JPS61145905A - Fm demodulation circuit - Google Patents

Abstract

PURPOSE:To realize stable FM demodulation with less number of components by providing a couple of differential circuits switched alternately with two FM signals opposite to each other and charging/discharging a capacitor in response to said switching. CONSTITUTION:FM signals opposite to each other in phase are fed respectively to input terminal 32, 34, transistor (TR)24, 26 repeat conduction/nonconduction alternately by an FM signal A1 and TRs 28, 30 repeat conduction/nonconduction alternately by an FM signal A2. A charge/discharge current flows to a capacitor 46 and charge/discharge voltages are generated at points B and C respectively. A switching current I flows to a collector of the TRs 26, 28 in response to the charge/discharge stated as above, the current flows to a resistor 56 by the current mirror effect of TRs 48, 50 and a demodulated output Vo is generated at an output terminal 58.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an FM demodulation circuit using a so-called pulse count demodulation method.

[Conventional technology]

The pulse count type FM (frequency modulation) demodulation circuit is
As shown in Figure 3, the FM signal applied to the input terminal 2 is input to the trigger circuit 4 and converted into a trigger pulse synchronized with the FM signal cycle, and this trigger pulse triggers the monostable multi-bi break 6. , which extracts the demodulated pulse of the FM signal from the output terminal 8. After smoothing the demodulated pulse of the FM signal through the low-pass filter 10,
A low frequency signal is obtained from its output terminal 12.

That is, when the FM signal shown in A in FIG. 4 is applied to the input terminal 2, the trigger circuit 4 generates a trigger pulse shown in B in FIG. 4 in synchronization with the rising and falling edges of the FM signal, and this trigger pulse causes multivi break 6
generates a demodulation pulse shown at C in FIG. The high level time t4 of this pulse is constant, but the time T is FM
Since the pulse signal corresponds to the frequency of the signal, when this pulse signal is passed through the low-pass filter 10, a low-frequency signal having a level corresponding to the FM signal is obtained.

(Problems to be Solved by the Invention) In such an FMlj type circuit, the trigger circuit 4 and the multivibrator 6 have a large number of components, the circuit configuration is complicated, and the manufacturing cost is high when configured with a semiconductor integrated circuit. As the cost increases, the circuit configuration becomes more complex and operation becomes unstable.

Therefore, this invention provides stable FM with a small number of components.
The present invention attempts to provide an FM demodulation circuit that realizes demodulation.

[Means for solving problems]

In other words, this invention provides two F
A pair of differential circuits that alternately switch depending on the M signal is provided, and a capacitor that can be switched to a charging state or a discharging state in response to the switching operation of each differential circuit is connected between the operating current outflow side of each differential circuit. This is what happens.

[For production]

Therefore, the present invention alternately switches a pair of differential circuits using an FM signal, charges and discharges a capacitor in response to this switching, and combines the potential change and the switching current of the differential circuit to generate an FM signal frequency ( Obtains a pulse output with a duty according to the period).

〔Example〕

Embodiments of the present invention will be described in detail below with reference to the drawings.

FIG. 1 shows an embodiment of the FM demodulation circuit of the present invention.

In FIG. 1, a pair of differential circuits 20 and 22 are installed in this FM demodulation circuit, the differential circuit 20 is a pair of transistors 24 and 26 that have a common emitter, and the differential circuit 22 has a pair of transistors that have a common emitter. Each transistor is composed of a pair of transistors 28 and 30. Input terminals 32.34 are formed at the bases of each of the transistors 24.30, and FM signals having mutually opposite phases are applied to the bases of the transistors 26.30.
A constant bias voltage □ is applied to the base of 28 from a voltage source 36.

The emitters of the transistors 24, 26 are grounded via a transistor 38: #3 and a resistor 40 for carrying an operating current, and similarly, the emitters of the transistors 28, 30 are grounded via a transistor 42 and a resistor 44 for carrying an operating current.
A capacitor 46 is connected between the emitters of transistors 24.26 and 28.30. Each transistor 3
A bias voltage Vll□ is applied to 8.42 from a common bias voltage source.

A power supply is connected between the collector of the transistor 24.30 and the ground point, and a driving voltage Vcc is applied to the collector side of the transistor 26.28.
A current mirror circuit consisting of a transistor 48.5o and a resistor 52.54 constituting an output circuit for taking out a demodulated output is installed. That is, transistor 5o
A load resistor 56 is connected between the collector and a ground point, and an output terminal 58 is formed.

Based on the above configuration, its operation will be explained with reference to FIG.

The input terminals 32 and 34 are supplied with FM signals shown in A in FIG. 2 that are in opposite phases to each other, and VWI indicates the midpoint voltage of the FM signals.

Transistors 24 and 26 are alternately conductive with the FM signal A1,
The transistors 28 and 30 alternately become conductive and non-conductive in response to the FM signal A2. Due to this switching operation, a charging/discharging current flows through the capacitor 46, and at point B in FIG. 1, a charging/discharging voltage shown at B in FIG.
At the zero point, a charging/discharging voltage shown as C in FIG. 2 is generated. The amplitude of these charge/discharge voltage waveforms is given by ■.

In response to such charging and discharging, a switching current ■ flows through the collectors of the transistors 26 and 28, and this current flows through the resistor 56 due to the current mirror effect of the transistors 48 and 50, and the output terminal 58 has the voltage shown in FIG. A demodulated output v0 shown in D is generated.

Here, the demodulated output ■ shown in D in FIG. In, the pulse width is TP, its period is 1/2 of the FM signal period T, the resistance value of the resistor 56 is RL, the output current flowing through the resistor 56 is , and the carrier frequency of the FM signal is rc.
Demodulation output■. is vo = I・RL・(CT
/2-TP )-TP )/ (T/2) = I-RL ・ (2/T) ・ (T/2-27F) = I-RL ・ (1--4Tp / T ) = I ・
RL・(1-4TPre)・・・・(
1), and the detection efficiency aVo/af is avo/ar=-41·RL -TP (2). Therefore, the demodulated output ■. ■. =-4■・R
L'TP .DELTA.f (3). However, Δ
f is the frequency deviation.

Also, if the capacitance of the capacitor 46 is C, then T, -〇-
Since V/I (■ is the peak value of B in FIG. 2), the demodulated output is ■. is, Vo= 4I-RL (C・V/I) ・Δf=
4Rt'C・■・Δf (4). However, in this case, the delay time of the transistor is ignored.

As is clear from equation (4), the demodulated output ■. is proportional to the frequency deviation Δf of the carrier frequency fe, so
A low frequency signal can be reproduced by passing it through a low pass filter.

In such an FM demodulation circuit, FM demodulation can be obtained with a small number of elements, and the voltage reduction characteristics are also improved. Also,
The capacitor 46 is approximately 30 pF and several MHz FMui.
Since fi is possible, a capacitive element on a semiconductor integrated circuit can be used, and the circuit configuration can be simplified.

〔Effect of the invention〕

As described above, according to the present invention, stable pulse count type FM demodulation can be realized with a small number of elements.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram showing an embodiment of the FM demodulation circuit of the present invention, Fig. 2 is an explanatory diagram showing its operating waveforms, Fig. 3 is a block diagram showing a conventional FM demodulation circuit, and Fig. 4 is its operation. It is an explanatory diagram showing a waveform. 20.22...differential circuit, 46...capacitor. Figure 2 Figure 3 Figure 4

Claims (1)

[Claims] A pair of differential circuits are provided that alternately switch depending on two FM signals having an antiphase relationship with each other, and between the operating current outflow sides of each differential circuit, the charging state or An FM demodulation circuit characterized by connecting a capacitor that can be switched to a discharge state.