JP3309455B2 - RF modulator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an R
More particularly, the present invention relates to an RF modulator having a test pattern generating circuit used for channel matching between a television receiver (hereinafter, simply referred to as a TV) and a VTR connected thereto.

[0002]

2. Description of the Related Art In a PAL VTR or the like, a built-in RF modulator is provided with a test pattern generating circuit for generating a predetermined test pattern signal for channel matching with a TV. . That is, a test pattern signal having a waveform as shown in FIG. 3 is generated by a test pattern generation circuit, this test pattern signal is AM-modulated by a carrier wave fc, and this is output as an RF output to T
V to the electronic tuning tuner, the carrier f
When the frequency c matches the frequency of the desired channel, a black and white vertical stripe test pattern can be clearly displayed on the CRT.

In a conventional RF modulator, FIG.
As shown in (1), in the normal mode, a clip / clamp circuit 8 which is a kind of a waveform shaping circuit for supplying an input video signal to the mixer 4, and in the test pattern mode, the test pattern signal is generated and supplied to the mixer 4. Both the test pattern generation circuit 7 and the test pattern generation circuit 7 are directly connected to the video-side input terminal of the mixer 4. Here, the current flowing from the mixer 4 into the test pattern generating circuit 7 is I _TP , and the current flowing from the mixer 4 into the clip / clamp circuit 8 is I _TP .
_V.

[0004]

However, in the conventional RF modulator having the above configuration, both the test pattern generation circuit 7 and the clip / clamp circuit 8 are provided in the mixer 4.
Connected directly to the video input of the
Since the ratio of the current I _V to the current I _TP cannot be properly set, the production variation of the modulation degree of the test pattern signal is large, the temperature stability of the modulation degree is poor, and the wiring capacity of the mixer 4 is increased. Since an extra capacity is added, there is a problem that the characteristics of the mixer 4 deteriorate.

Accordingly, the present invention is to provide an RF modulator which has a small variation in the production of the modulation degree of the test pattern signal, has a good temperature stability of the modulation degree, and can prevent the deterioration of the characteristics of the mixer. Aim.

[0006]

An RF modulator according to the present invention comprises a test pattern generating circuit for generating a predetermined test pattern signal, a selection switch for selecting one of an input video signal and a test pattern signal, and a selection switch for selecting one of the input video signal and the test pattern signal. A test pattern generating circuit comprising: a waveform shaping circuit that performs processing on the amplitude axis with respect to a signal waveform of a selected output by the switch; and a mixer that mixes an output of the waveform shaping circuit and a carrier to generate an RF output. , A first current source and a second
Current source, horizontal sync signal pulse and white signal pulse
ON / OFF control of the first and second current sources based on
A current source switch circuit and first and second current sources at one end
A connected first resistor, one end of the first resistor and a reference potential
A capacitor connected between the point and the other end of the first resistor
And a base connected to one end of the first resistor.
And a continuous emitter follower transistor .

[0007]

The input side of the waveform shaping circuit selects either the input video signal or the test pattern signal, supplies the selected output to the mixer via the waveform shaping circuit, mixes the output with the local oscillation output, and outputs the RF output. Since only the waveform shaping circuit is directly connected to the video-side input terminal of the mixer, the production variation of the modulation degree of the test pattern signal is reduced, and the temperature of the modulation degree is reduced. Since the stability is improved and the test pattern signal is injected from a portion unrelated to the mixer, deterioration of the characteristics of the mixer can be prevented. In the test pattern generation circuit,
A low-pass filter is formed by the first resistor and the capacitor.
Formed and tested by the action of this low pass filter
Drops higher order components of frequency spectrum in pattern mode
Due to higher order components of the spectrum
Suppress voice carrier test pattern spectrum level difference
It can be less than the detection limit of harmful sound.

[0008]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 is a block diagram showing one embodiment of the present invention. In FIG. 1, the RF modulator according to the present invention includes seven terminals to, an audio signal is input to the terminal, a tank circuit 1 for audio carrier oscillation is connected to the terminal, and a video signal is connected to the terminal. A test pattern changeover switch SW1 and an oscillator 2 for generating a clock of, for example, 500 KHz are connected to the terminals, and a carrier fc is input to the terminals.
An RF output is derived from the terminal.

An audio signal input from a terminal is subjected to FM modulation by an FM modulator 3 and supplied to a mixer 4.
The video signal input from the terminal is applied to the terminal a of the selection switch SW2. The test pattern changeover switch SW1 specifies a normal mode that functions as an original RF modulator when turned on (closed), and specifies a test pattern mode when turned off (opened). This test pattern mode is a test pattern generation mode for adjusting the frequency of the carrier fc to the frequency of a desired channel.

The switch switching circuit 5 controls the switching of the selection switch SW2 in accordance with the ON / OFF state of the test pattern switching switch SW1.
When W1 is on, the input of terminal a is selected, and when W1 is off, the input of terminal b is selected. When the test pattern changeover switch SW1 is off, the clock from the oscillator 2 is supplied to the pulse generator 6 via the terminal. The pulse generator 6 generates a negative logic horizontal synchronizing signal pulse (a) and a white signal pulse (b) as shown in FIG. These pulses (a) and (b) are supplied to the test pattern generation circuit 7.

The test pattern generation circuit 7 is for generating a test pattern signal as shown in FIG. 3 based on the horizontal synchronizing signal pulse (a) and the white signal pulse (b). The circuit configuration will be described later.
In the test pattern signal waveform shown in FIG. 3, S _H is a pulse for a horizontal synchronizing signal, and _SW is a pulse for a white signal. This test pattern signal is input to the terminal b of the selection switch SW2.

The video signal or test pattern signal selected by the selection switch SW 2 is supplied to the mixer 4 via the clip / clamp circuit 8. The clip / clamp circuit 8 is a type of a waveform shaping circuit, and performs a process on an amplitude axis for a signal waveform of an input signal. The mixer 4 includes the video signal having passed through the clip / clamp circuit 8 and the FM modulator 3
From the audio FM signal or clip and clamp circuit 8
Is AM-modulated by the carrier wave fc. The AM modulated wave from the mixer 4 is led out from the terminal via an amplifier 9 as an RF output.

Next, the circuit operation of the RF modulator having the above configuration will be described. First, in the normal mode in which the test pattern changeover switch SW1 is on, the audio signal input from the terminal is FM-modulated by the FM modulator 3 and supplied to the mixer 4. The video signal inputted from the terminal is supplied to the mixer 4 via the clip / clamp circuit 8 after being selected by the selection switch SW2.
It is AM-modulated by the carrier fc together with the voice FM signal, and is derived from the terminal as an RF output.

On the other hand, a test pattern changeover switch SW
In the test pattern mode in which 1 is off, the test pattern signal generated by the test pattern generation circuit 7 is selected by the selection switch SW2, passes through the clip / clamp circuit 8, is AM-modulated by the carrier 4 fc by the mixer 4, and the terminal Output. R based on this test pattern signal
By supplying the F output to the electronic tuning tuner of the TV, if the frequency of the carrier fc matches the frequency of the desired channel, a black and white vertical stripe test pattern is clearly displayed on the CRT.

FIG. 4 is a circuit diagram showing an example of a specific circuit configuration of the test pattern generation circuit 7. In FIG. 4, a transistor Q _{1 serving} as a first current source and a transistor Q _{2 serving as a second} current source are connected between one end of the resistor R ₁ and ground (reference potential point) via resistors R ₂ and R ₃ . Connected. Each base of the transistors Q _1, Q _2, the voltage V ₂ is applied by the voltage source 12. Transistor Q
₁ is in the steady state OFF condition, the transistor Q ₂ is in a steady on state. At one end of the resistor R _1, the base of the transistor Q ₃ of the emitter follower is connected, the emitter test pattern output V _O of the transistor Q ₃
Becomes The other end of the resistor R _2, the voltage due to the voltage source 11 V
₁ is applied.

The transistors Q ₁ and Q ₂ are connected to the pulse generator 6 (see FIG. 1) by the current source switch circuit 13.
On / off control is performed based on the horizontal synchronizing signal pulse (a) and the white signal pulse (b) as shown in FIG. The circuit configuration of the current source switch circuit 13 will be described below. A current source I ₁ and a current source I ₃ , a current source I ₂ and a current source I ₆ are provided between a power supply (Vcc) line and the ground.
Are connected in series. The current source I ₁ is turned on / off in synchronization with the white signal pulse (a), and the current source I 1
₂ is turned on / off in synchronization with the horizontal synchronizing signal pulse (b).

The connection point between the current sources I ₁ and I ₃ is connected to the base of a transistor Q ₄ . Transistor emitter of Q ₄ are connected to ground, its collector of resistor R ₄ to R ₆ which are connected in series between ground and Vcc line,
It is connected to the common connection point between the resistor R ₅ resistor R _6. Similarly, the connection point of the current source I ₂ and the current source I _6, the base of the transistor Q ₉ is connected. Transistor Q ₉
The emitter is grounded, the collector of the resistor R ₇ to R ₉ which are connected in series between ground and Vcc line, the resistance R
₈ and is connected to the common connection point of the resistors R _9.

The differential transistor pair Q ₅ and Q ₆ have emitters connected in common with each other and have a Vcc via a current source I _4.
Connected to line. The base of transistor Q ₅ is connected to the resistor R ₄ to the common connection point of the resistors R _5, its collector connected to the emitter of the transistor Q ₂ to which previously described. The differential transistor pair Q ₇ and Q ₈ have their emitters connected in common to each other, and have Vcc connected via a current source I _5.
Connected to line. The base of transistor Q ₇ is connected in common with the base of the transistor Q _6, the voltage V ₃ is applied to the common connection point. In addition, the transistor Q ₁ collector of the transistor Q ₇ is, previously described
Connected to the emitter. The base of transistor Q ₈ is connected to a common connection point between the resistor R ₇ resistor R _8.

Next, the test pattern signal waveform generated in the test pattern generation circuit 7 having the above configuration will be described. The transistor Q ₁ is in a steady during the OFF state, the transistor Q ₂ is assumed in the stationary on state, and the base of the transistor Q ₁ to Q ₃ - to the emitter voltage and V _BE. First, in the waveform diagram of FIG. 2, the test pattern output V at timing c at which neither the horizontal synchronizing signal pulse (a) nor the white signal pulse (b) is generated.
_OO is

V _OO = (V ₁ −V _be ) − (V ₂ −V _be ) · (R ₁ / R ₃ )

On the other hand, the test pattern output V _OH at the timing a when the horizontal synchronizing signal pulse (a) is generated is

V _OH = (V ₁ −V _be ) − (V ₂ −V _be ) · {(R ₁ / R ₂ ) + (R ₁ / R ₃ )} The test pattern output V _OW at the timing b at which the white signal pulse (b) is generated is

## _EQU3 ## V _OW = (V ₁ −V _be )

FIG. 3 shows this state. As is apparent from this figure, by changing the resistance values of the resistors R _{1 to} R ₃ and the pulse width of the white signal pulse (b) in FIG. 2, an arbitrary shape of the test pattern signal waveform can be generated. If resistors having the same temperature characteristics are selected as the resistors R _{1 to} R ₃ , and they are arranged close to each other when forming an IC, the following equation is obtained.
And the terms R ₁ / R ₂ and R ₁ / R _{3 in} the equations of the formula ₂ are constants irrelevant to temperature changes and resistance value variations.
Therefore, the test pattern output V _O is substantially constant with respect to the resistance value variation and the temperature change, and a stable test pattern modulation characteristic can be obtained.

A selection between a video signal and a test pattern signal is made on the input side of the clip / clamp circuit 8, and a video signal or a test pattern signal is supplied to the mixer 4 via the clip / clamp circuit 8. With the configuration, the clip-side input terminal of the mixer 4
Since only the clamp circuit 8 is directly connected, no extra capacitance is added, and the test pattern signal can be injected at a portion irrelevant to the mixer 4, so that the deterioration of the characteristics of the mixer 4 can be prevented.

In FIG. 5 showing the frequency spectrum in the test pattern mode, as shown by the solid line a, higher-order components of the spectrum are applied to the voice carrier, so that the frequency components of the test pattern near the voice carrier are disturbed. It will be output as sound. Therefore,
As shown by the broken line in FIG. 4, a capacitor C ₁ between one end and the ground (reference potential point) of resistor R ₁ connected to form a low-pass filtered by the capacitor C ₁ and resistor R _1.

Thus, as shown by a broken line b in FIG.
Since higher-order components of the spectrum can be dropped, the difference between the voice carrier-test pattern spectrum level difference caused by the higher-order components of the spectrum is detected by the interference detection limit (for example, 5
0 dB). In the measurement by the present inventor, the sound carrier - difference test pattern spectrum level, in case of connecting the capacitor C ₁ 56 dB, and the measurement result of 49dB is obtained when not connected to the capacitor C _1.

[0025]

As described above, according to the present invention,
The selection between the video signal and the test pattern signal is performed on the input side of the waveform shaping circuit, and the video signal or the test pattern signal is supplied to the mixer via the waveform shaping circuit. Only the waveform shaping circuit is directly connected to the input end, so that no extra capacitance is added and the test pattern signal can be injected in a part unrelated to the mixer, so that the production variation of the modulation degree of the test pattern signal And the temperature stability of the modulation is improved, the deterioration of the characteristics of the mixer can be prevented , and the effect of the low-pass filter allows
High-order components of the frequency spectrum in the strike pattern mode
Due to higher order components of the spectrum because it can be dropped
Voice carrier test pattern spectrum level difference
Can be less than the detection limit of interference sound.

[Brief description of the drawings]

FIG. 1 is a block diagram showing one embodiment of an RF modulator according to the present invention.

FIG. 2 is a waveform diagram of a horizontal synchronizing signal pulse (a) and a white signal pulse (b).

FIG. 3 is a waveform diagram of an example of a test pattern signal.

FIG. 4 is a circuit diagram showing an example of a specific circuit configuration of a test pattern generation circuit.

FIG. 5 is a frequency spectrum in a test pattern mode.

FIG. 6 is a block diagram showing a main part of a conventional example.

[Explanation of symbols]

 Reference Signs List 3 FM modulator 4 Mixer 6 Pulse generator 7 Test pattern generation circuit 8 Clip / clamp circuit 13 Current source switch circuit

Claims (2)

(57) [Claims] 1. A test pattern generating circuit for generating a predetermined test pattern signal, a selection switch for selecting one of an input video signal and the test pattern signal, and a signal waveform of a selection output by the selection switch. and a mixer for the RF output is mixed with the waveform shaping circuit forming the processing on the amplitude axis, and an output carrier wave of said wave-shaping circuit, said test pattern generating circuit includes a first current source, the 2
Current source, horizontal sync signal pulse and white signal pulse
ON / OFF control of the first and second current sources based on
Current source switch circuit, and the first and second current sources
Is connected to one end of the first resistor, and one of the first resistor is connected to the first resistor.
A capacitor connected between the end and a reference potential point;
A voltage source connected to the other end of the first resistor, and the first resistor;
An RF modulator comprising: an emitter follower transistor having a base connected to one end of a resistor. 2. The apparatus according to claim 1, wherein said first and second current sources are connected to said first current source.
Between the one end of the resistor and the reference potential point.
And a predetermined voltage is applied to each base.
2. The RF modulator according to claim 1, comprising first and second transistors .