JPH0516728Y2 - - Google Patents

Description

[Detailed description of the invention] (Technical field of the invention) The invention relates to an automatic gain control circuit (hereinafter referred to as AGC circuit) for a radio receiver, and is particularly applicable to a radio receiver that receives SSB waves to achieve effective reception. Aircraft AGC
Regarding circuits.

(Prior art and its problems) The receiver's AGC circuit protects the linearity of the receiver's amplification stage against level fluctuations of the received wave received in the propagation path, and also keeps the receiver's detected output signal level almost constant. This is a circuit that maintains the temperature. normal
The AGC circuit detects the received wave, extracts the change in the amplitude of the received wave as a change in DC voltage, and uses that voltage to control the gain of the received wave amplification circuit of the receiver, thereby generating the output signal of the amplifier. keeping the level constant. For example, if the wave to be received is accompanied by a carrier wave, a change in the amplitude of the carrier wave is detected and used as the AGC control voltage. Also,
For SSB waves that do not have a carrier wave, the AGC control voltage is obtained by detecting changes in the amplitude of sideband waves.

In the case of this SSB wave, if the modulation signal is a telegraph line and the peak level of the sideband wave is almost constant, there is no particular problem in detecting the amplitude of this sideband wave and using the resulting voltage as the AGC control voltage. do not have. However, if the sideband consists of a voice signal, a noise level will be detected especially in the silent section of the voice, and this detection level will increase the receiver gain,
The noise will be amplified and output to be approximately the same as the average level of the audio signal.

To solve these drawbacks, conventional AGC circuits applied to SSB lines without a carrier wave have a time constant that is larger than the time constant of AGC circuits applied to other lines. However, although this solution has the effect of suppressing the noise level output during silent sections (noise suppression),
Gain control in response to changes in the received wave level, which is the original function of the AGC circuit, has become difficult, and it has become impossible to obtain any effect, especially against short-period fading.

The ideal operation of an AGC circuit applied to an SSB line is that the charging/discharging time constant should be small so that it operates well against short-period fading, and the discharging time constant should be small to suppress noise during silent periods. is sufficiently large.

However, as mentioned above, there is no prior art that satisfies these two conditions at the same time.

For this reason, current receiver AGC circuits have two types of time constants depending on the reception situation: FAST (small time constant for fast response) and SLOW (large time constant for slow response). are selected manually.

A conventional example having such a configuration is shown in FIG. In the figure, 1 is an input terminal, 2 is an AGC amplification section, 3 is a detector, 4 and 5 are capacitors, 6 is a resistor, 7 is a manual switch, 8 is a DC voltage amplification section, 9
1 is an intermediate frequency amplification section, and 10 is an output terminal. An intermediate frequency (IF) signal is input from input terminal 1,
The intermediate frequency amplification section 9 amplifies the signal. This amplified output is
After being amplified by the AGC amplifier section 2, it is detected by the detector 3. This detected voltage charges the capacitor 4 or 5 along the path shown in the figure. The charges stored in the capacitor 4 or 5 are discharged via the resistor 7 (through a path). The voltage generated across the resistor 7 is amplified by a DC voltage amplification section 8, and then the gain of the intermediate frequency amplification section 9 is controlled. Here, it is assumed that the time constant of the discharge path is changed by the switch 6. The time constant (C ₁ ×R) due to the capacitor 4 and the resistor 7 is set smaller than the time constant (C ₁ ′×R) due to the capacitor 5 and the resistor 7. The smaller time constant is called FAST, and the larger one is called SLOW.

Here, the signal level output to the output terminal 10 will be explained with reference to FIG. In FIG. 3, a schematically indicates the receiver input level, and the shaded area indicates the noise level. Figure b shows the output signal level response when FAST is selected.
Since the time constant is small, it follows changes in the receiver input level well, but the noise level is amplified almost to the standard output level during the silent period. Figure c shows the response of the output signal level when SLOW is selected. Noise in the silent section is suppressed as shown by the hatched area, but at the same time it is unable to follow changes in the received signal, and the signal is also greatly suppressed as shown by the hatched area.

As mentioned above, in the conventional technology, sufficient
The AGC effect cannot be obtained, and time constant selection takes time.

(Purpose of the invention) The present invention was devised in view of the above-mentioned drawbacks of the prior art.
The present invention provides an AGC circuit for a receiver that has an effect similar to FAST, but has a SLOW effect during silent periods. Its feature is that it has a discharge path with a small time constant in addition to a path with a large time constant.

(Structure and operation of the idea) Hereinafter, the present invention will be explained in detail using the drawings.

FIG. 2 is a diagram illustrating an embodiment of the present invention, and shows parts related to the explanation of time constants.
The feature of this embodiment is that the time constant (C ₁ ×R) of the discharge path ′ constituted by the capacitor 11 and the resistor 7 is set to a large value, and a capacitor 12 is added in parallel to the detector 3 .

The present embodiment operates as follows. When the received signal level changes in the direction of increasing, the capacitor 11 is discharged by the charging path. In this charging state, the detector 3 is in a forward bias state, so the capacitor 12 is charged to a voltage approximately equal to the forward voltage drop of the detector 3. When the received signal level changes in the direction of decreasing, the detector 3 becomes reverse biased and the capacitor 11
The charges charged in the capacitor 12 are discharged in a path through the capacitor 12, and the capacitor 12 is reversely charged. This reverse charging rapidly progresses to an equilibrium state determined by the capacitance ratio of capacitor 11 and capacitor 12, and the input voltage of DC voltage amplification section 8 decreases by the amount of charge consumed by capacitor 11 for reverse charging. After reaching this equilibrium state, the capacitance of capacitors 11 and 12 is
Discharge occurs with a large time constant determined by C ₁ , C ₂ and the value R of the resistor 7.

FIG. 4 shows the impulse response of the AGC circuit according to this embodiment. Figure a shows the impulse at the receiver input, and Figure b shows its response. In the figure b, a shows the response of conventional SLOW, c shows the response of conventional FAST, and b shows the response of this embodiment.
Between P ₁ and P ₂ of curve b, capacitor 12
It can be seen that during the charging period, it operates almost FAST, and after that it operates SLOW.

On the other hand, the AGC response of this circuit is shown in Figure 3d.
It can be seen from the figure that there is little signal suppression in the sound section, and a large noise suppression effect in the silent section.

From the above, the setting of the _P2 point in FIG. 4 can be determined based on the fading depth of the receiver input, the desired noise suppression effect, the allowable degree of signal suppression, etc. Empirically, the section P ₁ - _{P 2} operating in FAST is L = 10 dB when the input voltage of the receiver is 40 dB/μV.
A good AGC effect is obtained by setting it to a certain degree.

(Effect of the invention) As explained in detail above, by inserting the capacitor 12 in parallel with the detector 3 and appropriately selecting the ratio of the capacitance C ₂ of the capacitor 12 to the capacitance C ₁ of the capacitor 11. , FAST, SLOW 2
It is possible to configure an AGC circuit that has a high degree of noise suppression in silent periods and a small degree of signal suppression in voiced periods.

In order to increase the degree of freedom in selecting the capacitance ratio, a plurality of capacitances may be prepared in advance as the capacitors 11 and 12 shown in FIG. 2, and these may be selected by a switch depending on the reception state.

[Brief explanation of the drawing]

FIG. 1 is a block circuit diagram showing an example of a conventional AGC circuit, FIG. 2 is a block circuit diagram showing an embodiment of the present invention, FIG. 3 is a time chart for explaining the operation of the conventional circuit and the present invention, and FIG. FIG. 4 is a characteristic diagram for explaining the characteristics of the circuit of the present invention. 1...Input terminal, 2...AGC amplifier section, 3...
Detector, 4, 5... Capacitor, 6... Resistor, 7
...Manual switch, 8...DC voltage amplification section, 9...
...Intermediate frequency amplification section, 10...Output terminal, 11,1
2... Capacitor.

Claims (1)

[Scope of utility model registration request] The amplifier is configured to automatically control the amplification gain of the amplifier using a DC output obtained by extracting the amplitude detection output of the amplified output obtained by amplifying the received wave by the amplifier, and the time constant circuit is configured to automatically control the amplification gain of the amplifier. The voltage is applied to the voltage storage capacitor of the time constant circuit through a parallel connection path of a diode and a capacitor that is in the forward direction with respect to the charging current, and the capacitance of the capacitor of the parallel connection path is equal to the voltage storage capacitor. An automatic gain control circuit for a receiver, wherein the time constant circuit has a discharge path with a large time constant and a discharge path with a small time constant.