JPS5964931A - Agc circuit of receiver - Google Patents

Abstract

PURPOSE:To improve the reception characteristics such as an S/N, distortion factor by controlling the output of a local oscillating circuit in response to the field intensity. CONSTITUTION:The RF reception input given through an antenna is amplified by an RF amplifier 1 and supplied to a mixer (frequency converting) circuit 2. The circuit 2 mixes the local oscillation signal given from a local oscillating circuit 3 to obtain a difference signal between both signals, i.e., a so-called IF signal. This IF signal is amplified by an IF amplifier 4 and then converted into an AF signal by an AM wave detecting circuit 5. While the output of the circuit 5 is converted into a DC through a rectifying/smoothing circuit 6 and then converted into the GC voltage. This GC voltage is applied to the circuit 3 to control the oscillation output of the circuit 3.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an AGC (automatic gain control) circuit of a receiver;
In particular, it relates to the AGC circuit of an AM receiver.

In the A and M receivers, so-called AGC is used to control the reproduced output level to be approximately constant despite fluctuations in electric field strength.
Circuits are essential. In conventional AGC circuits, a common method is to control the amplifier gains of R and F (high frequency) circuits according to the detection output level.

There is a limit to the improvement in receiver characteristics, such as spurious characteristics, S/N characteristics, and distortion rate characteristics, that can be obtained by such a conventional AGC circuit, and further improvement in characteristics is limited to 1.
Gain control of a 1.1" amplifier is no longer difficult.

Therefore, an object of the present invention is to provide an AGC circuit using a new method that can improve receiver characteristics.

The AGC circuit according to the present invention is characterized in that the voltage injected into the intermediate frequency conversion circuit is changed by controlling the output of the local oscillation circuit according to the electric field strength.

The present invention will be explained below using the drawings.

FIG. 1 is a circuit diagram of an embodiment of the present invention, in which RF reception input from an antenna is amplified by an R1'' amplifier 1 and input to a mixer (frequency conversion) circuit 2.Mixer circuit 2
Now, local oscillation circuit (hereinafter simply abbreviated as local oscillation circuit) 3
The local oscillator signal and the R, J'' signals are mixed to obtain a so-called 11='' (intermediate frequency) signal of the difference frequency between the two. This IF multiplied signal is amplified by the 1F amplifier 4 and AI'
Vi detection circuit 5 has an older brother A 1. ” (audio frequency) signal.

In this example, the output of the detection circuit 5 is converted into a DC voltage by the rectifying and smoothing circuit 6, and this AGC voltage is used to control the local oscillation signal level of the local oscillator circuit 30 (.tau.). The rectifying and smoothing circuit 6 includes a rectifying diode (I), a smoothing capacitor C1+U2, and a resistor of 1%, as shown in the figure. It is also %2. This smoothed output becomes the base input of the transformer/stor Q1 via the resistor 1(3), and the collector output of this transformer/stor Q1 is the resistor 1(l).
is applied to the base of the local oscillator circuit: 3 transformer 7 star Q2.

The local oscillator circuit 3 consists of the relevant transformer 7 star Q2 + resistor D, ~
1t7. Konotensa C3-C6,i, Limakonotensa C
It has a well-known configuration consisting of a variable capacitor C8 and a local oscillator coil T1, and the emitter output of the oscillator Q2 is injected into the emitter of the mixer transistor Q of the mixer circuit 2 via the converter C0. Ru. In addition, resistance 1 (
・8 is the emitter resistance of the transistor Q3. By applying the R] signal to the base of the transistor Q3, an output including the 11·′ signal is derived from the collector of the transistor Q3.

The collector output of the l□ 77 /star Q1, that is, the A and GC voltages are applied to the terminal of the local oscillator transformer 7 star Q2. Therefore, if the electric field strength increases, the detection output will increase and the base current of the transistor Q1 will also increase.

Therefore, more of the current flowing into the local transformer/star Q2 is bypassed through the resistor and the transformer/star Q, and flows to the ground. Therefore, the gain of the local oscillator transistor Q2 decreases, and the local oscillator signal output decreases. As a result, in the mixer circuit 2, the emitter press-fit voltage to the mixer transistor Q3 becomes small, resulting in a decrease in the conversion gain of the mixer, resulting in a decrease in the IF signal output, and the AGC action is performed. It turns out.

FIG. 2 is a circuit diagram of another embodiment of the present invention, in which parts equivalent to those in FIG. 1 are designated by the same reference numerals, and the description thereof will be omitted. In this example, the operating type 'df voltage (-
1-13) in accordance with the A and CiC voltages to control the local oscillator signal output level and change the voltage injected into the mixer circuit.

A voltage stabilizing circuit 7 is provided which generates the local oscillator circuit 3' supply voltage (+13), which also includes a Darling top-connected transformer/stor Q4 + Q51 resistor R0 and capacitor C1o. Connect the collector outputs of the power supply transistor Q and the power supply transistor Q to resistor 1 (,
It is designed to be applied via.

As the electric field strength increases, the base current of the transistor Q1 increases, and the current flowing through the resistor () becomes thicker through the resistor It4 and the transistor Q1 and is shunted to ground. As the emitter potential of /star Q4 decreases and the power supply voltage (+13) of local oscillator circuit 3 decreases, the local oscillator output level also decreases, and mixer circuit 2
The injection voltage of −・ decreases and the AGC
action becomes possible.

No. 1 G shows a circuit diagram of another embodiment of the present invention.
Components equivalent to those in FIG. 1 are designated by the same reference numerals, and their description will be omitted. In this example, the output of the local oscillator transistor Q2 is attenuated by the AGC voltage, and this attenuated output is transferred to the buffer 8.
The signal is injected into the mixer circuit 2 via the .degree. C. filter example for use in a multi-band receiver.

The collector of the local oscillator transformer 7 star Q2 has a coil T for MW (medium wave), and a coil T for 5W-1 (short wave-1).

5W-2 (short wave-2) coils T3 are connected in series. The secondary side of the SW-1 coil 'J゛1 is equipped with a tuning circuit consisting of a capacitor (-:, , c7. A tuning circuit consisting of capacitors 015 to C1□ is provided on the secondary side of the 5W-2 coil '■°, and each capacitor c
The output terminals of 7+C'+2 and C10 are selectively selected by a band changeover switch S2 and can be connected to variable capacitors C, . In addition, each coil ゛F1~'1゛3
The intermediate tap on the secondary side of the transformer 7 is connected to the bunt changeover switch S1 via capacitors 018 to C2o, respectively.
It can be alternatively connected to the emitter of Star CJ2.

At the base of transistor Q2 is a resistor R・61 + "6
□.

A base bias circuit consisting of a diode 1→2 is provided, and its emitter output is connected to a resistor R51°I(・
, 2 and connected to the resistor I via the capacitor C21.
T. , and is input to an attenuator consisting of resistors 1 (, 4).The output of this attenuator is added to the base of a buffer transistor Q6 via a capacitor C22.The output of this buffer circuit 8 is input via a capacitor C0. The resistor I is also injected into the mixer circuit 2. Also, 11 is the emitter resistance of the transformer 7 star Q6, and I (12 is the base bias resistance of the transformer/meta Q6). The base current of resistor It4 and transistor Q are also equal to each other.

The series impedance with the local oscillator transformer/meta Q2 decreases, and the local oscillator output from the local oscillator transformer/meta Q2 is attenuated more greatly.

Therefore, the voltage injected into the mixer circuit 2 is reduced and the AGC operation becomes possible.

The effect of improving reception characteristics by the circuits shown in FIGS. 1I to 3 will be explained based on the experimental results shown in FIGS. 4 to 6. First, referring to Fig. 4 (A) to (F), use approximately 16001<Ilz as the local oscillator signal (receiving input level is O(113m, 30d13+n, 7461
3m, 100cl13m.

1 ] These are spurious characteristics showing the harmonic components appearing in the received output when changing the frequency to 113 m and 120 dBm, respectively. The first peak in level is fO8c
(local oscillation frequency), and the second peak to the fourth to fifth peaks indicate harmonics of (N+1)·fosc (N is an integer). As can be seen from these (A) to (F), the spurious response is improved as the input increases. This is because harmonic components are suppressed by lowering the voltage injected into the mixer circuit, and as a result, spurious characteristics at medium-high inputs are improved.

Figure 5 shows the received input level vs. injection level, output level,
The S/N and distortion rate characteristics are shown respectively, and FIG. 6 shows the received input level versus output level, 8/N and distortion rate characteristics when a conventional AGC circuit is used.

In the example shown in Figure 6, the injection level to the mixer circuit is set to 98.
This is the case where it is set to 7nV.

As can be seen from Figures 5 and 6, the distortion rate during strong input has been significantly improved compared to the conventional example, and the injection level can be changed depending on the electric field strength (the conversion gain of the mixer is reduced, and as a result, the IF (signal reduction) greatly contributes to this improvement.

As described above, according to the present invention, by using a completely new method of controlling the voltage injected into the mixer stage using the AGC voltage, it is possible to improve the reception signal-to-noise ratio, distortion rate, etc., which could not be improved with conventional AGC circuits. Since the characteristics will be even better, a high-performance AM receiver can be obtained.

Note that the circuit examples described above are not limited to these, and various modifications can be made.

[Brief explanation of drawings]

Figures 1 to 3 are circuit diagrams of embodiments of the present invention, Figures 4 and 5 are diagrams showing actual and experimental data of reception characteristics by the circuit of the present invention, and Figure 6 is a conventional AGC circuit. FIG. Explanation of symbols of main parts 2 Mixer circuit 3... Local oscillator circuit 6... Rectifier smoothing circuit 7 ・Power supply circuit 8... Buffer Applicant Pioneer Co., Ltd. agent Patent attorney Motohiko Shizawa

Claims (3)

[Claims] (1) An AGC circuit for a receiver, characterized in that the voltage injected into the intermediate frequency conversion circuit is changed by controlling the local oscillation circuit output according to the electric field strength. (2) Claim 1, characterized in that the gain of the local oscillation circuit is controlled according to the electric field strength.
AGC circuit described in section. (3) The AGC circuit according to claim 1, wherein the power supply voltage of the local oscillation circuit is controlled according to the electric field strength.