GB2100084A - Automatic station select apparatus for an FM tuner - Google Patents

Abstract

Conventional automatic station selection apparatus for an FM tuner are liable to malfunctions caused by temperature distortion of the characteristics of electronic components. With a view to mitigating this disadvantage, an automatic station selection apparatus for an FM tuner is provided which includes reference frequency signal generator (46) which supplies a reference frequency equal to the IF frequency to the IF stage (16) of the FM tuner during a short time period upon appearance of the automatic station select indication. An output voltage from the FM discriminator (18) receiving the reference frequency signal is stored at C1 before initiation of the station select operation. When, during the station select operation, the output voltage from the FM discriminator (18) reaches the stored voltage then the station select operation is ceased, whereby correct station selection operation is performed notwithstanding the distortion in characteristics of the FM discriminator (18). <IMAGE>

Description

SPECIFICATION Automatic station select apparatus for FM tuner The present invention relates to an FM tuner and in particular to an automatic station select apparatus for an FM tuner.

Various automatic station select apparatus for an FM tuner have developed, one of which is shown in Japanese Utility Model Provisional Publication No. 57-34727 (Japanese Utility Model Application No. 55-109930) published on February 24, 1 982. Such automatic station select apparatus includes tuning frequency control means connected to the front-end stage of an FM tuner, for gradually changing the tuning frequency of the frontend stage by changing the local frequency of the front-end stage in response to an automatic station select indication signal produced from a key board manually operated. The tuning frequency control means continues its operation until the intensity of an output signal from the IF stage exceeds a predetermined level.

Another type of automatic station select apparatus has been proposed which is adapted to watch not only the intensity of the IF signal but also the intensity of an output signal of the FM discriminator of the tuner during the tuning frequency changing operation (search or scan operation) and to cease the tuning frequency changing operation when the IF level exceeds a predetermined level and the output level of the FM discriminator falls within a predetermined range. This type of automatic station select apparatus is more advantageous in operation than that disclosed in the Japanese Utility Model Provisional Publication No. 57-34727 since the former can operate more correctly than the latter.

However, it is still a problem that even the latter type of automatic station select apparatus is subject to maifunction under large variation of the ambient temperature of the temperature of electronic elements used therein because of temperature distortion in characteristics of the electronic elements.

The present invention is intended to mitigate such a drawback and inconvenience as mentioned above of the prior or proposed automatic station select apparatus.

It is, therefore, a primary object to provide a new and improved automatic station select apparatus to be incorporated in an FM tuner which can correctly select a broadcast station even in the face of variations or distortions in characteristics of electronic elements therein due to temperature variation of the electronic elements.

According to the present invention there is provided an automatic station select apparatus for an FM tuner including a front-end stage for converting a received broadcast wave signal into a signal containing an IF frequency signal component while changing its tuning frequency in accordance with a tuning frequency control signal, an IF stage for producing an amplified IF frequency signal in response to the signal from said front-end stage, and an FM discriminator for discriminating the amplified IF frequency signal, the automatic station select apparatus including control means for producing the tuning frequency control signal in response to an automatic station select start signal until a stop signal, field strength maximum signal producing means connected to the IF stage, for producing a field strength maximum signal when the intensity of the amplified IF frequency signal exceeds a predetermined level, the automatic station select apparatus further comprises: timing signal producing means for producing a timing signal lasting during a predetermined time period from when the automatic station select start signal is produced; reference signal supply means for supplying a reference frequency signal to the IF stage during the timing signal; storing means for storing the signal level of the output signal produced from the FM discriminator during the timing signal; comparing means for producing a detect signal when difference in level between the output signal from the FM discriminator and the stored signal level reaches a predetermined value after the decay of the timing signal; and stop means for producing the stop signal in response to the field strength maximum signal and the detect signal.

The features and advantages of an automatic station select apparatus according to the present invention will be more clearly appreciated from the following description taken in conjunction with the accompanying drawings in which like reference numerals disignate similar or corresponding units, members and elements in some figures and in which: Figure 1 is a block diagram showing a prior automatic station select apparatus; Figure 2A through 5 are graphs showing various wave forms of signals appearing in the apparatus of Fig. 1; Figure 6 is a block diagram showing a preferred embodiment of the present invention; and Figures 7A through 7E showing waveforms of some signals appearing in the apparatus of Fig. 6.

Referring to Fig. 1 there is shown a prior or proposed automatic station select apparatus which is incorporated into an FM tuner. The FM tuner is comprised of a front-end stage 1 O which receives broadcast wave signals through an antenna 1 2. As is well known in the art, the front-end stage includes a local oscirator (now shown) for producing a local frequency and a mixer for mixing the local frequency with the received broadcast wave signal so as to produce a mixture signal containing an IF frequency component with an IF frequency of, for example, 10.7 MHz.

The local oscillator is adapted to vary its frequency in accordance with a tuning frequency control signal. The local oscillator may be a voltage-controlled oscillator (VCO). The mixture signal is applied to an IF filter 14 which passes therethrough only the IF component. The IF component passed through the IF filter 14 is supplied to an IF amplifier 16.

The IF component thus amplified is supplied to an FM discriminator 18 which produces a voltage signal representative of deviations in frequency of the IF component from the IF frequency. The voltage signal is supplied to a demodulator (not shown) by way of an output terminal 20.

The automatic station select apparatus includes a controller 22 which produces the tuning frequency control signal in response to an automatic station select indication signal produced from a key board 24. The controller 22 changes the intensity of the tuning frequency control signal in accordance with an output frequency from the local oscillator (indicated as VCO OUTPUT in Fig. 1) so as to set the local frequency at a desired value thereby performing scan or search operation.

The controller 22 is adapted to produce a muting signal during the scan or search operation in order to avoid noise signal to be reproduced.

The field strength maximum signal producing means is composed of an IF level detector 26 which produces a voltage signal representative of the intensity of the amplified IF component and a voltage comparator 28 which produces the field strength maximum signal when the voltage signal from the IF level detector exceeds a predetermined reference voltage E,.

A low pass filter 30 is provided which produces a DC signal having a DC voltage representative of the intensity of the output signal from the FM discriminator. A DC level detector 32 is provided which produces a detect signal when the DC voltage from the low pass filter 30 falls within a predetermined range. The DC level detector 32 may be a window-comparator. The field strength maximum signal and the detect signal are applied to an AND gate 34 which produces a stop signal when it receives both of the field strength maximum signal and the detect signal. When the stop signal is supplied to the controller 22, the controller 22 ceases its scan or search operation thereby to select a broadcast station.

When, in operation, the output signal level of the low pass filter 30 varies as shown by a curve A in Fig. 2A in terms of frequency deviation from the IF frequency f0 in accordance with variation in frequency of the received broadcast wave signal. The curve A is called as S curve. On the other hand, the output signal of the IF level detector 26 varies in voltage as shown by curve B in Fig. 2B in terms of frequency deviation from the IF frequency in accordance with variation in frequency of the received broadcast wave signal.

Therefore, the voltage comparator 28 produces a logic "1" signal when the output signal from the IF level detector 26 exceeds the voltage E1 as shown by a curve C in Fig.

2C. The logic "1" signal from the voltage comparator 28 is the field strength maximum signal mentioned above. The DC level detector 32 produces a logic "1" signal when the level of the output signal of the low pass filter 30 falls within a range from + AV to - AV as shown by a curve D in Fig. 2D. The logic "1" signal from the DC level detector 32 is the aforementioned detect signal. When the AND gate 34 receives both of the field strength maximum signal and the detect signal, it produces the stop signal as shown by a curve E in Fig. 2E.

Since, as being well known in the art, distance in frequency between the adjacent two broadcast wave carriers is 50KHz at minimum, the automatic station select apparatus must distinguish a frequency distance of 50KHz. Thus, the stop signal should appear in a range of f + 50KHz. It is, however, to be noted that the FM discriminator 1 8 is subject to an inherent frequency deviation fe of its actual central frequency from the IF frequency due to error in the initial adjustment. The FM discriminator is further subject to frequency deviation fd of its actual central frequency from the IF frequency due to temperature drift.Thus, when the width in frequency of the stop signal is represented by f,, the following relations should exist: /f=5OKHz2fe+fd+fs (1) 52fd + fe (2) The above relations will be more clearly undetstood while taken in conjunction with Fig.

3.

If, in this instance, the frequency deviation fe is assumed to be 2KHz, then the following relations are obtained: fd+f5O2 + so - 2 = 48KHz (3) f,-f,L2KHz (4) If the frequency width fs is assumed to be 24 Hz, the frequency deviation fd is equal to or smaller than 24KHz (f,24KHz) in view of the relation (3).

The frequency width f, of the stop signal per se is subject to deviation of about 5KHz due to temperature characteristics of electronic elements or the like and therefore the frequency deviation fd is to be equal to or smaller than 19KHz (fd'19KHz).

It is now to be noted that the FM discrimi nator contains resonance circuit made of coil and capacitor which circuit is subject to a temperature drift of about 100 ppm/ C (1.07 KHz/ C). The FM discriminator is actually subject to humidity drift and accordingly it is impossible to keep the frequency deviation fd within the above-mentioned range.

When, as shown by curves X, Y and Z in Fig. 4A, the characteristics of the FM discriminator 1 8 varies due to temperature variation, the detect signal appears as indicated by pulses D and D' in Fig. 4B, respectively. As seen from Figs. 4A and 4B, no output voltage is produced from the DC level detector 32 when the characteristics of the FM descriminator 1 8 is distorted as shown by the curve Y.

Thus, the absolute values of the + AV and - AV are, in practice, made relatively large as shown in Fig. 5A, and the DC level detector can produce the detect signal even in the case shown by curves Y as shown by hatchings D", whereby the stop signal can be produced even in the case of Y. However, it is a problem that the automatic station select apparatus of Fig. 1 may select the adjacent station of a station to be select.

In order to solve the above mentioned problem, the present invention provides an improved automatic.station select apparatus a preferred embodiment of which is shown in Fig. 6.

The automatic station select apparatus of Fig. 6 includes a sample hold circuit 40 which has an input buffer amplifier OP1, a gate G, a hold capacitor C, and an output buffer amplifier OP2. The sample hold circuit 40 is adapted to hold a voltage supplied thereto from the low pass filter 30 when the gate G, is triggered by a timing signal supplied from a timing signal generator 42. The timing signal generator 42 may be a monostable multivibrator which is triggered by the leading edge of the muting signal from the controller 22. A comparator 44 compares the output voltage of the sample hold circuit 40 and a voltage from the low pass filter 30 and produces a detect signal when difference in voltage between the voltages supplied thereto reaches a predetermined value such as zero.A reference frequency signal generator 46 is provided which produces a reference frequency signal having a reference frequency equal to the IF frequency during a period when the timing signal lasts. The reference frequency signal is supplied to the input terminal of the IF amplifier 1 6. Other protions or elements of the apparatus of Fig. 6 is identically the same as that of Fig. 1.

When, in operation, the automatic station select indication is applied from the key board 24 to the controller 22, the controller 22 first produces the muting signal which lasts until the stop signal is produced from the AND gate 34, as shown in Fig. 7A. The timing signal generator 42 produces the timing signal lasting during a first predetermined time period of, for example, 35 ms (milisecond) in response to the muting signal. The timing signal is applied to the gate G, of the samplehold circuit 40 which then holds a voltage level of the output signal from the low pass filter 30.The timing signal is further applied to the reference frequency signal generator 43 which then applies such reference frequency as shown in Fig. 7C to the input terminal of the IF amplifier 1 6. The reference frequency signal is amplified by the IF amplifier 1 6 and thereafter supplied to the FM discriminator 1 8 which produces a voltage representative of the frequency deviation of the input frequency signal applied thereto. When the FM discriminator 1 8 is not suffered from the temperature drift and correctly operates, the output voltage of the FM discriminator 1 8 is zero.If, however, the FM discriminator 1 8 is adversely affected by the temperature drift etc., the FM discriminator 1 8 produces a certain voltage which is representative of the distortion in the characteristics of the FM discriminator due to the temperature drift etc. The certain voltage is held by the sample-hold circuit 40.

After the lapse of a second predetermined time period of, for example 50 ms, longer than the first predetermined time period, the controller 22 starts the search or scan operation to vary the tuning frequency of the frontend stage 10, as shown in Fig. 7D. When, during the search operation, the output voltage of the low pass filter 30 reaches the stored voltage in the sample-hold circuit 40, the comparator 44 produces the detect signal so that the AND gate 34 produces such stop signal as shown in Fig. 7E.

It is to be noted that the reference frequency may be f0 + Af, if desired. In this case, the comparator 44 is to be adapted to produce the detect signal when the difference between the stored voltage in the sample-hold circuit 40 and the output voltage of the low pass filter 30 becomes + Af.

It is now to be understood that the automatic station select apparatus according to the present invention can correctly operate even in the face of the distortion in the characteristics of the FM discriminator due to temperature drift, humidity drift, or the like, whereby it can select one station among stations equidistantly positioned by such a narrow distance in frequency as 25 KHz as well as 50 KHz.

It is further to be noted that the automatic station select apparatus according to the present invention can be applied not only to the frequency synthesizer FM tuner but also to the voltage synthesizer FM tuner.

It will be understood that the invention is not to be limited to the exact construction shown and described and that various changes and modifications may be made without departing from the scope of the invention, as defined in the appended claims.

Claims (5)

1. An automatic station select apparatus for an FM tuner including a front-end stage for converting a received broadcast wave signal into a signal containing an IF frequency signal component while changing its tuning frequency in accordance with a tuning frequency control signal, an IF stage for producing an amplified IF frequency signal in response to said signal from said front-end stage, and an FM discriminator for discriminating the amplified IF frequency signal, said automatic station select apparatus including control means for producing said tuning frequency control signal in response to an automatic station select start signal until a stop signal appears, field strength maximum signal producing means connected to said IF stage, for producing a field strength maximum signal when the intensity of the amplified IF frequency signal exceeds a predetermined level, the improvement which comprises: timing signal producing means for producing a timing signal lasting during a predetermined time period from when said automatic station select start signal is produced; reference signal supply means for supplying a reference frequency signal to the IF stage during said timing signal; storing means for storing the signal level of the output signal produced from said FM discriminator during said timing signal; comparing means for producing a detect signal when difference in level between the output signal from the FM discriminator and the stored signal level reaches a predetermined value after the decay of said timing signal; and stop means for producing said stop signal in response to said field strength signal and said detect signal.

2. The automatic station select apparatus as defined in claim 1, in which said reference frequency signal has a frequency equal to the IF frequency, and said predetermined value is zero.

3. The automatic station select apparatus as defined in claim 1, in which said storing means includes: a capacitor; a low pass filter connected to said FM discriminator, for passing therethrough only lower frequency components from said FM discriminator; switch means connected between said low pass filter and said capacitor, for relaying therethrough said low frequency components to said capacitor in response to said timing signal; and relay means connected to said capacitor, for relaying therethrough the voltage across said capacitor to said comparing means.

4. An automatic station select apparatus for an FM tuner, substantially as hereinbefore described with reference to and as illustrated in Figs. 6 and 7 of the accompanying drawings.

5. An FM tuner having an automatic station select apparatus as claimed in any preceding claim.