JPS6211824B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a squelch circuit for an FM-AM receiver.

In the US, 30~50MHz, 150~170MHz,
450-512MHz FM commercial communications (police, fire,
Narrowband automatic scanning receivers for receiving information (security, weather forecasts, etc.) are being sold to the general public in addition to business personnel. On the other hand, narrowband automatic scanning receivers that receive 108-138 MHz AM wave aircraft radios (air traffic control and public telephones connected to aircraft) have recently begun to be sold to the general public.

When incorporating receiver circuits with different modulation methods into one receiver and using it in the same way as a conventional automatic scanning receiver, the following problems may occur.

In other words, the noise generation level when there is no signal is different between FM and AM (high when receiving FM and low when receiving AM), so if you try to use one squelch control volume for both FM and AM reception, The threshold squelch operating point when there is no signal (the point at which squelch operation starts when there is no signal) is set to FM.
It is difficult to set the squelch control volume at the same position both during reception and during AM reception. However, if the threshold squelch operating point during no signal cannot be set at the same position on the squelch control volume during FM reception and AM reception, there is a risk that the operation during scanning will be abnormal.

For example, in a 4-channel FM-AM receiver, the first channel is the FM band and the second channel is the FM band.
AM band, 3rd channel is FM band, 4th channel
If the channel is selected as AM band, even if the squelch control volume is set to a predetermined position, if you manually scan the reception channel, sound may appear depending on the channel even when there is no signal. It disappears.
In other words, a squelch operation is performed on the first and third channels of the FM band, and the sound disappears.
Since the squelch operation is not performed on the second channel and the fourth channel of the AM band, and the sound appears, the operation during scanning of the reception channel feels unnatural to the user, which is inconvenient. Further, when scanning reception channels automatically, channels on which a squelch operation is being performed are scanned, but scanning operations are stopped on channels on which a squelch operation is not being performed. Therefore, even though we want to automatically scan channels with no signals, the scanning operation stops at the second or fourth channel of the AM band in the above example, which is an inconvenience. occurs.

The present invention describes an embodiment of a squelch circuit for an FM-AM receiver that solves the above problems.
This will be explained according to FIG.

In Fig. 1, 1 is an antenna, 2 and 3 are
High frequency amplifier circuit for FM and first stage mixing circuit, 5
and 6 is a high frequency amplifier circuit for AM and a first stage mixing circuit, 4 is a first stage local oscillation circuit for both AM and FM,
8 is a first-stage intermediate frequency filter; 9 is a first-stage intermediate frequency amplification circuit; 10, 11, and 12 are second-stage mixing circuits, local oscillation circuits, and intermediate frequency filters; 13;
14 is the second stage intermediate frequency amplification circuit for FM and FM
Detection circuit, 15 and 16 are second stage intermediate frequency amplification circuit and AM detection circuit for AM, 17 is automatic AM detection circuit.
Switch circuit that closes when receiving and opens when receiving FM.
18 is an AGC signal transmission line that returns from the AM detection circuit 16 to the AM high frequency amplification circuit 5, first stage mixing circuit 6, and first stage intermediate frequency amplification circuit 9, VR ₁ is a variable resistor for volume adjustment, and 7 is a low frequency amplification circuit. circuit, 19 is the speaker, 20 is when the received input signal is small
Due to the noise signal from the FM detection circuit 14, or when the received input signal is large, the AGC signal or AM
This squelch circuit is controlled by a carrier signal from the second stage intermediate frequency amplification circuit 15, and includes an active bandpass filter 21, a rectifier circuit 22,
It is composed of an AGC signal amplification circuit 23, a squelch control volume VR ₂ for both FM and AM, a squelch switching circuit 24, and the like.

In addition, opening switch 17 when receiving FM is as follows:
This is to prevent the detuned signal from being slope-detected by the AM detection circuit 16 and being applied to the squelch circuit 20 .

Furthermore, the second-stage mixing circuit 10 and local oscillation circuit 11, the second-stage intermediate frequency amplification circuit 13 for FM, and
The FM detection circuit 14, the active bandpass filter 21, and the squelch switching circuit 24 may be built in an IC in the same package.

25 is a scan switch circuit controlled by the output of the squelch switching circuit 24; 26;
is a scanning IC controlled by the scan switch circuit, which is set to the no-signal operation state when the automatic scanning operation mode is set;
A plurality of crystals corresponding to the number of channels connected to the crystal connection part 27 provided on the output side of the IC 26
Scanning is performed sequentially from X ₁ to Xn. When a broadcast wave of a certain channel is received, by continuing to ground the crystal corresponding to that channel, the first stage local oscillation circuit 4 is excited at a specific frequency, and the band changeover switch circuits 28, 29 are excited.
An operating voltage is supplied to the high frequency amplification circuit and the first-stage mixing circuit for the band corresponding to the band, and settings are made so that the specific broadcast of the corresponding band can be listened to. 30 is a channel display section composed of a plurality of light emitting diodes, etc.;
1 is a block including a scanning operation automatic/manual changeover switch and a channel changeover switch for manual scanning operation.

As can be seen from the block diagram in Fig. 1, in the circuit shown in Fig. 1, only the high frequency amplifier circuits 2 and 5 and the first stage mixing circuits 3 and 6 are selectively switched and used in each of the FM and AM bands, and the other circuits are used. Commonly used for FM and AM.

The configuration and operation of the scanning circuit and band switching circuit are described in Japanese Patent Application No. 49-44115 (Japanese Unexamined Patent Publication No. 137414-1973), and the configuration of the automatic/manual switching switch for scanning operation and the channel switching switch. Regarding the operation, please refer to the patent application No. 51-64798.
(Japanese Unexamined Patent Publication No. 52-146510), etc., it has been explained in detail, so further explanation will be omitted here.

Here, the specific circuit configuration and operation near the squelch circuit 20 , which is the main part of the present invention, are shown in FIG.
This will be explained in more detail with reference to FIG.

In FIG. 2, Q ₁ and Q ₂ are transistors that constitute the second stage AM intermediate frequency amplification circuit 15, IFT is an intermediate frequency transformer, D ₁ is a diode that constitutes the AM detection circuit 16, and D ₂ is a switch. The diode that constitutes the circuit 17 is connected in series between the capacitors C ₁ and C ₂ that constitute the AM detection circuit 16, and the anode of the diode D ₂ also has a “High” level voltage when receiving AM. , one end of a line 32 is connected, which is controlled by a band changeover switch circuit 28 so that the voltage is 0V when receiving FM. As described above, the diode D ₂ becomes non-conductive during FM reception, thereby preventing the detuned signal from being slope-detected by the AM detection circuit 16 and being applied to the squelch circuit 20 . 33 is an operational amplifier 3 that constitutes the FM detection circuit 14 and the active bandpass filter 21.
4. An IC that includes a squelch switching circuit 24 and the like, and the terminal of the IC, that is, the output terminal of the FM detection circuit 14, is connected to the volume adjustment variable resistor VR ₁ and the active bandpass filter 21.
D ₃ is a diode that constitutes the rectifier circuit 22. Q ₃ is an AGC that constitutes the AGC signal amplification circuit 23
A signal amplification transistor whose base is connected to the AGC signal transmission line 18, whose emitter is connected to ground 26 via a resistor _R1 , and whose collector is connected to a resistor _R2.
It is connected to the power supply line 35 via a resistor _R3 , and to the terminal of the IC 33 and the slider 36 of the squelch volume _VR2 (the input terminal of the squelch switching circuit 24).

Note that the terminal of the IC 33 is a scan control signal output terminal, and the output of this output terminal is applied to the scan switch circuit 25. The terminal of the IC 33 is a squelch signal output terminal, and the output of the output terminal is applied to the low frequency amplifier circuit 7 to control ON/OFF of the low frequency amplifier circuit.

The operation of the circuits of FIGS. 1 and 2 constructed in this manner will be described next.

When there is no signal, the noise that appears at the terminal of IC33, that is, the output terminal of the FM detection circuit 14, during both FM reception and AM reception, is suppressed by the variable resistor for volume adjustment.
An active bandpass filter 2 is guided by VR ₁ and is composed of resistors R ₄ , R ₅ , R ₆ , a capacitor C ₃ , and an operational amplifier 34 in an IC 33 .
It will lead you to 1. This bandpass filter 21 extracts noise components outside the audio band. The noise component is negatively rectified by diode _D3 , and IC33
is applied to the input terminal of the squelch switching circuit 24.

Here, +0.8V is the threshold voltage of the terminal that is the input terminal of the squelch switching circuit 24 inside the IC33, and when a voltage of +0.8V or more is applied to the terminal, the squelch signal output terminal of the IC33 becomes open and the squelch signal is output. The operation is no longer performed, [the squelch is opened (released)], and the low frequency amplifier circuit 7 is activated. One terminal voltage is +0.8V
When the voltage is below, the terminal is shorted to the ground 26, a squelch operation is performed, and the low frequency amplifier circuit 7 becomes inactive.

Note that when there is no signal, there is no need to consider voltage generation by the carrier signal.

Furthermore, when there is no signal or a small input, the AM side
Since AGC is not effective, the high frequency amplification circuit 5 and first stage intermediate frequency amplification circuit 9 are at approximately the maximum gain state even during AM reception, and both FM and AM reception
It may be considered that the noise voltages from the detection circuit 14 are equal. Therefore, the threshold squelch operating point when there is no signal can be easily set at the same position of the squelch control volume _VR2 both when receiving FM and when receiving AM. Therefore, it is possible to eliminate the unnaturalness during scanning as described above.

By the way, in the case of a noise squelch method that uses noise to perform a squelch operation, it is possible to set the squelch operation point to a weak electric field, that is, to start the squelch operation in a weak electric field, but it cannot be set to a strong electric field. The limit is a high frequency carrier input level (2 to 3 μV/m) with an approximate S/N ratio of 20 to 30 db. In addition, in the case of the noise squelch method, if the squelch operating point is set too deep (to a strong electric field),
The phase distortion component of the modulated wave generated within the receiver exceeds the amount of out-of-band noise within the receiver, causing squelch malfunction due to reception of the modulated wave, i.e., a phenomenon in which a squelch that has been released once is applied again (voice lock phenomenon), resulting in squelch. It has already been found that the operation becomes unstable, and in the case of a narrowband receiver, this voice lock phenomenon is extremely severe, making it extremely difficult to widen the variable range of the squelch operating point. On the other hand, if an AGC signal or a carrier signal is used, the squelch operating point can be set in a strong electric field.

Therefore, in the embodiments shown in FIGS. 1 and 2, as will be explained in detail later, the squelch operating point can be set to a strong electric field by using the AGC voltage.

Figure 3 shows the relationship between the high frequency carrier input level and
Changes in AGC voltage from AM detection circuit 16 and FM
5 is a characteristic diagram showing changes in noise voltage from the detection circuit 14. FIG. As can be seen from Figure 3, the high frequency carrier input level of the noise changes up to around 10 μV/m, so in the noise squelch method, the squelch operating point can be set up to around this range, but in reality the voice lock phenomenon occurs. Therefore, the limit is 2 to 3 μV/m. Since the change in the noise voltage is large in the small input portion, it is very easy to set the squelch operating point by using this noise voltage. On the other hand, there is no change in noise voltage in the large input section where the high frequency carrier input level is around 100μV/m,
Since the AGC voltage changes linearly, it is suitable to use the AGC voltage in this vicinity to set the tight squelch operating point (the point at which the squelch opens when tight squelch is set), and this AGC voltage By using , it is possible to set a deep tight squelch operating point, that is, to set a squelch operating point in a strong electric field. And the tight squelch operating point is AGC
It can be set within the range where the voltage changes linearly, but in the case of the AGC squelch method or the carrier squelch method, the temperature characteristics are poor, so if the squelch operating point is set too deep, the squelch may not open. The limit is 100μV/m.

In order to adjust the tight squelch operating point, in the circuit shown in Figure 2, for example, the voltage dividing resistors R ₂ and R ₃ connected to the collector of the AGC signal amplification transistor Q ₃ should be set to appropriate values. Just make it into something.

Next, in the circuits of FIGS. 1 and 2, when the high frequency input level increases, the noise component from the FM detection circuit 14 decreases, the negative rectified voltage of the diode D ₃ decreases, and at the same time the transistor Q ₁ ，
The second stage intermediate frequency signal is amplified by Q ₂ ,
Although the AGC voltage decreases, the change in the AGC voltage is inverted and DC amplified by the transistor _Q3 , and the positive voltage is applied to the terminal of the IC 33, that is, the input terminal of the squelch switching circuit 24. This IC3
The squelch operating point is set by controlling both the voltage applied to the terminal No. 3, which is the voltage obtained by rectifying the noise from the FM detection circuit 14, and the voltage obtained by inverting and DC amplifying the AGC voltage from the AM detection circuit 16. When the slider 36 of the squelch control volume VR ₂ is set to the furthest side (non-ground side), no squelch operation is performed and the sound does not disappear. Further, when the slider 36 is moved to the farthest side (earth side), a tight squelch state occurs, that is, a state where the squelch operating point is the deepest. Therefore, by adjusting the slider 36 between -, the depth of the squelch operating point can be varied. Now, if you move the slider 36 of the squelch control volume VR ₂ to the side to make it into a tight squelch state and increase the high frequency input signal, the effect of the noise voltage disappears at around 10μV/m, and the effect of the AGC voltage changes. Once within this range, pull the IC33 terminal towards the + voltage direction, and eventually the squelch will open when the voltage exceeds +0.8V (the squelch will no longer operate).

That is, in the circuits shown in FIGS. 1 and 2, the threshold squelch operating point is opened by noise, and the tight squelch operating point is opened by AGC voltage.

In this way, in the circuits shown in Figures 1 and 2, the squelch operating point (shallow squelch operating point) in the small input section is FM.
Since the noise from the detection circuit 14 and the squelch operating point (deep squelch operating point) of the large input section are made to depend on the AGC voltage, the variable range (dynamic range) of the squelch operating point can be greatly expanded. I can do it.

Furthermore, there is a concern that the S/N ratio may deteriorate due to noise from the FM detection circuit 14 during AM reception.
When the S/N ratio is around 10 db, FM noise is suppressed and has almost no effect on the S/N ratio during AM reception. Naturally, at higher input levels, there is no FM detection output, and only AM detection output.

FIGS. 4 and 5 are a main part block diagram and a main part circuit connection diagram showing another embodiment of the present invention. Fourth
The circuit shown in Figures 1 and 5 uses the carrier signal from the AM second-stage intermediate frequency amplification circuit 15 instead of using the AGC voltage to obtain a deep tight squelch operating point. , which is different from Fig. 2.

Next, the configuration and operation of FIGS. 4 and 5 will be explained.

In FIGS. 4 and 5, the same parts as in FIGS. 1 and 2 are designated by the same numbers as in FIGS. 1 and 2.

C ₄ and D ₄ are capacitors and voltage doubler rectifier circuit 3 connected in series between the collector of transistor Q ₂ of second-stage intermediate frequency amplifier circuit 15 for AM and ground 26.
8, the first rectifier diode D ₅ , R ₆
is the connection point 3 of the capacitor C ₄ and diode D ₄
7 and a second rectifier diode and resistor for voltage doubler rectification connected in series between the terminal of IC33,
R ₇ is a resistor connected between the resistor R ₆ and the power supply line 35. And in Figures 4 and 5,
AGC signal amplification circuit 23 is not provided. The other configurations are exactly the same as in FIGS. 1 and 2.

In the circuits shown in FIGS. 4 and 5, the operation when there is no signal is the same as that in FIGS. 1 and 2. Also, when the high frequency input level increases, the FM detection circuit 14
The noise component from the transistor is reduced, and at the same time the negative rectified voltage of the diode _D3 is reduced.
The second stage intermediate frequency signal is amplified by Q ₁ and Q ₂ ,
The carrier component is the first and second rectifying diodes.
The voltage is doubled and rectified by D ₄ and D ₅ and a positive voltage is applied to the terminal of the IC 33. Then, the squelch control volume VR ₂ controls both the voltage obtained by rectifying the noise from the FM detection circuit 14 and the voltage obtained by rectifying the carrier component from the second stage intermediate frequency amplification transistor Q ₂ to set the squelch operating point. are doing.

In the circuit shown in FIG. 5, in order to adjust the tight squelch operating point, for example, resistors R ₆ and R ₇ may be set to appropriate values.

The other operations are the same as those in FIGS. 1 and 2, so their explanation will be omitted.

In this way, in the circuits shown in Figures 1, 2, 4, and 5, the FM detection circuit is operated even during AM reception.
Since the noise from the FM detection circuit is used to open the threshold squelch operating point when there is no signal,
The threshold squelch operating point when there is no signal can be set at the same position on one squelch control volume during both FM reception and AM reception.
Also, since the AGC voltage or carrier signal is used to open the tight squelch operating point during both FM reception and AM reception, it is possible to set a deep tight squelch operating point. Therefore, unnaturalness during scanning can be eliminated, and the variable range (dynamic range) of the squelch operating point can be greatly expanded.

Furthermore, the circuits shown in Figs. 1, 2, 4, and 5 use a squelch switching circuit with one input terminal for both FM and AM, and do not use a voltage comparator with two input terminals. The number of parts can be reduced, and the circuit can be simplified.

Although FIGS. 1, 2, 4, and 5 show embodiments in which the first-stage local oscillation circuit is constructed using crystals in a number corresponding to the number of channels, the present invention Needless to say, the invention is not limited to the embodiments, and can be applied to a so-called synthesizer type FM-AM receiver in which the first stage and/or second stage local oscillation circuits are constructed using a PLL circuit.

As described above, according to the squelch circuit of the FM-AM receiver according to the present invention, the FM detection circuit and the AM reception system circuit are set to the operating state during both FM reception and AM reception, and the A squelch control means is provided with a signal obtained by rectifying the noise and a signal related to the carrier wave level from the AM receiving system circuit, and is based on the signal obtained by rectifying the noise from the FM detection circuit. The squelch control means sets a threshold squelch operating point, and the squelch control means sets a tight squelch operating point based on a signal related to the carrier level from the AM receiving system circuit. It is now possible to set the threshold squelch operating point when there is no signal at the same position on the squelch control means (squelch control volume), and it is also possible to set a deep tight squelch operating point, eliminating unnaturalness during scanning. It is possible to expand the variable range (dynamic range) of the squelch operating point.

[Brief explanation of the drawing]

Figures 1 and 2 are a block diagram and main circuit wiring diagram showing an embodiment of the FM-AM receiver according to the present invention, and Figure 3 shows changes in AGC voltage and noise voltage with respect to high-frequency carrier input level. The characteristic diagram shown, FIGS. 4 and 5 are a main part block diagram and a main part circuit connection diagram showing other embodiments of the FM-AM receiver according to the present invention. 14...FM detection circuit, VR ₂ ...squelch control volume.

Claims (1)

[Scope of Claims] 1. An FM detection circuit and an AM reception system circuit that are set to an operating state during both FM reception and AM reception;
A squelch control means is provided with a signal obtained by rectifying the noise from the FM detection circuit and a signal related to a carrier level from the AM receiving system circuit, and the squelch control means is provided with a signal obtained by rectifying the noise from the FM detection circuit. Based on the obtained signal, the squelch control means sets a manual squelch operating point, and the squelch control means sets a tight squelch operating point based on a signal related to the carrier level from the AM receiving system circuit. A squelch circuit for an FM-AM receiver, characterized in that the squelch circuit is designed to 2. The squelch of the FM-AM receiver according to claim 1, wherein the AM receiving system circuit includes an AGC circuit, and the signal related to the carrier wave level is an AGC voltage from the AGC circuit. circuit. 3. The device according to claim 1, wherein the AM receiving system circuit is an intermediate frequency circuit, and the signal related to the carrier wave level is a carrier signal.
FM-AM receiver squelch circuit.