JPH04292019A - Squelch circuit for fm receiver - Google Patents

Abstract

PURPOSE:To expand the setting range of a squelch control point from no input of small electric field intensity to high input at which S/N of the output of a demodulation circuit becomes more or less 35 dB, and besides, to greatly increase response speed related to the detection of the squelch control point. CONSTITUTION:This circuit is constituted so that the output of a receiver is interrupted when the input of the receiver becomes below a difinite level. Especially, a noise output signal Sx based on a noise component Sn obtained from the demodulation circuit 2 is given to the comparison input part Ca of one side of a comparator C. Besides, a carrier output signal Sy based on a carrier component Sc obtained from an intermediate frequency amplification circuit 3 is given to the comparison input part Cb of the other side of the comparator C, and a comparison processing circuit 4 which outputs a squelch control signal Ss for interrupting the receiver output from the output part Cc of the comparator when the noise output signal Sx becomes larger than the carrier output signal Sy is provided.

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 comparison input part of the comparator 40, and the reference voltage Es from the reference level circuit 41 is applied to the other comparison input part. As a result, if the noise component Sn increases and the DC signal Ed becomes larger than the reference voltage Es, the comparator 4
By outputting a squelch control signal Ss from 0 and changing the bias of the low frequency amplifier circuit 35, squelch control is performed to stop the operation.

0004

However, the above-described conventional squelch circuit 37 has the following problems.

That is, the magnitude of the noise component Sn with respect to the magnitude of the electric field strength usually changes as shown in FIG. 2(B). Therefore, the setting range of the squelch control point is limited to a range of low electric field strength from no input to about 0 [dBμV]. Therefore, for example, as shown in FIG.
μV], the amount of change in the noise component Sn is relatively small. That is, when comparing the reference voltage Es and the noise component Sn, the peak value of the noise component Sn overlaps with the reference voltage Es at any electric field strength, making detection of the squelch control point unstable.

For this reason, in practice, as described above, the noise component Sn is detected by the detection diode D10... and further converted into a DC signal Ed by the time constant circuit 39. However, the time constant of the time constant circuit 39 becomes quite large. Therefore, there is a delay in the response required to detect the squelch control point, and for example, in the case of an FM transceiver, there is a problem that the ringing time is significantly delayed.

The present invention solves the problems that exist in the conventional technology, and changes the setting range of the squelch control point from no input with small electric field strength to a range in which the output of the demodulation circuit has a S/N ratio.
It can expand up to a high input of about 35dB, and
An object of the present invention is to provide a squelch circuit for an FM receiver that can dramatically increase the response speed for squelch control point detection.

[0008]

[Means for Solving the Problems] The present invention provides a squelch circuit 1 for an FM receiver that cuts off the receiver output when the receiver input falls below a certain level.
In particular, the noise output signal Sx based on the noise component Sn obtained from the demodulation circuit 2 is input to one comparison input section C of the comparator C.
A carrier output signal Sy based on the carrier component Sc given to a and obtained from the intermediate frequency amplification circuit 3 is given to the other comparison input section Cb of the comparator C, and the noise output signal S
When x becomes larger than the carrier output signal Sy,
It is characterized by comprising a comparison processing circuit 4 which outputs a squelch control signal Ss for cutting off the receiver output from the output section Cc of the comparator C. Therefore, squelch circuit 1
includes a detection circuit 5 that detects the carrier component Sc, a DC conversion circuit 6 that converts the output of the detection circuit 5 into DC, and a DC conversion circuit 6.
The operational amplifier 7 is provided with an operational amplifier 7 that amplifies the output of the signal to obtain a carrier output signal Sy, and the operational amplifier 7 is also provided with a gain adjustment circuit 8 that varies the degree of amplification and an operating point adjustment circuit 9 that shifts the operating point with respect to the input.

[0009]

[Operation] According to the squelch circuit 1 of the FM receiver according to the present invention, the noise output signal Sx is based on the noise component Sn obtained from the demodulation circuit 2, and the carrier output signal Sy is based on the carrier component Sc obtained from the intermediate frequency amplification circuit 3. By monitoring both the squelch control point K (see Fig. 2(C))
Detection is performed.

As shown in FIG. 2(A), the magnitude of the carrier component Sc increases in proportion to the magnitude of the electric field strength. Therefore, the carrier component Sc is detected by the detection circuit 5 and its output is converted to a DC circuit. 6 converts the current to direct current, the operational amplifier 7 performs amplification processing, the gain adjustment circuit 8 in the operational amplifier 7 variably adjusts the degree of amplification, and the operating point adjustment circuit 9 offsets the operating point relative to the input. A carrier output signal Sy shown in 2(C) can be obtained. That is, the change characteristics of the noise output signal Sx and the change characteristics of the carrier output signal Sy with respect to the electric field strength intersect at a predetermined electric field strength. Therefore, if this intersection is set as the squelch control point K, the squelch control point K will be such that the output of the demodulation circuit 2 has an S/N ratio of 35 from no input with small electric field strength.
This means that it is possible to set up to a high input of about dB. Note that the noise output signal Sx shown in FIG. 2(C) is a signal obtained by detecting the noise component Sn.

Therefore, in the squelch circuit 1 according to the present invention, the comparator C in the comparison processing circuit 4 compares the magnitudes of the noise output signal Sx and the carrier output signal Sy applied to each comparison input section Ca and Cb. Output signal S
When x becomes larger than the carrier output signal Sy,
Squelch control signal Ss from output part Cc of comparator C
was output.

At this time, as shown in FIG. 2C, if the proportional change range of the carrier output signal Sy with respect to the electric field strength crosses the noise output range of the noise output signal Sx, the noise output signal Sx with respect to the change in the electric field strength and the relative amount of change in the carrier output signal Sy becomes large. That is, as shown in FIG. 3(A), when a part of the noise component Sn (noise output signal Sx) and the carrier output signal Sy overlap when the electric field strength is -5 [dBμV], the electric field strength becomes ), the carrier output signal Sy increases significantly as well as the noise component Sn decreases.
Both are completely separated. On the other hand, if the electric field strength decreases to -7 [dBμV] shown in the same figure (C), the noise component Sn
As the carrier output signal Sy increases, the carrier output signal Sy decreases greatly, and the two completely overlap.

Therefore, even when the noise component Sn is used, the squelch control point K can be reliably detected. Therefore, there is no need to convert the noise component Sn into a DC signal through a time constant circuit with a large time constant or a hysteresis circuit with a large hysteresis width, and there is also no response delay in the circuit.

[0014]

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, a low frequency amplification circuit 13, and a speaker 14, and is a known FM receiver main body. Configure.

[0017] Reference numeral 1 designates a squelch circuit, particularly a squelch control section 1s. The squelch control section 1s includes a comparator C forming a comparison processing circuit 4. and,
One comparison input section Ca of the comparator C is connected to the demodulation circuit 2 via a noise processing circuit 15, a noise filter 16, and a capacitor 17.

On the other hand, the other comparison input section Cb of the comparator C is connected to the operational amplifier 7, the AM removal filter 18 and logarithmic amplification section 19 which constitute the DC conversion circuit 6, and the detection diodes 5a, 5b, 5c which constitute the detection circuit 5. and connect it to the intermediate frequency amplification circuit 3. In this case, the demodulation circuit 2 includes a multiplier 2a and a phase shifter 2b, and the intermediate frequency amplification circuit 3 includes a first intermediate frequency amplification section 3a, a second intermediate frequency amplification section 3b, and a third intermediate frequency amplification section 3c. , each detection diode 5a... is connected to each intermediate frequency amplification section 3a.... Further, the output side of the AM removal filter 18 is connected to one input section 7a of the operational amplifier 7. A variable resistor 21 is connected between the comparison input section 7a and the output section 7c of the operational amplifier 7.
are connected to form a gain adjustment circuit 8, and a power supply line 23 is connected to the other input section 7b of the operational amplifier 7 via a variable resistor 22 to form an operating point adjustment circuit 9.

Although the noise processing circuit 15 has a detection function, it does not include a time constant circuit with a large time constant.

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

First, the carrier component Sc taken out from the intermediate frequency amplification circuit 3 is detected by the detection diodes 5a, 5b, 5.
The output is logarithmically amplified by the logarithmic amplifier 19, and further, the AM component is removed by the AM removal filter 18 and converted into the DC output signal So. This DC output signal So is amplified by the operational amplifier 7 and applied to the comparison input section Cb of the comparator C as a carrier output signal Sy. At this time, by adjusting the variable resistor 21 in the gain adjustment circuit 8, the amplification degree of the operational amplifier 7 can be variably adjusted, and by adjusting the variable resistor 22 of the operating point adjustment circuit 9, the input to the operational amplifier 7 can be adjusted. The operating point for the DC output signal So provided from the AM removal filter 18 can be shifted. That is, by adjusting the variable resistor 21,
The rising angle of the carrier output signal Sy shown in FIG. can be done.

On the other hand, the demodulated signal from the demodulation circuit 2 passes through the capacitor 17 and the noise filter 16 to remove the noise component S.
It is taken out as n. The noise component Sn is detected by the noise processing circuit 15 and applied to the comparison input section Ca of the comparator C as a noise output signal Sx. FIG. 2C shows the noise output signal Sx processed in this way.

Therefore, as shown in FIG. 2C, the variable resistor 21 in the gain adjustment circuit 8 is adjusted so that the proportional change range of the carrier output signal Sy with respect to the electric field strength crosses the noise output range of the noise output signal Sx. to adjust the amplification degree, and the variable resistor 2 in the operating point adjustment circuit 9
2 to adjust the offset voltage (reference voltage) applied to the input section 7b of the operational amplifier 7 and shift the operating point, the intersection of each output signal Sx and Sy becomes the squelch control point K.
becomes. In this case, when the electric field strength is high, the operating point adjustment circuit 9 is adjusted to increase the offset voltage, and
The gain adjustment circuit 8 may be adjusted to reduce the amplification degree. Furthermore, when the electric field strength is small, the operating point adjustment circuit 9 may be adjusted to reduce the offset voltage, and the gain adjustment circuit 8 may be adjusted to increase the amplification degree.

As a result, when the electric field strength is large, the carrier output signal Sy becomes larger than the noise output signal Sx, so the squelch control signal Ss is output from the comparator C.
is not output and the FM receiver operates with the squelch canceled. However, when the electric field strength is small, the noise output signal Sx becomes larger than the carrier output signal Sy, so the squelch control signal Ss is output from the output section Cc of the comparator C, and for example, the bias of the low frequency amplifier circuit 13 is changed. By stopping the operation, a squelch operation is performed to cut off the receiver output.

Although the embodiments have been described in detail above, the present invention is not limited to these embodiments. For example, it is not necessary to provide a noise processing circuit. Moreover, any means other than those illustrated can be used as means for obtaining the carrier output signal and the noise output signal. Other details such as the circuit configuration can be arbitrarily changed without departing from the gist of the present invention.

[0026]

As described above, the squelch circuit of the FM receiver according to the present invention applies the noise output signal based on the noise component obtained from the demodulation circuit to one comparison input section of the comparator, and also provides the noise output signal based on the noise component obtained from the intermediate frequency amplification circuit. A carrier output signal based on the carrier component is applied to the other comparison input section of the comparator, and a squelch control signal is provided from the output section of the comparator to cut off the receiver output when the noise output signal becomes larger than the carrier output signal. Since it is equipped with a comparison processing circuit that outputs the output, the setting range of the squelch control point is from no input with small electric field strength to when the output of the demodulation circuit has an S/N ratio of 35.
It has the remarkable effect of being able to expand up to a high input of about dB, and dramatically increasing the response speed for squelch control point detection.

[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] Characteristic diagram showing the magnitude of the signal at each part relative to the electric field strength,

FIG. 3 is a waveform diagram showing the relationship between noise components and carrier output signals with respect to electric field strength;

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 Comparison processing circuit 5 Detection circuit 6 DC converter circuit 7 Operational amplifier 8 Gain adjustment circuit 9 Operating point adjustment circuit C Comparator Ca comparison input section Cb Comparison input section Cc output section Sn Noise component Sc carrier component Sx Noise output signal Sy Carrier output signal

Claims (4)

[Claims] Claim 1: In a squelch circuit for an FM receiver that cuts off the receiver output when the receiver input falls below a certain level, the noise output signal based on the noise component obtained from the demodulation circuit is sent to one of the comparators. A carrier output signal based on the carrier component obtained from the intermediate frequency amplification circuit is applied to the comparison input section and is applied to the other comparison input section of the comparator, and when the noise output signal becomes larger than the carrier output signal, the output signal of the comparator is A squelch circuit for an FM receiver, comprising a comparison processing circuit that outputs a squelch control signal for cutting off receiver output from an output section. [Claim 2] A detection circuit that detects a carrier component;
2. The squelch circuit for an FM receiver according to claim 1, further comprising a direct current converting circuit for converting the output of the detection circuit into direct current, and an operational amplifier for amplifying the output of the direct current converting circuit to obtain a carrier output signal. 3. The squelch circuit for an FM receiver according to claim 2, wherein the operational amplifier includes a gain adjustment circuit that varies the degree of amplification. 4. The squelch circuit for an FM receiver according to claim 2, wherein the operational amplifier includes an operating point adjustment circuit for shifting an operating point with respect to the input.