Automatic fine tuning detection circuit

A system for carrying out automatic fine tuning detection comprising a generating section for generating a video intermediate frequency signal from a radio wave, a filtering section connected to the generating section for filtering the video intermediate frequency signal supplied from the generating section, the filtering section having low and high frequency regions having different gains from each other, a subtracting section connected to both the generating section and filtering section for subtracting an output signal of the filtering section from the video intermediate frequency signal supplied by the generating section, an extracting section connected to the filtering section for fetching the output signal of the filtering section and subsequent extraction of a video carrier wave signal component from the output signal and a multiplying section connected to the subtracting section, the extracting section and the generating section for multiplying an output signal of the subtracting section by an output signal of the extracting section and subsequent conversion into a voltage signal to be fed back to the generating section to thereby control a frequency of the video intermediate frequency signal at a predetermined value.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to an automatic fine tuning detecting circuit, and 
more particularly to an automatic fine tuning detecting circuit involved 
in an automatic frequency controller for tele-receiver tuning in which the 
automatic fine tuning detecting circuit fetches a carrier wave component 
from a video signal of intermediate frequency for achievement of a video 
detection. 
2. Description of the Related Art 
A circuit configuration including a conventional automatic fine tuning 
detecting circuit as known in the art is as illustrated in FIG. 1. An 
antenna 1 receives a radio wave for television so that the radio wave is 
converted into an radio frequency electrical signal that will subsequently 
be transmitted to a tuner 2. The tuner 2 converts the radio frequency 
electrical signal into a video intermediate frequency signal. The video 
intermediate frequency signal is then transmitted into a SAW filter 3 for 
receiving a restriction of bandwidth thereof or filtering. The filtered 
video intermediate frequency signal is transmitted into a video 
intermediate frequency amplifier circuit 4 for amplification thereof. The 
amplified signal is then transmitted into both a carrier amplifier 5 and a 
video detecting circuit 6. The carrier amplifier 5 is provided with a 
filter 8 having a center frequency that corresponds to a frequency of the 
video intermediate frequency signal. This conventional automatic fine 
tuning detecting circuit utilizes a pseudo-synchronous detecting system in 
which a carrier wave signal is fetched from the video intermediate 
frequency signal for synchronous detection. The carrier amplifier 5 
fetches the carrier wave signal from the video intermediate frequency 
signal so that the filter 8 may serve as a selective transmitter. The 
fetched carrier wave signal is then transmitted into the video detecting 
circuit 6 that serves as a multiplier wherein a synchronous detection of 
the carrier wave signal with the amplified signal from the video 
intermediate frequency amplifier circuit 4 is carried out for detection of 
a video signal and subsequent output thereof from the video detecting 
circuit 6. The above described video signal detection is carried out at a 
0 degree of phase difference between the output signals from the carrier 
amplifier 5 and from the video intermediate frequency amplifier circuit 4. 
By contrast, the automatic fine tuning detection is carried out by an 
automatic fine tuning detecting circuit 15 that is provided with an 
automatic fine tuning detecting filter 14. The automatic fine tuning 
detecting filter 14 is connected to the filter 8 of the carrier amplifier 
5 through capacitors 13 so that the filter 14 exhibits a signal differing 
in phase by 90 degrees from the signal on the filter 8. The automatic fine 
tuning detecting circuit 15 multiplies an output signal supplied from the 
carrier amplifier 5 with the signal appearing on the automatic fine tuning 
detecting filter 14. The automatic fine tuning detection differs from the 
video detection in a phase difference by 90 degrees between them. In the 
automatic fine tuning detection, when the output from the carrier 
amplifier 5 has a frequency difference from the predetermined center 
frequency of the video intermediate frequency signal, the automatic fine 
tuning detecting circuit 15 generates an output signal having a variation 
in voltage whose amount corresponds to an amount of the frequency 
difference. The output signal having the voltage variation is then fed 
back to the tuner 2 through a filter 16 so that the video intermediate 
frequency signal outputted from the tuner 2 is automatically controlled at 
the center frequency. The difference in frequency may be detected by the 
automatic fine tuning detecting circuit 15 by multiplying the output 
signal from the carrier amplifier 5 with the signal that has passed 
through a phase circuit comprising the capacitors 13 and the filter 14. 
The detected frequency difference is then converted into a voltage. The 
phase circuit is so constructed that filter 14 has a signal having a 
difference in phase by 90 degrees from the signal appearing on the filter 
8 to permit a variation in frequency of the signal to generate a variation 
in phase thereof. Namely, the phase circuit may be regarded as a 
quadrature circuit that is disclosed in the Japanese patent publication 
No. 62-11550. 
The above described automatic fine tuning detecting circuit has 
disadvantages as mentioned below. The conventional automatic fine tuning 
detecting circuit requires two filters in which one is provided to the 
carrier amplifier 5 for pseudo-synchronous detection and the another is 
provided to the automatic fine tuning detecting circuit for the automatic 
fine tuning detection. The conventional automatic fine tuning detecting 
circuit requires at least two adjustments by an individual. The two 
filters have the same center frequency but have a difference in phase by 
90 degrees from each other. Those may cause an interference between the 
two filters. This results in an inferior of circuit detection performance 
under a condition of a weak field. 
SUMMARY OF THE INVENTION 
Accordingly, it is a primary object of the present invention to provide a 
novel automatic fine tuning detecting circuit free from any disadvantage 
as described above. 
It is a further object of the present invention to provide a novel 
automatic fine tuning detecting circuit having a small number of elements 
constructing the circuit. 
It is a another more object of the present invention to provide a novel 
automatic fine tuning detecting circuit that requires a small amount of 
adjustments of elements involved in the circuit. 
It is a another more object of the present invention to provide a novel 
automatic fine tuning detecting circuit that is free from any interference 
between filters involved in the circuit. 
The above and other objects, features and advantages of the present 
invention will be apparent from the following descriptions. 
The invention provides a system for carrying out automatic fine tuning 
detection. The system comprises an antenna for receiving a radio wave and 
subsequent conversion into a radio frequency electrical signal, a tuner 
connected to the antenna for converting the radio frequency electrical 
signal into the video intermediate frequency signal, an amplifier circuit 
connected to the filter for amplifying the video intermediate frequency 
signal, a filtering section connected to the amplifier circuit for 
filtering the amplified video intermediate frequency signal supplied from 
the amplifying circuit wherein the filtering section has low and high 
frequency regions having different gains from each other, a subtracting 
section connected to both the amplifying circuit and the filtering section 
for subtracting an output signal of the filtering section from the video 
intermediate frequency signal supplied by the amplifying circuit, an 
extracting section connected to the filtering section for fetching the 
output signal of the filtering section and subsequent extraction of a 
video carrier wave signal component from the output signal wherein the 
extracting section comprises a carrier amplifier circuit and a filter 
having the same center frequency as the predetermined frequency of the 
video intermediate frequency signal generated by the generating section 
and a multiplying section being connected to the subtracting section, the 
extracting section and the tuner for multiplying an output signal of the 
subtracting section by an output signal of the extracting section and 
subsequent conversion into a voltage signal to be fed back to the 
generating section to thereby control a frequency of the video 
intermediate frequency signal at a predetermined value.

PREFERRED EMBODIMENTS OF THE INVENTION 
The present invention provides a novel automatic fine tuning detecting 
circuit that comprises a filter circuit for filtering a video intermediate 
frequency signal wherein the filter circuit has high and low frequency 
parts having different gains from each other, a subtracting circuit for 
subtracting an output of the filter circuit from the video intermediate 
frequency signal, an extracting circuit for extracting a video carrier 
wave signal component from the output signal of the filter circuit and a 
multiplying circuit for multiplying the output signal of the extracting 
circuit by the output of the subtracting circuit to apply a local part 
oscillation frequency control voltage into a tuner. 
In operation of the novel automatic fine tuning detecting circuit according 
to the present invention, the video intermediate frequency signal is 
inputted into the filter circuit to pass through the filter circuit. The 
subtracting circuit subtracts the signal passed through the filter from 
the video intermediate signal that has not passed through the filter to 
generate signals having a phase difference by 90 degrees from each other, 
one of which is inputted into a carrier amplifier for carrier extraction 
thereof to extract a carrier signal for pseudo-synchronous detection. The 
extracted carrier is then inputted into the multiplying circuit to be 
multiplied by the another signal having the phase difference by 90 degrees 
from the one signal for automatic fine tuning detection. The phase circuit 
acting for the automatic fine tuning detection may also act as a filter 
for the carrier extraction. As compared to the conventional automatic fine 
tuning circuitry, the novel automatic fine tuning circuitry requires a 
small number of elements. The novel automatic fine tuning detecting 
circuit requires a single adjustment for the single filter. The novel 
circuitry is free from the requirement to provide two filters that have 
the same center frequency but have a difference in phase by 90 degrees 
from each other. Those features may permit the novel circuitry to be free 
from any problem with an interference between the filter. This permits the 
novel circuitry to exhibit excellent circuit performances even under a 
condition of a weak field. 
The description will focus on a circuit configuration of a novel automatic 
fine tuning detection system in a first embodiment according to the 
present invention with reference to FIG. 2. The novel automatic fine 
tuning detection system has an antenna 1 for receiving a radio wave to be 
converted into a radio frequency electrical signal. The novel automatic 
fine tuning detection system also has a tuner 2 into which the radio 
frequency signal received by the antenna 1 is transmitted for conversion 
of the radio frequency signal into a video intermediate frequency signal. 
The novel system also has a SAW filter for filtering the video 
intermediate frequency signal for bandwidth restrictions thereof. The 
novel system also includes a video intermediate frequency amplifier 
circuit 4 for amplifying the filtered video intermediate frequency signal 
transmitted from the SAW filter 3. The novel system also includes a low 
pass filter 10 constructing a linear filter and a subtracting circuit 9 in 
which the amplified video intermediate frequency signal is transmitted 
into both the subtracting circuit 9 and the low pass filter 10. The low 
pass filter 10 filters the amplified video intermediate frequency signal. 
The novel system also includes a video detecting circuit and a carrier 
amplifier 5 being provided with a filter 8 having a center frequency that 
corresponds to a frequency of the video intermediate frequency signal. The 
filtered video intermediate frequency signal of the low pass filter 10 is 
then supplied into the subtracting circuit 9, the carrier amplifier 5 and 
the video detecting circuit 6. The filtered video intermediate frequency 
signal of the low pass filter 10 or the output signal of the low pass 
filter 10 serves as a reference signal in this novel system. The 
subtracting circuit 9 subtracts the output signal of the low pass filter 
10 from the amplified video intermediate frequency signal fetched from the 
video intermediate frequency amplifying circuit 4 to thereby generate an 
output signal having a phase difference by 90 degrees from the output 
signal supplied from the low pass filter 10. The novel system further 
includes an automatic fine tuning multiplier 7 into which the output 
signal of the subtracting circuit 9 is transmitted. The carrier amplifier 
5 provided with a filter 8 which extract a carrier wave signal from the 
filtered video intermediate frequency signal supplied from the low pass 
filter 10 for subsequent transmission of the extracted carrier wave signal 
into the video detecting circuit 6 and the automatic fine tuning 
multiplier 7. The video detecting circuit 6 constitutes a multiplying 
circuit for performing a synchronous detection of the carrier wave signal 
extracted by the carrier amplifier 5 with the output signal of the low 
pass filter 10 to thereby detect a video signal. The detected video signal 
is outputted from the video detecting circuit 6. The automatic fine tuning 
multiplier 7 multiplies the output signal of the carrier amplifier 5 by 
the output signal of the subtracting circuit 9 for automatic fine tuning 
detection. This automatic fine tuning detection may be carried out in the 
quadrature system that requires a filter for converting a variation in 
frequency of signals into a variation in phase thereof. The filter 8 
provided to the carrier amplifier 5 may serve as the filter that may 
convert any variation in frequency of the signal into a variation in 
phase. An output of the automatic fine tuning multiplier 7 is then fed 
back through a filter 11 into the tuner so that the center frequency of 
the video intermediate frequency signal is controlled at a predetermined 
value. 
This automatic fine tuning detection may also be described by referring the 
following equations. 
The low pass filter 10 and the filter 8 provided to the carrier amplifier 5 
have transfer functions L(f) and B(f) given by equations (1) and (2) 
respectively. 
EQU L(f) =1/(1+j(f/f.sub.c)) (1) 
EQU B(f)=(j(f/f.sub.0)(1/Q))/(1+j(f/f.sub.0)(1/Q)-(f/f.sub.0).sub.2)(2) 
where f.sub.c is the cutoff frequency of the low pass filter 10, f.sub.0 is 
the center frequency of the video intermediate frequency carrier wave 
signal, Q is the degree of selection of the filter 8. 
The output signal E.sub.0 (f) of the video intermediate frequency amplifier 
circuit 4, the output signal E.sub.1 (f) of the low pass filter 10 and the 
output signal E.sub.2 (f) of the output signal of the subtracting circuit 
9 as well as the output signal E.sub.3 (f) of the carrier amplifier 5 are 
expressed by the following equations respectively. 
##EQU1## 
The output signal E.sub.1 (f) has a difference in phase by 90 degrees from 
the output signal E.sub.2 (f). The output signal E.sub.1 (f) is inputted 
into the carrier amplifier 5 to be converted into the output signal 
E.sub.3 (f). Then, the output signal E.sub.3 (f) may be expressed by the 
following equation. 
##EQU2## 
When f=f.sub.0, a difference in phase between the output signals E.sub.3 
(f) and E.sub.1 (1) is zero. In this case, the phase property of the 
transfer function B(f) is defined by the phase property of the phase of 
the output signal E.sub.3 (f). The phase property of the transfer function 
B(f) is expressed by the following equation. 
##EQU3## 
where .phi. is the phase of the filter 8 having the transfer function 
B(f). When f=f.sub.0, the phase of the filter 8 is zero. The output 
signals E.sub.1 (f) and E.sub.2 (f) have a difference in phase by 90 
degrees from each other. Then, a difference .theta. in phase between the 
output signals E.sub.2 (f) and E.sub.3 (f) is expressed by the following 
equation. 
##EQU4## 
When f=f.sub.0, the phase difference .theta. is 90 degrees. The phase 
difference is converted by the automatic fine tuning multiplier 7 into a 
voltage that will be fed back through the filter 11 into the tuner 2 to 
thereby achieve the automatic fine tuning operation. 
The description will focus on a circuit configuration of a novel automatic 
fine tuning detection system in a second embodiment according to the 
present invention with reference to FIG. 3. The novel automatic fine 
tuning detection system of the second embodiment has a structural 
difference in providing a high pass filter 12. The novel system has an 
antenna 1 for receiving a radio wave to be converted into a radio 
frequency electrical signal. The novel automatic fine tuning detection 
system also has a tuner 2 into which the radio frequency signal received 
by the antenna 1 is transmitted for conversion of the radio frequency 
signal into a video intermediate frequency signal. The novel system also 
has a SAW filter for filtering the video intermediate frequency signal for 
band width restrictions thereof. The novel system also includes a video 
intermediate frequency amplifier circuit 4 for amplifying the filtered 
video intermediate frequency signal transmitted from the SAW filter 3. The 
novel system also includes a high pass filter 12 constructing a linear 
filter and a subtracting circuit 9 in which the amplified video 
intermediate frequency signal is transmitted into both the subtracting 
circuit 9 and the high pass filter 12. The high pass filter 12 filters the 
amplified video intermediate frequency signal. The novel system also 
includes a video detecting circuit and a carrier amplifier 5 provided with 
a filter 8 having a center frequency that corresponds to a frequency of 
the video intermediate frequency signal. The filtered video intermediate 
frequency signal of the high pass filter 12 is then supplied into the 
subtracting circuit 9, the carrier amplifier 5 and the video detecting 
circuit 6. The filtered video intermediate frequency signal of the high 
pass filter 12 or the output signal of the high pass filter 12 serves as a 
reference signal in this novel system. The subtracting circuit 9 subtracts 
the output signal of the high pass filter 12 from the amplified video 
intermediate frequency signal fetched from the video intermediate 
frequency amplifying circuit 4 to thereby generate an output signal having 
a phase difference by 90 degrees from the output signal supplied from the 
high pass filter 12. The novel system further includes an automatic fine 
tuning multiplier 7 into which the output signal of the subtracting 
circuit 9 is transmitted. The carrier amplifier 5 provided with the filter 
8 extract a carrier wave signal from the filtered video intermediate 
frequency signal supplied from the high pass filter 12 for subsequent 
transmission of the extracted carrier wave signal into the video detecting 
circuit 6 and the automatic fine tuning multiplier 7. The video detecting 
circuit 6 constitutes a multiplying circuit for performing a synchronous 
detection of the carrier wave signal extracted by the carrier amplifier 5 
with the output signal of the high pass filter 12 to thereby detect a 
video signal. The detected video signal is outputted from the video 
detecting circuit 6. The automatic fine tuning multiplier 7 multiplies the 
output signal of the carrier amplifier 5 by the output signal of the 
subtracting Circuit 9 for automatic fine tuning detection. This automatic 
fine tuning detection may be carried out in the quadrature system that 
requires a filter for converting a variation in frequency of signals into 
a variation in phase thereof. The filter 8 provided to the carrier 
amplifier 5 may serve as the filter that may convert any variation in 
frequency of the signal into a variation in phase. An output of the 
automatic fine tuning multiplier 7 is then fed back through a filter 11 
into the tuner so that the center frequency of the video intermediate 
frequency signal is controlled at a predetermine value. 
A difference .theta. in phase between the output signals E.sub.2 (f) and 
E.sub.3 (f) is expressed by the following equation. 
##EQU5## 
The phase difference is converted by the automatic fine tuning multiplier 
7 into a voltage that will be fed back through the filter 11 into the 
tuner 2 to thereby achieve the automatic fine tuning operation. 
Whereas modifications of the present inventions will no doubt be apparent 
to a person having ordinary skill in the art, to which the invention 
pertains, it is to be understood that the embodiments shown and described 
by way of illustrations are by no means intended to be considered in a 
limiting sense. Accordingly, the claims are intended to cover all 
modifications which fall within the spirit and scope of the invention.