JPS6051295B2 - Waveform shaping circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a waveform shaping circuit for converting a non-square wave signal such as a sine wave signal into a square wave signal, for example, a waveform shaping circuit for demodulating a received signal in an FSK demodulator. This is intended to stabilize the duty.

The demodulation method used in the FSK demodulator includes an input signal (
There is a method of reproducing an FSK signal by shaping a sine wave (sine wave) and converting it into a square wave, and then measuring the time every half cycle to determine whether the frequency is higher or lower.

Figure 1 shows the demodulated output of the device, which is 1.2KH2
is the center frequency. 1.2±O as FSK signal,
The frequency changes within a range of 1KH2, 1.1KH2 corresponds to demodulated output H (high), and 1.3KH2 corresponds to L (low). The box on the horizontal axis shows the half period of each frequency. By the way, if the output duty cycle of the shaped square wave is not stable, the time measured every half cycle will be different, which may make it impossible to accurately demodulate the FSK signal. For example, civil engineering ±O, IKH
2(7) When demodulating an FSK signal using the above demodulation method,
When the duty is 4.16% or more, the center 1.2KH
A signal higher (lower) than 2 will be determined as a low (high) signal, resulting in malfunction. This will be explained with reference to FIG. Figure a shows a waveform with a frequency of 1.1KH2 and a duty of 50%, but if this deviates by 5%, that is, if the duty becomes 45% (55%) as shown in the figure, the part marked * will become 1. At .2, the period becomes less than the period of wave J of H2. Conversely, the waveform of 1.3KH2 (duty 50%) shown in figure c
) changes and the duty becomes 45% (55%) as shown in d in the same figure.
), the part marked with * becomes more than the wave period of 1.2KH2. Possible causes of fluctuations in the output duty of the waveform shaping circuit include fluctuations in the input signal level, fluctuations in the threshold value due to fluctuations in the power supply voltage, and the like.

However, since the conventional waveform shaping circuit 1 is generally composed of a waveform shaping section 2 and a duty setting section 3 as shown in FIG. Since the operating point (operating point) is fixed, if the relative relationship between the slice desired potential of the input signal and the threshold changes due to the above reasons,
Square wave signal 0 obtained by shaping the input signal such as FSK signal
It is inevitable that the duty of the driver will fluctuate from time to time. In the present invention, the duty of the output square wave is always kept constant by providing an automatic control system that cancels the duty fluctuation of the output of the waveform shaping section. Explain in detail.

FIG. 4 is a schematic block diagram of the present invention, in which a part of the output of the waveform shaping section 2 is received, and the average high level voltage VH and The duty detection unit 4 outputs an average low level voltage proportional to the proportion of time at low level in one cycle, and the voltages H and VL are compared, and if there is a difference between the two, the direction in which the difference decreases is determined. and a duty comparison section 5 that controls the threshold value of the waveform shaping section 2, forming an automatic control system that cancels out duty fluctuations.

FIG. 5 is an embodiment of the present invention that embodies FIG. 4.

The waveform shaping unit 2 includes a coupling capacitor C3, series resistors R3 and R4 that define the center level of the input signal IN whose DC component is blocked by the capacitor, and a differential amplifier whose inverting input is the potential at the connection point of the series resistors. A1, and the output V of the duty comparator 5 is connected to the non-inverting input of the amplifier A1.
, a stable square wave signal 0UT with a duty of 50% can be obtained. Duty detection section 4
is an inverter 11 that inverts a part of the square wave signal 0UT.
The first charging section 4a charges the capacitor C1 through the resistor R1 with the high level output of the inverter 11, and the resistor R with the high level output of the inverter 12 which further inverts the output of the inverter 11.
2=through R1 to capacitor C2=. It consists of a second charging section 4b that charges C1. The output of the charging section 4a is the above-mentioned average low level voltage ■, and the output of the charging section 4b is the average high level voltage ■8. Duty comparison section 5
is the above voltage ■H, VL. The main part is a differential amplifier A2 which receives as input, and its output becomes the threshold voltage ■s of the waveform shaping section 2. This voltage ■, always keeps the output 0UT duty constant even if the power supply voltage or input level changes (in this example, 50
%). The duty around output 0 is 50%, which means that the output of the detection section 4 is VH=
VH/VL or VH<VL
If so, amplifier A2 generates a modified voltage s to cancel it. The easiest way to do this is to use ■DD as a positive power source and ■s as a negative power source, and set the connection point of resistors R3 and R4 to 0V by hand, and set it so that when ■8=■L, Vs=0. In reality, the gain of the amplifier ~ is assumed to be GA2, so that a control error of {circle around (2), /G,, 2} occurs in a steady state.

However, GA2′. If it is 00, it becomes ・, so V: 3
It is safe to assume that VH always changes so that VH=■L. In the above example, the duty of output 0UT is 50.
%, but if a voltage divider circuit 6 is inserted between the duty detection section 4 and the comparison section 5 as shown in Fig. 6, the duty of the output 0UT can be kept constant at any value. can be kept.

That is, the voltage dividing circuit 6 includes a variable resistor VRl that divides the output VL of the first charging section 4a and a variable resistor VR2 that divides the output VH of the second charging section 4b. are the outputs V″L and V″6 of the variable resistors VRl and VR2, so the output V becomes 0 with V″5=V″9. Therefore, if the voltage division ratio of the variable resistors ■Rl and ■R2 is made different, ■H and half■ can be obtained. Therefore, the output 0UT can be stabilized at a value other than the duty of 50%. FIG. 7 shows the stability of the output duty with respect to input level fluctuations. Curve A is for the circuit of the present invention in FIG. 5, and curve A is for the conventional circuit in FIG. 3.

As is clear from the figure, the curve A is approximately uniform over the entire area, whereas the stable range of the curve opening is limited to a certain part. In FIG. 8, the power supply voltage range is limited to a part. FIG. 8 shows the stability of the output duty with respect to fluctuations in the power supply voltage. Curve A is also for the circuit of the present invention, and curve A is for the conventional line. Even in this case, the curve A is not affected by power supply fluctuations, whereas the duty of the curve A changes with the power source voltage. As described above, according to the present invention,
Since the output duty of the waveform shaping circuit can always be kept constant, if this is applied to an FSK demodulator that requires precision, there is an advantage that accurate demodulated output can always be obtained.

[Brief Description of the Drawings] Fig. 1 is an explanatory diagram of the FSK demodulation method, Fig. 2 a to d is an explanatory diagram of malfunction due to duty fluctuation, Fig. 3 is a schematic block diagram of a conventional waveform shaping circuit, and Fig. 4 5 is a schematic block diagram of a waveform shaping circuit of the present invention, FIG. 5 is a circuit diagram showing one embodiment of the present invention, FIG. 6 is a main part circuit diagram showing another embodiment of the present invention, and FIGS. FIG. 8 is a characteristic diagram showing the stability of the output duty with respect to input level fluctuations and power supply voltage fluctuations. In the figure, 1 is a waveform shaping circuit, 2 is a waveform shaping section, 4 is a duty detection section, 4a is a first charging section, 4b is a second charging section, and 5 is a duty comparison section.

Claims (1)

[Claims] 1. A waveform shaping section that converts a non-square wave signal into a square wave signal of the same frequency; a duty detection section that outputs an average high-level voltage proportional to the proportion of time when the voltage is low in one cycle and an average low-level voltage proportional to the proportion of the time when it is low level in one cycle; and an average high-level voltage from the duty detection section. and a duty comparison section that compares the average low-level voltage and the average low-level voltage, and if there is a difference between the two, controls the threshold value of the waveform shaping section in a direction that reduces the difference. 2 The duty detection section includes a first charging section that is charged with the high level of a signal obtained by inverting the output square wave signal of the waveform shaping section and outputs an average low level voltage, and a high level of a signal obtained by further inverting the inverted signal. 2. The waveform shaping circuit according to claim 1, further comprising: a second charging section that is charged with a second charging section and outputs an average high-level voltage.