JP3194273B2 - 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 RF modulator, and more particularly to an RF modulator capable of switching to an operation mode corresponding to each signal of PAL / SECAM and the PAL / SCAM.
The present invention is suitable for use in an RF modulator capable of automatically compensating for a difference in the degree of modulation in each operation mode of the ECAM.

[0002]

2. Description of the Related Art As is well known, a television signal is transmitted in an NTSC system adopted in Japan and the United States, in a SECAM system adopted in France,
And three PAL systems adopted in Europe other than France.

Therefore, a video tape recorder used in France is provided with a positive modulation RF modulator, which is a SECAM modulation method, and a video tape recorder used in other countries uses a negative modulation RF modulator. . As a result, each video tape recorder can generate a television signal suitable for the system adopted in the country, and the generated television signal, that is, the video playback video is viewed on a television receiver of each system. It becomes possible.

[0004] In this way, each country enjoys the image reflected on the television receiver in each country.
In countries adjacent to France, there is a great deal of exchange with France, and in order to be able to receive French broadcasts, dual-type televisions that can receive both PAL and SECAM television signals are available. Television receivers are often used.

[0005] Conversely, the dual-type television receiver is used in France in a region adjacent to a foreign country in order to receive television broadcasts in neighboring countries. Therefore, in such an area, even in a video tape recorder, the PA
There has been a demand for a dual-type video tape recorder that can handle both L-type and SECAM-type television signals.

Therefore, in order to meet such a demand,
A video tape recorder, which has an RF modulator configured to switch between the PAL system and the SECAM system, and can freely process both the SECAM TV signal and the PAL TV signal, has been put into practical use. It has been widely used in these areas for years.

FIG. 6 shows a conventional PAL / SE.
FIG. 2 is a circuit diagram illustrating a configuration of a main part of a CAM switching type RF modulator. The RF modulator shown in FIG. 6 is constituted by a discrete circuit, and supplies a high-frequency signal oscillated by an oscillation circuit to a transformer 30 and a video signal source 3
The high-frequency television signal RF _out is generated by _{superimposing the} video signal given from 1 on the high-frequency signal.

In this circuit, the capacitor 3
3, by controlling the idling current of the Schottky diode 32 by changing the values of the resistor 34, the DC power supply 35, and the like, and varying the modulation degree of the output RF signal, the modulation degree of the PAL system and the SECAM system Of the degree of modulation.

[0009]

A conventionally used RF modulator as shown in FIG. 6 requires a large number of external components due to a discrete configuration (for example, 13
About). In addition, three expensive switches, such as a three-circuit two-contact switch, are required for mode switching for audio input, mixer operating point, and voice oscillation switching. Furthermore, since the modulation degree is adjusted by controlling the idling current of the Schottky diode 32, the variation is large. Therefore, it took a lot of trouble and cost to adjust this. The present invention has been made in view of the above-described problems, and has as its object to provide an inexpensive RF modulator that is small in size, does not require adjustment work, and is inexpensive. I do.

[0010]

An RF modulator according to the present invention comprises a double-balanced mixer for mixing a video signal supplied from a video signal source and a high-frequency signal supplied from an RF carrier generation source; A switching circuit that switches the path through which one of the two output currents from the mixer flows between the PAL method and the SECAM method, and the other of the two output currents that flows
Switching the load resistance value on the path
And change the size according to the change.
By generating voltage, video signals and high-frequency signals
And the RF level at the time of mixing PAL system and S
R caused by the difference in the modulation degree from the ECAM method
A voltage generation circuit for compensating for the deviation of the F level.

[0011]

A switching circuit for switching a path through which one of two output currents from a double-balanced mixer flows, and a load corresponding to the switching of the switching circuit
And a voltage generating circuit for generating a magnitude of the voltage varies by a change in the resistance value is provided, the magnitude of the voltage generated by the voltage generation circuit by adding or subtracting in the PAL system and SECAM system, the above double-balanced mixer Video communication
RF level when mixing signal and high frequency signal
As a result, an output of substantially the same level can be obtained regardless of the PAL system / SECAM system.

[0012]

FIG. 1 is a block diagram showing a main part of an RF modulator according to an embodiment of the present invention. As is clear from FIG.
The RF modulator according to the present embodiment includes a switching circuit 2 that changes the flow of a signal current of a mixer 1 between a SECAM system and a PAL system, and a voltage generation circuit 3 that generates a voltage when the signal current of the mixer 1 flows. It has.

[0013] The mixer 1, as shown in principal part circuit diagram of the RF modulator of Fig. 2, first comprising a first differential connection circuits 1a, and the transistors Q3 and Q4 Metropolitan consisting of transistors Q1 and Q2 A double-balanced mixer composed of two differential connection circuits 1b is used.

The switching circuit 2 includes a differential connection circuit including transistors Q5 and Q6, a switch SW3 whose switching state of its movable contact is controlled by a DC voltage supplied from an external circuit, and a base for the transistors Q5 and Q6. It is composed of base voltage generating circuits 6 and 7 for generating a voltage.

On the other hand, the voltage generation circuit 3 includes resistors RL1 and R
L2, which is connected between the power supply line and the collector of the transistor Q1 constituting the first differential connection circuit 1a. The collector of the transistor Q5 that forms the switching circuit 2 is connected to the contact point A between the resistors RL1 and RL2, and the voltage extracted from the resistor RL and the collector of the transistor Q1 is an RF signal voltage. It is output to the outside as _Vout .

[0016] Further, the high-frequency signal supplied from the RF carrier generating source 5 is adapted to be applied to the bases of the transistors Q1 to Q4, the constant current I ₁ flows to the first differential connection circuit 1a It has been made like that.
This is switched between I ₁ and I ₁ ′ by the switch SW1. On the other hand, a video signal is supplied to the base of the first transistor constituting the differential connection circuit 8, a DC constant voltage is supplied to the base of the second transistor, and the video signal is supplied to the second differential connection circuit 1b. current I ₂ is adapted to be flowed. In this case, the current is switched between I ₂ and I ₂ ′ by the switch SW2.

In the thus configured RF modulator of the present embodiment, it is possible to switch the degree of modulation of the RF output in accordance with the externally applied DC voltage and output the same. That is, the modulation method differs between the SECAM method and the PAL method. As shown in the modulation method explanatory diagram of FIG. 3, the SECAM method performs positive modulation and the PAL method performs negative modulation.

Therefore, the currents I ₂ > I ₁ flowing through the second differential connection circuit 1b by switching the switches SW1, SW2, etc.
The negative modulation can be realized by setting so that the video signal shown in (a) is modulated as shown in the waveform diagram of (b) as shown in the modulation explanatory diagram of FIG. can do. Conversely, by setting the current to be I ₂ <I ₁ , positive modulation can be realized. In this case, the video signal is modulated as shown in the waveform chart (c). Can be.

Therefore, when the current I ₂ is modulated by a video signal, either positive modulation or negative modulation can be determined by selecting the magnitude relationship between the currents I ₂ and I ₁ . The modulation degree differs between the SECAM method and the PAL method. The modulation degree is set to 90% in the SECAM method, and 80% in the PAL method.
Is set to

FIG. 5 shows the relationship between the modulation degree and the RF level. When the modulation degree is m, the maximum voltage level is V _max , and the minimum voltage level is V _min , the modulation degree m
Is determined by the equation _{m = (V max -V min)} ÷ V max × 100 ... (1). Therefore, now (V _max −
If V _min) is constant, the maximum voltage level V _max when the modulation degree of 80% and a V _max = 80dBμ = 10mV, the maximum voltage level V _max when the modulation degree of 90% 8.
89mV, and the difference [Delta] V between the maximum voltage level V _max and the minimum voltage level V _min is a [Delta] V = 8mV. Therefore, a level difference of 1.02 dB occurs between the two. By the way, since the RF output level in the RF modulator is strictly regulated to ± 3 dB in total, it is desired that the RF output level be constant irrespective of positive modulation or negative modulation.

Therefore, in the RF modulator of this embodiment, the level difference of 1.02 dB is compensated by correcting the mixer gain in the double-balanced mixer 1 according to the difference in the PAL / SECAM system. I have. That is, for example, assuming that the video input, the voltage RE across the resistor provided in the differential connection circuit 8 and the magnitudes of the currents I _{1 and} I ₂ are constant, the mixer output i _o1 is also constant.

Therefore, the externally supplied DC
The case where the switch SW3 is switched to the base voltage generating circuit 6 which is the PAL system side by the voltage will be described. In this case, since the transistor Q5 is turned on , the current io1 + current io2 flows through the load resistor RL2.
However, the load resistor RL1 has a collector of the transistor Q1.
Only the current io1 flows. Therefore, the output voltage Voutp of the PAL system is as follows: Voutp = io1 × RL1 (2)

On the other hand, the switch SW3
Is switched to the side of the base voltage generating circuit 7 of the SECAM system, the transistor Q5 is turned off and the transistor Q6 is turned on, so that the current i _o2 does not flow through the resistor RL2 and the collector of the transistor Q1 Current i _o1 will flow through both load resistors RL1 and RL2. Therefore, the output voltage V _outs in the SECAM system is as follows: V _outs = i _o1 × (RL1 + RL2) (3) Here, if (RL1 + RL2) ÷ RL1 ≒ 1.125 (4), the RF output level can be corrected by 1.02 dB, and substantially the same output level can be obtained regardless of the PAL system / SECAM system. .

In this embodiment, since the switching between the PAL system and the SECAM system is performed as described above, it is not necessary to perform the adjustment performed at the time of the switching. Therefore, all functions can be incorporated in the IC, and parts can be reduced by eliminating the need for expensive three-circuit two-contact switches, etc., thereby enabling cost reduction, space saving and unit miniaturization. Can be achieved. Further, a DC voltage for switching between the PAL system and the SECAM system can be supplied from the system control microcomputer, and the switching control can be easily performed.

Further, unlike the conventional method of adjusting the modulation degree by adjusting the idling current of the Schottky diode, the modulation degree is changed by switching the current in the double-balanced mixer 1. Thus, it is possible to set the modulation degree with less variation. For example, since the current ratio can be kept at 2% or less in the same chip, the modulation degree of the PAL system and the SECAM system can be set well in each system. In addition, since the same RF level can be obtained irrespective of the positive / negative modulation, it is possible to conform to the safety standard (FTZ) without any problem and to obtain stable mass productivity. In the above embodiment, the example in which the load resistors RL1 and RL2 are connected in series to constitute the voltage generating circuit 3 is shown. However, the present invention is not limited to this. A voltage corresponding to the magnitude may be generated.

In such an RF modulator, the frequency to be handled is a very high frequency, for example, 540 to 630 MHz. In the present embodiment, however, resistors R1 and R2 for generating a base voltage are used. Since the capacitors C1 and C2 are connected in parallel, the high-frequency impedance can be reduced. That is, the high-frequency impedance is, for example, 1
When 0 pF and the frequency are 540 MHz, it can be set to about 29Ω.

[0027]

According to the present invention, as described above, one of the two output currents from the double-balanced mixer flows.
A switching circuit for switching a path, and the switch
A voltage generating circuit for generating voltages of different magnitudes corresponding to the switching of the switching circuit, and adjusting the magnitude of the voltage generated by the voltage generating circuit between the PAL system and the SECAM system. When the system is switched from the SECAM system to the SECAM system, or vice versa, the system is switched from the SECAM system to the PAL system.
It is possible to automatically correct a change in the RF level due to a difference in the degree of modulation in the AM system.
Therefore, it is not necessary to externally provide a circuit for adjusting the RF output level to the integrated circuit,
This makes it possible to omit expensive three-circuit two-contact switches and other components. Therefore, the cost can be reduced, the unit can be downsized, and the space can be saved.

[Brief description of the drawings]

FIG. 1 is a configuration diagram illustrating a main configuration of an RF modulator according to an embodiment of the present invention.

FIG. 2 is a circuit diagram of a main part of the RF modulator of FIG.

FIG. 3 is an explanatory diagram showing a difference between positive modulation and negative modulation.

FIG. 4 is a waveform diagram showing a difference between when a video signal is positively modulated and when it is negatively modulated.

FIG. 5 is an explanatory diagram showing a difference in a modulation degree between the PAL system and the SECAM system.

FIG. 6 is a circuit diagram showing an example of a conventional circuit for correcting the degree of modulation.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Double balance type mixer 2 Switching circuit 3 Voltage generation circuit 4 Video signal source 5 RF carrier generation source RL1, RL2 Load resistor

Claims (1)

(57) [Claims] 1. A double-balanced mixer for mixing a video signal supplied from a video signal source and a high-frequency signal supplied from an RF carrier generation source, and one of two output currents from the double-balanced mixer. A switching circuit for switching the path through which the current flows between the PAL system and the SECAM system, and a load on the path through which the other of the two output currents flows
The resistance value changes according to the switching of the above switching circuit.
And generate voltages of different magnitudes according to the change.
When mixing video signals and high-frequency signals,
And a voltage generation circuit for correcting an RF level deviation of the PAL system and a RF level deviation caused by a difference in modulation degree between the PAL system and the SECAM system.
F modulator.