KR880001799B1 - Pulse frequency modulator for video signal transmission - Google Patents

Abstract

The method is for improving the linearity and modulation degree of PFM in optical telecommunication. A condenser (C) discharges to flow current (Ii) proportional to an input signal (Vi) through a TR2 in which the gradient of Ii/C represents a variation of the oscillation frequency. A mono-stable oscillator (106) provides a pulse having constant width to turn on the TR2, then the condenser (C) is charged. The oscillation is continued by repeating these processes. An auxiliary restart circuit (105) is added to restart the oscillation automatically in case of interruption of the oscillation.

Description

Pulse frequency modulator for video signal transmission

1 is a schematic configuration diagram of a transmission device for optical communication.

2A is a schematic configuration diagram of an apparatus of the present invention.

Figure 2b is a detailed circuit diagram of the device of Figure 2a.

3a is a waveform diagram generated at each part of FIG. 2b.

Figure 3 (b) is a graph of the voltage to current ratio for explaining the operation of the present invention.

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a pulse frequency modulation device installed in a video signal transmission device for transmitting a video signal. In particular, in the analog video signal transmission method used for optical fiber communication, the modulation degree and linearity of the PFM (pulse frequency modulation) method The present invention relates to a pulse frequency modulation device for image signal transmission that has been greatly improved.

In general, color video signal transmission using optical fiber is largely divided into digital transmission and analog transmission, and digital transmission is widely used for long-distance use. However, since high-speed analog digital signal must be modulated and demodulated by PCM (pulse code modulation). A / D) converters and high-speed digital analog (D / A) converters are expensive because they must be used.

Although the analog method is widely used for short-range video signal transmission, the signal-to-noise ratio (SNR) and modulation linearity are poor, and thus, it is not used for long-distance transmission.

An object of the present invention is to provide a pulse frequency modulator having excellent modulation and modulation linearity characteristics by transmitting an image signal through a PFM method of an analog transmission method, which is an economical transmission method.

In the conventional pulse frequency modulation method, a method of applying a VCO, a bistable oscillator, or a high frequency oscillator is used.

In the VCO method, an active oscillator is configured to modulate using a varactor's voltage-to-capacitance change. Therefore, the signal-to-noise ratio (SNR) characteristic is limited because the linear region is narrow and the frequency shift is small due to the varactor's characteristics.

The application of bistable oscillator is modulated by the change of the circuit's resistance and the capacitor's time constant, which also has a narrow linear range and low frequency shift due to the characteristics of the resistor and capacitor.

In addition, this method is unavoidable in the low frequency band for stable operation.

The high frequency oscillation circuit is applied by modulating the input signal with two different high frequency oscillators (approximately 10 times the frequency band used) by the frequency shift and then transmitting the difference. Although it can be secured, the circuit is complicated and there is a problem in handling the high frequency.

The present invention is a pulse frequency modulator that composes a simple circuit and compensates for these different disadvantages, which is controlled by a short pulse forcibly to control charge discharge, and by using a high-speed digital IC in the charge discharge control circuit. The frequency can be obtained, and the linear characteristics of the modulator can be improved by using the voltage-to-current amplification characteristics of the transistors rather than the characteristics of the resistors and the capacitors (RCs) in the discharge circuit, thereby obtaining a wide frequency shift.

In addition, the initial oscillation condition becomes a problem in this circuit, including the detection of the presence of an oscillation signal and forcibly oscillating. This modulator has a very simple circuit configuration and can be modulated with excellent linear characteristics from 10MHz to 90MHz, and it can be used to construct an optical image signal transmission device at a low price.

First, referring to FIG. 1, an overall open circuit diagram of an optical communication transmission apparatus using the PFM modulator 10 according to the present invention is shown, wherein the color image signal input terminal 1 is an image signal processor 4 Through the voice signal input terminal 2 and the mixer 7 through the voice signal processor 5 and the frequency modulator 8, and the data signal input terminal 3 through the data buffer 6 and the frequency modulator 9 are connected to the mixer 7 so that the color image signal, the audio signal and the data signal are mixed in the mixer 7 and applied to the PFM modulator 10, and the PFM modulator 10 pulses. Frequency modulation is applied to the optical transmitter 11 with its output. Then, the semiconductor laser 12 connected to the optical transmitter 11 generates a predetermined optical signal and transmits the optical signal to the optical fiber. The characteristics of the PFM modulator 10 should be improved in order for the characteristics of the optical communication apparatus to be improved.

Referring now to the configuration of the PFM modulator 10 of the present invention with reference to FIG. 2A, the output of the discharge circuit 102 to which the signal input terminal 101 is connected and the output of the braking operation circuit 105 are the charging circuit 103. The output of the comparator 104 is output to the output terminal 107 through the monostable oscillator 106 and the output of the monostable oscillator 106 is the charging circuit 103. This configuration is of feedback type and the devices used here are ultra-fast devices.

Referring to Figure 2b will be described the operation of the configuration of Figure 2a as follows.

은 단안정 발진기(IC₂)의 클럭입력(C)에 연결되어 있고, 그 출력(Q)은 리세트(R)입력과 가변저항(R₅)을 통하여 트랜지스터(Tr₃)의 베이스에 연결되어 있다. 출력

은 가변저항(R₄)을 통해 트랜지스터(Tr₂)의 베이스에 연결 되어 있다. 트랜지스터(Tr₂)의 콜렉터는 충방전 콘덴서(C)에 연결 되어 있고 트랜지스터(Tr₂와 Tr₃)의 에미터는 서로 접속되어 저항(R₈)을 통하여 전원(V)에 연결 되게 있다.A signal input terminal 101 is the collector of the transistor is connected to the base of (Tr ₁₎ the transistor (Tr ₁₎ is connected to the "+" input of the charge and discharge capacitor (C) and the comparator (104). '-' Terminal of the comparator 104 is connected to the variable resistor (R ₃ ), the output

Is connected to the clock input (C) of the monostable oscillator (IC ₂ ), and its output (Q) is connected to the base of the transistor (Tr ₃ ) through the reset (R) input and the variable resistor (R ₅ ). have. Print

Is connected to the base of the transistor Tr ₂ through the variable resistor R ₄ . The collector of the transistor Tr ₂ is connected to the charge / discharge capacitor C and the emitters of the transistors Tr ₂ and Tr ₃ are connected to each other so as to be connected to the power supply V through the resistor R ₈ .

은 발진기(IC₇)의 리세트 입력에 연결되어 있다. 발진기(IC₇)의 출력(out)은 트랜지스터(Tr₄)를 통해 트랜지스터(Tr₄)의 콜렉터에 연결된 것이다.The output Q of the comparator 104 is connected to the input of the emitter coupling logic (ECL) -TTL conversion element IC ₅ through the D flip-flops IC ₃ and IC ₄ , and the conversion element IC ₅ . The output of is connected to the input terminal B ₁ of the monostable oscillator (IC ₆ ), and its output

Is connected to the reset input of the oscillator (IC ₇ ). The output (out) of the oscillator (IC ₇₎ is connected to the collector of the transistor (Tr ₄₎ through a transistor (Tr _4).

The operation of the present invention will be described in detail with reference to FIGS. 2B and 3.

부분의 파형을 나타낸 그림이다.Figure 3a is shown in Figure 2b

The figure shows the waveform of the part.

In the second b, the discharge current Ii proportional to the input voltage Vi flows to the emitter of the transistor Tr ₁ while the capacitor C discharges while the transistor Tr ₂ is turned off.

In this case, the change in voltage over time at points A of FIGS. 2B and 3 may be represented by the following equation.

At this time, if the voltage-to-current amplification degree of the transistor Tr ₁ is k, that is, Ii = kvi ... (2), the slope of tanθ1 changes according to the magnitude of the input voltage vi. This means a change in oscillation frequency.

In the comparator output B of Figs. 2b and 3, when the output pulse generated by the voltage technique occurs at the point A by the reference voltage vr, it is changed into a pulse having a constant width at the point of the monostable oscillator output C. As a result, the transistor Tr ₂ is turned on and charging of the capacitor C starts. At this time, the discharge continues during the transmission delay time T ₁ of the comparator and the monostable oscillator output, and the transistor Tr ₂ is 'off' after the output pulse width T ₂ time of the monostable oscillator is started again.

This process is repeated to oscillate.

When the relationship between the charging time and the voltage is divided at the point A, the transistor Tr ₂ is turned on and the charging current Id is charged to the capacitor C.

At this time, the change of voltage with respect to time becomes as follows.

tan θ2 = (Id-Ii) / C,... … … … … … … … … … … … … … … … … … … … … … … … (3)

If one cycle between charge and discharge is T3, the relationship between charge and discharge can be expressed by the following equation.

(Id-Ii) T2 / C = Ii (T3-T2) / C

T3 = ID T2 / Ii... … … … … … … … … … … … … … … … … … … … … … … … … … … (4)

Therefore, the oscillation frequency f becomes as follows.

f = 1 / T3 = Ii (Id T2)... … … … … … … … … … … … … … … … … … … … … … … … (5)

If you substitute (2) into (4),

f = kvi / (Id T2)... … … … … … … … … … … … … … … … … … … … … … … … … … (6)

Here, the pulse width T2 of the monostable oscillator does not have a frequency dependency and is only affected by the delay characteristics of the device. Id can be seen as a constant current source because transistors Tr ₂ and Tr3 operate as switches. Therefore, in the equation (6), the oscillation frequency f is ensured linearity with respect to the input voltage vi. The maximum oscillation frequency is affected by the periods T1 and T2.

At this time, the maximum oscillation frequency can be increased up to 90MHz by using a fast D flip-flop.

Next, the automatic operation circuit 105 will be described. When the power supply is turned on or the oscillation is stopped due to any influence during the oscillation, the oscillation cannot be started because the charge / discharge capacitor C of the modulator is not charged. To prevent this, the oscillation signal is divided by 4 from the D flip-flops IC ₃ and IC ₄ , and then the monostable oscillator IC ₆ is driven to control the output of the oscillator IC ₇ .

At this time, the output pulse width of the monostable oscillator IC ₆ should be larger than the oscillation pulse width of the modulation circuit. If oscillation continues in the modulation circuit, the reset terminal of oscillator IC ₇ goes low and its output goes low. However, if the oscillation does not occur in the modulation circuit, a pulse is oscillated at the output of the oscillator IC ₇ so that the capacitor C is charged and oscillation starts. 3b is a diagram showing the relationship between the voltage and the oscillation frequency of the input signal when the charge and discharge capacitor (C) is 53PF.

As described above, the present invention can be utilized as a wideband PFM modulator that can be modulated with excellent linear characteristics from 10 MHz to 90 MHz as an extremely simple circuit configuration, and can transmit a video signal at a low cost, especially in an optical communication field.

Claims (1)

The video signal input terminal 101 is connected to the base of the transistor Tr ₁ of the discharge circuit 102, and the collector of the transistor Tr ₁ and the capacitor C of the charging circuit 103 are the terminals of the comparator 104. Connected to positive, output of comparator 104

Is connected to the clock terminal C of the monostable oscillator 106, and the output Q of the monostable oscillator 106 is connected to its reset terminal R and at the same time the output Q,

) Is connected to the bases of the transistors Tr ₂ and Tr ₃ through the level shifting resistors R4 and R5 of the charging circuit 103, and the output Q, of the monostable oscillator 106,

) Is connected to the output terminal 107, the output (Q) of the comparator 104 is connected to the terminal (CP) of the counter (IC ₃ ) of the automatic operation circuit 105, the output of the counter (IC ₄ )

Is connected to the input terminal B ₁ of the monostable oscillator (IC ₆ ) through the conversion element (IC ₅ ), the output of the monostable oscillator (IC ₆ )

The output (OUT) of being connected to reset only (R) of the oscillator (IC _7), the oscillator (IC ₇₎ is characterized in that through the transistor (Tr ₄₎ connected to the capacitor (C) of the charging circuit 103 Pulse frequency modulation device for video signal transmission including an automatic circuit.

Images