JPH04292020A - Squelch circuit for fm receiver - Google Patents

Abstract

PURPOSE:To drastically improve response speed related to the detection of a squelch control point. CONSTITUTION:A detection circuit 4 to detects a noise component Sn obtained from a demodulation circuit 2 or a carrier component Sc obtained from an intermediate frequency amplification circuit 3 and a rectification circuit 5 to convert the output signal So of the detection circuit 4 into a DC signal Sd are provided. Especially, the output signal So is imparted to the input part 6a of one side of an operational amplifier 6, and simultaneously, the output signal So is imparted to the input part 6c of the other side of the operational amplifier 6 after removing the DC component of the output signal So by interposing a coupling capacitor Cc to the input part 6b of the other side of the operational amplifier 6, and the DC signal Sd is obtained from the output part 6c of the operational amplifier 6.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention is an FM system that cuts off the receiver output when the receiver input falls below a certain level.
Regarding the squelch circuit of the receiver.

0002

2. Description of the Related Art Generally, in an FM receiver, when the receiver input (field strength) is large, the noise is extremely small due to the amplitude limiting effect. However, when the receiver input falls below a predetermined level, noise increases significantly. For this reason,
In an FM receiver, the magnitude of the noise component is detected, and the squelch control section is activated based on the detection result, and the receiver output is controlled by, for example, changing the bias of the low frequency amplifier circuit and stopping its operation. A squelch circuit is provided to disconnect the signal.

FIG. 4 shows a schematic configuration of a conventional squelch circuit. In the same figure, the FM receiver indicated by the reference numeral 30 has an antenna 3.
1, comprising a high frequency amplification circuit 32, an intermediate frequency amplification circuit 33, a demodulation circuit 34, a low frequency amplification circuit 35, and a speaker 36,
It constitutes a known FM receiver main body. Further, 37 is a squelch circuit, particularly a squelch control section that obtains a squelch control signal Ss. The control section 37 extracts the noise component Sn from the demodulation circuit 34 using the noise filter 38, detects the noise component Sn using the detection diodes D10 and D11, and also detects the noise component Sn using the detection diodes D10 and D11.
It is converted into a DC signal Ed via a time constant circuit 39 consisting of. Then, the DC signal Ed is applied to one input part of the comparator 40, and the reference voltage Es from the reference level circuit 41 is applied to the other input part. As a result, if the noise component Sn increases and the DC signal Ed becomes larger than the reference voltage Es, the squelch control signal Ss is output from the comparator 40 and, for example, by changing the bias of the low frequency amplifier circuit 35, Squelch control is performed to stop the operation.

0004

[Problems to be Solved by the Invention] However, the conventional squelch circuit 37 described above has
The noise component Sn is detected by the time constant circuit (smoothing circuit) consisting of the capacitor C10 and the resistor R10.
39, the time constant in the time constant circuit 39 becomes considerably large, and due to the response delay required to detect the squelch control point, e.g.
Transceivers have had problems such as large delays in ringing time.

The present invention solves the problems existing in the conventional technology, and aims to provide a squelch circuit for an FM receiver that can dramatically increase the response speed for squelch control point detection. shall be.

[0006]

[Means for Solving the Problems] The present invention includes a detection circuit 4 that detects a noise component Sn obtained from a demodulation circuit 2 or a carrier component Sc obtained from an intermediate frequency amplification circuit 3, and a detection circuit 4 that detects a noise component Sn obtained from a demodulation circuit 2 or a carrier component Sc obtained from an intermediate frequency amplification circuit 3; The squelch circuit 1 of the FM receiver is equipped with a DC converting circuit 5 for converting the DC signal Sd into a DC signal Sd, and detects that the receiver input has fallen below a certain level based on the magnitude of the DC signal Sd, and cuts off the receiver output. When configuring, in particular, the output signal So is applied to one input section 6b of the operational amplifier 6, and the DC component of the output signal So is connected to the other input section 6a of the operational amplifier 6 through, for example, a coupling capacitor Cc. The present invention is characterized in that it includes a direct current converting circuit 5 configured to remove and apply a direct current signal Sd, thereby obtaining a direct current signal Sd from the output section 6c of the operational amplifier 6.

[0007]

[Operation] According to the squelch circuit 1 of the FM receiver according to the present invention, for example, the noise component Sn with a frequency of around 70 kHz obtained from the demodulation circuit 2 is detected by the detection circuit 4, and the output signal So of the detection circuit 4 is detected by the detection circuit 4. The DC signal Sd is converted into a DC signal Sd by the DC conversion circuit 5. As a result, the squelch circuit 1 monitors the magnitude of the DC signal Sd, and when the receiver input becomes less than a certain level, that is, the magnitude of the DC signal Sd becomes more than a certain level, the squelch circuit 1 performs a squelch operation to cut off the receiver output. Take control.

At this time, since the output signal So is applied to the input section 6a of the operational amplifier 6 in the DC conversion circuit 5 via the coupling capacitor Cc, the input signal Si input to the input section 6a is as shown in FIG. 2(B). There is only the AC component Sa shown in FIG. On the other hand, since the output signal So is applied to the input section 6b of the operational amplifier 6 without passing through the coupling capacitor, the input signal Sj input to the input section 6b is as shown in FIG.
) is a signal containing an AC component Sa and a DC component Sf.

Therefore, if the difference between the input signal Si and the input signal Sj is amplified by operating the operational amplifier 6 as a complete differential amplifier, the alternating current component is canceled out, so that the output section 6c of the operational amplifier 6 has the voltage shown in FIG. Only the DC signal Sd shown in (C) is obtained. At this time, since the DC conversion circuit 5 does not include a time constant circuit with a large time constant or a hysteresis circuit with a large hysteresis width, there is no response delay in the circuit.

0010

Embodiments Next, preferred embodiments of the present invention will be described in detail with reference to the drawings.

First, the configuration of the FM receiver and squelch circuit 1 according to the present invention will be explained with reference to FIG.

In the figure, P indicates an FM receiver, which includes an antenna 11, a high frequency amplification circuit 12, an intermediate frequency amplification circuit 3, a demodulation circuit 2 including a multiplier 2a and a phase shifter 2b;
It is equipped with a low frequency amplification circuit 13 and a speaker 14, and is a known FM
Configure the receiver body.

[0013] Reference numeral 1 denotes a squelch circuit, particularly a squelch control section 1s. The squelch control section 1s includes a detection circuit 4, a direct current conversion circuit 5, and a comparator 15, and the input side of the detection circuit 4 is a noise filter 16 and a capacitor 17.
It is connected to the demodulation circuit 2 via. Note that the detection circuit 4 includes detection diodes D1 and D2. On the other hand, the DC conversion circuit 5
is equipped with an operational amplifier 6, and one input section (inverting input section) 6
a is connected to the output side of the detection circuit 4 via a series circuit of a coupling capacitor Cc and a resistor R1, and the other input section (non-inverting input section) 6b is connected to the output side of the detection circuit 4 via a resistor R2. Connecting. Furthermore, operational amplifier 6
The output section 6c is one input section 15a of the comparator 15.
The other input section 15b of the comparator 15 is connected to a reference level setting section (variable resistor) 18. A reference signal (reference voltage) Se is provided from the reference level setting section 18. Note that R3, R4, R5, and R6 represent resistance.

Next, the overall operation of the squelch circuit 1 according to the present invention will be explained with reference to FIG.

First, a demodulated signal obtained from the demodulation circuit 2 is extracted as a noise component Sn via a capacitor 17 and a noise filter 16, and is detected by a detection circuit 4. The output signal So of the detection circuit 4 is converted into a DC signal by a DC converting circuit 5, and the output signal So is converted into a DC signal by a DC converting circuit 5.
A DC signal Sd is obtained at the output section 6c. In this case, it is desirable that the operational amplifier 6 performs completely differential amplification with respect to the noise component Sn.

Therefore, the magnitude of this DC signal Sd decreases as the electric field strength increases, and increases as the electric field strength decreases. Therefore, the comparator 15 monitors the magnitude of the DC signal Sd, and When the voltage is smaller than the preset reference voltage Se, the squelch control signal S
s is not output, and the FM receiver operates with the squelch canceled. However, when the magnitude of the DC signal Sd becomes larger than the reference voltage Se, the squelch control signal Ss is output from the output section 15c of the comparator 15, and, for example, the bias of the low frequency amplifier circuit 13 is changed, and its operation is By stopping the signal, squelch control is performed to cut off the receiver output.

On the other hand, FIG. 3 shows a modified embodiment. In the embodiment shown in FIG.
However, in the modified embodiment, the carrier component Sc obtained from the intermediate frequency amplification circuit 3 is used. In addition, in FIG. 3,
Components that are the same as those in FIG. 1 are designated by the same reference numerals, and detailed description thereof will be omitted.

In a modified embodiment, the intermediate frequency amplification circuit 3
are a first intermediate frequency amplification section 3a, a second intermediate frequency amplification section 3b,
Since the third intermediate frequency amplification section 3c is provided, each of the detection diodes D3, D4, D5 constituting the detection circuit 4 is connected to the respective intermediate frequency amplification section 3a. Therefore, the carrier component Sc... is detected by the detection circuit 4, and its output is applied to the input side of the DC conversion circuit 5 as the output signal So via the logarithmic amplification section 20. Processed similarly. However, since the magnitude of the carrier component Sc increases in proportion to the electric field strength, the comparator 15 monitors the magnitude of the DC signal Sd obtained from the DC conversion circuit 5, and monitors the magnitude of the DC signal Sd obtained from the DC converting circuit 5 to ensure that the magnitude of the carrier component Sc is lower than or equal to a preset reference voltage Se. When the squelch control signal Ss is reached, the squelch control signal Ss is output. In this way, the DC conversion circuit 5 in the modified embodiment is also configured and functions in the same manner as the embodiment shown in FIG.

Although the embodiments have been described in detail above, the present invention is not limited to these embodiments. For example, the operational amplifier may be a differential amplifier configured with semiconductor elements such as transistors. Other details such as the circuit configuration can be arbitrarily changed without departing from the gist of the present invention.

[0020]

As described above, the squelch circuit of the FM receiver according to the present invention provides an output signal obtained by detecting noise components etc. to one input section of the operational amplifier, and also provides an output signal obtained by detecting noise components etc. to the other input section of the operational amplifier. For example, since it includes a DC converting circuit that removes and applies the DC component of the output signal through a coupling capacitor and obtains a DC signal from the output section of the operational amplifier, the time constant circuit in the DC converting circuit is This makes it possible to dramatically increase the response speed for squelch control point detection, and has the remarkable effect that, for example, the calling time in an FM transceiver or the like can be significantly shortened.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a squelch circuit of an FM receiver according to the present invention;

[Figure 2] Waveform diagram of signals at various parts in the same squelch circuit,

FIG. 3 is a block configuration diagram of a squelch circuit of an FM receiver according to a modified embodiment of the present invention;

FIG. 4 is a block diagram of a squelch circuit of an FM receiver according to the conventional technology;

[Explanation of symbols]

1 Squelch circuit 2 Demodulation circuit 3 Intermediate frequency amplification circuit 4 Detection circuit 5 DC converter circuit 6 Operational amplifier 6a Input section 6b Input section 6c Output section Sn Noise component Sc carrier component So Output signal Sd DC signal

Claims (2)

[Claims] Claim 1: A detection circuit that detects a noise component obtained from a demodulation circuit or a carrier component obtained from an intermediate frequency amplification circuit, and a DC conversion circuit that converts an output signal of the detection circuit into a DC signal. In the squelch circuit of the FM receiver, which detects that the receiver input has become below a certain level and cuts off the receiver output, the output signal is applied to one input section of the operational amplifier, and the output signal is applied to one input section of the operational amplifier. 1. A squelch circuit for an FM receiver, comprising a direct current converting circuit that removes and applies a direct current component of the output signal to the other input section to obtain a direct current signal from the output section of the operational amplifier. 2. The squelch circuit for an FM receiver according to claim 1, wherein the output signal is applied to the other input section of the operational amplifier via a coupling capacitor.