WO1982001291A1

WO1982001291A1 - Radio manual tuning circuit

Info

Publication number: WO1982001291A1
Application number: PCT/US1981/001020
Authority: WO
Inventors: Inc Motorola; L Ecklund
Original assignee: Inc Motorola
Priority date: 1980-09-29
Filing date: 1981-07-29
Publication date: 1982-04-15
Also published as: IT8149344A0; IT1142870B; EP0060844A1

Abstract

A manual tuning circuit for an AM and/or FM radio (10) in which the magnitude of the radio audible signals is intentionally varied in accordance with the magnitude of an error signal related to the absolute difference between the frequency of a desired received modulated carrier signal and the frequency to which the radio front end (11-17) is tuned. In this manner, precisely tuning the radio front end to precisely the desired frequency can be obtained by tuning for the maximum volume of radio audible signals. Preferably, this is accomplished through the use of full wave rectification of either the FM discriminator (35) DC output voltage or full wave rectification of the control voltage for a voltage controlled oscillator (VCO) (26) in a phase locked loop (24) which tracks the IF output signal of an AM/FM radio IF stage (16).

Description

RADIO MANUAL TUNING CIRCUIT

Background of the Invention

The present invention is generally related to the field of manual tuning circuits which are adaptable for tuning AM and/or FM radio receivers.

Typically, AM and FM radios have manual tuning circuits which make it difficult to precisely tune the radio to a desired received signal. The problem exists for the commonly used superheterodyne radio receivers which utilize a tuning front end comprising a manually tunable local oscillator, a mixer and a fixed tuned narrow band IF stage to accomplish frequency selection. The tuning problem is most acute when receiving large amplitude input RF carrier signals since this would generally result in having the radio audio output at essentially the same amplitude even if the radio front end was slightly mistuned from the desired received signal.

While in most applications, a slight mistuning of the radio receiver from the incoming modulated carrier signal is not substantially detrimental, in certain situations this mistuning can -be extremely objectionable since the slight mistuning may result in the narrow band IF stage unintentionally destroying some of the side band information which comprises the modulation of the carrier signal. It has been found that this a serious problem for several proposed AM stereo systems. Thus many -of these systems have attempted to provide various tuning indications which would allow the operator to properly manually tune the receiver front end to the

O FI precise frequency of the incoming desired modulated RF carrier. Some prior systems have attempted to provide a visual indication to the operator as to the frequency coincidence between the received signal and the fre- quency to which the radio front end is tuned, and they have utilized various visual tuning indications compris¬ ing tuning meters or a series of low and high tuning indicator lights. These systems differ from the normal stereo pilot light indicator which merely indicates if the pilot signal magnitude is above a threshold value. These visual tuning systems are generally costly to implement and require the utilization of a substantial amount of space on the radio front control panel. Also they are deemed to be undesirable for use in automotive radio environments since they, would potentially require distracting the driver of the automobile away from his driving duties since they require his visual concentra¬ tion for tuning the radio. Various automatic tuning systems have proved unacceptable for automotive radios for exactly tuning the front end to the exact incoming frequency since these systems are generally accomplished by implementing a very strong AFC (automatic frequency control) function which results in having the radio undesirably switch between weak and strong stations due to minor signal amplitude fades which randomly occur due to automobile movement.

Thus prior art systems have been essentially unable to implement tuning systems which allow for the precise tuning of the radio front end to a received signal fre- quency while accomplishing this in a manner suitable for use in the automotive radio environment. This is espec¬ ially true for AM stereo systems which would typically require extremely precise alignment between the received signal frequency and the tuning of the radio front end since minor misalignment would result in the destruction of the stereo separation information.

OI.ΪPI Summary of the Invention

An object of the -present invention is to provide an improved manual tuning circuit adaptable for tuning AM and/or FM radio receivers in an automobile and which overcomes the above-mentioned deficiencies of prior manual tuning circuits.

A more particular object of the present invention is to provide an improved manual tuning circuit which enables the operator to manually tune the radio to substantially the exact frequency of a received signal, and preferably this is accomplished by providing an audible indication which varies continuously in inten¬ sity in accordance with the degree of mistuning.

In one embodiment of the present invention, a man¬ ual tuning circuit for enabling tuning to substantially the exact frequency of a received signal is provided. This manual tuning circuit comprises: manually tunable receiver front end means for receiving modulated carrier signals over a frequency band and selecting one of said received modulated carrier signals having a predeter- mined frequency in said band for further processing by tuning said front end means to a first frequency in said band, said first frequency being the center frequency of an effective narrow passband for said front end means; signal processing circuit means coupled to said front end means for receiving said selected one of said modu¬ lated carrier signals and providing audible signals related to the modulation of said selected one of said carrier signals; error signal means, including part of at least one of said front end and signal processing circuit means, for providing an error signal having a magnitude related to the absolute frequency difference ' between said first frequency and said predetermined fre¬ quency of said selected one of said carrier signals; and

O ?I indicator means coupled to said error signal means for receiving said error signal and providing in response thereto a humanly perceptible indication which varies continuously in intensity in accordance with the magni- tude of said error signal over at least one predetermin¬ ed range of error signal magnitudes, whereby the opera¬ tor tuning said front end means can readily identify when the tuned first frequency of said receiver front end means substantially corresponds to the predetermined frequency of the selected received modulated carrier signal by tuning said receiver front end means for maximum or minimum intensity of said humanly perceptible indication.

Basically, the manually tunable receiver front end means comprises a manually tunable local oscillator, a mixer and a fixed tuned narrow band IF stage which together form the basic tuning front end of a super¬ heterodyne tuning circuit which tunes the receiver by implementing an effective narrow passband centered at a frequency corresponding to the adjustable frequency of the local oscillator output signal. The signal process¬ ing circuit means corresponds to the carrier signal demodulator circuitry, the audio signal processing circuitry and the receiver speakers. Essentially, the error signal is produced by com¬ paring the frequency of the IF output signal provided by the mixer with the fixed IF reference center frequency to which the IF stage is tuned. For an FM receiver, an FM discriminator provides a variable DC control voltage which varies in accordance with the arithmetic differ¬ ence between these two frequencies. Alternatively, for either AM or FM receivers a phase locked loop can be utilized which is locked to the output signal of the IF stage wherein the loop comprises a comparator, a low pass filter (LPF), and a voltage controlled oscillator (VCO) which are utilized such that the comparator com¬ pares the output frequency of the VCO with the IF output signal and wherein the frequency of the VCO is made to correspond to the fixed IF reference center frequency when the IF output signal corresponds to the fixed IF reference center frequency. In this manner, the phase locked loop also provides a variable DC control signal (as a control signal input to the VCO) which is related to the arithmetic difference between the IF output sig- nal and the fixed IF reference center frequency. This variable DC control signal is then received by a full wave rectifier means that produces as an output an error signal which has a magnitude related to the absolute frequency difference between these two frequencies, and this difference can be shown to be equal to the absolute frequency difference between the frequency to which the front end means is tuned and the frequency of the selected received carrier signal.

Through the utilization of the above-noted struc- ture, the present invention provides a manual tuning circuit for AM or FM radio receivers, or other communi¬ cation receivers, in which the receiver front end can be precisely manually tuned to the frequency of the received signal by merely adjusting the tuning of the front end for a maximum or minimum intensity of a human¬ ly perceptible indication (preferably audible signals) produced by the communication receiver. Since the tuning of the receiver front end can now be made to coincide to the selected received signal frequency, this ensures that substantial mistuning between the IF center frequency and the desired mixer output frequency will not occur and that therefore destruction of sideband information contained as modulation information of the selected carrier signal will not be destroyed due to mistuning of the receiver front end. This advance has made it possible to construct easily tunable AM stereo

OMPI receivers according to the teachings of several proposed AM stereo systems whereas without the use of this inven¬ tion, properly constructing such a radio receiver was significantly more difficult and precise tuning of such a receiver was significantly more complex.

Brief Description of the Drawings

For a more complete understanding of the invention reference should be made to the drawings, in which:

Figure 1 is a block and schematic diagram of a stereo radio receiver embodying the principles of the present invention;

Figure 2 is a graph illustrating the relationship between the magnitude of a control signal varying in accordance with the arithmetic amount of mistuning of the radio receiver shown in Figure^" 1; Figure 3 is a graph illustrating the relationship for an error voltage related to the absolute amount of frequency mistuning for the radio receiver shown in Figure 1; and

Figure 4 is a graph illustrating a possible modi- fication of the error voltage versus frequency mistuning relationship shown in Figure 3.

Description of the Preferred Embodiment

Figure 1 illustrates a superheterodyne AM and/or FM automobile radio receiver 10 having an improved manual tuning circuit which enables the operator of the radio receiver to easily manually tune to exactly the fre¬ quency of a desired received signal. This is accom¬ plished by having the operator tune the receiver for maximum or minimum intensity of a humanly perceptible indication which varies continuously in intensity in accordance with the magnitude of an error signal related to the absolute frequency difference between the fre¬ quency of the carrier signal which is desired to be received and the frequency to which the radio receiver is tuned.

In the receiver 10, radiated electromagnetic modulated carrier signals are received by an antenna 11 which supplies corresponding electrical RF signals to a radio RF stage 12 that in turn supplies amplified RF signals to a mixer stage 13 which also receives a local oscillator input signal from a manually tunable local oscillator 14. The mixer stage 13 provides an output difference frequency signal at an output terminal 15 which is coupled as the input to a fixed tuned narrow passband intermediate frequency (IF) filter stage 16 providing an output signal at a terminal 17. The opera¬ tion of the components 11 through 17 is conventional and represents the basic tuning principles of superhetero¬ dyne tuning wherein desired received radio frequency carrier signals are selected by a tunable receiver front end stage through the use of a mixer circuit that pro¬ duces an output signal having the difference frequency between a received carrier signal frequency and a tunable local oscillator signal frequency wherein this difference frequency must be substantially at the fixed center frequency of a narrov; band highly selective subsequent IF filter stage in order for the IF stage to pass the mixer output signal.

It is contemplated that the RF stage 12 is an optional circuit which can either be fixed tuned or made tunable in accordance with the frequency of the local oscillator output signal provided by the oscillator 14. Essentially, the mixer stage 13 receives informa¬ tion modulated RF carrier signals over a predetermined RF frequency band from the RF stage 12 and the compon-

C^' .rl ents 13 through 17 result in selecting one of the received modulated carrier signals having a predetermin¬ ed frequency in this band for further processing wherein this is accomplished by effectively tuning the front end of the radio receiver to a first frequency within the received band with this first frequency being the center frequency of an effective RF narrow passband for the front end of the radio receiver with the IF stage 16 ___ determining this narrow passband. The preceding discus- sion is merely a brief recitation of the superheterodyne radio tuning principle and is well-known to all radio engineers. Basically, the present invention concerns a tuning circuit for the radio which allows adjusting the frequency of the output signal of the local oscillator 14 such that the front end of the radio receiver will be tuned to exactly the frequency of a desired received carrier signal. In this manner, the received carrier signal will then be located at the center frequency of the effective front end narrow passband, and this will result in minimum loss of sideband information of the received carrier signal while allowing the front end to be designed to have a narrow passband which aids in adjacent channel rejection.

In typical radio receivers, when a strong signal is received, it is possible to have the front end of the radio receiver mistuned since the received signal is so strong that a significant output from the IF stage will be obtained even if there is some mistuning between the local oscillator 14 and the received carrier frequency. Normally, this does not present a significant problem, but in many proposed AM stereo systems, the problem becomes acute because this πύstuning results in having the narrow passband of the IF stage destroy some side¬ band information of the received carrier signal. In proposed AM stereo systems, this results in an appre-

O PI ciable loss in the stereo separation information with the result being either a garbled or non-separated stereo signal. The present invention is directed to solving this problem by enabling the operator of the radio receiver to precisely adjust the frequency of the local oscillator signal provided by the oscillator 14 such that the receiver front end will be tuned precisely to the frequency of the desired received RF carrier signal. The manner in which this is accomplished will now be discussed.

The output terminal 17 of the IF stage 16 is coupled as an input to a stereo demodulator 18 (shown dashed) which essentially receives the IF signal pro¬ vided at the terminal 17, detects the modulation of this IF signal, and produces left and right audio output sig¬ nals at audio output terminals 19 and 20, respectively. The output. terminals 19 and 20 are coupled as inputs to audio gain control circuits which are illustrated by a schematic block 21 which provides audio output signals to left and right speakers 22 and 23, respectively, that in turn provide audible signals related to the modula¬ tion of the carrier signal selected by the receiver front end comprising the components 11 through 17. The operation of the components 18 through 23, as described above, is conventional and known to all radio engineers. It is contemplated that the stereo demodulator 18 includes a phase locked loop 24 which is locked to the IF signal provided at the terminal 17. In Figure 1, this is indicated by an IF terminal 17' (at which the IF signal at the terminal 17 is provided) within the demod¬ ulator 18 being coupled as an input to a phase/frequency comparator 25 which also receives as an input the output signal of a voltage controlled oscillator (VCO) 26 and provides a difference output signal to a low-pass filter (LPF) 27 which provides a variable DC output control

OMPI signal to a control terminal 28 that supplies a frequency control voltage to the VCO 26. It should be noted that in most proposed AM stereo systems, it is necessary to provide a phase locked loop which is locked to the IF signal coming from the IF stage 16 in order to properly determine the phase of the IF signal which is necessary to extract the proper stereo information and provide the proper left and right channel separation. U.S. patent 4,128,586, to Parker et al., entitled "Compatible, AM Stereo Broadcast System" and assigned to the same assignee as the present invention illustrates such a phase locked loop which is locked to the IF signal for this purpose. While the present invention contemplates that the phase locked loop 24 be part of the stereo demodulator 18, this is not necessarily the case since the present invention will function even if an additional phase locked loop is provided exterior to the demodulator as long as the loop is locked to the IF signal. Also such a phase locked, loop could be used in AM or FM radios.

The operation of the phase locked -loop 24 is conventional and results in providing at the terminal 28 a variable DC control voltage for the VCO 26 that varies in accordance with the phase/frequency difference between the received IF frequency of ^"the signal at terminal 17 and the frequency of the output signal of the VCO 26. The control voltage at the terminal 28 is designated by the notation C_v and Figure 2 illustrates a graph of this voltage as a function of the arithmetic frequency difference between the IF signal at the termi¬ nal 17' and the output signal of the VCO 26. The opera¬ tion of the phase locked loop 24 and the characteristic for the control voltage Cy shown in Figure 2 are known and represent the typical operation of tracking phase locked loops. The present invention involves coupling the terminal 28 of the phase locked loop 24 to a full wave rectifier 30 which receives the signal C_v as an input and provides an output error signal e_v as an output to a control terminal 31 of the audio gain control circuits 21. Figure 3 illustrates the magnitude of the error signal e_v as a function of the frequency difference between the IF signal frequency and the output of the VCO 26. It is significant to note that due to the use - of the full wave rectifier 30, the magnitude of the error signal e_v is related to the absolute frequency difference (the absolute value of the frequency differ¬ ence)between the VCO output frequency signal and the IF signal frequency, whereas the magnitude of the control voltage C_v is related to the arithmetic difference (the difference which includes both magnitude and sign) between these frequencies. It is contemplated that the magnitude of the error signal e_v at the terminal 31 is utilized to control the gain of the audio gain con- trol circuits 21 such that for zero frequency difference between the IF signal at the terminal 17' and the output of the VCO 26 (which corresponds to the vertical axes in Figs. 2 and 3) maximum gain for the audio signals provided to the speakers 22 and 23 is provided which results in maximum intensity of the audible signals provided by these speakers.

In essence, the full wave rectifier 30, in combina¬ tion with the audio gain control circuits 21 and the speakers 22 and 23, provides the operator of the radio receiver 10 with a humanly perceptible (audible) indica¬ tion which varies continuously in intensity in accord¬ ance with the magnitude of the error signal e_v . This is accomplished over at least one predetermined range of error signal magnitude wherein this continuous variation is represented by sloped portions ^"32 and 33 of the e_v characteristic illustrated in Figure 3.

OMPI The present invention, by varying the gain of the audible signals produced in response to the modulation of the selected received carrier signal, provides an audible indication of how close the tuning of the front end stage of the receiver 10 is to the desired received carrier frequency. This enables the operator of the radio receiver 10 to precisely tune the front end to the desired received signal frequency by tuning the receiver for a maximum intensity of the audible signals produced by the radio. This in effect results in adjusting the frequency of the local oscillator such that the desired received carrier signal is translated by the mixer 13 to an intermediate frequency which is precisely at the center frequency of the narrowband IF stage 16. In this manner, the narrow passband of the IF stage 16 will not result in destroying sideband information on the receiv¬ ed carrier signal but will still enable adjacent channel rejection. Since precise tuning is essential for the proper reception and decoding of AM stereo signals according to most proposed AM stereo systems, the pres¬ ent invention provides a manual tuning circuit which is readily adaptable for use in such AM stereo systems. While the thrust of the present invention is to provide a manual tuning circuit which allows precise tuning for an AM stereo system, the present invention also -contemplates the use of similar concepts for pro¬ viding such precise tuning in an FM stereo system. Such a system is shown by the dashed connections in Figure 1. In such a system, an FM discriminator 35 would receive its input from the IF output terminal 17 and provide decoded audio frequency signals as an output at a termi¬ nal 36. As is commonly understood, the output of the FM discriminator would comprise both a DC signal related to the arithmetic frequency difference between the IF sig- nal at the terminal 17 and the center design^" frequency

CMPI of the IF passband, and an AC signal related to the modulation information contained on the selected carrier signal. In Figure 1, the AC audio modulation informa¬ tion is illustrated as being provided at the terminal 36, v/hile the DC frequency difference signal is provided at a terminal 37. Figure 1 illustrates that the modu¬ lation information at the terminal 36 is coupled as an input to the stereo demodulator 18 while the DC fre¬ quency difference signal is coupled as an input to the ^' full wave rectifier 30. All of the connections for the FM discriminator 35 are shown dashed to indicate that this is an alternative radio tuning circuit connection as contrasted with the above described AM radio tuning system. For the FM tuning system, the operation of the cir¬ cuitry is essentially the same as the above-described AM system in that the FM discriminator 35 at its DC output terminal 37 provides a control voltage which is essen¬ tially identical to the control voltage provided at the terminal 28. Therefore the full wave rectifier 30, in the above described FM system, will -again provide an error signal at the terminal 31 to adjust the audio gain of the audible signals provided by the speakers 22 and 23 such that by tuning for maximum intensity of the audible signals, the operator of the receiver 10 can precisely adjust the frequency of the local oscillator 14 to match that of the incoming desired carrier signal. This would permit FM radios to be designed without using extremely strong AFC circuits which are responsible for objectionable reception caused by switching between strong and weak stations due to temporary fades of the strong signal.

Figure 4 illustrates that the characteristic for the error voltage e_v at the terminal 31 need not con- prise the shape of the characteristic shown in Figure 3 since it may be desired to have a center flat portion of this characteristic about the zero frequency difference axis such that for relatively small deviations between the local oscillator frequency and the received carrier signal frequency no variation in the audio gain is obtained. The characteristic in Figure 4 can be readily implemented through the use of either limiting the volt¬ age characteristic shown in Figure 3, or through the use of different types of full wave rectifiers comprising the rectifier 30. It should be noted that the structure of the full wave recitifer 30 is not illustrated since there exist many types of known full wave rectifiers which can receive the arithmetic difference control voltage characteristic shown in Figure 2 and provide the characteristics shown in either Figure 3 or Figure 4.

In summary, it should be noted that essentially the present invention provides for varying the audible signals produced by the radio receiver 10 such that by tuning the local oscillator 14 for maximum intensity of these audible signals, the mixer 13 will translate a selected received carrier signal to a frequency substan¬ tially at the center of the passband of the IF stage 16 such that the skirts (passband edges) of the IF filter stage 16 will not substantially destroy sideband infor- ation contained in the received carrier signal. The present invention accomplishes this by comparing the mixer output signal produced at the terminal 15 which has a frequency closest to the fixed IF center frequency with the actual fixed IF reference center frequency about which the passband for the IF filter stage 16 is designed. For an AM stereo system this is accomplished through the use of a phase locked loop which provides a variable DC control voltage related to the frequency difference between the actual IF signal produced by a received carrier signal at the terminal 17 and the

CMFI designed IF center frequency provided as the nominal output of the VCO 26. For FM systems, an FM discrimina¬ tor 35 can be utilized to provide a similar DC control signal. In both cases, comparing the difference between the actual IF signal frequency and the fixed IF center frequency provides a DC control voltage. Since the pre¬ ceding frequency difference is equal to the difference between the frequency of the selected RF carrier being received and the effective RF frequency to which the front end is tuned, this enables the precise tuning of the radio front end.

While I have shown and described specific embodi¬ ments of this invention, further- modifications and improvements will occur to those skilled in the art. One such modification would be to utilize the error voltage at the terminal 31 to control the intensity of a visual display. While controlling the intensity of an audible indication is preferable for the use of the ^" present invention in the automotive environment, since no visual distraction of the driver is required, cer¬ tainly the use of a visual display which varies in intensity with the magnitude of the error signal e_v is contemplated by the present invention. All such modifi¬ cations which retain the basic underlying principles disclosed and claimed herein are within the scope of this invention.

OHPI

Claims

1. A manual tuning circuit for enabling tuning to the exact frequency of a received signal, said manual tuning circuit comprising: manually tunable receiver front end means for receiving modulated carrier signals over a frequency band and selecting one of said received modulated carrier signals having a predetermined frequency in said band for further processing by tuning said front end means to a first frequency in said band, said first frequency being the center frequency of an effective narrow passband for said front end means; signal processing circuit means coupled to said front end means for receiving said selected one of said modulated carrier signals and providing audible signals related to the modulation of said selected one of said carrier signals; error signal means, including part of at least one of said front end and signal processing circuit means, for providing an error signal having a magnitude related to the absolute frequency difference between said first frequency and said predetermined frequency of said selected one of said carrier signals; and indicator means coupled to said error signal means for receiving said error signal and providing in response thereto a humanly perceptible indication which varies continuously in intensity in accordance with the magnitude of said error signal over at least one prede¬ termined range of error signal magnitudes, whereby the operator tuning said front end means can readily identify when the tuned first frequency of said receiver front end means substantially corresponds to the predetermined fre¬ quency of the selected received modulated carrier signal by tuning said receiver front end means for maximum or minimum intensity of said humanly perceptible indication.

2. A manual tuning circuit according to claim 1 wherein said front end means comprises a manually tun¬ able local oscillator, a mixer stage, and an IF stage, said mixer stage receiving an output signal of said local oscillator and receiving said modulated carrier signals in said band and providing a mixer output signal to the IF stage which is constructed to have a narrow passband tuned about a fixed IF reference center fre¬ quency and provides an IF output signal in response to said mixer output signal.

3. A manual tuning circuit according to claim 2 wherein said error signal means comprises circuitry for comparing the frequency of said IF output signal with the fixed IF reference center frequency and providing a variable DC output signal in accordance with the arith¬ metic frequency difference.

4. A manual tuning circuit according to claim 3 wherein said error signal means includes a full wave rectifier means for receiving said arithmetic difference signal and providing, in response thereto, an output signal having a magnitude related to the absolute fre¬ quency difference between the IF output signal and the fixed IF reference center frequency.

5. A manual tuning circuit according to claim 4 wherein said modulated carrier signals comprise FM carrier signals and wherein said error signal means circuitry comprises an FM discriminator.

6. A manual tuning circuit according to claim 4 wherein said modulated carrier signals comprise AM stereo carrier signals containing both left and right channel information, and wherein said error signal means

OMPI includes a phase locked loop which tracks the frequency of said IF output signal and provides, as the input to a VCO in the phase locked loop, said arithmetic difference signal.

7. A manual tuning circuit according to claim 4 wherein said error signal means includes a phase locked loop for tracking said IF output signal and having a frequency/phase comparator which receives said IF output signal, a lowpass filter, and a voltage controlled oscillator all arranged in a loop configuration, and wherein the output frequency of the voltage controlled oscillator (VCO) corresponds to the fixed IF reference center frequency in response to zero difference being sensed by said frequency/phase comparator, said loop providing, as a control input to said VCO, said arith¬ metic difference signal.

8. A manual tuning circuit according to any of the claims 1, 2, 3, 4, 5, 6 or 7 wherein said indicator means includes gain controlled amplifier means which receive said error signal and provide signal gain in accordance with the magnitude of said error signal.

9. A manual tuning circuit according to claim 8 wherein said gain control amplifier means comprises at least one audio amplifier stage amplifying demodulated signals obtained from said IF output signal, said aud¬ ible signals thereby being related to the modulation of the selected one of said carrier signals, the magnitude of said audible signals thereby being controlled in accordance with said absolute frequency difference, and maximum gain of said audio amplifier stage being provid¬ ed for zero frequency difference between said tuned first frequency and the predetermined frequency of the selected received modulated carrier signal, whereby an audible indication is provided which is continuously varied in intensity in accordance with the magnitude of the error signal to thereby indicate the degree of coincidence between the tuned first frequency and the selected carrier signal frequency.

OMPI