JPH01114183A - Power supply superimposing circuit for monitor camera system - Google Patents

Abstract

PURPOSE:To obtain a power supply superimposing signal separation circuit with less power consumption and capable of balancing transmission by connecting each electrode of NPN and PNP transistors(TRs) as desired so as to couple the electrodes. CONSTITUTION:Emitters of NPN and PNP TRs 17a, 17b are connected via a resistor 19a, a Zener diode 21 and a resistor 19b. Capacitors 18a, 18b are connected between the emitter and base of the TRs 17a, 17b. The power to the camera is supplied to a power voltage application terminal 10 to which a collector of the TR 17a is connected and the collector of the TR 17b is connected to ground. Thus, nearly constant voltage is obtained between terminals 9, 9 to which a cable is connected. The input impedance at a video frequency band depends on the sum of the resistors 19a, 19b when the capacitors 18a, 18b are sufficiently large. The input video signal from the terminal 9 is outputted to a video signal output terminal 11 via a balance/unbalance conversion circuit 12. Since the base current is very small, the power consumption through the resistors 19a, 19b is very small. The voltage drop between the terminals 10, 9 is very small.

Description

[Detailed Description of the Invention] [Technical Field] The present invention relates to a power superimposing circuit for a surveillance camera system in which a pair of parallel two-wire cables is used to supply power to a surveillance camera and to transmit video signals from the camera to a monitor television. It is.

[Background technology]

FIG. 1 shows a block diagram of a system that supplies power to the camera and transmits video signals from the camera to the monitor television using a pair of parallel two-wire cables.

1 is a monitor television, 2 is a power supply circuit, 3 is a balanced-unbalanced conversion circuit, 4 is a power supply superimposed signal separation circuit, 5 is a power supply separation signal mixing circuit, 6 is a surveillance camera, 7 is an unbalanced-balanced conversion circuit, 8 is a parallel two-wire cable. Here, the video signal from the camera is converted into a balanced signal by the unbalanced-balanced conversion circuit 7, and then sent out via the power supply separation signal mixing circuit 5. The signal is inputted to the power supply superimposed signal separation circuit 4 via the parallel two-wire cable 8, converted into an unbalanced signal by the balanced-unbalanced conversion circuit 3, and sent to the monitor television 1. Power is supplied to the surveillance camera using a DC power supply.

After being generated by the circuit 2, it is superimposed on the cable by the power superimposed signal separation circuit 4, and after being separated from DC by the power separation signal mixing circuit 5, it is supplied to the monitoring camera 6. Here, the output impedance of the video signal frequency band sent from the power source separation signal mixing circuit 5 and the input impedance of the power source superimposed signal separation circuit 4 must match the characteristic impedance of the parallel two-wire cable. Otherwise, the deterioration of the transmission frequency characteristics will cause deterioration of the image on the monitor television. In addition, in order to maintain balanced transmission of signals on the parallel two-wire cable 8, the power superimposed signal separation circuit 4
It is necessary that the impedances between each of the pair of input terminals and the ground be equal, and similarly, the impedance between each of the output terminals of the power supply separation signal mixing circuit 5 and the ground must be equal. Otherwise, the balanced transmission of the signal will be disrupted. Further, it is a condition that the DC voltage at the input terminal of the power supply superimposed signal separation circuit 4 is also constant.

As a method generally used as a power supply superimposed signal separation circuit in a balanced transmission system, there is a method shown in FIGS. 2 and 3. 10 is a power supply voltage application terminal, 11 is a video signal output terminal connected to a monitor television, 12 is a balanced/unbalanced conversion circuit, 13a and 13b are resistors that determine the input impedance, and 14a and 14b are DC shielding capacitors.

15 is an AC signal shorting capacitor. The balanced/unbalanced conversion circuit 12 has high input impedance and includes resistors 13a,
13b allows the input impedance to be determined. In this circuit, compared to the voltage applied to the power supply voltage application terminal 10,
Since the voltage between terminals 9 and 9 is reduced by resistors 13a and 13b, a higher voltage must be applied. Furthermore, heat generation due to power consumption in the resistors 13a and 13b poses a problem. For example, if the resistance value is 75Ω and the camera current consumption is 300mA, the voltage drop across the resistance is 22.5V and the power consumption is 6.
．． 75W, which makes this method impractical.

FIG. 3 shows another method. 15a and 15b are choke coils, and 16a and 16b are impedance matching resistors. The problem with this circuit system is that since the video signal frequency ranges from several tens of Hz to several MHz, it is difficult to increase the impedance of the choke coil in the low frequency range, and the shape also becomes large. be.

[Purpose of the invention]

SUMMARY OF THE INVENTION An object of the present invention is to provide a power superimposed signal separation circuit for a parallel two-wire cable balanced transmission system that consumes less power, provides a constant voltage with less voltage drop, and can perform impedance matching in the video signal frequency band.

[Disclosure of the invention]

The present invention provides a balanced signal transmission system in which a power supply voltage is superimposed on a signal line, in which a capacitor is connected between the emitter bases of an NPN transistor and a PNP transistor, and a capacitor is connected between the bases of each other via a resistor and a Zener diode. The collector of one transistor is connected to a power source, the other end is connected to a power source, and each emitter is connected to a signal line connection terminal. An embodiment according to the invention is shown in FIG.
17a is an NPN) transistor, 17b is a PNP transistor, 18a and 18b are transistors connected between the emitter and base, 19'a and 19b are resistors, 20a,
20b is a bias current supply resistor, and 21 is a Zener diode.

Power supply to the camera is applied to the power supply voltage application terminal 10, and a voltage is generated between the terminals 9 and 9. The voltage between terminals 9 and 9 is Vo=Vz + Ra Ina+ Ra l118
+ VEBA + VERBv2: Zener voltage of Zener diode IsA: Base current of 17a l5ll: Base current of 17b vEIIA = Emitter-base voltage of 17a (= 0
．． 7V) Visa: 17b emitter-base voltage (#0.7V
), and a nearly constant voltage can be obtained.

The input impedance in the video frequency band is Ztn=RA+R,+r if the values of the capacitors 18a and 18b are sufficiently large.

r2: Zener diode AC resistance Zill#RA
+R.

is given by Therefore, the input impedance is R
It is determined by A+R.

The video signal input from the terminal 9 is output via the balanced/unbalanced conversion circuit 12 to the video signal output terminal 11 connected to a monitor television.

The power consumed by the resistor 19a or 19b is P=RI
Slightly more than B2.

The voltage drop between the terminals 9 and 9 with respect to between the power supply voltage application terminal and the ground is only the collector-emitter voltage of the two transistors, and this is also small.

〔Effect of the invention〕

In the power supply superimposition circuit according to the present invention, a capacitor is connected between the emitter and the base of each of the NPN type transistor and the PNP type transistor, the base of each transistor is connected via a resistor and a reference voltage source, and the collector of one transistor is connected to the base of each transistor through a resistor and a reference voltage source. is grounded, the power supply voltage is applied to the collector of the other transistor, and the emitter of each transistor is connected to the input terminal, so the input impedance can be set to a constant value for AC, and the voltage between the applied voltage and the input terminal The drop can be reduced, and therefore power consumption can be reduced.
Since the input terminal has a high impedance with respect to ground, there is an effect that balanced transmission can be performed.

[Brief explanation of the drawing]

1 to 3 show conventional examples, in which FIG. 1 is an explanatory diagram of a camera power superimposition system, and FIGS. 2 and 3 are power superimposed signal separation circuits. FIG. 4 shows a power supply superimposed signal separation circuit showing an embodiment of the present invention. 17a... NPN type transistor, 17b... PNP type transistor, 18a, 18b... Capacitor, 19a
, 19b...Resistor, 21... Zener diode.

Claims (1)

[Claims] In a signal balanced transmission system in which a power supply voltage is superimposed on a signal line, a capacitor is connected between the emitter bases of an NPN transistor and a PNP transistor, and the bases of each are connected via a resistor and a Zener diode. A power supply superimposition circuit for a surveillance camera system, characterized in that a collector of the transistor is connected to a + power supply, the other end is connected to a - power supply, and each emitter is connected to a signal line connection terminal.