JPS6356729B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION This invention relates primarily to a squelch circuit suitable for use in a wireless microphone receiver, and in particular to a squelch circuit that reliably eliminates transient noise that occurs when a wireless microphone switch is opened or closed. This relates to a squelch circuit that can be removed.

The squelch circuit itself frequently opens and closes to eliminate transient noise that occurs when the wireless microphone's power switch and transmitter switch are opened and closed, and due to minute fluctuations in the electric field strength that occur when the microphone is carried around. In order to eliminate the resulting switching noise, a squelch circuit having an appropriately large time constant and hysteresis specification, as shown in FIG. 1, has conventionally been used.

To briefly explain Fig. 1, antenna 2
The signal received by is amplified by the high frequency amplification section 4,
The mixer 6 mixes the signal with the signal supplied from the local oscillator 8 and converts it into an intermediate frequency signal. The intermediate frequency signal is amplified by an intermediate frequency amplifying section 10, and a detector 12
The audio signal is detected by the squelch circuit 14, and is supplied to the low frequency amplification section 18 via the switch section 16 of the squelch circuit 14. The output of the low frequency amplification section 18 is supplied to a speaker 20.

On the other hand, the signal taken out from the intermediate frequency amplification section 10 is supplied to a rectifier circuit 25 made up of diodes 22 and 24 of the squelch circuit 14 and rectified. The rectified signal passes through a first time constant circuit 29 consisting of a capacitor 26 and a resistor 28 to the transistor 3.
Hysteresis circuit 3 configured by 0,32
3. The output of the hysteresis circuit 33 is supplied to the switch unit 16 via a second time constant circuit 37 consisting of a resistor 34 and a capacitor 36.

FIG. 2 shows the relationship between the operating time of the squelch circuit 14 and the control voltage generated at its output point B. When the transmitter wireless microphone is switched on at _t1 , the resistor 34 and capacitor 36
The output voltage increases exponentially due to the action of the second time constant circuit 37 consisting of. When the voltage reaches the on level _Vi2 at _t2 , the switch section 16 is turned on and the audio is broadcast. The period from t ₁ to t ₂ is approximately 0.2
The time is between 0.3 seconds and 0.3 seconds. When the wireless microphone is switched off at t ₃ , the voltage at output point B decreases relatively quickly due to the action of the first time constant circuit 29 consisting of a capacitor 26 and a resistor 28, but there is still a slight delay. remains, and due to the hysteresis characteristics of the hysteresis circuit 33, the on-level
At time _t5 when the off level V _i1 , which is lower than V _i2 , is reached, the switch section 16 is turned off.

In the above-mentioned conventional squelch circuit, when the switch of the wireless microphone is turned on and broadcasting starts, the noise is almost completely eliminated because there is a delay between t ₁ and t ₂ before the switch section 16 is turned on. t ₃ to t ₅ when the wireless microphone is switched off.
Since the switch section 16 remains on during this period (usually 2 to 3 ms), there is a drawback that noise is generated. Note that the reason why a variable resistor is used as the resistor 28 of the first time constant circuit 29 is to adjust the squelch levels V _i2 and V _i1 with respect to the antenna input.
Furthermore, the reason for using the hysteresis circuit 33 is to prevent the squelch circuit itself from turning on and off frequently due to slight changes in the electric field strength that occur when the wireless microphone is carried around.

The present invention uses a level detection circuit to detect a sudden drop in electric field strength that occurs when a wireless microphone is switched off at a point A near the input point of the squelch circuit, and thereby detects a sudden drop in electric field strength that occurs when a wireless microphone is switched off at a point A near the input point of the squelch circuit. 32 is forcibly turned on to short-circuit the capacitor 36, as shown in Fig. 2.
The purpose of the present invention is to provide a novel squelch circuit which turns off the switch section 16 at _t4 and prevents noise from being broadcast even when the switch of the wireless microphone is turned off.

FIG. 3 shows an embodiment of the squelch circuit according to the present invention. In this figure, parts equivalent to those in the circuit of FIG. 1 are given the same reference numerals, and explanations thereof will be omitted. A level detection circuit 38 is provided between the point A of the first time constant circuit 29 and the base of the second transistor 32 of the hysteresis circuit 33. This level detection circuit 38 is a transistor 42
When the wireless microphone is on, transistor 42 is off because capacitor 40 connected between its base and point A is charged. When the wireless microphone is switched off and the voltage at point A drops rapidly, capacitor 40 is discharged and base current flows through resistor 41 to transistor 42, turning transistor 42 on. As a result, the voltage at the collector C of the transistor 42 increases.
This voltage is applied to the protective resistor 44 and the reverse current blocking diode 4.
6 to transistor 32, forcing transistor 32 to turn on at t ₄ in FIG.
The capacitor 36 is discharged, and at the same time the switch section 1
Turn off 6. The transistor 32 is the detection circuit 3
The period during which the circuit is forcibly turned on by the output of t 8 is determined by a third time constant circuit consisting of a capacitor 40 and a resistor 41, and the period is set to be slightly longer than t ₅ . It is determined. After _t5 , the normal squelch operation keeps the transistor 32 on and the switch section 16 off.

V _B in FIG. 2 is the transistor 4 of the detection circuit 38.
2 shows the change in base voltage. At _t4 , when the base voltage exceeds the threshold value _Vt , the transistor 42 is turned on, and the voltage _Vc at point C rises in a stepwise manner.
At _t6 , when the base voltage becomes smaller than the threshold value _Vt , the transistor 42 is turned off, and the voltage _Vc at point C has decreased. Therefore, in this example, transistor 32 is forced on for a period from t ₄ to t ₆ .

Therefore, according to the squelch circuit of the present invention, since the level detection circuit 38 detects a sudden drop in the voltage level at point A, noise is not generated when the wireless microphone is switched off. The possible period is the conventional t ₃ to t ₅ (2
~3ms) to approximately one-tenth of t3 to _t4 ( _0.2 ~0.3ms)
During this time, the switch noise is completely unnoticeable, and the noise that occurs when the wireless microphone is switched off can be almost completely eliminated. Note that the noise generated when the wireless microphone is switched on is removed by the function of the second time constant circuit 37, similar to the conventional one shown in FIG.

In addition, in the squelch circuit of the present invention, when the wireless microphone is turned on, the potential at point A increases, so the detection circuit 38 does not respond at all.
Therefore, there is no concern that the initial operation of the squelch circuit will be affected. Furthermore, since the level detection circuit 38 only responds to a sudden level drop at point A, it does not respond to a relatively gradual level drop caused by a dead point caused by the movement of the microphone. on,
No need to worry about it turning off.

Although this invention has been described with respect to a squelch circuit used in a wireless microphone receiver equipped with a hysteresis circuit 33, this invention
It can also be applied to squelch circuits that do not have a hysteresis circuit.

[Brief explanation of the drawing]

Fig. 1 is a block diagram of a receiver using a conventional squelch circuit, and Fig. 2 shows voltages and voltages at various parts to explain the operation of the conventional squelch circuit shown in Fig. 1 and the squelch circuit of the present invention shown in Fig. 3. FIG. 3, which is a diagram showing the relationship with time, is a circuit diagram of the main part of an embodiment of the squelch circuit according to the present invention. 14... Squelch circuit, 16... Switch section, {3
4...Resistor, 36...Capacitor} Time constant circuit, 3
8...Detection circuit.

Claims (1)

[Claims] 1. A switch section 16 interposed between a detection section 12 that detects an intermediate frequency signal and a low frequency amplifier 18;
a rectifier 25 that rectifies the intermediate frequency signal; a smoother 29 that smoothes the rectified output of the rectifier 25;
The smoothed output of this smoothing section 29 is inputted, and the first part is configured to be in a non-conducting state when the smoothed output is smaller than a predetermined threshold value, and to be in a conducting state when the smoothed output is larger than the threshold value. A transistor 30 and a second transistor that is conductive when the first transistor 30 is non-conductive and is non-conductive when the first transistor 30 is conductive.
a first capacitor 36 which is connected between the output terminal of the second transistor 32 and a reference potential point and whose voltage across the capacitor 36 is supplied to the switch section 16 as a control signal for opening and closing it. and a series circuit of a second capacitor 40 and two resistors connected between the output of the smoothing section 29 and the power supply, respectively, and the potential at the interconnection point of both of the resistors is inputted to the smoothing section. a third transistor 42 that turns on the second transistor 32 in response to a potential change at the interconnection point due to a discharge of the second capacitor 40 based on a sudden change in the smoothed output of the squelch circuit 29; .