JPH066141A - Adjustment method for voice detection circuit - Google Patents

Abstract

PURPOSE:To obtain the detection characteristic with less disturbance by selecting a coil being a component of a phase shift circuit having a larger impedance in the three-times mode frequency so as to reduce the effect of a phase characteristic onto a main detection characteristic. CONSTITUTION:The detection characteristic is measured by using three kinds of coils 43a-43c having respective impedance characteristics for a coil 43 being a component of a phase shift circuit 4 of a voice detection circuit. The impedance of the coils 43a-43c is respectively nearly 1.9, 2.3, 3.9kohms in the three- times mode frequency of a piezoelectric resonator 42. A little disturbance is in existence in the detection characteristic when the coils 43a, 43b are used and no disturbance is in existence for the coil 43c. Moreover, the distortion factor is less in the case of the coil 43c than the other coils 43a, 43b. Thus, the damping in the three-times mode of the piezoelectric resonator 42 is strengthened by selecting a coil having a larger impedance in the three-times mode frequency for the coil 43 being the component of the phase shift circuit 4 and the effect of the phase characteristic onto the main detection characteristic is reduced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for adjusting a quadrature type voice detection circuit used for detecting (demodulating) an FM signal of, for example, a television or an FM radio.

[0002]

2. Description of the Related Art An example of this type of voice detection circuit is shown in FIG. This voice detection circuit is configured as follows.
That is, the FM audio intermediate frequency signal which is the input signal is input to the limiting amplifier 2, where a limiter is applied to bring the signal into a state close to a square wave, and the signal is divided into two and one remains in the original phase. 90 ° ± θ ° (θ is an FM modulation component and 0 ≦ θ ≦
90) The phase is shifted and input to the multiplier 6. The multiplier 6 multiplies both input signals and outputs a pulse signal having a width proportional to the phase difference between the two. A detection output is obtained by averaging this through the low-pass filter 8 and amplifying it by the amplifier 10.

The phase shift circuit 4 is a two-terminal type piezoelectric resonator 42.
A circuit in which a coil 43 is connected in series with a resistor 44 and a circuit in which a resistor 44 is connected in parallel and a capacitor 41 are connected in an L shape as shown in the figure. The capacitor 41 changes the phase of the input signal by 90 °.
The circuit of the piezo-resonator 42, the coil 43 and the resistor 44 functions to shift the frequency change of the input signal into the phase change.

The coil 43 and the resistor 44 are connected to the piezo-resonator 42 as described above because the coil 43 shifts the resonance frequency of the circuit to a lower frequency than the resonance frequency of the piezo-resonator 42 itself. The resistor 44 dulls the impedance characteristic of the piezoelectric resonator 42 in the vicinity of the antiresonance frequency, whereby the phase changes substantially linearly with respect to the frequency between the vicinity of the resonance frequency of the piezoelectric resonator 42 and the vicinity of the antiresonance frequency. This is to obtain the characteristics to

[0005]

However, in the voice detection circuit using the phase shift circuit 4 as described above, the phase characteristic of the piezoelectric resonator 42 in the higher mode (mainly the triple mode) is the main one. There is a problem that the detection characteristics are disturbed. FIG. 8 shows an example of such a detection characteristic, and the detection output and the distortion rate are disturbed in the circled portion. In this specification, the main frequency is 4.5 MHz, and the frequency in the triple mode is 13.5 MHz.

The mechanism that causes the detection characteristics to be disturbed as described above is as follows. That is, the audio intermediate frequency signal is first limited by the limiting amplifier 2 and becomes a state close to a square wave, but this always causes a higher-order mode component (however, this is the limiting amplifier 2).
Limited to the range of frequency characteristics). Here, the impedance characteristics of the piezoelectric resonator 42 and the coil 43 connected in series to it in the phase shift circuit 4 are as shown in FIG. 6 and FIG. 7, respectively. Piezoelectric resonator 42-3
The impedance at the double mode frequency (that is, 13.5 MHz) is about 129 Ω as shown in FIG. 6, while the impedance at the triple mode frequency of the coil 43 is self as shown in FIG. Since the resonance frequency is high, it is only about 1.9 KΩ. With this difference, the triple mode of the piezoelectric resonator 42 is not sufficiently damped by the impedance of the coil 43.

In summary, the piezoelectric resonator 42 has a frequency characteristic.
The intermediate frequency signal including the higher-order mode, which has passed through the limiting amplifier 2 extending to a frequency range higher than the triple mode frequency of, has a high self-resonance frequency in series with the piezoelectric resonator 42 and When the coil 43 having a low impedance at the triple mode frequency is connected, the triple mode is not damped and is disturbed by the phase inversion and detected. The result shows the detection characteristics as shown in FIG.

In view of the above, the present invention reduces the influence of the phase characteristics of the piezoelectric resonator at the triple mode frequency on the main detection characteristics in the voice detection circuit as described above, and provides good detection characteristics with little disturbance. The main purpose is to provide an adjustment method that can be obtained.

[0009]

In order to achieve the above-mentioned object, the adjusting method of the present invention measures the detection characteristics of the above-mentioned audio detection circuit so that the disturbance of the detection characteristics is lower than a desired allowable range. If it is large, it is characterized in that the coil forming the phase shift circuit is replaced with a coil having a larger impedance at the triple mode frequency of the piezoelectric resonator.

[0010]

As described above, the coils forming the phase shift circuit are
When the impedance of the piezoelectric resonator at the triple mode frequency is changed to a larger value, the increased impedance enhances the damping of the piezoelectric resonator at the triple mode, and the phase characteristics of the piezoelectric resonator affect the main detection characteristics. As a result, good detection characteristics with less disturbance can be obtained.

[0011]

EXAMPLE In this example, the coil 43 constituting the phase shift circuit 4 of the voice detection circuit shown in FIG. 1 has three types of coils 43a to 43c having impedance characteristics as shown in FIGS. Was used to measure the detection characteristics of the voice detection circuit of FIG.

As shown in FIGS. 2 to 4, the coils 43a to 43c have impedances of about 1.9 KΩ at the triple mode frequency of the piezoelectric resonator 42 (ie, 13.5 MHz in this example). It is about 2.3 KΩ and about 3.9 KΩ. The impedance characteristic of the coil 43a is almost the same as that shown in FIG.

The detection characteristics when the above coils 43a to 43c are used are shown in FIG. Regarding the detection output, it can be seen that in the circled portion, there is disturbance in the case of the coil 43a, slight disturbance in the case of the coil 43b, and no disturbance in the case of the coil 43c. In addition, regarding the strain rate, the values in the case of the coils 43a and 43b are large in the circled portion,
It can be seen that the value in the case of the coil 43c is considerably smaller than that.

That is, it can be seen that the disturbance of the detection characteristic is reduced by using the coil having the higher impedance at the triple mode frequency. This is because when the coil 43 constituting the phase shift circuit 4 is replaced with a coil having a larger impedance at the triple mode frequency, the increased impedance enhances damping of the piezoelectric resonator 42 in the triple mode, and the phase thereof is increased. This is because the influence of the characteristics on the main detection characteristics can be reduced.

Therefore, the detection characteristic of the voice detection circuit as described above is measured, and when the disturbance of the detection characteristic is larger than the desired allowable range, the coil 43 constituting the phase shift circuit 4 is measured.
Is replaced with one having a larger impedance at the triple mode frequency, the disturbance of the detection characteristic can be put in a desired allowable range very easily.

By thus reducing the disturbance of the detection characteristic, the distortion of the output audio signal is eliminated. Particularly in recent years, the high fidelity has been advanced in televisions and the like, and the effect is great.

Note that the specific permissible range of the disturbance of the detection characteristic differs depending on the set (device such as television) in which this audio detection circuit is used.

[0018]

As described above, according to the present invention, by the extremely simple method of exchanging the coil which constitutes the phase shift circuit, the main detection characteristic due to the phase characteristic at the triple mode frequency of the piezoelectric resonator is obtained. It is possible to obtain an excellent detection characteristic with less disturbance by reducing the influence of.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an example of a quadrature type voice detection circuit.

FIG. 2 is a diagram showing an example of impedance characteristics of a coil used in the phase shift circuit in FIG.

3 is a diagram showing an example of impedance characteristics of another coil used in the phase shift circuit in FIG.

FIG. 4 is a diagram showing an example of impedance characteristics of still another coil used in the phase shift circuit in FIG.

5 is a diagram collectively showing the detection characteristics of the voice detection circuit of FIG. 1 when the coils having the impedance characteristics shown in FIGS. 2 to 4 are used.

6 is a diagram showing an example of impedance characteristics and phase characteristics of the piezoelectric resonator in FIG.

FIG. 7 is a diagram showing an example of impedance characteristics of a conventionally used coil.

8 is a diagram showing a detection characteristic of the voice detection circuit of FIG. 1 when a coil having an impedance characteristic as shown in FIG. 7 is used.

[Explanation of symbols]

 2 Limiting amplifier 4 Phase shift circuit 6 Multiplier 8 Low pass filter 10 Amplifier 41 Capacitor 42 Piezoelectric resonator 43, 43a to 43c Coil 44 Resistance

Claims (1)

[Claims] 1. A quadrature type voice detection circuit including a circuit in which a coil is connected in series to a two-terminal type piezoelectric resonator and a resistor is connected in parallel to the phase shift circuit, and the detection characteristic is measured. Then, if the disturbance of the detection characteristic is larger than a desired allowable range, the coil forming the phase shift circuit is replaced with a coil having a larger impedance at the triple mode frequency of the piezoelectric resonator. How to adjust the voice detection circuit.