Method of Aligning a Radio Receiver and Means for Indicating the Alignment of a Radio Receiver

Home , Intermediate frequency, Radio receiver, Signal

Europâisches Patentamt

Office européen des brevets Bl

@ Date of publication of patent spécification: 07.1 0.81 (g) Int. Cl.3: H 04 B 1/26, H 03 J 3/14 @ Application number: 79300029.0

(g) Date offiling: 09.01 .79

(54) Method of aligning a radio receiver and means for indicating the alignment of a radio receiver.

© Priority: 09.02.78 US 876386 @ Proprietor: MOTOROLA, INC. 1303 East Algonquin Road Schaumburg Illinois 60196 (US) (43) Date of publication of application: 22.08.79 Bulletin 79/1 7 @ Inventor: Gould, Larrie Alan 5622 Woodhollow @ Publication of the grant of the European patent: Arlington Texas 7601 6 (US) 07.10.81 Bulletin 81/40 Inventor: Reed, Kim Warren 6036 East Belknap FortWorth Texas 761 17 (US) @ Designated Contracting States: CH DE FR GB NL @ Représentative: Newens, Léonard Eric et aJ, F.J. CLEVELAND CO. 40/43 Chancery Lane © Références cited: London WC2A 1 JQ (GB) DE-A-2 624 787 DE-B-1 034 716 GB-A-559 555 US - A - 3 896 386

O Note: Within nine months from the publication of the mention of the grant of the European patent, any person may give notice to the European Patent Office of opposition to the European patent granted. Notice of opposition shall be filed in a written reasoned statement. It shall not be deemed to have been filed until the opposition fee has been m paid. (Art. 99(1 ) European patent convention). Courier Press, Leamington Spa, England. Background of the Invention Briefly, according to the invention, an FM The present invention pertains to the radio radio frequency receiver having a tuned receiver art and, more particularly, to a means intermediate frequency (IF) stage followed by a for, and a method of aligning a radio receiver. detector stage may be aligned by first The radio receiver art has developed several alternately applying a noise signal and then a techniques for the aligning of radio receivers. predetermined frequency signal to the IF stage. Alignment is a process whereby various stages The receiver is then adjusted such that the of the receiver are properly tuned to the desired output of the detector stage reponsive to the frequency. For example, when a modulated applied noise signal bears a predetermined signal of predetermined frequency is applied to relationship to the detector stage output the radio carrier frequency input stage of a responsive to the applied predetermined receiver, the local oscillator stage, tuned frequency signal. Preferably, the frequency of intermediate frequency stage, and detector the receiver's local oscillator is adjusted until stage must all be adjusted to maximally the voltage at the detector output is the same reproduce the signal at the detector (or whether the noise signal or the predetermined demodulator in FM radio receiver) output. frequency signal is applied. A typical alignment procedure known in the The means for indicating receiver alignment prior art is as follows. A predetermined includes a signal generator for generating a frequency signal is applied at the radio predetermined frequency alignment signal, a frequency input, or antenna connection, of the noise source for generating a random noise receiver being aligned. The local oscillator is signal and a switching means which alternately adjusted such that the output from the mixer is couples the predetermined frequency alignment the input signal converted to the receivers signal and the noise signal to the receiver's intermediate frequency which, in a common FM intermediate frequency stage. A comparator radio is 10.7 MHz. The intermediate frequency compares the output of the detector stage stage is then adjusted such that the alignment responsive to the predetermined frequency signal is directly in the centre of its passband. alignment signal being coupled to the IF stage For receivers employing a fixed tuned IF stage, with the output of the detector responsive to however, the receiver may be misaligned due to the noise signal being applied to the IF stage. the center band of the filter being at other than An error signal is produced indicative of the the derived intermediate frequency. This results tuning of the receiver stage with respect to the in misalignment. Finally, a DC tuning meter is predetermined frequency alignment signal. connected to the output from the detector stage Preferably the comparator comprises a and the detector is adjusted such that the capacitor which has one terminal coupled to the output reading is zero volts DC. Also, due to output of the detector and the other terminal component value drifts caused by aging and coupled through a meter to reference, or ground thermal effects, a reading of zero volts DC may, potential. The switching means further includes in fact, correspond to a misaligned receiver. a means which short circuits the meter during The above described prior art alignment the time that one of the predetermined procedure suffers from numerous dis- frequency alignment signal and noise signal is advantages. Firstly, it is a complicated and applied to the IF stage. In this manner, the tedious procedure, not generally suited to fluctuation of the meter is indicative of the efficient manufacturing operation. Also, for difference between the magnitudes of the receivers employing fixed intermediate detector stage outputs corresponding to the frequency tuned stages, the above procedure predetermined frequency alignment signal and does not provide optimum alignment. In the noise signal being coupled to the IF stage, addition, it is desirable for purposes such as and, thus, is indicative of the relative alignment field alignment to use a zero centered DC tuning of the radio receiver. meter. For radio designs utilizing a quadrature detector, an output non-zero DC signal appears Brief Description of the Drawings at the detector when the detector is properly Fig. 1 is a block diagram illustrating the basic aligned. This fact precludes the use of zero structure and operation of the preferred centered DC tuning indicators when used in the embodiment of the invention; and prior art aligning procedure. Fig. 2 shows a series of waveforms representative of operation of the preferred Summary of the Invention embodiment shown in Fig. 1. It is an object of this invention, therefore, to provide a simple, efficient and precise method Detailed Description of the Preferred Embodi- for the aligning of a radio receiver. ment of the Invention It is a further object of the invention to Fig. 1 illustrates, in block diagram form, a provide an improved means for indicating the conventional FM radio receiver, indicated relative degree of receiver alignment. generally at 10 in combination with the preferred embodiment of the alignment in- operation of the means for indicating receiver dicator. The receiver is comprised of a alignment, may be understood as follows. A conventional RF input stage 12 adapted for signal source 80 is coupled to the input of the receiving signals from an antenna 11. The RF stage 12. The signal source 80 is adjusted to output from the RF stage 12 feeds to the first generate a predetermined frequency alignment input 14a of a conventional mixer circuit 14. carrier. The alignment carrier signal is con- Applied to the second input 14b of mixer stage verted, via the operation of mixer 14 and local 14 is the output from the local oscillator stage oscillator 16, to the intermediate frequency 16. Local oscillator stage 16 is of conventional whereby the converted signal appears at design, and is tunable over a defined range the mixer output 14c. In its first mode suitable for converting the signal received at the of operation, that is when its contacts mixer first input 14a to the intermediate are shorted together, switch 42 couples the frequency of the receiver. converted alignment signal through the IF stage The converted output appearing at the mixer 20 and detector stage 24 whereby the output output 14c is coupled through switch 22 to the from the detector responsive to the applied input 20a of a conventional tuned intermediate alignment signal appears at the detector output frequency stage (IF) 20. Intermediate frequency 24b. When switch 42 is in its second mode of stage 20 is of standard design employing operation, i.e. its contacts being open circuited, circuits having tuned characteristics about a a random, white noise signal appears at the center frequency f, In a typical _FM radio input 20a of the IF stage 20. It is commonly receiver, fc is selected to be 10.7 MHz. The known to one of ordinary skill in the art that the filtered, amplified and limited signal from IF input 20a of IF stage 20 will contain white noise stage 20 appears at the stages output 20b and when open circuited. An alternate method of passes to the input 24a of a conventional applying white noise to the IF stage would be to detector stage 24. Acting in the known manner, inhibit operation of the local oscillator stage 16 the detector 24 which produces an output or to open circuit the RF stage 12 input. A signal representative of the frequency change of further way to apply white noise to the IF stage signals applied to it, recovers the information; 20 would be to switch the IF stage input 20a e.g. audio signal, contained in the Input RF directly to a white noise source, such sources signal, reproducing this information signal at its being commercially available and shown by output terminal 24b. white noise source 15 in Fig. 1. The alignment indicating circuitry, according During the period that switch 42 is in its to the preferred embodiment of the invention, second mode and white noise is applied through includes switching means, indicated generally the IF stage 20 and detector stage 24, switch at 40, which includes a first switch 42, coupled 44 is closed thereby shorting out the tuning between the mixer output 14c and the IF input meter 60. Thus, in this mode, capacitor 50 20a, a second switch 44 and a switching integrates the processed noise signal through control oscillator 46. Each switch 42, 44 is of the IF 20 and detector stage 24. Assuming a , the single pole-single throw type and responds symmetric detector stage 24, the net, to control signals generated by switching integrated voltage appearing at the detector control oscillator 46 to appropriately either output 24b during this second mode operation short, or open its contacts. Thus, the switches of switch 42 charges capacitor 50 to a voltage 42, 44 may be comprised of relays or, Vn. This voltage is directly representative of the preferably, suitable semiconductor devices such center frequency of the IF stage 20. as bipolar transistors. In this, the preferred With switch 42 in its first mode, the embodiment of the invention, the two switches converted alignment signal is applied through 42, 44 are connected 180° out of phase, such the IF stage 20 and detector stage 24 thereby that when one switch is open, the other is producing an output signal VS. This voltage closed. appears directly across both the capacitor 50 A series connected integrating capacitor and tuning meter 60 since, in this mode, switch 50 and tuning meter 60 are coupled between 44 is open circuited. Thus, the voltage the detector output 24b and reference, or appearing across tuning meter 60 is equal to ground potential 70. As will be understood VS-Vn which is a different error, voltag more fully hereinbelow, capacitor 50 is of representative of the difference between the selected value to integrate the signals outputs of detector 24 resulting from appli- alternately applied to it as a result of the cation of the white noise signal and the switching means 40. predetermined frequency alignment signal. Tuning meter 60 is of conventional design, Since the integrated noise signal Vn reading centre scale with zero volts DC applied is representative of the center frequency to it. In this, the preferred embodiment of the of the IF 20 and detector 24, deviations invention, tuning meter 60 is selected to be of the alignment signal from the centre deflected fuil scale, in either direction, with an frequency of IF 20 and detector 24 will applied current of 25 microamps. produce an output signal Vs differing in The method of aligning a radio receiver magnitude from that of the noise signal Vn. according to the instant invention, and Thus, meter 60 will deflect to indicate magnitude of misalignment of the receiver as being applied to the IF stage, it should be well as an indication of whether the receiver is apparent that an alignment signal could be tuned above or below the alignment signal. applied directly to the IF input and the receiver Thus, the receiver may be aligned as, for aligned accordingly. example, by adjusting local oscillator 16 such that the reading on meter 60 is a zero volts DC. Fig. 2 illustrates representative waveforms appearing at the output 24b of detector 24 1. A method of aligning a radio receiver (10) during operation of the alignment method. Here, having a tuned intermediate frequency (IF) the time T1 is representative of the time during stage (20) followed by a detector stage (24), which which the first switch 42 is in its first mode or detector stage produces an output closed state of operation (6 milliseconds in the representative of the change in frequency of a preferred embodiment of the invention) with the signal applied at its input, characterised in that time t2 representative of switch 42 being in its the method comprises the steps of: second mode or open state of operation (2 milli- a) alternately applying a white noise signal and seconds in the preferred embodiment of the a predetermined frequency alignment signal invention). Thus, referring to Fig. 2a, which is (80) to said IF stage; and representative of the alignment frequency signal b) aligning said receiver such that the output of being at a frequency below the center of the said detector stage (24b) responsive to the IF 20 passband, during the time t1 capacitor 50 applied white noise signal bears a is charged to the voltage Vg. Then, during time predetermined relationship to said detector interval t2, white noise is fed through IF 20 and stage output (24b) responsive to the applied the meter circuit 60 is shorted. This produces a predetermined frequency alignment signal. noise voltage, which is centered about V". The 2. The method of claim 1 wherein said ratio difference between the two signals Vs-Vn is a receiver (10) has a mixer stage (14) feeding said net negative voltage which drives meter 60 of IF stage (20) said mixer stage having a signal Fig. 1 in the negative direction. input (14a) for receiving said predetermined In Fig. 2b, the alignment signal is at a frequency alignment signal and a local oscillator frequency above the center passband of IF 20 input (14b) for receiving a local oscillator signal and detector 24. Here, the net error signal (16), said local oscillator signal (16) being Vs-Vn is positive, which drives meter 60 to a tunable over a predetermined frequency range positive reading. The capacitor 50 and the to convert signals received at said mixer signal meter 60 comprise a comparator means for input to the frequency of said IF stage; and comparing the output of the detector 24 under characterized in that step a) further comprises the two conditions of operation. the steps of: By adjusting the duty cycle of time periods t1 1) providing a first switch means (42) which and t, the reading of meter 60 may be responds to a received control signal to calibrated in terms of frequency of couple the converted output of said mixer misalignment, in hertz per unit of voltage error. (14) to said IF stage (20) in a first mode and In addition, by proper selection of the to couple white noise to the input of said IF capacitor 50, the sampling rate, the duty cycle stage in a second mode; and and the mechanical dampening of the meter 60, 2) providing a controlling oscillator means (46) the meter assumes a stable reading indicative of to predeterminedly activate said first receiver alignment. switch means (42) between its first and In summary, the method of aligning a radio second modes. receiver, according to the instant invention, may 3.The method of claim 2 characterized in that be efficiently performed while maintaining a step b) further comprises the steps of: high degree of precision. 1) comparing the detector stage output (24b) The means for indicating the degree of responsive to said first switch means (42) receiver misalignment, according to the being in its first mode to the detector stage invention is precise, and inexpensive to output (24b) responsive to said first switch implement. Further, for receivers employing means being in its second mode; and detectors which inherently have a DC offset at 2) varying the tuning of said local oscillator (16) the output 24b, such as quadrature detectors, to establish a predetermined relationship the instant indicator means allows a zero center between said compared outputs. DC meter to be employed due to the DC 4. The method of claim 3 characterized in blocking action of capacitor 50. that step b) 1) further comprises: While a preferred embodiment of the i) providing a predetermined value capacitor invention has been described in detail, it should (50); be apparent that many modifications and ii) coupling one terminal of said capacitor to variations thereto are possible, all of which fall the output of said detector stage (24b); within scope of the invention. iii) providing a meter means (60): For example, whereas in the instant iv) coupling said meter means between the embodiment of the invention an alignment free end of said capacitor and reference signal was converted through the mixer before potential (70); and switch (44), v) providing a second means que le procédé comprend les opérations responsive to said controlling oscillator (46), consistant à: to apply a short circuit across said meter (60) a) alternativement appliquer un signal de bruit in response to said first switch means (42) blanc en un signal d'alignement de fréquence being in its second mode, whereby the signal prédéterminée (80) audit étage FI; et said (60) is indicative of measured by meter b) aligner ledit récepteur de façon que le signal the relative alignment of said radio receiver de sortie dudit étage démodulateur (24b) qui (10). répond au signal de bruit blanc appliqué relative 5. Indicator means for indicating the présente une relation prédéterminée avec le alignment of a radio receiver (10), having a signal de sortie dudit étage démodulateur tuned intermediate frequency (IF) stage (20) (24b) qui répond au signal d'alignement de detector which followed by a stage (24), fréquence prédéterminée appliqué. detector stage produces an output (24b) 2. Procédé selon la revendication 1, où ledit of the in of representative change frequency a récepteur de radiodiffusion (10) possède un characterised signal applied at its input (24a), étage mélangeur (14) alimenant ledit étage FI by: (20), ledit étage mélangeur ayant une entrée signal generator means (80) for generating a (14a) de signal qui reçoit ledit signal predetermined frequency alignment signal and a d'alignement de fréquence prédéterminée, et for noise source means (15) generating a une entrée (14b) d'oscillateur local qui reçoit un random noise signal; signal d'oscillateur local (16), ledit signal for switching means (42) alternately d'oscillateur local (16) pouvant être accordé sur coupling, at a predetermined rate, the une gamme de fréquence prédéterminée de (80) predetermined frequency alignment signal manière . à transformer des signaux reçus à and the noise signal to said IF stage; and ladite entrée de signal du mélangeur en la for the comparator means (50) comparing fréquence dudit étage FI; et caractérisé en ce output of said detector stage (24b) responsive que l'opération a) comprend en outre les to the predetermined frequency alignment opérations consistant à: the IF with the signal being coupled to stage 1) prévoir un premier moyen de commutation output of said detector stage (24b) responsive (42) qui répond à un signal de commande the noise said IF to signal being applied to stage reçu en couplant le signal de sortie and producing an error signal (60) indicative of transformé dudit mélangeur (14) audit étage with said the tuning of said IF stage respect to FI (20) dans un premier mode et en couplant predetermined frequency alignment signal. un bruit blanc à l'entrée dudit étage FI dans means 6. The indicator of claim 5 un deuxième mode; et in that the characterized comparator means 2) prévoir un moyen oscillateur de commande comprises: (46) afin d'actionner de façon prédéterminée of capacitor means (50), predetermined ledit premier moyen commutateur (42) entre value, having one terminal coupled to the son premier et son deuxième mode. output of said detector (24b); 3. Procédé selon la revendication 2, means for the meter (60), indicating caractérisé en ce que l'opération b) comprend magnitude of said error signal, coupled between en outre les opérations consistant à: the free end of said capacitor and a reference 1) comparer le signal de sortie de l'etage and potential; démodulateur.(24b) qui répond au fait que le further wherein said switching means premier moyen commutateur (42) est dans comprises: son premier mode avec le signal de sortie de means for short circuiting (44) said meter l'etage démodulateur (24b) qui répond au fait time that of said means (60) during the one que le premier moyen commutateur est dans predetermined frequency alignment signal and son deuxième mode; et noise signal is coupled to said IF stage, 2) faire varier l'accord dudit oscillateur local whereby the deflection of said meter is (16) afin d'établir une relation prédéterminée indicative of the difference between the entre lesdits signaux de sortie comparés. magnitudes of the detector stage outputs 4. Procédé selon la revendication 3, corresponding to the predetermined frequency caractérisé en ce que l'opération b) 1) comprend and the noise alignment signal signal being en outre le fait de: coupled to said IF stage. i) prévoir un condensateur (50) de valeur prédéterminée; ii) coupler une borne dudit condensateur à la sortie dudit étage démodulateur (24b); 1. Procédé d'un de d'alignement récepteur iii) prévoir un moyen indicateur (60); radio-diffusion (10) un (FI) de possédant étage iv) coupler ledit moyen indicateur entre fréquence intermédiaire accordé (20) suivi par l'extrémité libre du condensateur et un un étage démodulateur (24), lequel étage potentiel de référence (70); et de sortie démodulateur produit un signal v) prévoir un deuxième commutateur d'un moyen représentatif du changement de fréquence (44), qui répond audit oscillateur de signal appliqué à son entrée, caractérisé en ce commande (46) en appliquant un court- circuit aux bornes dudit indicateur (60) en ein Ausgangssignal erzeugt, das repräsentativ réponse au fait que le premier moyen für die Änderung in der Frequenz eines an ihren commutateur (42) est dans son deuxième Eingang angelegten Signals ist, dadurch mode, si bien que le signal mesuré par ledit gekennzeichnet, daß das Verfahren die Schritte indicateur (60) est indicatif de l'alignement aufweist: relatif dudit récepteur de radiodiffusion (10). a) Abwechselndes Anlegen eines weißen 5. Moyen indicateur permettant d'indiquer Geräuschsignals und eines vorbestimmten l'alignement relatif d'un récepteur de Frequenzabgleichsignals (80) an die ge- radiodiffusion (10), possédant un étage (FI) de nannte IF-Stufe und fréquence intermédiaire accordé (20) suivi par b) Abgleichen des Empfängers derart, daß der un étage demodulateur (24), lequel étage Ausgang (24b) der genannten Detektorstufe démodulateur produit un signal de sortie (24b) bei Anlegen des weißen Geräuschsignals in représentatif du changement de fréquence d'un einer vorbestimmten Beziehung steht zu dem signal appliqué à son entrée (24a), caractérisé genannten Detektorstuffenausgangssignal par: (24b) bei Anlegen des vorbestimmten Fre- un moyen (80) générateur de signal qui quenzabgleichsignals. produit un signal d'alignement de fréquence 2. Verfahren nach Anspruch 1, wobei der ge- prédéterminée et un moyen (15) faisant nannte Rundfunkempfänger (10) auch eine fonction de sortie de bruit qui produit un signal Mischstufe (14) besitzt, die die genannte IF- de bruit aléatoire; Stufe (20) speist, wobei die genannte Misch- un moyen de commutation (42) qui couple stufe einen Signaleingang (14a) zum Empfang alternativement, à un rythme prédéterminé, le des genannten Abgleichsignals mit vorbe- signal (80) d'alignement de fréquence stimmter Frequenz und einen Ortsoszill- prédéterminé et le signal de bruit à l'etage FI; et atoreingang (14b) zum Empfang eines un moyen comparateur (50) qui compare le Ortsoszillatorsignals (16) besitzt, wobei das ge- signal de sortie de l'étage démodulateur (24b) nannte Ortsoszillatorsignal (16) über einem répondant au signal d'alignement de fréquence vorbestimmten Frequenzbereich äbstimmbar prédéterminée couplé à l'etage FI avec le signal ist, um am genannten Mischersignaleingang de sortie de l'étage démodulateur (24b) empfangene Signale auf die Frequenz der ge- répondant au signal de bruit appliqué audit nannten IF-Stufe umzuwandeln; und dadurch étage FI et qui produit un signal d'erreur (60) gekennzeichnet, daß Schritt a) ferner die indicatif de l'accord dudit étage FI relativement Schritte umfaßt: audit signal d'alignement de fréquence 1) Anorden eines ersten Schaltermittels (42), prédéterminée. das auf ein empfangenes Steurersignal an- 6. Moyen indicateur selon la revendication 5, spricht, um das umgewandelte Ausgangssig- nal des Mischers in caractérisé en ce que le moyen comparateur genannten (14) einer comprend: ersten Betriebsart an die genannte IF-Stufe in un moyen faisant fonction de condensateur (20) anzukoppeln und einer zweiten Be- (50), de valeur prédéterminée dont une borne triebsart weißes Geräusch an den Eingang est connectée à la sortie dudit démodulateur der genannten IF-Stufe anzukoppeln, und (24b); 2) Anordnen eines Steueroszillatormittels (46) un moyen indicateur (60), destiné à indiquer zur vorbestimmten Aktivierung des ge- l'amplitude dudit signal d'erreur, couplé entre nannten ersten Schaltermittels (42) zwischen l'extrémité libre dudit condensateur et un seiner ersten und zweiten Betriebsart. potentiel de référence; et 3. Verfahren nach Anspruch 2, dadurch daß Schritt où ledit moyen de commutation comprend gekennzeichnet, b) ferner die en outre: Schritte umfaßt: un moyen permettant de mettre en court- 1 ) Vergleichen des Detektorstufenausgangs- circuit (44) ledit moyen indicateur (60) pendant signals (24b) bei Ansprechen auf das sich in que l'un des signaux, à savoir ledit signal seiner ersten Betriebsart befindliche ge- d'alignement de fréquence prédeterminée et nannte erste Schaltermittel (42) mit dem ledit signal de bruit, est couplé audit étage FI, si Detektorstufenausgangssignal (24b) bei An- das sich in seiner zweiten bien que la déviation dudit indicateur est sprechen auf Be- indicative de la différence existant entre les triebsart befindliche genannte erste Schalter- amplitudes des signaux de sortie de l'étage mittel, und démodulateur correspondant au signal 2) Variieren der Abstimmung des genannten d'alignment de fréquence prédéterminée et au Ortsoszillators (16), um eine vorbestimmte signal de bruit qui sont couplés audit étage FI. Beziehung zwischen den genannten verg- lichenen Ausgangssignalen herzustellen. 4. Verfahren nach Anspruch 3, dadurch gekennzeichnet, daß Schritt b) 1) ferner die 1. Verfahren zum Gesamtabgleich eines Rund- Schritte umfaßt: funkempfängers (10) mit einer abgestimmten i. Anordnen eines Kondensators (50) mit Zwischenfrequenz-(IF)- Stufe (20), der eine einem vorbestimmten Wert, Detektorstufe, (24) folgt, welche Detektorstufe ii. Koppeln einer Klemme des genannten Kondensators mit dem Ausgang der ge- Ausgangssignals (24b) der genannten Detektor- nannten Detektorstufe (24b); stufe bei Ansprechen auf das an die IF-Stufe an- iii. Anordnen eines Meßmittels (60); gekoppelte Abgleichsignal mit vorbestimmter iv. Ankoppeln des genannten Meßmittels Frequenz mit dem Ausgangssignal (24b) der ge- zwischen das freie Ende des genannten Kon- nannten Detektorstufe bei Ansprechen auf das densators und Bezugspotential (70); und an die genannte IF-Stufe angelegte Geräusch- v. Anordnen eines zweiten Schaltermittels signal und Erzeugen eines Fehlersignals (60), (44), das auf den genannten Steueroszillator das die Abstimmung der genannten IF-Stufe (46) anspricht, um einen Kurzschluß über das bezüglich des Abgleichsignals mit vorbe- genannte Meßmittel (60) bei Ansprechen auf stimmter Frequenz anzeigt. das sich in seiner zweiten Betriebsart befind- 6. Anzeigemittel nach Anspruch 5, dadurch liche genannte erste Schaltermittel (42) gekennzeichnet, daß die Vergleichsmittel auf- anzulegen, wodurch das durch das genannte weisen: Meßmittel (60) gemessene Signal den rela- Kondensatormittel (50) von vorbestimmten tiven Abgleich des genannten Rund- Wert mit einer Klemme, die mit dem Ausgang funkempfängers (10) anzeigt. des genannten Detektors (24b) gekoppelt ist; 5. Anzeigemittel zum Anzeigen des relativen Meßmittel (60) zur Anzeige der Größe des Abgleichs eines Rundfunkempfängers (10) mit genannten Fehlersignals, die zwischen das freie einer abgestimmten Zwischenfrequenz-(IF)- Ende des genannten Kondensators und ein Be- Stufe (20) gefolgt von einer Detektorstufe (24), zugspotential gekoppelt sind; welche Detektorstufe ein Ausgangssignal (24b) und wobei die genannten Schaltermittel erzeugt, das repräsentativ ist für die Änderung ferner aufweisen: in der Frequenz eines an ihren Eingang (24a) an- Mittel zum Kurzschließen (44) der genannten gelegten Signals gekennzeichnet durch Meßmittel (60) während derjenigen Zeit, Signalgeneratormittel (80) zum Erzeugen während der das genannte Abgleichsignal mit eines Abgleichsignals mit vorbestimmter Fre- vorbestimmter Frequenz oder das Geräuschsig- quenz und ein Geräuschquellenmittel (15) zum nal an die genannte IF-Stufe angekoppelt sind, Erzeugen eines willkürlichen Geräuschsignals; wodurch der Ausschlag des genannten Meß- Schaltmittel (42) zum abwechselnden mittels die Differenz zwischen den Größen der Koppeln mit einer vorbestimmten Rate, des Detektorstufenausgangssignale anzeigt, die Abgleichsignals (80) mit vorbestimmter Fre- dem an die genannte IF-Stufe angekoppelten quenz und des Geräuschsignals an die genannte Abgfleichsighal mit vorbestimmter Frequenz IF-Stufe; und und Geräuschsignal entsprechen. Vergleichermittel (50) zum Vergleichen des