practice, however, distorted reception of weak signals, and in many instances unintelligible reception of the weak signals, is found in a receiver with the phase-locked loop.FM signals are transmitted relatively short distances due to the characteristics of the waves. A radio receiver in the fringe area of the signal station transmitter particularly obtains a very erratic signal and often the noise submerges the low strength FM signal.

The present inventor realized that the characteristic of the tuner bandpass curve and its relationship to the bandwidth demodulator characteristic creates the loss of the weak signal.

The IF bandpass curve or skirt is a substantially U-shaped characteristic having a distinct curved and rounded bottom portion. Thus, the IF bandpass curve does not include the idealized shape of sharp sides interconnected to a flat bottom but rather has reasonable steep sides joined by a shallow curved continuously changing bottom portion at the lowest signal strength. The bandwidth of the phase-locked loop detector which equals or extends beyond the IF curve and encompasses the high strength signal portion of the tuner bandwidth is operable to detect the RF signals with a distortion free transmission of the demodulated audio signal. However, in the weak or fringe signal portions where the IF curves off, the total signal excursions may move beyond the IF characteristic.The receiver does not then respond with the normal limiting characteristic and the background noise level may increase relative to the normal noise suppression operation.

The weak transmitted signal is then lost within the noise signal, resulting in the unintelligible sound reception in the fringe areas of the transmitter or the like.

The present invention is particularly directed to an FM receiver apparatus using a phase-locked loop type demodulator means having a loop filter means for recovering of the modulated signal and which is designed and constructed with a fixed bandwidth of the demodulator means. Generally in accordance with the present invention, the phaselocked loop demodulator means is provided with a variable bandwidth control unit which is selectively introduced into the circuit to produce a bandwidth related to the strength of the received signal and more particularly the bandpass characteristic of the tuning unit. The bandwidth therefore generally tracks the tuning unit bandwidth curve.As the strength of the tuned signal decreases and moves into the curved portion of the tuned bandwidth skirt, the demodulation unit is changed to reduce the bandwidth from a fixed bandwidth set to exceed the tuned RF signal curve, such that the phaselocked loop detector bandwidth constricts and moves downwardly, restricting it preferably substantially to the bandpass curve of the tuned signal in the curved weak signal portion.

More particularly in accordance with a preferred embodiment of the present invention, the phase-locked loop detector includes a high quality voltage controlled oscillator driving a tuned signal phase comparator connected to a loop filter which includes an active loop amplifier. The output of the filter is connected to drive the voltage control and rebalance the phase comparator in accordance with any phase differences between the tuned signal and the oscillator signal as well as to generate a demodulated FM output signal. The loop filter serves to set the bandwidth of the demodulation in accordance with the amplifier gain, the resistance capacitance filter components and the like. The loop filter of commercially available phase-locked loop demodulation unit includes a fixed capacitance establishing a large signal bandwidth.

A second capacitor means is connected in series with an electrically responsive existence means and the series branch is connected in parallel with the fixed bandwidth filter capacitor. An RF signal amplitude monitoring means is connected to the IF input signal amplifier and the output is coupled to and actuates the resistance means to vary the capacitance and resistance in parallel with the fixed capacitor to reduce the bandwidth, particularly during receipt of the weak signals created within the curved portion of the IF bandpass characteristic curve. Although the change can be provided in any desired progressive or stepped controlled system, a particularly satisfactorily switching system employs a fixed capacitor in series with an FET switch of a P channel type operating in the enhancement mode.

The input electrode of the field effect transistor is coupled to the amplitude monitoring means. As the RF signal input decreases, the conductance of the FET will increase and the resistance decreases, resulting in a greater current flow through the paralleled capacitor. In accordance with well known looped filter construction, this reduces the bandpass characteristic of the loop filter. By proper selection of the signal transmitted to the FET, the bandpass characeristic of the loop filter can be made to generally follow or track the IF bandpass characteristic, and to equal it during the period of reception of a weak signal.

The present invention has been found to provide a dramatic change in the characteristic of a high quality FM tuner and may establish a completely new standard for high fidelity FM receivers.

For example, the standard for single channel, high quality FM tuners, permit three per cent distortion with a 1.5 microvolt (uv) RF input signal into a 300 ohm impedance. The tuner with the present invention operates with three per cent distortion with an RF input signal of 0.8 microvolts into a 300 ohm impedance. Further, when used with a stereo system, the present invention also correspondingly improves the sensitivity with the present invention, the separation of the signals in a stereo receiver is generally reduced in accordance with the effectiveness in the improvement of the quality of transmission of weak signal. This however has been found to provide an improved stereo reception in that the merging of the weak sound signals actually improves the quality of the sound by submerging of noise signals.

The present invention has thus been found to provide a simple reliable and relatively inexpensive means of dramatically increasing the quality of present day high quality FM tuner receiver apparatus.

In the drawings: Fig. 1 is a simplified block diagram of an FM tuner having a phase-locked loop demodulator unit and incorporating an embodiment of the present invention; Fig. 2 is a graphical illustration of the bandwidth characteristics of a standard FM tuner having a standard phase-locked loop demodulation unit; Fig. 3 is a graphical illustration similar to Fig. 2 illustrating the bandwidth characteristic of the phase-locked loop demodulator unit constructed in accordance with the teaching of the present invention:: Fig. 4 is an illustration of a circuit chip constructed to operate as a high fidelity receiver employing a phase-locked loop detection unit modified in accordance with the teaching of the present invention; Fig. 5 is a graphical illustration of the distortion versus the RF signal input of a circuit such as shown in Fig. 4 with and without the present invention, with 50% modulation of the carrier frequency; Fig. 6 is a graphical illustration similar to Fig. 5 of the distortion versus the RF frequency input signal at 100% modulation; Fig. 7 is a graphical illustration for a circuit such as shown in Fig. 4 illustrating the signal-to-noise characteristic with the RF input signal level for mono-transmission quieting; Fig. 8 is a graphical illustration similar to Fig. 7 illustrating the characteristic for stereo transmission;; Fig. 9 is a graphical illustration showing the distortion in a stereo system with the varying RF input signal; and Fig. 10 is a graphical illustration of the capture ratio of FM receivers including the change produced by the present invention.

Referring to the drawing and particularly Fig. 1, a simplified high fidelity audio receiver 1 is shown in block diagram incorporating the subject matter of the present invention. The illustrated receiver 1 is an FM audio signal receiver. An antenna 2 is adapted to receive transmitted high frequency FM signals. The antenna 2 is connected to the input of the receiver 1 and particularly an FM tuner 3 which is shown in block diagram. The tuner 3 may be any high fidelity unit, a greater variety of which are commercially available. The tuner 3 will of course include the usual RF amplifier, mixer and local oscillator stages to select the proper RF signal for the desired channel frequency, amplify such signals and convert them to a lower intermediate frequency (IF) for subsequent processing and detection. The converted RF signal is amplified in a suitable IF amplifier 4 to increase the signal to an amplitude suitable for detection and demodulating. In accordance with the present invention. the high quality receiver is of the type having a phase-locked loop demodulation unit 5 which is operable to amplify and process the IF signal and particularly to separate the modulating audio signal from the IF signal. Thus, an FM signal includes the high frequency carrier wave, the frequency of which is modulated above and below a center frequency in accordance with the strength of relatively low frequency modulating wave.The output of the phase-locked loop demodulation unit 5 is a signal corresponding to the modulating waves and is applied to a conventional output circuit which includes an audio amplifier 6 to increase the level of the demodulated signal sufficiently to produce the desired volume in an audio output device, such as another speaker system 7. High quality fidelity FM receivers of this type just described have been widely developed and are commercially available with the basic construction shown in Fig. 1. The phase-locked loop demodulation unit 5 in particular provides high quality reproduction of the modulation signal forming the information contact of the high frequency carrier wave of the received FM signal.As shown in block diagram and as well known in the art, the phase-locked loop demodulation unit 5 includes a phase comparator or detector 8 having its input connected to the output of the IF amplifier 4. The phase comparator 8 includes a second input connected to a phase controlled oscillator 9. The output of the phased detector or comparator 8 is proportional to the difference in phase of the two input signals. The output of the phase comparator 8 is connected as an input to a loop filter 10 which is operable to separate the modulation signal from the carrier signal to produce a suitable audio signal voltage applied to the audio amplifier 6. In addition, the output of the loop filter 10 provides a voltage control signal connected to control the output of the voltage controlled oscillator.The output is such that the voltage controlled oscillator 9 drives the phase comparator 8 to rebalance the phase comparator such that the output continuously tracks the phase difference in the two input signals voltages. The voltage controlled oscillator is set to the center or carrier frequency to produce a zero or reference output in the absence of a modulated RF signal. The modulation of the carrier frequency signal thereof results in the audio signal which varies above and below a reference signal level as a replica of the original modulating signal.

In accordance with well known construction of phase-locked loop demodulation units, a fixed capacitor 11 is connected into the circuit of the loop filter 10 and establishes the loop filter bandpass or bandwidth characteristic. The bandwidth of the loop filter 10 is a substantially linear output of voltage versus frequency such as shown at 12 in Fig. 2. The FM tuning circuit for any given selected frequency transmits an IF signal generally as shown in Fig. 2. Thus, the FM tuner is set to transmit the selected carrier frequency and a band of frequency to each side of such carrier frequency to include all of the frequencies within 100% modulation.A typical IF curve of frequency versus signal strength, shown by curve 13 includes relatively straight side edge portions 14 and 15 interconnected by a shallow shaped base portion 16 to the opposite sides of the carrier frequency to form a generally U-shaped skirt pattern. The curve 13 would idealistically be formed as a sharp rectangular trough as shown at 17 in Fig. 2, but in attainable structures is the shape of the curve 13-1 6. In order to adequately detect the complete and total bandwidth or range of the IF input signals, the loop filter 10 is designed with a capacitor 11 which establishes a bandwidth and bandpass characteristic 12 of Fig.

2. Thus, the filter bandpass characteristic 12 is a substantially linear curve particularly through the IF curve and somewhat to the opposite sides thereof with the higher frequency portion passing through the corresponding curved portion of the IF curve. The extension to the opposite sides of the IF curve 13 is universally used in commercial receivers in order to transmit the higher strength signals in total IF bandwidth with minimum distortion.

The curvature in the portion 1 6 of the IF characteristic curve 13, however, significantly reduces the bandwidth in the areas of relatively weak RF signals, for example, in the fringe area of the transmitter range of the particular selected frequency station. The weak signals in such fringe signal range will often have excursions in excess of the IF band, resulting in amplification and tranmission of noise signals by the phase-locked loop filter and particularly the loop demodulator unit. This results in essentially complete submersion of the information sound frequency's signals with unintelligible sound reception. Thus at approximately the center points 18 to each side of the central frequency carrier, the transmission band is reduced by approximately 50%, creating a significant noise area in which the information signals are suppressed.As more fully developed hereinafter, an RF signal input of less than one and one half (1.5) microvolts (uv) into a conventional 300 ohm coupling input impedance used in high quality FM receivers results in unacceptable sound reception. The present invention is based on the inventor's realization that a phase-locked loop type demodulation unit may be used to improve the characteristic and to transmit the local RF signals within the curved portion 1 6 of tuner bandwidth curve 13 while maintaining the same or improved transmission of the high strength RF signals. In accordance with the teaching of the present invention, the bandwidth of the phaselocked loop demodulating unit 5 is modified in accordance with the characteristic of the tuner bandpass curve 13 and in particular to provide a modified bandpass characteristic such as typically shown in Fig. 3.Referring particularly to Fig. 3, the bandpass characteristic of the phase-locked loop demodulating unit 5 is shown having a first expanded bandwidth characteristic curve 19 shown extending somewhat beyond the IF bandwidth in the high strength RF signal portion.

The bandwidth curve is significantly modified as the IF signal decreases into the weak or low strength level, and in particular within the curved portion 16, to a bandpass curve 20 wherein the linear portion tracks and matches the IF curve portion 16. The inventor has found the change produces a dramatic and unexpected improvement in the quality of sound transmission in commercially available high fidelity receiver equipment. Referring again to Fig. 1 , the phaselocked loop demodulating unit 5 is shown including a bandpass control means including a capacitor 21 in series with a voltage responsive control means 22, the conductance and resistance of which varies with an input voltage signal. The series connected capacitor and control means define a control branch 23 connected in parallel with the bandwidth setting fixed capacitor 11.The control means 22 in turn, as presently described, is controlled by the amplitude or the strength of the received RF signal.

In the embodiment of the invention shown in Fig. 1, the control means 22 is controlled by a voltage proportional to the strength of the output of the IF amplifier 4. A level detector 24 is connected to the output of the IF amplifier 4. A signal meter drive unit 25 and a mute drive unit 26 are connected to the level detector 24 and provide corresponding output signals. These signals are employed in a conventional receiver in accordance with well known connections. In the present invention, the signal meter drive unit is also connected by a signal line 27 to the input of the control means 22 to control the conductivity of the and thereby the current through the bandwidth modifying capacitor 21.In particularly, the response of means 22 and the RF signal related voltage at line 27 are selected to increase the conductivity and reduce the resistivity control of the means 22 and effectively change the resistance and capacitance in the feedback network of the loop filter with a decreasing IF signal. The change in the feedback network will decrease the bandpass of the loop fidelity, causing the characteristics to progressively change from curve 19 to curve 20 in Fig. 3.

The bandwidth characteristic curve 19 of the FM demodulating unit 4 provides minimum distortion of the IF signal as long as a substantial RF signal is received. The low distortion is responsible for the high fidelity quality of an FM receiver. When a weak signal is received, however, background noise level would normally rise significantly and the information signal would be actually lost in the amplified noise signal with the transmission of practically unintelligible audio signal. With the present invention, the reduced bandwidth curve 20 of the loop filter 10 prevents the signal from deviating beyond the IF curve 13 and as a result the noise factor is significantly minimized, such that the low level IF signal is transmitted as an improved and reasonably high quality audio signal.

The present invention is thus particularly directed to the concept and the structure of an FM receiver employing a phase-locked loop type demodulating unit in which the loop filter includes a variable control means for changing of the bandpass or bandwidth of a phase-locked loop demodulating unit. Although the variable feedback network can be provided in any suitable manner, a fixed capacitor 21 in series with a voltage or other amplitude responsive control mean 22 provides a simple, reliable and inexpensive control which can be applied to a conventional phase-locked loop system including existing systems.

For example, Fig. 4 illustrates a commercially available FM receiver manufactured by Fulet Company of Taiwan and which is sold by NAD Fulet NAD New Audio Dimensions having a main office in Boston, Massachusetts, under Model No.

4020A. Thus, as shown in Fig. 4, the FM tuner includes an integrated electronic chip 28 having the necessary output pin connectors for interconnection to the standard signal inputs and outputs circuit elements, as shown. A loop bandpass fixed capacitor 29 is connected to the loop filter terminal 30 and presets the bandwidth of the loop filter and demodulating unit within chip 28, as in block diagram 1. A fixed capacitor 31 in series with a field effect transistor 32 is connected in parallel with the capacitor 29. The field effect transistor 32 is shown as a P-channel type having its input electrode 33 connected to the signal meter drive output terminal 34 of chip 28.A voltage dividing network 35, including a series resistor and a pair of ground resistors to the opposite sides thereof, is connected in the signal line to the electrode 33 to provide an appropriate proportional signal to the field effect transistor.

The chip 28 operates to produce a demodulated audio output signal at the output terminal 37 for driving of the audio or sound generating circuit in response to an RF signal input at terminal 38. The field effect transistor 32 operates in the enhancement mode and thus varies the resistance in the branch circuit and current supply from the chip through the capacitor 31 and in parallel with capacitor 29. The applied signal varies directly with the amplitude of the IF signal. As the signal decreases, the voltage signal decreases and the field effect transistor becomes more conductive and increases the capacitance connected in the feedback network of the loop filter. The increase in the capacitance in the feed back loop, in accordance with well known understanding, reduces the bandwidth of the phase-locked loop demodulator 5.

By proper selection of the components of the coupling network, the transistor, and capacitors, the feedback network change can be selected to progressively reduce the bandwidth with the decrease in the strength of the IF signal. Thus, the location and breadth of the bandwidth changes to move down and to approach the tuner bandwidth such that at the very weakest portion of the signal such as at the 50% point on the high side of the IF curve, the demodulator bandwidth essentially spans and or corresponds to the IF bandwidth as shown by curve 20 in Fig. 3.

The significant results from this improvement are graphically depicted in Figs.5~9.

Referring particularly to Fig. 5, a graphical illustration is given of the percentage distortion with a varying RF input signal. The results shown are those obtained from a commercial FM tuner similar to that such shown in Fig. 4. The curves are illustrated for a single or mono-channel sensitivity and with 50% modulation of the carrier signal. The carrier signal frequency was selected with 90 megahertz (MHZ) and the carrier modulation of 1 kilohertz (KHZ). The full line characteristic curve 39 is that obtained in a standard circuit incorporating an automatic variable capacitance in accordance with this invention, while the dashed line characteristic curve 40 is that obtained with the same standard circuit operator with the usual single, fixed capacitor 11. The three percent (3%) distortion point for the RF input is of course the significant factor.

Thus, high fidelity tuners generally use as a standard a 3% distortion characteristic with an RF input signal of 1-1/2 microvolts applied to a receiver source impedance 300 ohms. This is the theoretical limit of the usable sensitivity in high fidelity high quality FM tuners. Without the structure of the present invention, the 3% distortion point 41 occurs whenever the RF input signal drops to 1-1/2 uv, as shown in Fig. 5. The addition of the present invention provides a dramatic improvement in that the 3% distortion does arise until the RF input signal decreases to 0.8 uv, with a significant increase in the fidelity of the sound at weak signal levels. The improvement to 0.8 microvolts is a magnitude of difference which establishes a completely new standard of performance.The improvement occurs over a substantial portion of the characteristic as a result of the progressive change to the high strength RF input of substantially 10 uv. Thus, at ten microvolts, the standard high quality FM tuner has the quality of the FM tuner incorporating the present invention. Fig. 5 thus depicts the significance of the present invention as applied to the FM transmitting stations which use a 50% modulation.

Fig. 6 is a graphical illustration of distortion similar to that of Fig. 5 in a system using 1 00% modulation in contrast to the 50% modulation of Fig. 5. The graphical difference, however, is similar. In this instance, 3% distortion arises at approximately 1 microvolt with variable bandwidth control as shown by curve. The conventional high quality FM tuner however again produces a distortion 3% at 1-1/2 microvolts as shown by curve 42.

The change in the signal-to-noise characteristic resulting from the invention is illustrated in Figs. 7 and 8. Fig. 7 is a graphical illustration showing signal-to-noise versus the RF input signal voltage for a mono-quieting, that is a single channel receiver. Assuming the same low level input signal of 1-1/2 microvolts, there is a significant reduction in the signal-to-noise ratio with the invention, as shown by the full line curve 43 versus the dashed line curve 44 of the standard receiver.

The present invention can also be readily and advantageously applied to stereo receiver equipment. A similar quieting illustration of the signal-to-noise ratio versus RF input level signal is shown in Fig. 8 for stereo quieting. In the circuit of Fig. 8, a similar carrier and modulating frequency was used, with a pilot frequency of 19 kilohertz +10%.

Fig. 9 illustrates the reduction in distortion in a stereo receiver. The stereo sensitivity with the present invention is significantly improved. For example, the 3% distortion does not arise until the input voltage drops to approximately 2.2 microvolts with the present invention as shown by curve 47 while without the present invention 3% distortion is reached at 3.3 microvolts input as shown in curve 48.

Fig. 10 illustrates the improvement in the capture ratio resulting from the present invention.

The capture ratio defines the capability of the circuit to select the strongest of two corresponding RF signals. This is particularly significant in the reception of weak signals. Thus, the RF signals received may include signals which have bounced off an element as well as direct, uninterrupted signals. The latter signal is of course normally of a greater strength. Fig.10 illustrates the capture ratios of the same receiver with and without a structure of the present invention in the curves 49 and 50 respectively. A capture ratio of 1 or less is an optimum response in that the stronger signal is then uniformly established. As shown in Fig. 10, the ratio departs from such desired characteristic as the signal strength drops below 100 uv, and departs more rapidly below 10 uv.

However, with the present invention, the ratio, does not significantly increase above 1 until the signal decreases to 10 uv, and then with decreasing signal strength, the ratio increases more slowly than without the present invention.

Fig. 10 also illustrates the capture ratio curves 51 and 52 for a standard ratio detector used in FM receivers and for a quadrature detector. The standard ratio detector cannot generally be changed to improve such characteristic. Although the quadrature detector may provide an improvement in the capture ratio through a bandwidth control, the quality will not generally exceed that of a curve 50 for a phase-locked loop demodulator without the inventor. Fig.10 again indicates the dramatic improvement in the receiver functioning during receipt of relatively weak signals.

Those familiar with the art of FM tuners will readily recognize the substantial difference in the receiving quality of a high quality FM tuner having a phase-locked loop demodulating unit using the variable bandwidth control of the present invention.

Although shown in a preferred embodiment employing a variable conductive control, such as a field effect transistor, any other suitable means which can be made responsive to the change and level of an input RF signal can be used. Further a bank of capacitors coupled through a bank of switches could be used to provide a step change in the capacitance connected into the circuit with a corresponding step change in the characteristic of the bandwidth of the phase-locked loop demodulating unit and if suitable a variable capacitance device is available a direct action might be used. Further, the bandwidth of the phase-locked loop demodulation may be changed in any other suitable manner such as modification within the phase comparator unit, the gain of the loop filter amplifier may be changed or the like.

The inventor does not know any other FM tuner having the capability of operating at the low signal inputs of the present invention while providing a high quality output signal of the characteristic provided by the use of the present invention. In a stereo system, the increase in sensitivity is accompanied by a decrease in the separation of the audio signals between the two channels. This however is desirable with relatively weak signals because the merged sounds tend to further submerge and eliminate the extraneous and undesirable noise in the audio signal.

Further, illustration of the improvement of the present invention is shown in a separate paper accompanying the application and to be made a part of the file wrapper. Generally, the photographic audio signal oscillarace traces are shown with and without the present invention which show significant elimination of the noise component and production of a smooth audio signal having minimal noise at various low signal levels including voltages of 0.7 and 0.9 microvolts.

The present invention thus provides a simple, reliable with a significant improvement in the art of high fidelity tuners of the phase-locked loop demodulating type.

Claims (14)

1. An electromagnetic modulated signal receiver apparatus for receiving a modulated FM signal having an RF carrier frequency and a modulating frequency and having a phase-locked loop demodulating means including a filter means having a capacitance to control the bandwidth of the demodulating means, the improvement in a bandwidth control capacitance means adapted to be connected to said phase-locked loop demodulating means, comprising a variable bandwidth means connected to said phase-locked loop demodulating means, and control means to control the bandwidth means to compensate for changes received signal characteristic with changes in the strength of the modulated FM signal.

2. The modulated signal receiver apparatus of Claim 1, wherein said bandwidth means includes at least one capacitor and said control means includes a variable resistance means connected in a series branch with said capacitor for selectively connecting said capacitor into circuit.

3. The modulated signal receiving apparatus of Claim 1 wherein said control means is a means having a conductance controlled by an electrical input, and means monitoring the strength of said RF signal connected to said input.

4. The modulated signal receiver apparatus of Claim 1, wherein said variable control means includes a fixed capacitor, said control means includes a field effect transistor of the P channel type operating in the enhancement mode, said transistor being connected in series with said capacitor and having a control input for controlling the conductivity of said transistor and thereby the connection of the capacitor to said phase-locked loop filter means.

5. The signal receiver apparatus of claim 4 wherein said control means includes monitor means monitoring the strength of the received FM signal and establishing a control signal in accordance with said strength, and said control input being connected to said monitor means and operable to progressively change the conductivity of said transistor in a direction to cause the bandpass of the filter means to decrease with a decrease in the strength of the FM signal.

6. The signal receiver apparatus of claim 1 having an FM tuner for selecting the RF carrier frequency input and said tuner having a bandpass curve including a rounded portion in the weak signal reception signal portion, and wherein said control means is operable to reduce the bandwidth of the filter means to equal the bandpass of the tuner in the weak signal portion of the receiver.

7. A phase-locked loop demodulating apparatus having an RF signal input adapted to receive an FM signal from receiver tuner means and having a loop filter to control the bandpass of the demodulating means comprising a variable capacitor means having an electrically responsive input means and a varying capacitance output, circuit means adapted to connect said capacitance into said loop filter to control the bandwidth of said detector, and control means adapted to monitor the strength of said RF signal and connected to said electrically responsive input means to change said capacitance output and thereby correspondingly change the bandwidth in response to a change in the strength of the RF signal.

8. The phase-locked loop demodulating apparatus of claim 7 wherein said variable capacitor means includes a fixed capacitor connected in series with a control means to control the current flow through said capacitor, and means to actuate said control means to reduce said bandwidth.

9. The phase loop demodulating apparatus of claim 7 wherein said bandwidth of said filter means equals the tuner bandwidth in at least one portion of the tuner bandpass.

10. The phase lock loop demodulating apparatus of claim 7 wherein said electrically responsive input means includes a solid state switch means having a transistor input means to control the conductivity of the switch means, and said control means is connected to said transistor input means.

11. The phase-locked loop demodulating apparatus of claim 10 wherein said switch means is a field effect transistor.

12. The phase-locked loop demodulating apparatus of claim 7 wherein said phase-locked loop filter means includes a fixed capacitor connected to establish a fixed bandpass curve, said variable capacitor or means includes a fixed capacitor in series with a voltage responsive control means, said control means having a control input responsive to the level of an input voltage controlling the resistivity of the control means, said monitor means establishing a voltage proportional to the RF signal voltage.

13. An FM signal receiver, comprising a tuner, an IF amplifier connected to said tuner and establishing an IF input signal, said tuner having a generally U-shaped signal bandpass curve including a substantially curved bottom portion defining the weakest IF signal portion, a phaselocked loop demodulation means having a dual input phase detector connected to said IF amplifier and to a voltage controlled oscillator and having low pass loop filter connected to the output of phase detector and the control input of the voltage controlled oscillator, said loop filter including a fixed capacitor connected to establish a filter bandwidth curve including linear portion substantially greater than the handpass curve of said tuner, a variable conductance control means having an electrically responsive input means to vary the conductance through said control means, means connecting said control means in a series with said control capacitor to define a bandwidth controlling capacitor branch connected in parallel with said fixed capacitor, a signal level detector connected to said IF amplifier, means connecting said signal level detector to said switch means to vary the conductance and operative connection of said controlling capacitor branch to establish a progressively reducing bandwidth of said low pass loop filter with a progressively reducing strength of said IF signal.

14. The FM signal receiver of claim 13 wherein said control means and said control capacitor are constructed and arranged to establish a filter bandwidth corresponding to the bandwidth of the tuner at least within the curved bottom portion of the tuner bandpass curve.

1 5. The FM signal receiver of claim 13 wherein said switch means is a field effect transistor having output terminals connected in series with said control capacitor and having a control terminal connected to said signal level detector.