JP2994727B2 - AGC circuit of radio receiver - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an AGC circuit used for, for example, an FM radio receiver, and more particularly to an AGC circuit capable of obtaining good intermodulation interference characteristics and adjacent station interference characteristics. It is about.

[Prior Art] FIG. 7 is an equivalent circuit block diagram showing an example of an AGC circuit in a conventional on-vehicle FM radio receiver. This AGC circuit performs AGC by detecting the output signal level of the mixer. The '88 Sanyo Semiconductor Data Book, Bipolar Integrated Circuit for Car Audio, published on March 1, 1988 (CQ Publishing Company) ) ", Pp. 124-136.

In FIG. 7, reference numeral 10 denotes an AGC circuit (AGC).
Is integrated on top. Reference numeral 14 denotes an antenna circuit (ANT) whose output is supplied to a first gate of an RF amplifier 16 and a pin diode 18 which is an attenuator (ATT) of the antenna circuit 14 is connected to the antenna circuit (ANT). ) Has been adjusted. Reference numeral 20 denotes a mixer (MIX), which receives an amplified antenna signal via an RF amplifier 16 and a local oscillation signal from a local oscillator (OSC) 22 via a buffer 24, and Frequency conversion to IF has been performed. Reference numeral 26 denotes a level detector, which detects the level of an IF signal which is an output signal of the mixer 20, and the detected level signal is input to the AGC 10.

The AGC 10 generates an AGC signal when the signal level becomes equal to or higher than a predetermined level, and controls the gain of the antenna circuit 14 and the RF amplifier 16.

That is, the signal level input to the radio receiver varies significantly depending on the distance from the transmitting station, propagation conditions, and the like, and such AGC is applied to adjust the level to an optimum range processed in the receiver. In particular, in the case of an in-vehicle receiver, the distance from the transmitting station changes over time, and the propagation condition varies significantly depending on the traveling position of the vehicle.
The antenna input level varies extremely, for example, in the range of about 1 μV to 3 V. Therefore, in order to obtain a satisfactory reception condition even at a low signal level, and to suppress the occurrence of harmonic distortion due to a non-linear element such as a transistor, an FET or a variable capacitance diode at the time of reception at a high signal level to obtain reception without interference. Such an AGC circuit is indispensable.

The AGC signal is applied to the pin diode 18 and the second gate of the RF amplifier 16 via the pin of the IC 12 and the pin, respectively, and the level of the RF signal is attenuated. Thus, AGC10
Functions to make the output level of the mixer 20 constant by the AGC signal.

In this case, the relationship between the mixer input frequency and the AGC sensitivity is
As shown in FIG. As shown in FIG. 8, the AGC sensitivity becomes lower as the detuning degree Δf is larger than the center frequency (Δf = 0) of the desired station.

If there is no AGC, when the ANT input and MIX input levels increase, third-order harmonic distortion occurs due to, for example, non-linearity of Tr or the like, and IM (intermodulation) interference characteristics deteriorate. Therefore, the AGC attenuates the RF signal, suppresses the generation of distortion, and improves the interference characteristics.

[Problems to be Solved by the Invention] Interference that must be considered in the above-mentioned AGC circuit includes intermodulation interference (3 signal characteristics) and adjacent station interference (2 signal characteristics).

The former is an interference in which, due to the signal level of the other station frequency on both sides of the desired station frequency, a harmonic component due to the non-linear characteristic of the element generated at the input of the large signal level is mixed into the desired station frequency. To prevent this, the signal level in a band apart from the desired station frequency is attenuated using the above-described conventional AGC circuit. For this reason, it is preferable that the AGC characteristics be broadband AGC.

On the other hand, the latter adjacent station interference occurs when there is a strong electric field interference station adjacent to the desired station and the desired station has a weak electric field.

That is, in the above-described conventional technique, when an AGC signal is generated from the mixer output, if there is an adjacent station having a large electric field strength with respect to the center frequency of the desired station, the eighth technique is used.
Due to the characteristics shown in the figure, the AGC 10 generates an AGC signal for the adjacent station, and at the same time, the signal level of the desired station is also attenuated, so that there is a problem that almost no mixer input can be obtained. .

In addition, AGC based on the mixer input
When the signal is generated, there is a problem that the AGC circuit malfunctions due to the local oscillation signal. That is, as described above, the front end of the FM receiver
There is a local oscillation circuit including a local oscillator 22 and a buffer 24, and its signal level is 300 mVrms to 1 Vrms, which is extremely large as compared with the mixer input level. Also, the frequency of the local oscillation signal is a high frequency of 50 MHz to 120 MHz, which is a frequency close to the mixer input signal. Therefore, GND line, Vcc
Local oscillation signal is AGC due to line or air propagation
Mixed into the circuit, and
There was a risk of generating AGC signals. Such a malfunction of the AGC circuit extremely lowers the gain of the RF amplifier, causing deterioration in sensitivity.

Therefore, when AGC is performed by detecting the mixer input level, it is necessary to lower the sensitivity of the AGC. However, lowering the sensitivity lowers the interference (rejection) characteristic especially at frequencies where the degree of detuning Δf is small. Occurs.

Conventionally, a keyed AGC is known as a further improved AGC circuit. According to the keyed AGC, a control level of the AGC circuit is compensated by using an IF unit output level of a radio receiver. .

Therefore, according to this conventional circuit, when the IF section output level is low, the control amount of the AGC is reduced or cut off,
The aforementioned sensitivity deterioration is prevented.

However, such a keyed AGC has a defect that the keyed AGC works even when the IF output level is low or in a no-signal state where there is no output level, and intermodulation interference becomes particularly large.

The present invention has been made to solve the above-mentioned problems of the prior art, and an object of the present invention is to effectively suppress both intermodulation interference and adjacent station interference, and furthermore, to generate a conventionally generated local deep frequency signal. Optimal AGC by reliably preventing contamination etc.
An object of the present invention is to provide an AGC circuit capable of setting a quantity.

[Means for Solving the Problems] In order to achieve the above object, an AGC circuit according to the present invention detects a received signal level and responds to the signal level.
In an AGC circuit that generates an AGC signal and controls the gain of an RF amplifier circuit, a filter circuit that passes a signal in a predetermined frequency band from the output signal of the mixer, and a detection sensitivity that changes according to the degree of detuning with respect to a predetermined center frequency. A first level detection circuit for detecting the output signal level of the mixer band-limited by the filter circuit, and having a constant detection sensitivity regardless of the degree of detuning with respect to a center frequency; A second level detection circuit for detecting,
An addition circuit that adds the first and second level detection signals and outputs the AGC signal as an AGC signal; an S meter that detects a desired station signal level of an IF unit to which the mixer output is supplied; Supply to the first level detection circuit,
AGC that lowers the output of the second level detection circuit with a decrease in the desired station frequency signal level to lower the mid-band AGC sensitivity
And a compensation circuit.

[Operation] Therefore, according to the AGC circuit of the present invention, it is possible to set the optimum detection sensitivity by the first level detection signal for the adjacent interference near the center frequency of the desired station, and An interference signal generated when the electric field strength is high can be effectively suppressed.

Further, for intermodulation interference having a detuning degree equal to or more than a predetermined value, a constant detection sensitivity can be set by the second level detection signal, and excessive sensitivity suppression of a desired station frequency due to excessive AGC is performed. Can be prevented.

Further, according to the present invention, the desired station frequency signal level of the IF section is detected by the S-meter, and the output of the first level detection circuit is adjusted by the S-meter output. IF set medium band AGC
It is possible to compensate by the unit output level.

Therefore, this compensation in the present invention provides the mid-band AGC by applying the key compensation provided to all conventional AGC circuits to only the first level detection signal obtained from the mixer output side divided into two as described above. In addition to the control by the output of the IF unit, there is an effect that the AGC of the wide band can always be given at a constant value.

Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

FIG. 1 is a circuit block diagram showing one embodiment of an AGC circuit according to the present invention. The same or corresponding parts as those in FIG. 7 are denoted by the same reference numerals, and description thereof is omitted.

In FIG. 1, reference numeral 30 denotes a bandpass filter, which can cut off a signal of 50 MHz or more in an output signal line of the mixer 20 and remove a local oscillation signal (about 50 MHz to 120 MHz).

32 has the first level detection circuit of the present invention, a predetermined center frequency as shown in FIG. 2, the detection sensitivity decreases depending on the detuning degree Δf example for the IF frequency f _1.

The first level detection circuit 32 forms a first AGC system that detects the output signal level of the mixer 20 band-limited by the band-pass filter 30 and performs AGC according to the detection sensitivity.

Reference numeral 34 denotes a second level detection circuit of the present invention, as shown in FIG. 3, which has a constant detection sensitivity regardless of the detuning degree with respect to a center frequency, for example, a tuning frequency. The second level detection circuit 34 forms a second AGC system that detects the input signal level of the mixer 20 according to the detection sensitivity.

Since a high frequency of about 60 MHz to 110 MHz is input to the second level detection circuit 34, the frequency characteristics of the level detection circuit 34 need to have a broadband characteristic of at least up to about 110 MHz. This frequency band corresponds to the local oscillation signal (50
(MHz to 120 MHz), there is a risk that the AGC 10 malfunctions due to the intrusion of the local oscillation signal. For this reason, in the present embodiment, the AGC sensitivity (detection sensitivity) in the second level detection circuit 34 is suppressed as compared with the first level detection circuit 32. The AGC sensitivity is set not only to a value that does not cause the above-described malfunction but also to a value that can eliminate the intermodulation interference described in the related art, including variations in elements, temperature characteristics, and the like.

An adder circuit 36 adds the first and second level detection signals and outputs an AGC control signal output from the AGC circuit 10. The addition circuit 36 is provided with an offset circuit,
The offset amount of the AGC control signal output from the adding circuit 36 is determined. In the two-series AGC circuit of the present invention having the above configuration, first, a local oscillation signal of 50 MHz (to 120 MHz) or more is removed from the output signal level of the mixer 20 by a band-pass filter, and then the first level detection circuit 32 The first level detection signal is output according to the characteristics shown in FIG.

On the other hand, the second level detection circuit 34 outputs a second level detection signal from the input signal level of the mixer 20 according to the characteristics shown in FIG.

The first and second level detection signals are added to an adder circuit.
The AGC control signal is added at 36, and the AGC control signal is supplied to the attenuator 18 and the RF amplifier circuit 16 of the antenna circuit 14.

Therefore, according to the two-sequence AGC according to the present invention, the overall characteristic is as shown in FIG. 4 obtained by superimposing FIGS.
In the middle band region of F = ± 1 MHz, the first AGC system has a high detection sensitivity, so that the first AGC system mainly works in accordance with the output signal level of the mixer 20, and conversely in a wide band where the degree of detuning is larger. The second AGC path corresponding to the input signal level of the mixer 20 works preferentially.

Therefore, according to the present invention, the band is divided into the middle band and the wide band according to the degree of detuning from the center frequency.
Applying AGC control enables AGC control that can optimally deal with any disturbance.

That is, for a wide band, the second level detection circuit
34 provides almost constant detection sensitivity, and can control intermodulation interference over a wide frequency band.
It is possible to prevent the center frequency from being excessively suppressed in sensitivity due to excessive application.

On the other hand, for adjacent station interference near the center frequency, the first
Since the detection sensitivity of the level detection circuit 32 is set to be high, such adjacent station interference can be effectively attenuated.

In this embodiment, the effect of the local oscillation signal is
This is eliminated by arranging a band-pass filter for cutting a signal of 50 MHz or more, and making the fixed detection sensitivity of the second level detection circuit higher than the detection sensitivity of the first level detection circuit at the time of detuning.

In the present invention, the two series of AGCs are further provided, and the AGC compensation is given to the first AGC system by the output level of the IF unit. It can be eliminated.

In FIG. 1, the IF unit receives an output from the front-end mixer 20 and performs FM demodulation and the like. In the present invention, the IF unit
Forty outputs, the output of the multi-stage limiter in the embodiment, are taken out as narrow band signals because they have a center frequency component. And this IF signal is S meter
The signal is sent to 42 and the signal level of the desired station is detected. Of course,
Although the output of the S meter 42 is used in other circuits of the radio receiver, in the present invention, the output of the S meter 42 is supplied to the first level detection circuit 32, and the output of the S meter 42 corresponds to the output level of the IF unit. Compensate the detection sensitivity.

FIG. 5 shows the AGC compensation effect of the first AGC system by the output of the IF unit. In the middle band, the characteristic is hatched from the state of the chain line to the state of the solid line according to the output level of the IF unit. Adjusted as you did.

The chain line in FIG. 5 shows the case where the output of the IF section is sufficiently large, and shows the detection sensitivity for the second AGC system substantially similar to that in FIG.

On the other hand, the solid line in FIG. 5 shows a state in which the output level of the IF section has decreased. In this state, the control by the first AGC system is reduced, thereby reliably preventing the conventional keyed AGC from over-applying AGC. It is possible.

FIG. 6 shows a more detailed circuit diagram of the AGC circuit according to the present invention, and the same members as those in FIG.

The bandpass filter 30 for band-limiting the output signal of the mixer 20 and supplying it to the first level detection circuit 32 is made of, for example, a ceramic filter or the like, and is equivalently shown by a resistor and a capacitor in the figure. When the AGC circuit according to the present invention is integrated into an IC, stray capacitance (parasitic capacitance) occurs as shown by a broken line. In such an integrated circuit, the frequency of the bandpass filter 30 is determined in consideration of the parasitic capacitance. .

The output of the bandpass filter 30 is supplied to the base of the transistor 50 of the first level detection circuit 32, and a signal according to the characteristics shown in FIG. 2 is supplied to the first input transistor 52 of the addition circuit 36.

On the other hand, the input signal level of the mixer 20 is supplied to the base of a transistor 54 provided in the first level detection circuit 34, and the output thereof is supplied to the second input of the addition circuit 36 as shown by the characteristic in FIG. It is supplied to the base of transistor 56.

Accordingly, the adder circuit 36 receives the input obtained by adding the collector currents of the transistors 52 and 56, thereby supplying the combined AGC control signal from the output terminal 36a to the attenuator 18 and the RF amplifier 16 of the antenna circuit 14 as described above. Then, gain adjustment is performed. The addition circuit 36 includes an offset circuit therein as described above.

The two-level detection circuits 32 and 34 are provided with reference power supply circuits 60 and 62, respectively, and supply a predetermined collector voltage to the transistors 50 and 54, respectively.

In the present invention, as described above, the AGC compensation signal is supplied from the S meter 42 to the first level detection circuit 32 according to the output level of the IF unit. In the embodiment, the output of the S meter 42 is The output is supplied from a transistor 66 to a reference power supply circuit 60 of the first level detection circuit 32.

Therefore, the detection sensitivity of the first level detection circuit 32 is compensated by the IF output level, that is, the output of the S meter 42.

[Effects of the Invention] As described above, according to the AGC circuit of the present invention, the first and second AGCs that detect the output and input level of the mixer
By providing a system and having different detection sensitivity characteristics, both intermodulation interference and adjacent station interference can be reliably removed. Further, according to the present invention, the first AGC system for performing the mid-band AGC is compensated according to the output level of the IF unit, so that the disadvantages of the conventional keyed AGC can be reliably removed.

[Brief description of the drawings]

FIG. 1 is a circuit block diagram showing an embodiment of an AGC circuit according to the present invention, FIG. 2 is an AGC sensitivity characteristic diagram in a first level detection circuit, and FIG. 3 is an AGC sensitivity characteristic in a second level detection circuit. FIG. 4, FIG. 4 is a diagram showing a combined AGC sensitivity characteristic according to the present invention, and FIG.
FIG. 6 is a specific circuit diagram showing a preferred embodiment of the AGC circuit according to the present invention, FIG. 7 is an equivalent circuit block diagram showing an example of a conventional AGC circuit, and FIG. 8 is a conventional example. FIG. 9 is a graph showing AGC sensitivity characteristics in FIG. 20 mixer 30 bandpass filter 32 first level detection circuit 34 second level detection circuit 36 addition circuit 42 S-meter

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) H03G 3/20

Claims (1)

(57) [Claims] An AGC circuit for detecting a received signal level, generating an AGC signal corresponding to the signal level, and controlling the gain of an RF amplifier circuit, a filter for passing a signal of a predetermined frequency band from an output signal of a mixer. A first level detection circuit having a detection sensitivity that varies according to the degree of detuning with respect to a predetermined center frequency and detecting an output signal level of a mixer band-limited by the filter circuit; A second level detection circuit that has a constant detection sensitivity regardless of the above and detects the input signal level of the mixer; and an addition circuit that adds the first and second level detection signals and outputs the same as the AGC signal. An S-meter for detecting a desired station signal level of an IF section to which the mixer output is supplied; and supplying the S-meter output to the first level detection circuit, An AGC circuit for a radio receiver, comprising: an AGC compensation circuit that lowers the output of a second level detection circuit as the number of signal levels decreases, thereby lowering the mid-band AGC sensitivity.