JPH0444441A - Fsk reception circuit - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an FSK receiving circuit for an FSK (Frequency Shift Keying) modem used for power distribution line transportation and the like.

[Conventional technology]

As shown in Fig. 3, the conventional FSK receiving circuit eliminates noise components (
The active filter 1 removes high frequency components) and the amplifier 2 amplifies the noise-removed FSK signal (Fig. 4 (B)).
It is demodulated by the K signal discrimination circuit 3, and the demodulated signal is input to the central processing unit (CPtJ) 4 of the computer.

[Problem to be solved by the invention]

The active filter 1 placed in front of the amplifier 2 removes noise of unnecessary band components included in the incoming FSK signal, and the amplifier 2 obtains an amplified FSK signal. However, when the incoming FSK signal has a wide dynamic range, Due to the saturation of the operational amplifier in amplifier 2, the output of amplifier 2 is distorted into a clipped trapezoidal waveform as shown in Figure 4(C).For this reason, harmonic components in unnecessary bands appear simultaneously, so that the FSK signal in the subsequent stage is distorted. This affects the FSK signal discrimination in the discrimination circuit 3.

Therefore, the present invention solves the above problems,
The object is to provide an FSK receiver circuit that achieves both noise removal and amplification regardless of the level of the incoming FSK signal.

[Means to solve the problem]

In order to solve the above problems, the means taken by the present invention is to configure an incomplete integrating circuit including a feedback capacitor and a feedback resistor using an operational amplifier or the like, and to respond to the increase or decrease in the output of the incomplete integrating circuit by using an operational amplifier or the like. A feedback resistance variable means is provided to increase or decrease the resistance value in the opposite direction.

[Effect]

According to this means, when an FSK signal containing a noise component is input to the incomplete integration circuit, the integration function of the incomplete integration circuit removes the noise component included in the FSK signal, and the amplification of the incomplete integration circuit is performed. The function produces an amplified FSK signal. When a low-level FSK signal enters, the output level of the incomplete integration circuit is low, so the feedback resistance variable means acts to increase the value of the feedback resistance.

This increases the gain of the incomplete integration circuit, resulting in a high-level amplified FSK signal from which noise has been removed. On the other hand, when a high-level FSK signal is input, the output level of the incomplete integration circuit is high, so that the feedback resistance variable means acts to lower the value of the feedback resistance.

This lowers the gain of the imperfect integration circuit, so that the incoming FSK signal is not amplified much as a result, and an FSK signal from which noise has been removed is obtained. Therefore, incoming FSK
Regardless of the signal level, F is at a constant level.
A signal is obtained at S-, and the FSK signal has noise removed.

〔Example〕

Next, an embodiment of the FSK receiving circuit according to the present invention will be described based on the accompanying drawings.

FIG. 1 is a circuit configuration diagram showing an embodiment of an FSK receiving circuit according to the present invention. In FIG. 1, the same parts as those shown in FIG. 3 are given the same reference numerals, and their explanations will be omitted.

The FSK receiving circuit in this embodiment is composed of a piquad circuit type bandpass filter circuit using three operational amplifiers 10, 11, and 12.

The first operational amplifier IO includes an input resistor R3 connected in series to the inverting input, a feedback capacitor C3 interposed between the output terminal and the inverting input terminal, and feedback resistors Rx and Rs connected in parallel to the feedback capacitor C1. and a feedback resistance value variable circuit 20.

The variable feedback resistance value circuit 20 includes a field effect transistor Q whose drain is connected to the midpoint between the feedback resistors Rz and Rz and whose source is grounded, and this gate and an operational amplifier 10.
It is composed of a diode D and a Zener diode Z that are interposed between the output terminal of the gate and the output terminal, and a resistor R4 that is interposed between the gate and ground.

The second operational amplifier 11 has an input resistor Rs and a feedback capacitor C! connected in series to the inverting input. It has Further, the third operational amplifier 12 has an input resistor R and a feedback resistor R1 connected in series to the inverting input, and the output terminal of the operational amplifier 12 is connected to the inverting input of the operational amplifier 10 via the feedback resistor R. It is connected. In this circuit configuration, the output of the operational amplifier 10, which essentially functions as an FSK receiving circuit, is supplied to the FSK signal discrimination circuit 3.

The operational amplifier 10 having a feedback capacitor C1 and feedback resistors R, , R3 functions as an incomplete integrating circuit. The feedback capacitor C1 and the operational amplifier 10 constitute an integrating circuit, and the feedback resistors R, , R, and the operational amplifier 10 constitute an amplifier, but the actual value of the feedback resistance is determined by the feedback resistance value variable circuit 2.
0, the amplification degree of the operational amplifier 10 (H
) changes. That is, the amplification degree (H
) is H=Vout/VIN=-((Rt+Ryu)/R1+R,R:l/RRos)
is given by

Here, VOUT is the output voltage of the operational amplifier IO, and VI
N is the voltage of the incoming FSK signal, and Ro is the drain-source resistance of the field effect transistor Q.

When a low-level FSK signal as shown in Figure 2 (A) comes in, the output amplitude of the operational amplifier 10 is small, so the gate voltage of the field effect transistor Q increases, and the drain-source resistance R□ decreases. Therefore, a large amount of shunt current flows to the ground side. This corresponds to a substantial reduction in the feedback current and a reduction in the feedback resistance. Therefore, in the above equation, it can be regarded as an increase in the feedback resistance R1, so the amplification degree (H) of the operational amplifier 10 increases, and as a result, the output terminal of the operational amplifier 10 becomes as shown in FIG. 2(B). An amplified FSK signal shown appears.

On the other hand, a large FSK as shown in Figure 2 (C)
When a signal comes in, the output amplitude of the operational amplifier 10 is large, so the gate voltage of the field effect transistor Q becomes low, and the drain-source resistance RDg becomes small.
The shunt current flowing to the ground side is reduced. This corresponds to a substantial increase in the feedback current and an increase in the feedback resistance. Therefore, in the above equation, it can be regarded as a decrease in the feedback resistance R□, so the amplification degree (H
) decreases, and as a result, the FSK signal is hardly amplified,
The output terminal of the operational amplifier 10 has an FS shown in FIG. 2 (D).
A K signal appears.

In this way, the variable feedback resistance value circuit 20 performs the inverse AGC function like an automatic control surface B (AGC), so that excessive amplification of the incoming FSK signal with a large dynamic range can be suppressed. This eliminates the need for clipping in the saturated region, and prevents the occurrence of high-frequency components.

In other words, an FSK signal with a wide dynamic range can also be supplied to the subsequent stage with almost constant amplitude without distortion.

〔Effect of the invention〕

As explained above, the present invention is characterized in that an incomplete integration circuit is employed in the FSK receiving circuit, and a feedback resistance value variable means is provided for changing the value of this feedback resistance depending on its output. However, it has the following effects.

In other words, since the imperfect integration circuit itself has an inverse automatic gain control function, it is possible to obtain an amplified FSK signal at a nearly constant level even if the incoming FSK signal level is excessive, and it is possible to prevent noise from occurring. Noise contained in the FSK signal can also be removed by the integration function of the incomplete integration circuit itself. This makes it possible to suppress as much as possible the adverse effects of noise on the FSK signal discrimination circuit connected to the subsequent stage.

[Brief explanation of the drawing]

FIG. 1 is a circuit configuration diagram showing an embodiment of an FSK receiving circuit according to the present invention. FIGS. 2(A), (B), (C), and (t)) are signal waveform diagrams showing signal waveforms of various parts in the same embodiment. FIG. 3 is a circuit configuration diagram showing an example of a conventional FSK receiving circuit. FIGS. 4A, 4B, and 4C are signal waveform diagrams showing signal waveforms of various parts in the conventional example. [Explanation of symbols] 3... FSK signal discrimination circuit 4... Central processing unit (CPU) 10.11.12... Operational amplifier 20... Feedback resistance value variable circuit R+, Rs, R-... Input resistance Cr, Ct... Feedback capacitor Rz, R3, Rt, Rs... Feedback resistance Q... Field effect transistor R... Resistor D... Diode] Figure (A) vth? figure

Claims (1)

[Claims] An FSK receiving circuit for removing high frequency components of an incoming FSK (Frequency Shift Keying) signal and amplifying the FSK signal, the circuit comprising: an incomplete integration circuit including a feedback capacitor and a feedback resistor; and,
An FSK receiving circuit comprising feedback resistance value variable means for increasing or decreasing the value of the feedback resistor in the opposite direction in response to an increase or decrease in the output of the incomplete integration circuit.