JPH07106870A - Video signal isolation amplifier circuit - Google Patents

Abstract

PURPOSE:To realize excellent DC stability, linearity and low power consumption while securing required insulation and frequency characteristics. CONSTITUTION:A carrier signal is fed to an input winding of a transformer T1 of a balanced modulation circuit 33, an input video signal is fed between a neutral point of an output winding of the transformer T1 and a neutral point of an input winding of a transformer T2 via an input circuit 30 and a modulated signal is outputted from an output winding of the transformer T2. Said carrier signal is fed to an input winding of a transformer T4 of a balanced demodulation circuit 34, a modulated signal is fed from the balanced modulation circuit 33 to the input winding of the transformer T3 and a demodulation signal is outputted between a neutral point of an output winding of the transformer T3 and a neutral point of an output winding of the transformer T4. An amplifier circuit 39 of an output circuit 36 amplifies the demodulation signal received from the balanced demodulation circuit 34.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a video signal isolation amplifier, and more particularly to a video signal isolation amplifier utilizing a balanced modulation circuit.

[0002]

2. Description of the Related Art In a device that handles video signals, when the ground potential on the signal supply side and the ground potential on the signal reception side are different, or inductive noise is generated around the cable connecting the signal supply side and the reception side. In some cases, it is desirable to be able to insulate the signal supply side and the signal reception side in order to prevent the effects of noise. For this reason, an insulation amplifier circuit capable of transmitting a video signal is required.

Conventionally, there are various circuit systems for the isolation amplifier. One method is to use a transformer. This system has a simple structure and can secure a sufficient withstand voltage, but the usable signal frequency range is limited by the characteristics of the transformer.

The most widely used method is a method of combining a V / F converter, an insulation transformer and an F / V converter. This method theoretically has no limitation on the usable frequency band. However, in reality, there are restrictions in design and manufacturing.

[0005]

In the conventional isolation amplifier using a transformer, the usable frequency is determined by the characteristics of the transformer. In reality, there is no wide-band transformer capable of transmitting from a direct current or commercial power supply frequency to a high frequency of several MHz or more. Therefore, this type of isolation amplifier has a problem that it cannot transmit a video signal.

Further, a V / F converter, an insulation transformer,
With an isolation amplifier combined with an F / V converter, it is difficult to secure DC stability and linearity when the specifications are set so that a video signal can be transmitted.
There is a problem that a considerable amount of power must be supplied.

The present invention has been made in view of the above points, and an object of the present invention is to provide a video signal isolation amplifier having good DC stability and linearity and low power consumption.

[0008]

The invention according to claim 1 is the first
And a second transformer, and a switching element connected between the output side winding of the first transformer and the input side winding of the second transformer, and the carrier signal is supplied to the input side winding of the first transformer. Is supplied, and an input video signal is supplied between the midpoint of the output side winding of the first transformer and the midpoint of the input side winding of the second transformer, from the output side winding of the second transformer. It is composed of a balanced modulation circuit for outputting a modulated signal, third and fourth transformers, and a switching element connected between the output side winding of the third transformer and the output side winding of the fourth transformer. , A carrier signal having the same phase and the same polarity as the carrier signal is supplied to the input side winding of the fourth transformer, and the modulated signal is supplied to the input side winding of the third transformer from the balanced modulation circuit. Output side of the output side of the 3rd transformer and the output side of the 4th transformer From between the midpoint of the line, and configured to have a balanced demodulator circuit for outputting a demodulated signal, and an amplifier circuit for amplifying the demodulated signals supplied from the balanced demodulator.

According to the second aspect of the present invention, the second transformer and the third transformer are constituted by one transformer in which the output side winding of the second transformer is the output side winding of the third transformer. .

According to a third aspect of the present invention, in the first transformer and the fourth transformer, the input side winding of the first transformer and the input side winding of the fourth transformer are the same winding. Composed of a transformer.

[0011]

According to the first aspect of the present invention, the balanced modulation circuit converts an input video signal into a modulated signal, which is transmitted to the balanced demodulation circuit through the first and second transformers, and the balanced demodulation circuit performs the video. Demodulate the signal. Therefore, it is possible to transmit a video signal in a required band while ensuring a sufficient withstand voltage. In addition, the circuit configuration is simple, and it is possible to realize good DC stability, linearity, and low power consumption.

According to the second aspect of the present invention, since the required transformers can be reduced to three, it is possible to reduce the cost and the occupied volume.

According to the third aspect of the present invention, the number of transformers required can be reduced to two, so that the cost and the volume occupied can be further reduced.

[0014]

1 is a circuit diagram of a video signal insulation amplifier circuit according to a first embodiment of the present invention. The video signal insulation amplification circuit of FIG. 1 includes an input circuit 30, a balanced modulation circuit 33, and a balanced demodulation circuit 3.
4 and an output circuit 36.

Here, an outline of the operation of the video signal insulation amplifier circuit of FIG. 1 will be described. Balanced modulation circuit 33 terminal P
_3, the terminal P _13, P ₁₄ of P ₄ and the balanced demodulator circuit 34, the carrier signal is supplied from the carrier signal generator 35.

The input video signal supplied from the input terminal CN _I is supplied to the input terminals P ₁ and P ₂ of the balanced modulation circuit 33 via the input circuit 30. The balanced modulation circuit 33 generates a modulated signal by modulating the carrier signal with a video signal, and supplies the modulated signal to the terminals P ₁₁ and P ₁₂ of the balanced demodulation circuit 34. The balanced demodulation circuit 34 demodulates the video signal and supplies it to the output circuit 36 from the terminals P ₁₅ and P ₁₆ . The output circuit 36 amplifies the video signal demodulated and output from the output terminal CN _O. In addition,
The input video signal and the output video signal are insulated from each other by the transformer T2 of the balanced modulation circuit 33 and the transformer T3 of the balanced demodulation circuit 34, which will be described later.

The respective parts of the video signal insulation amplifier circuit of FIG. 1 will be described below. The input circuit 30 includes a resistor R ₁ ,
It consists of a fixed attenuator 31 consisting of R ₂ and R ₃ and a common mode balun 32. Video signal supplied from the input terminal CN _I can be attenuated by the fixed attenuator 31 to an amplitude that does not cause distortion in a balanced modulator circuit 33, it is supplied to the signal input terminal P _1, P ₂ of the balanced modulation circuit 33.

If the input video signal supplied unbalanced by the coaxial cable is supplied as it is to the input terminals P ₁ and P ₂ of the balanced modulation circuit 33 which is a balance element, the common-mode signal suppression ratio becomes worse. Therefore, the common mode balun 32 is provided to improve the common-mode signal suppression ratio.

Next, the balanced modulation circuit 33 will be described.
It FIG. 2 shows an explanatory diagram of the balanced modulation circuit 33. Balanced modulation
The circuit 33 includes a first transformer T1 and a second transformer T
2. Diode D which is a switching element₁~ D_FourFrom
Composed. Diode D₁~ D _FourHas all the characteristics
Use one.

Input side winding terminals P ₃ , P of the transformer T1
The _4, sufficient amplitude carrier signal to cause the switching diode D ₁ to D ₄ is supplied from the carrier signal generator 35. Terminal P _{1 at} the midpoint of the output side winding of the transformer T1
Between the terminal P _{2 at} the midpoint of the input side winding of the transformer T2 and
A video signal is supplied as a modulation signal from the input circuit 30. The carrier signal is set to have a frequency sufficiently higher than that of the modulated signal. In addition, the amplitude of the modulation signal is the diode D
_The amplitude is made sufficiently small with respect to the carrier signal for switching _{1 to} D ₄ .

The operation of the balanced modulation circuit 33 will be described below. When a carrier signal is supplied to the terminals P ₃ and P ₄ ,
Depending on the polarity of the carrier signal generated in the output winding of the transformer T1, or the diode D _1, D ₃ is conductive, or, diode D _2, D ₄ are rendered conductive. The modulated signal is at terminals P ₁ and P
₂ is not supplied to the input side winding of the transformer T2, no current flows, and the output side winding of the transformer T2 (terminal P
_No signal appears at ₁₁ and P ₁₂ ).

When the modulation signal is supplied to the terminals P ₁ and P ₂ , while the diodes D ₁ and D ₃ are conducting, as shown in FIG.
In the middle, a current due to the modulation signal flows between the point M ₁ of the input side winding of the transformer T2 and the middle point (broken line i _{1 in the} figure). While the diodes D ₂ and D ₄ are conducting, the transformer T
A current due to the modulation signal flows between the point M ₂ and the middle point of the second input side winding (broken line i _{2 in the} figure).

Therefore, the modulation signals of the same polarity (in FIG. 2,
Is terminal P₂Against P₁Is positive),
Diode D₁, D₃While the diode is conducting and the diode D
₂, D _FourWhile the line is conducting, the broken line arrow i₁, I₂
As shown in, the current flowing in the input side winding of the transformer T2
Of the signal generated in the output side winding of the transformer T2 due to the different direction of
The polarity of the number is reversed.

As described above, the terminal P₃, P_FourIs supplied to
Terminal P depending on the polarity of the carrier signal ₁, P₂Is supplied to
The modulated signal whose polarity is switched is
P₁₁, P₁₂Generated in the meantime.

Terminal P₁, P₂The modulation signal supplied to
If the flow component is not included, the terminal P₁₁, P₁₂Generated in between
The modulated signal is a carrier suppressed amplitude modulated signal (DSB)
Become. Also, the terminal P₁, P₂The modulation signal supplied to
If a flow component is included, the terminal P ₁₁, P₁₂The cover that is generated between
The carrier wave component corresponding to the DC component is added to the modulated signal.
The resulting amplitude modulated signal.

Next, the balanced demodulation circuit 34 will be described. FIG. 3 shows an explanatory diagram of the balanced demodulation circuit 34. The balanced demodulation circuit 34 has the same configuration as the balanced modulation circuit 33,
Third transformer T3, the fourth transformer T4, composed of diode D ₁₁ to D ₁₄ is a switching element. As the diodes D _{11 to} D ₁₄ , those having uniform characteristics are used.

Terminals P ₁₃ , P of the input side winding of the transformer T4
_A carrier signal generator 35 supplies to ₁₄ a carrier signal of sufficient amplitude to switch the diodes D ₁₁ -D ₁₄ . Input side winding of transformer T3 (terminal P ₁₁ ,
The modulated signal is supplied from the balanced modulation circuit 33 to P ₁₂ ).

The operation of the balanced demodulation circuit 34 will be described below. When a carrier signal is supplied to the terminals P ₁₃ and P ₁₄ ,
Depending on the polarity of the carrier signal generated in the output winding of the transformer T4, the diode D _13, or D ₁₂ is conductive, or, diode D _14, D ₁₁ are turned on. The modulated signal is the terminal P ₁₁ ,
If it is not supplied to P ₁₂ , no signal appears at the output terminals P ₁₅ and P ₁₆ .

The modulated signal is the terminal P₁₁, P₁₂Supplied to
If the diode D₁₃, D₁₂While the
3, the output winding of the transformer T3 is at the midpoint and M₁₂Between
Signal is output terminal P₁₅, P₁₆Will be output in between. Also,
Iodo D₁₄, D₁₁Output of transformer T3 while
Midpoint of power winding and point M₁₁The signal between and is the output terminal
P ₁₅, P₁₆Will be output in between.

[0030] Assuming that the same polarity signal is present at the output winding of the transformer T3, the diode D _13, D ₁₂
Is ON and the diodes D ₁₄ and D ₁₁ are ON, the polarities of the signals output between the terminals P ₁₅ and P ₁₆ are opposite.

In practice, as mentioned above, the modulated signal is
This is a signal in which the polarity of the original modulation signal is switched according to the polarity of the carrier signal of the carrier signal generator 35, and the terminals P ₁₃ , P
₁₄ is the same as this (that is, the phase and polarity match)
A carrier signal is being supplied.

Therefore, the terminal P₁₃, P₁₄To be supplied to
Depending on the polarity of the transmission signal, diode D ₁₃, D₁₂And Daio
De D₁₄, D₁₁At the same time that the switch T2
Carrier signal with the same polarity of the modulated signal generated on the output side winding
Has been switched by. Therefore, the output terminal P₁₅, P
₁₆Between the terminals P of the balanced modulation circuit 33₁, P₂Supply in between
A demodulated signal with the same polarity as the original modulated signal
It Demodulation of the modulated signal by the balanced demodulation circuit 34 is
Generally, it is called synchronous detection.

The demodulated signals generated at the output terminals P ₁₅ and P ₁₆ are signals including the DC component of the video signal supplied to the terminals P ₁ and P ₂ of the balanced modulation circuit 33. However, since the balanced modulation circuit 33 and the balanced demodulation circuit 34 have inherent losses, the demodulated video signal is a signal attenuated by this loss.

The frequency of the carrier signal depends on the video signal to be handled.
Set sufficiently higher than the upper limit of the effective frequency components included.
Set. The frequency of the carrier signal is f_CAnd video signal
The frequency of (modulated signal) is f_sThen the frequency of the modulated signal
The range of wavenumber is f_C-F_S~ F _C+ F_SBecomes Equilibrium change
Of the transformers T1 and T2 of the adjustment circuit 33 and the balanced demodulation circuit 34
The transformers T3 and T4 have a frequency f of this modulated signal._C−
f_S~ F_C+ F_SIn, if the signal can be transmitted faithfully
Good. Frequency f of carrier signal_cBy setting
Balance signal that can transmit video signals of 100 MHz or more.
It is possible to realize the adjustment circuit 33 and the balanced demodulation circuit 34.
is there.

As can be seen from the circuit of FIG. 1, the video signals supplied to the terminals P ₁ and P ₂ of the balanced modulation circuit 33 and the video signals output from the terminals P ₁₅ and P _{16 of the} balanced demodulation circuit 34 are , And transformers T2 and T3 for insulation. The transformers T2 and T3 need only be able to transmit the modulated signal,
Since the passing power is also very small, it is easy to manufacture a transformer having a withstand voltage of about 1500V. In the case of a device that handles video signals, it is usually sufficient to have a withstand voltage of several tens of V when connecting devices having different ground potentials or when preventing the influence of inductive noise.

Next, the output circuit 36 will be described. The output circuit 36 is comprised of an attenuator 37, a low pass filter 38, amplifier 39, resistor R _23. Output terminal of the amplifier circuit 39 via the resistor R _23, the output terminal CN _O of the video signal
It is connected to the. The attenuator 37 includes resistors R ₁₁ , R ₁₂ , R
₁₃ and R ₁₄ , and serves to reduce the influence of reflection on the low-pass filter 38 side. The low-pass filter 38 suppresses the residual carrier component contained in the signal demodulated by the balanced demodulation circuit 34, and reduces the noise outside the band. The resistor R ₁₃ is a load resistor that matches the impedance of the circuit.

The amplifier circuit 39 includes a differential amplifier A ₂₁ and a resistor R.
₂₁ and R ₂₂ . As the amplifier A ₂₁ , one having a sufficient gain in the band of the video signal to be handled is used. Amplifier circuit 39, the amplifier circuit 3 from the input terminal CN _I of the video signal
It has a gain that compensates for the amount of attenuation in the signal path to the 9 inputs. Accordingly, the output terminal CN _O of the video signal,
The same level of the signal and the video signal supplied to the input terminal CN _I is outputted. The resistor R ₂₃ is a resistor having a value equal to the impedance of the circuit.

As described above, the video signal insulation amplifier circuit of this embodiment can transmit a video signal from DC to 100 MHz or more, and can realize a circuit having a sufficient withstand voltage. Further, since the balanced modulation circuit 33 and the balanced demodulation circuit 34 are less likely to vary in terms of direct current and have good linearity, it suffices to ensure the direct current stability and linearity of the amplifier circuit 39, and to obtain sufficient direct current stability. Linearity can be easily realized.

Further, a V / F converter, an insulation transformer,
Unlike the system in which the F / V converter is combined, a DC power supply is not required on the input side of the video signal (the side of the balanced modulation circuit 33), and only a high-frequency carrier signal with low power is required. Since the power consumption of the entire circuit is mostly for the amplifier circuit 39, V
Power consumption can be significantly reduced as compared with the system in which the / F converter, the insulation transformer, and the F / V converter are combined.

Further, the circuit configuration is simple, and since the balanced modulation circuit 33 and the balanced demodulation circuit 34 are all composed of passive components, they are essentially reliable and have low noise. Moreover, since the circuit configuration is simple, the cost can be reduced.

Further, since the coupling capacitance between the input side and the output side is only a small capacitance between the windings of the transformer, the coupling capacitance between the input side and the output side is smaller than that of the conventional system, and the coupling capacitance with respect to the alternating current is small. Excellent insulation performance.

Next, a specific example of the circuit shown in FIG. 1 will be described. Here, it is assumed that a video signal having a specified amplitude of 1 Vpp including a signal component from DC to 10 MHz is handled. The impedance of the video signal is 75Ω.

The fixed attenuator 31 of the input circuit attenuates the 1 Vpp video signal into a ⅕ 0.2 Vpp signal. The carrier signal generator 35 includes a balanced modulation circuit 33 and a balanced demodulation circuit 3.
4 is supplied with a carrier signal of 10 dBm. The frequency of the carrier signal is preferably about 5 times the upper limit frequency of the video signal, and in this example, it is 50 MHz.

The transformers T1 to T4 are transformers having the same specifications and have a winding without an intermediate terminal and a winding with an intermediate terminal. The number of turns of the winding without the intermediate terminal is equal to the number of turns between one end of the winding with the intermediate terminal and the intermediate terminal. For example, the trifilar is wound four times with a 0.2φ formal copper wire. In addition, a high frequency transformer is used as the core of the transformer. Further, as the withstand voltage, a sufficient withstand voltage of about 1500 V can be obtained. The frequency characteristics of the transformer are:
It suffices that a signal of 40 MHz to 60 MHz can be transmitted without distortion, and a transformer having this performance can be easily manufactured.

The diodes D _{1 to} D ₄ and D _{11 to} D ₁₄ are
For high frequency switching, use one with uniform characteristics. For example, a characteristic selection product of 1SS43 is used.

The attenuation amount of the attenuator 37 of the output circuit 36 is about 3 dB. Further, the load resistance R ₁₃ and the output resistance R ₂₃
Is 75Ω according to the impedance of the video signal. Further, the gain of the amplifier circuit 39 is set to about 30 dB in accordance with the attenuation amount of the entire signal path.

In the above example, a sufficient frequency characteristic is obtained for a video signal of DC to 10 MHz. Also,
DC stability, linearity, and S / N ratio have obtained satisfactory performance for video signals. Moreover, the power consumption of all the circuits could be suppressed to a low level of about 200 mW.

FIG. 4 is an explanatory view of a video signal insulation amplifier circuit of the second embodiment of the present invention. The input circuit and the output circuit of the second embodiment are the same as the input circuit 30 and the output circuit 36 of the first embodiment shown in FIG.

In FIG. 1, the transformer T2 is arranged so that the output side winding of the transformer T2 becomes the output side winding of the transformer T3.
It is clear that 2 and T3 can be combined in one transformer. In the second embodiment, the transformers T2 and T3 of the first embodiment are combined into one transformer T5. In the second embodiment, the number of transformers required for the first embodiment can be reduced by one, the cost can be reduced and the volume occupied can be reduced accordingly. The operation of the circuit is exactly the same as that of the first embodiment.

FIG. 5 is an explanatory diagram of a video signal insulation amplifier circuit of the third embodiment of the present invention. The input circuit and the output circuit of the third embodiment are the same as the input circuit 30 and the output circuit 36 of the first embodiment shown in FIG.

In FIG. 4, the same carrier signal is supplied to the input side winding of the transformer T1 and the input side winding of the transformer T4. Therefore, the transformers T1 and T4 can be replaced with one transformer. In the third embodiment,
The transformers T1 and T4 are replaced with one transformer T11. Transformer T11, the input winding of the carrier signal (the terminal P _3, P _4), the diode _{D 1 ~D 4, D 11 ~D} 14
Phase-divided winding for switching (output side winding)
It is a transformer with two windings and a total of three windings.

The transformer T12 is a combination of the transformers T2 and T3 of the first embodiment, like the transformer T5 of the second embodiment.

In the third embodiment, only two transformers T11 and T12 are used, and the required transformer can be reduced by one compared with the second embodiment, and the cost and the occupied volume can be reduced. It can be reduced. The operation of the circuit is exactly the same as that of the first embodiment.

The windings of the terminals P ₂₁ and P ₂₂ of the transformer T12 are windings provided when the circuit of FIG. 5 is used as a modulator for a general communication device, not as an insulating amplifier circuit. In this case, the demodulated signals output from the terminals P ₁₅ and P ₁₆ can be used as a negative feedback signal for monitoring the modulator or improving the linearity of the modulator.

[0055]

As described above, according to the invention of claim 1,
The balanced modulation circuit converts an input video signal into a modulated signal, transmits the modulated signal to the balanced demodulation circuit via the first and second transformers, and demodulates the video signal by the balanced demodulation circuit. It is possible to transmit a video signal in a required band while maintaining the above-mentioned characteristics, has a simple circuit configuration, and can achieve good DC stability and linearity and low power consumption. .

According to the invention of claim 2, the required transformer can be reduced to three, so that the cost and the occupied volume can be reduced.

In the invention of claim 3, the required transformer can be reduced to two, so that the cost and occupied volume can be further reduced.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a video signal insulation amplifier circuit according to a first embodiment of the present invention.

FIG. 2 is an explanatory diagram of a balanced modulation circuit.

FIG. 3 is an explanatory diagram of a balanced demodulation circuit.

FIG. 4 is an explanatory diagram of a video signal insulation amplifier circuit according to a second embodiment of the present invention.

FIG. 5 is an explanatory diagram of a video signal isolation amplifier circuit according to a third embodiment of the present invention.

[Explanation of symbols]

30 Input Circuit 31 Fixed Attenuator 32 Common Mode Balun 33 Balanced Modulation Circuit 34 Balanced Demodulation Circuit 35 Carrier Signal Generator 36 Output Circuit 37 Attenuator 38 Low-pass Filter 39 Amplification Circuit T1 to T4, T5, T11, T12 Transformer D ₁ ~ D ₄ , D ₁₁ ~ D ₁₄ Switching diode

Claims (3)

[Claims] 1. A first transformer and a second transformer, and a switching element connected between an output side winding of the first transformer and an input side winding of the second transformer. The carrier signal is supplied to the input winding, and the input video signal is supplied between the midpoint of the output winding of the first transformer and the midpoint of the input winding of the second transformer. It is connected between the balanced modulation circuit that outputs the modulated signal from the output side winding of the transformer, the third and fourth transformers, and the output side winding of the third transformer and the output side winding of the fourth transformer. And a carrier signal having the same phase and the same polarity as the carrier signal is supplied to the input side winding of the fourth transformer, and the carrier wave of the third transformer from the balanced modulation circuit is supplied to the input side winding of the third transformer. The modulation signal is supplied, and the output coil of the third transformer has a middle point and a fourth point. Video signal insulation characterized by having a balanced demodulation circuit for outputting a demodulation signal from the midpoint of the output side winding of the transformer and an amplification circuit for amplifying the demodulation signal supplied from the balanced demodulation circuit. Amplifier circuit. 2. The second transformer and the third transformer are constituted by one transformer in which an output side winding of the second transformer is an output side winding of the third transformer. The video signal insulation amplifier circuit according to claim 1. 3. The first transformer and the fourth transformer are composed of one transformer in which an input side winding of the first transformer and an input side winding of the fourth transformer are the same winding. The video signal insulation amplifier circuit according to claim 2, wherein