JPH0218766B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to an FM detection circuit, and more particularly, to a circuit for detecting and extracting an FM-modulated video signal in a VTR or the like.

Prior Art FIG. 7 shows a block system diagram of an example of a conventional FM detection circuit (quadruple detection circuit). In the figure, the FM signal input to the terminal 1 is converted into a rectangular wave by the limiter circuit 2 and supplied to the exclusive OR gate (multiplication circuit such as a balanced modulator) 3, while the FM signal is converted to the FM carrier by the delay circuit 4. For example, the pulse signal is delayed by 64 ns, which is 1/4 of the wavelength of As a result, the second high frequency component of the FM signal is removed and taken out from the terminal 6 as a detection output.

Problems to be Solved by the Invention The conventional circuit described above requires a low-pass filter 5 to remove the second harmonic component of the FM signal generated in the detection process, and has the problem that the circuit cannot be easily configured. It was hot.

By the way, it is used as a frequency-voltage converter (F-V converter) in the servo system for speed control of a DC motor.
When using an FM detection circuit, the high-frequency side characteristics of its response characteristics are influenced by the low-pass filter used to remove harmonic components. There was a problem with deterioration due to the area filter. In order to improve this high frequency characteristic, it is necessary to set the FM carrier frequency of the FM signal high, but if the FM carrier frequency is set high, when this FM detection circuit is used as an F-V converter of a servo circuit, In this case, the configuration of the servo circuit itself and the servo mechanism parts becomes complicated, which causes new problems such as increased costs.

The present invention does not generate the second harmonic component of the FM signal, thereby eliminating the need for a low-pass filter, and
The object of the present invention is to provide an FM detection circuit that can achieve high linearity of demodulated output.

Means for Solving the Problems FIG. 1 shows a circuit diagram of an embodiment of the circuit of the present invention. In the figure, 2 is a rectangular wave conversion circuit (limiter) that converts the input FM signal into a rectangular wave, and 7 is a limiter 2.
An inverting circuit (inverter) that inverts the output of Tr ₁ ,
R ₁ , R ₂ , C ₁ , C ₂ , D ₂ , D ₃ and Tr ₄ , R ₃ , R ₄ ,
C ₃ , C ₄ , D ₅ , and D ₆ are first and second detection circuits that detect the outputs of the limiter 2 and the inverter 7, respectively;
Tr ₃ , Tr ₆ and terminal 6 are a combining circuit that combines and extracts the outputs of the first and second detection circuits.

Function The first and second detection circuits are rectangular wave conversion circuits 2
and the output of the inverting circuit 7 (Fig. 2 B, C), the voltage rises with a time constant during a certain level period of each output, and rises during a different level period following that level period (Fig. 2). E, F) are held, and the synthesis circuit extracts the voltage level (G in the figure) held during the output of the detection circuit.

Example In Fig. 1, the FM signal a entering terminal 1
(Fig. 2A) is converted into a rectangular wave b (Fig. 2B) by the limiter circuit 2, and the rising edge and falling edge are detected by the differentiating circuit of the capacitor C ₁ and the resistor R ₁ , and the signal d (Fig. D). In addition, diode D ₁
is for limiting negative voltage pulses that occur on falling edges.

The transistor Tr ₁ is momentarily turned on by the rising edge of the signal d, and the charge previously stored in the capacitor C ₂ is transferred to the transistor Tr 1 .
It is discharged through Tr ₁ , and its collector voltage e (E in the figure) becomes zero V. Since the transistor Tr ₁ is then turned off, the capacitor C ₂ is charged through the resistor R ₂ and the diode D ₃ , and its collector voltage e gradually increases (at a time determined by the resistor R ₂ and the capacitor C ₂ (The collector voltage e gradually rises with a constant value.) During this period (rising period), signal b is H
diode _D2 is reverse biased and no current flows through it.

When the signal b becomes L level, a current flows through the diode _D2 , and the diode _D3 becomes reverse biased and the current is cut off. This makes capacitor C ₂
Charging ends at this point, and the charging voltage e is maintained until the next transistor _Tr1 is turned on.

Therefore, the signal b output from the limiter circuit 2
During the H level period of signal b, the collector voltage e (output voltage of the first detection circuit) rises with the above-mentioned time constant, and during the L level period following the H level period of signal b, the collector voltage e maintains the rising voltage.

On the other hand, the output signal b of the limiter circuit 2 is inverted by the inverter 7 to become the signal c (C in the same figure), and the voltage f (F in the same figure) is taken out from the collector of the transistor Tr ₄ through the various circuits having the above configuration. Ru. signal e,
f is supplied to transistors Tr ₃ and Tr ₆ via buffer transistors Tr ₂ and Tr ₅ , respectively, and is combined there, and the maintained level of both signals is taken out and made into a demodulated signal g (G in the same figure). , taken out from terminal 6.

In this way, in this embodiment, the signal e due to charging/discharging and voltage holding by the capacitor C ₂ , resistor R ₂ , diodes D ₂ and D ₃ and the signal e generated by the capacitor C ₄ , resistor R ₄ ,
FM by combining the signal f from diodes D ₅ and D ₆
Since the voltage g is obtained at a level corresponding to the frequency of the signal, there is no leakage of the second harmonic component of the FM signal as in the circuit shown in FIG. 7, and there is no need to use a low-pass filter. Of course, the deterioration of the high-frequency characteristics of the demodulated signal due to the low-pass filter is eliminated, and the high-frequency characteristics are improved.

Here, let us consider the charging curves of the signals e and f (the output voltage curves at the rising time of the first and second detection circuits). The charging curve of signal e is resistor R ₂ ,
Capacitor C ₂ , charging curve of signal f is resistor R ₄ ,
The exponential curve (characteristic shown in Figure 3 A) determined by each capacitor _C4 is theoretically a 1/t (t is time) curve (characteristic shown in Figure 3 A). It is necessary that there be. This is because the frequency f and the period T have a relationship of f=1/T.

If the charging curve is not 1/t, the effect will appear on the linearity of the demodulated output, so if highly accurate linearity of the demodulated output is required, it is necessary to make the charging curve as close to the 1/t curve as possible.

FIG. 4A shows a basic circuit diagram of an exponential curve generation circuit. With this, a curve based on the time constants of the resistor R and the capacitor C can be obtained. As shown in Figure 3B, Figure B shows that in the initial stage, a curve with a time constant of (r ₁ r ₂ )·C can be obtained, and when diode D is turned off,
A curve with a time constant of r ₁ ·C can be obtained, and a curve close to 1/t can be obtained. C in the same figure uses diodes d ₁ to d _o-1 and resistors r ₁ to _{r o} ,
It is possible to obtain a curve closer to the 1/t curve than the one shown in FIG.

Therefore, considering the circuit shown in FIG _{. 1} , _the circuit shown in _FIG . 6 If you replace the circuit with only resistor R ₄ shown in Figure A with the circuit with resistors R ₄ ′, R ₄ ″ and diode D ₈ shown in Figure B, you will get a circuit with a simple configuration.
A charging curve closer to the 1/t curve can be obtained.

Effects of the Invention As described above, the FM detection circuit according to the present invention includes a rectangular wave converting circuit that converts an input FM signal into a rectangular wave, an inverting circuit that inverts the output of the rectangular wave converting circuit, and an output of the rectangular wave converting circuit. first and second detection circuits each supplied with the output of the inverting circuit, each of which rises with a time constant during a certain level period of the respective output, and holds a voltage that rises during a different level period following that level period; Since it consists of a circuit that is supplied with the output of the first detection circuit and a circuit that takes out the voltage level that is maintained during the output, there is no leakage of the second harmonic component of the FM signal. Since there is no need to use a low-pass filter, it has the advantage of simplifying the circuit configuration.
Furthermore, the high-frequency characteristics of the demodulated output are improved, so
When this FM detection circuit is used as an F-V converter of a servo circuit, the servo response characteristics are improved, and when it is used as a video signal detection circuit, the high frequency characteristics during detection are improved.

In addition, an FM detection circuit in which the first and second detection circuits are configured to obtain a curve (1/t curve) corresponding to the reciprocal of time as the output voltage curve at the time of rise has good linearity of the demodulated output. obtain. Furthermore, the first and second detection circuits are (n-1)
(n is an integer of 2 or more) are connected in series in the forward direction, and a power supply is connected to the anode at one end of the series connection, while n diodes are connected to the anode of each diode and the cathode at the other end of the series connection. One end of each resistor was connected, and the other end of the resistor was made common and grounded via a capacitor.
The FM detection circuit has a simple circuit configuration that allows for 1/t
It is possible to obtain a curve close to the curve.

[Brief explanation of drawings]

1 and 2 are a circuit diagram of an embodiment of the circuit of the present invention and a signal waveform diagram for explaining its operation, FIG. 3 is a capacitor charging voltage vs. time characteristic diagram, and FIGS. 4 to 6 are 1 A circuit diagram for explaining the circuit configuration for obtaining the /t curve, FIG. 7 is a block system diagram of an example of a conventional circuit. 1...FM signal input terminal, 2...Limiter circuit, 6...Output terminal, 7...Inverter, Tr ₁ ~
Tr ₆ ...transistor, _C1 to _C4 ...capacitor,
_R1 to _R4 ...Resistor, _D1 to _D6 ...Diode.

Claims (1)

[Claims] 1. A rectangular wave converting circuit that converts an input FM signal into a rectangular wave, an inverting circuit that inverts the output of the rectangular wave converting circuit, and an output of the rectangular wave converting circuit and an output of the inverting circuit. first and second detection circuits that are supplied to the respective outputs, rise with a time constant during a certain level period of the respective outputs, and hold the rising voltages for a different level period following the level period; 2
1. An FM detection circuit comprising: a circuit that is supplied with the output of each of the detection circuits and extracts the voltage level held during the output. 2. The FM detection circuit according to claim 1, wherein the first and second detection circuits are configured to obtain a curve corresponding to a reciprocal of time as an output voltage curve at the time of rising. . 3 The first and second detection circuits have (n-1) diodes (n is an integer of 2 or more) connected in series in the forward direction, and a power source is connected to the anode at one end of the series connection. , comprising a circuit in which one end of each of n resistors is connected to the anode of each of the diodes and the cathode of the other end of the series connection, and the other ends of the resistors are common and grounded via a capacitor. An FM detection circuit according to claim 1 or 2, characterized in that: