JPH10327087A - Television signal reception circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a television TV reception circuit where a front end circuit of a television receiver is integrated and the output of an image frequency is suppressed. SOLUTION: A received broadcast signal is given to an integrated circuit 20 consisting of an analog circuit via a TV variable frequency tuning circuit 10. The integrated circuit 20 is provided with an AGC amplifier 21 and 1st and 2nd frequency mixers 22A, 22B, the 1st and 2nd frequency mixers 22A, 22B receive local signals with a phase difference of 90 deg. from a ϕ/2 phase shift circuit 24 shifts a phase of an oscillated frequency fL0 and provide an output of an intermediate frequency that is a difference component between a frequency of the local signal and a frequency of a received channel. The intermediate frequency outputted from the 1st and 2nd frequency mixers 22A, 22B is fed respectively to a +π/4 phase shift circuit 25A and a -ϕ phase shift circuit 25, outputs of them are added by an adder circuit 26 at an equal level and the sum is outputted to an intermediate frequency amplifier circuit 27 as a synthesized signal.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a television receiving circuit suitable for receiving a broadcast channel selected from a plurality of channels and outputting it as an intermediate frequency, and more particularly, to efficiently removing an image frequency. And an integrated television receiver circuit that is capable of performing such operations.

[0002]

2. Description of the Related Art A receiving circuit for selecting and receiving a desired channel from a large number of television (hereinafter referred to as TV) broadcast channels includes a high-frequency amplifier for amplifying a selected broadcast signal and an intermediate high-frequency output. It includes a frequency mixing circuit for converting to a frequency, and is, for example, a configuration circuit as shown in the block diagram of FIG. In this figure, reference numeral 1 denotes a single tuning type filter for selecting a desired frequency from the channel frequency of a broadcast wave received by an antenna, and 2 denotes an automatic gain control type amplifier for controlling the level of a received signal, which is not shown. The demodulation output controls the output level to be within a predetermined level range. Numeral 3 relates to a double-tuned filter for limiting the pass band of the TV signal, so that the pass frequency of the selected broadcast wave together with the single-tuned filter can be changed by a tuning signal (not shown). It has been done.

[0003] Reference numeral 4 denotes a trap circuit for removing an image frequency that is mixed when the received broadcast frequency is converted into an intermediate frequency. The frequency to be removed is set so that no frequency is generated. 5
Converts the input high-frequency signal to an intermediate frequency (58.75 M
Hz), a frequency conversion circuit (mixing circuit) for converting the signal into a local signal oscillator L.
o, a frequency mixer MIX, and an intermediate frequency amplifier IFA for selecting an intermediate frequency.

As shown in FIG. 5, the above-described TV receiver circuit has a local oscillation frequency (hereinafter, also referred to as a local frequency) with respect to a desired channel frequency (hereinafter, also referred to as a tuning frequency) fD. ) FLO is set, and is controlled so as to output the difference frequency as an intermediate frequency fIM by being mixed by the frequency mixer MIX. By the way, in the frequency mixer MIX, the local frequency fL
If the image frequency fIM is mixed at a point separated by an intermediate frequency fIF from O, an undesired intermediate frequency fIF (IM) having the same frequency component is output for this image frequency fIM. In order to prevent the image frequency component fIM which causes image interference from entering the circuit, a trap circuit 4 for removing the image frequency is provided in the case of FIG. Therefore,
In the case of the above-mentioned TV receiver circuit, it is necessary to change a single tuning circuit, a double tuning circuit and an image trap circuit every time a tuning channel is changed, so that the tuning control circuit becomes complicated and all the tuning control circuits become complicated. There has been a problem that it is difficult to make adjustments to reduce image interference in the reception band.

[0005] Therefore, there has been proposed a receiving circuit of a system for canceling an image frequency by using two frequency mixers as the mixing circuit 5 formed as an integrated circuit. FIG. 6 is a diagram showing the principle of such a mixing circuit.
To the second frequency mixer MIX (Q) and the second frequency mixer MIX (I). The first and second frequency mixers MIX (I) (Q)
Are supplied with local frequencies fLO (I) and fLO (Q) having a phase difference of 90 degrees formed by the π / 2 phase shifter PH, respectively, and the first and second frequency mixers MIX (I) From (Q), an intermediate frequency corresponding to the difference frequency between the channel selection frequency and the local frequency is output. Image frequency f input by synthesis
You can cancel IM. (IEEE
Transactions on Consumer Electronics, Vol. 38, No.
3, August 1992)

[0006]

However, the above-described image canceling and mixing circuit is practically used in a radio frequency receiving circuit having a relatively low frequency band, but is not practical in a TV receiving circuit. It is difficult to use. The reason is that in the case of a broadcast signal having a frequency band of several MHz as seen in a TV broadcast wave, due to variations in the resistance R when forming an integrated circuit, variations in capacitance, and parasitic stray capacitance, In particular, it is difficult to configure the π / 2 phase shifter PH to accurately shift the local oscillation frequency by π / 2 when receiving all broadcast channels. Was considered difficult.

[0007]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, a television receiving circuit according to the present invention has an automatic gain circuit for controlling the level of an input signal, and an output from the automatic gain circuit. The first and second frequency mixing circuits and the first and second frequency mixing circuits
A π / 2 phase shift circuit for supplying a local oscillation frequency having a phase difference of 2, and an output of the first frequency mixing circuit to + π / 4
A first π / 4 phase shift circuit for shifting the phase, a second π / 4 phase shift circuit for shifting the output of the second frequency mixing circuit by -π / 4, and the first and second π An integrated circuit that integrates a synthesis circuit that adds the outputs of the ４ phase shift circuit at the same level, and outputs a television signal input to the integrated circuit via a variable frequency tuning circuit as an intermediate frequency. It was done.

According to the present invention, in particular, the π / 2 phase shift circuit includes a phase comparator for comparing a phase-shifted output with an output before the phase shift, and a π / 2 phase whose output of the phase comparator is fed back. By configuring the control circuit, the π / 2 phase shift circuit can be an automatic phase shift circuit that can accurately shift the phase at all frequencies, and the first and second input signals are input to the synthesis circuit. A level detector for detecting the level difference between the signals is provided, and based on the detection output of the level detector, the first and second signal levels are added to the same level to complete the removal of the image frequency. I am trying to be.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described with reference to FIG. In this figure, a desired broadcast channel is selected in a TV variable frequency tuning circuit 10 for a broadcast signal input via an antenna, a cable, or the like. Then, the selected signal whose frequency is set to fD, for example, is input to the integrated circuit 20 composed of an analog circuit. First, the broadcast signal input to the integrated circuit 20 is A
The signal is amplified by the GC amplifier 21 so as to have a predetermined signal level and supplied to the first and second frequency mixers 22A and 22B. The first frequency mixer 22A and the second frequency mixer 22B receive a local signal having a phase difference of 90 degrees from a π / 2 phase shift circuit 24 that shifts the oscillation frequency fLO of the local oscillator 23. And the difference component between the frequency of the local signal and the frequency of the selected channel is output as the intermediate frequency fIF.

The intermediate frequencies output from the first and second frequency mixers are then supplied to a + π / 4 phase shift circuit 25A and a -π / 4 phase shift circuit, respectively.
Are added at the same level and output as a composite signal to the intermediate frequency amplifier circuit 27. Reference numeral 28 denotes a PLL (Phase-Locked Loop) circuit for forming a local oscillation frequency used in a TV receiver, which forms an accurate local oscillation frequency based on a reference signal frequency source and generates the oscillation. The pass frequency of the variable frequency tuning circuit is finely adjusted by the information corresponding to the frequency and the control by the AFT system. Reference numeral 29 denotes a system controller which performs a tuning operation of the TV receiver and controls the whole.

According to the present invention, the local frequencies fLO (1) and fLO (2) which are shifted by 90 degrees with respect to the frequency fD selected as described in the above embodiment are used as the frequency mixer 22A.
22B, and a mixing method of multiplying by the tuning frequency fD is employed. Therefore, as described below, even when an image interference frequency fIM exists, the interference wave generated by this frequency component is canceled so as not to be output. can do.

The desired tuned signal is converted to ACos (ωst + φ
s), the frequency that causes image interference is BCos (ωit + φ
i), assuming that the local frequency is Cos (ωLt + φL), the output frequency fMIX (1) of the first frequency mixer 22A becomes
(1).

(Equation 1) Then, when only the difference signal component (ωS−ωL) is extracted from the operation output of Expression 1 through a low-pass filter or the like, an intermediate frequency fIF (1) as shown in Expression (1) of Expression 2 is output.

(Equation 2)

Next, in the second frequency mixer 22B, the frequency Sin (ωLt) obtained by shifting the local frequency by π / 2 is used.
+ Φ L), the mixed output frequency fmix (2) becomes as shown in Expression 3 when the calculation is performed in the same manner as Expression 1.

(Equation 3) Also in this case, when the difference signal component is extracted through the low-pass filter, the intermediate frequency signal fIF
(2) is output.

(Equation 4)

Comparing the signal components of Equations (2) and (4), both signal components are formed by two signal components having a phase difference of π / 2, so that the phase difference between the two is π / 2. Such a phase circuit, that is, in the case of FIG. 1, one signal is supplied to a phase shift circuit 25A that shifts the phase by + π / 4,
When the other signal is supplied to the phase shift circuit 25B that shifts the phase by -π / 4, Equation (1) in Equation 2 becomes as Equation (2) in Equation 2, and the phase shifts by π / 2. Then, (2) in the above equation (2) and the second term in the equation (4) have different polarities but indicate the same signal. Therefore, the signals fIF (1) (π / 2) and fIF ( When the signal of 2) is synthesized by the adder circuit, the desired reception frequency fD (ωD) is obtained as shown in Expression 5.
And a signal A * Sin {(ωL−ωs) t + φL−φs} having only the difference signal of the local frequency fL (ωL) as the intermediate frequency.
Can be obtained.

(Equation 5)

Since the signal component of the image frequency fIM serving as an interference wave has been removed from the output of the adder circuit 26,
No image signal component is output to the output of the intermediate frequency amplifier 27, so that the image quality of the demodulated video signal does not deteriorate. Further, when the above-described integrated circuit is configured by a push-pull system (balanced circuit) based on, for example, a differential amplifier, and performs signal processing, it is possible to eliminate second-order harmonic distortion of a signal component. . Further, even when a frequency that causes some image interference on the input side is input, the image intermediate frequency can be sufficiently removed, so that there is an advantage that the channel selection circuit on the input side can be simplified.

In the above-described image canceling receiving circuit, the π / 2 phase shift circuit 24 converts the local frequency having a phase difference of exactly 90 degrees as shown in Expressions 1 to 5. Although it is necessary to provide such an integrated circuit, it is necessary to provide an integrated circuit that completely shifts the phase by π / 2 in a frequency region having a high frequency band such as a TV broadcast wave. This requires a complicated circuit configuration and is not practical. Therefore, in the embodiment of the present invention, a phase comparison circuit 24A that compares the phase difference between the signal shifted by the π / 2 phase shift circuit 24 and the signal before the phase shift is performed.
And a π / 2 phase control circuit 24B for controlling the amount of phase shift by the constant current source circuit, and by feeding back the phase error signal output from the phase comparison circuit 24A to the π / 2 phase shift control circuit 24B. Two local signals having a phase difference of π / 2 at all local frequencies are output.

FIG. 2 is a block diagram showing an embodiment of the automatic .pi. / 2 phase shift circuit 24 as described above. Reference numeral 31 denotes a .pi. / 2 constituted by a time constant circuit charged by a variable current source. An output buffer for extracting a local signal having a phase difference of π / 2 from each other as a balanced signal and outputting the balanced signal, and a phase shift control circuit for supplying the signal to the frequency mixers 22A and 22B. , An input buffer 34, a phase comparator 35, a low-pass filter for outputting an average value of the phase difference signal, and a phase error amplifier 36.

The phase comparator 34 can be constituted by a multiplying circuit for multiplying two input local frequencies, and a phase error (Sin) in which the phase difference between the two local signals is shifted from π / 2 by the operation output. Δφ) is detected. The signal is averaged by a low-pass filter 35 and output as a DC signal. The DC signal is amplified by the error amplifier 35 and the output constitutes a π / 2 phase shift control circuit. For example, a current flowing into an integration circuit is controlled. Thus, the feedback control is performed so that the phase shift amount of the π / 2 phase shifter 31 always becomes π / 2.

Also, in the case of the TV receiver circuit of FIG. 1 shown in the present embodiment, the levels of the signals inputted to the synthesizing circuit 26 for adding the mixed signals and removing the image components are the same. Otherwise, the image components cannot be completely canceled. Therefore, as shown in FIG. 3, a + π / 2 phase shift circuit 41A of a type that balances input and output of the frequency mixers 22A and 22B is used as the synthesis circuit 26.
A -π / 2 phase shift circuit 41B is provided, and the intermediate frequency components extracted by the + π / 2 phase shift circuits 41A and -π / 2 phase shift circuits 41B are supplied to the level variable circuits 42A and 42B. These level variable circuits 42A and 42B
Is adjusted so as to obtain the same output signal by controlling the gain of the differential amplifier of the gain adjustment type, and the adjustment value has a circuit configuration capable of manually adjusting the gain of the differential amplifier.

The signals input to the level variable circuits 42A and 42B are detected by a level comparator 43, and the detected values are controlled so that the output levels of both the level variable circuits 42A and 42B become equal. Is also good. In this case, when the level to be detected is detected on the output side of the level variable circuits 42A and 42B, the output levels can be automatically matched by a feedback system. The outputs of the variable level circuits 42A and 42B are combined and added in a tank circuit 44 having an appropriate bandwidth and tuned to an intermediate frequency and composed of a coil L and a capacitor C, and supplied to the intermediate frequency amplifier 27.

[0021]

As described above, the TV receiving circuit according to the present invention includes a signal processing circuit for converting a TV signal received on a semiconductor integrated substrate into an intermediate frequency, so that the elements constituting the circuit are absolutely required. The receiving circuit incorporates an image-cancelling circuit, which reduces the size of the receiving circuit and removes the disturbing image signal with high accuracy. Therefore, the trap circuit required for the channel selection circuit can be omitted.

Further, if an automatic π / 2 phase shift circuit or an automatic level adjustment circuit is employed to eliminate the image frequency, the influence of the variation of each element of the semiconductor integrated circuit is eliminated, so that the adjustment work of the reception circuit is largely performed. This has the advantages that the cost can be reduced, the cost can be reduced, and the design of the TV receiver circuit becomes easy.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating an embodiment of a television receiving circuit of the present invention.

FIG. 2 is a block diagram showing a principle diagram of a phase shift circuit used in a television receiving circuit.

FIG. 3 is a block diagram of an adding circuit for canceling an image frequency that can be used for a television receiving circuit.

FIG. 4 is a block diagram of a conventional television receiving circuit.

FIG. 5 is an explanatory diagram of an image frequency.

FIG. 6 is a block diagram of a mixing circuit having an image cancel function.

[Explanation of symbols]

10 Variable frequency tuning circuit, 21 AGC amplifier, 22
A, 22B frequency mixer, 23 local oscillator, 24 automatic phase shifter, 25A, 25B π / 4 phase shifter, 26 adder

Claims (3)

[Claims] 1. An automatic gain circuit for controlling a level of an input signal, and first and second inputs of an output of the automatic gain circuit.
A frequency mixing circuit, a π / 2 phase shift circuit for supplying a local oscillation frequency having a phase difference of π / 2 to the first and second frequency mixing circuits, and an output of the first frequency mixing circuit A first π / 4 phase shifter that shifts the phase of the second frequency mixing circuit by + π / 4, a second π / 4 phase shifter that shifts the output of the second frequency mixing circuit by −π / 4, An integrated circuit that integrates a synthesis circuit that adds the output of the second π / 4 phase shift circuit, and outputs a television signal input to the integrated circuit via a variable frequency tuning circuit as an intermediate frequency. A television signal receiving circuit. 2. The π / 2 phase shift circuit includes a phase comparator for comparing a phase-shifted output with an output before the phase shift, and a π / 2 phase control circuit to which the output of the phase comparison is fed back. The television receiving circuit according to claim 1, wherein the television receiving circuit is configured. 3. The synthesizing circuit according to claim 1, further comprising:
And a combiner for adding the first and second signal levels at the same level based on the detection output of the level detector. The television receiving circuit according to claim 1.