JPH03171833A - Agc circuit for receiver - Google Patents

Abstract

PURPOSE:To reduce the interference due to IM disturbance by providing a trap circuit which takes the output of an IF amplifying circuit as the input and has the intermediate frequency as the trap frequency and a control signal generating circuit which generates the output corresponding to the output level of the trap circuit and controlling the RF input by the control signal. CONSTITUTION:A trap circuit 21 which takes the output of an IF amplifying circuit 18 as the input and has the intermediate frequency of an intermediate frequency signal corresponding to its selection characteristic as the trap frequency and a control signal generating circuit 22 which generates the output corresponding to the level of the signal obtained from this trap circuit 21 are provided, and the output of the control signal generating circuit 22 is used as the control signal of an RF input control circuit 12. Consequently, the characteristic of the frequency to the output level of the control signal has a wide band where the output level in the vicinity of the intermediate frequency is slightly reduced and output levels on both sides are reduced but are not attenuated very much. Thus, the AGC action in the wide band is available and the interference due to IM disturbance is reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an AC;C (automatic gain control I1l) circuit for controlling the RF (high frequency) input level of a receiver. [Prior art and its problems] An AGC circuit is essential in the front end of a receiver.

FIG. 4 shows a block diagram of a receiver equipped with such an AGC circuit. That is, 1 indicates a receiving antenna, an RF signal from this antenna 1 is applied to an RF input control circuit (attenuator) 2 that performs an AGC function, and its output is applied to an RF buffer amplifier 3. The output of this buffer amplifier 3 passes through an RF tuning circuit 4 and is amplified by an RF amplifier 5.

The output of this RF amplifier 5 is applied to the multiplexer circuit 6,
It is mixed with the local signal from the local signal oscillation circuit 7. The intermediate frequency (I
F) The signal is selectively amplified by the IF amplifier circuit 8, demodulated into an audio signal by the next stage detection circuit 9, and output to the output terminal O.
brought to ut.

Then, the IF which is selectively amplified by the IF amplifier circuit 8
The output is converted into a DC control signal Va according to the IF output level by the AGC circuit 10, and is used as a control signal for the attenuator 2.

In the AGC circuit of the receiver, which is more powerful than the conventional one, the AGC circuit detects the signal level that has passed through the filter provided in the RF tuning circuit 4 and the IF amplifier circuit 8 in the RF stage.
Since the GC control signal Va is generated, the relationship between the level of the AGC control signal Va and the frequency exhibits an extremely narrow band specification as shown in FIG.

For this reason, the AGC function is effective against received desired waves, but has no effect on interference waves other than the desired reception. Therefore, when the desired received wave is in a medium electric field and the adjacent station is in a strong electric field, or at night when the electric field is relatively strong, there is a drawback that the interference phenomenon due to IM (intermodulation) interference becomes noticeable.

In order to prevent such drawbacks, as shown in FIG. 4, the output A from the RF amplifier 5 is
, or AGC using the output B from the ξ kisser circuit 6.
It is also conceivable to generate the control signal Va.

However, with such a structure, another drawback is that the local signal from the local signal oscillation circuit 7 mixes into the output A or output B, which interferes with the AGC operation, and that processing is difficult due to the high frequency signal. will occur.

[Purpose of the invention]

Therefore, an object of the present invention is to provide an AGC circuit for a receiver that can eliminate interference caused by IM interference without being affected by the 800 effect caused by local signals as described above.

(Summary of the Invention) In order to achieve the above-mentioned object, the AGC circuit of the receiver according to the present invention uses the output of the IF amplifier circuit as an input, and calculates the intermediate frequency of the intermediate frequency signal corresponding to the selection characteristic of the AGC circuit of the receiver. A trap circuit having a trap frequency of It is characterized by the fact that it is used.

[Example] Examples of the present invention will be described below with reference to the drawings. FIG. 1 shows an embodiment of the invention.

That is, 11 indicates a receiving antenna, and this antenna 1
The RF reception signal from 1 is introduced via a capacitor C1 to an RF input control circuit 12 consisting of diodes DI and D2, which are variable impedance elements. Each impedance element DI, D2 of this RF input control circuit 12 is AGCv
Its impedance is controlled by the I control signal Va to control the antenna reception signal level. Note that capacitor C2
is a bypass capacitor.

The output from this RF input control circuit l2 is the capacitor C3.
A hum noise blocking coil L and an active element FET (field effect transistor) Q drop the commercial power frequency from the antenna l1 to the reference potential point (earth) through the
is applied to the RF buffer amplifier 13 having a voltage of 1. This RF buffer amplifier 13 itself receives the received signal from the antenna 1l in a non-tuned manner, and the load of this amplifier 13 is a tuning transformer T and a variable capacitance element VD}.
An RF tuning circuit 14 consisting of a Lima capacitor C4 and a resistor Rl is connected and is configured to selectively derive frequencies within the desired receive band. In the variable capacitance element VD, the tuning voltage VT is connected to the resistor R2.
The capacitance value is controlled by applying the voltage through the capacitor. Note that the capacitor C5 is for bypass.

The selectively derived RF tuning output is amplified in the RF amplifier 15, applied to the mixer circuit 16, and mixed with the local signal from the local signal oscillation circuit 17.

The IF signal obtained by this mixer circuit 16 is selectively amplified by an IF amplification circuit 18 consisting of an IF lance for removing local signals, a narrowband filter, an amplifier circuit, and a limiter, and then the IF signal is selectively amplified by an IF amplifier circuit 18 that includes an IF lance for removing local signals, a narrow band filter, an amplifier circuit, and a limiter. The signal is demodulated into an audio signal by the circuit 19 and provided to the output terminal ○ut.

The signal is then selectively amplified by the IF amplifier circuit I8, and the output from the IF transformer at the previous stage is applied via the IF buffer amplifier 20 to a trap circuit 2l whose trap frequency is the intermediate frequency of the intermediate frequency signal.

As shown in FIG. 2, this trap circuit 2l exhibits maximum attenuation at the IF frequency of 10.7 MHz, and the I
The amount of attenuation decreases toward the outside of the F frequency.

The output of this trap circuit 2l is the control signal generating circuit 22.
is applied to the trap circuit 21, and generates a DC output according to the level of the signal obtained by the trap circuit 21. The DC output Va of this control signal generation circuit 22 is the RF input control circuit l.
The input RF signal from the antenna 11 is applied to the impedance elements DI and D2 constituting the antenna 11, the impedance of which is controlled, and the input RF signal from the antenna 11 is attenuated.

FIG. 3 shows the output level versus frequency characteristic of the control signal Va applied to the RF input control circuit l2.
That is, due to the action of the trap circuit 21, the selected frequency f
The output level around o (10.7MHz) decreases slightly,
The output level on both sides is further reduced, but has a broadband characteristic that does not attenuate much.

By selecting the attenuation characteristic of the trap circuit 21, this broadband characteristic can be changed arbitrarily. That is, as described above, the amount of attenuation of the trap circuit 21 decreases as it approaches both sides of the IF frequency.

Further, in the illustrated example, an IF amplification circuit for selectively amplifying an IF signal is shown as a single block with reference numeral 18, but this IF amplification circuit is usually composed of multiple stages and derives the signal to the trap circuit 21. The broadband characteristics can be changed arbitrarily by selecting the IF amplification stage. Therefore, a broadband A with arbitrary frequency characteristics
It becomes possible to carry out the GC function, and it is possible to significantly reduce interference caused by IM interference. [Effects] As is clear from the above explanation, the AGC circuit according to the present invention includes a trap circuit that receives the output of the IF amplifier circuit as an input and uses the intermediate frequency of the intermediate frequency signal as the trap frequency, and It is equipped with a control signal generation circuit that generates an output according to the signal level, and the output of this control signal generation circuit is used as a control signal for the RF input control circuit, so that it has wideband AGC characteristics. Therefore, interference caused by IM interference can be significantly reduced. Moreover, according to this invention, the IF is used as the human power to control the AGC.
Since the output of the amplifier circuit is used, the extent to which the local signal leaks into the AGC control signal is extremely small due to the selection characteristics of the IF amplifier circuit, and the local signal is
The inconvenience of interfering with GC action can also be prevented.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an embodiment of the present invention, FIG. 2 is a characteristic diagram of a trap circuit used in the one shown in FIG. 1, and FIG. 3 is a characteristic diagram showing AGC characteristics of the one shown in FIG. 1. FIG. 4 is a block diagram showing an example of a conventional device, and FIG. 5 is a characteristic diagram showing AGC characteristics of the device shown in FIG. 11... Antenna, l2... RF input control circuit, 1
3...RF buffer amplifier, 14...RF tuning circuit, 15...RF amplifier, 16...mixer circuit,
l7... Local signal oscillator, l8... Intermediate frequency amplification circuit, 19... Detection circuit, 20... IF buffer amplifier, 2l... Trap circuit, 22... Control signal generation circuit.

Claims (1)

[Claims] an RF input control circuit that controls the level of an input RF signal;
A mixer circuit that mixes a local signal from a local signal oscillator with the RF signal that has passed through this RF input control circuit, and an intermediate frequency amplifier circuit that tunes to the intermediate frequency signal obtained by this mixer circuit and amplifies the intermediate frequency signal. and,
The receiver is equipped with a detection circuit that takes the output of the intermediate frequency amplification circuit as an input and demodulates it into an audio signal, the receiver that takes the output of the intermediate frequency amplification circuit as the input and uses the intermediate frequency of the intermediate frequency signal as a trap frequency. further comprising a trap circuit and a control signal generation circuit that generates an output according to the level of the signal obtained by the trap circuit, and the output of the control signal generation circuit is used as a control signal for the RF input control circuit. A receiver AGC circuit featuring: