JPS60160713A - Automatic gain adjusting circuit - Google Patents

Abstract

PURPOSE:To quicken the leading of an AGC voltage by inserting a diode between an output of an amplifier outputting a gain control voltage and a capacitor for a filter so as to charge the filter capacitor via said diode. CONSTITUTION:An output of an amplitude detector 8 enters an automatic gain adjusting amplifier 12 and is extracted as a control voltage of a mixer and an intermediate amplifier (not shown in a figure). A parallel circuit comprising a resistor 18 and a diode 22 is inserted between an output terminal 14 of the gain adjusting amplifier 12 and the filter capacitor 16, and when a potential at the terminal 14 is higher than the potential at a terminal 20 in excess of a forward voltage drop of the diode 22, since the capacitor 16 is charged rapidly via the diode 22, the charging speed is quickened and the leading speed of the AGC voltage is improved.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an automatic gain adjustment circuit, and particularly to improving its response characteristics.

FIG. 1 shows a general automatic gain adjustment circuit. That is, the AM (amplitude variation 1Jl) high frequency signal received by the antenna 2 is mixed with the local oscillation frequency in the mixer circuit 4 and changed to an intermediate frequency, and then amplified by the intermediate frequency amplifier 6. The frequency of this intermediate frequency amplifier 6 is detected by an AM detector 8, taken out as a low frequency output from an output terminal 10, and added to an automatic gain adjustment amplifier 12 to form a gain control voltage (AGC voltage). .

This automatic gain control output is applied to a filter capacitor 16 connected to terminal 14, and the control voltage generated across capacitor 16 is applied to mixer circuit 4 and intermediate frequency amplifier 6 as their gain control inputs. As a result, the gains of the mixer circuit 4 and the intermediate frequency amplifier 6 are adjusted according to the received signal level, so the level adjustment is performed in response to fluctuations in the received signal level, and it is possible to obtain audio output at a constant level. can.

In such an automatic gain adjustment circuit, when the capacitance of the capacitor 16 is increased in order to reduce distortion of the signal waveform due to the signal modulation component contained in the control voltage, the rise of the AGC voltage is delayed due to the filter time constant.
Automatic gain adjustment response becomes slow.

Therefore, it has been proposed to add a quick charging circuit to the capacitor 16 to increase the charging speed.

However, in the conventional quick charging circuit, if the AGC voltage required in a steady state is, for example, about O08■, the voltage at which AGC operation starts, that is, about 0.4 V, which is half of the voltage value, is applied to the capacitor 16. Precharge,
This is to increase the charging speed to the AGC voltage required in a steady state.

FIG. 2 shows how the AGC voltage rises in this case.

The rise in the period of time T is the same as in the case where the quick charging circuit is not installed. Therefore, the speed is slow, and a sufficient improvement in response speed cannot be expected.

In particular, if the charging speed of the capacitors 1 and 6 is slow, problems such as cracking of the sound and the inability to perform accurate detuning and tuning operations occur. In other words, the filter time constant is related to the distortion rate of the signal waveform, and this and the rising speed of the AGC voltage are two sides of the same coin.
It was necessary to sacrifice either the voltage rise speed.

This invention increases the capacitance of the capacitor and
The purpose is to improve the rise speed of the C voltage.

The present invention is characterized in that a filter capacitor is installed via a diode at the output section of an automatic gain adjustment amplifier that forms a gain control voltage from an amplitude detection output, thereby increasing the charging speed.

Hereinafter, the present invention will be described in detail with reference to embodiments shown in the drawings.

FIG. 3 shows an embodiment of the automatic gain adjustment circuit of the present invention,
Components that are the same as those in the automatic gain adjustment circuit of FIG.

A resistor 18 is connected to the output terminal 14 of the automatic gain adjustment amplifier 12.
A filtering capacitor 16 is connected between the terminal 20 formed through the capacitor 18 and a ground point, and a diode 22 is connected in parallel to the resistor 18 for rapidly charging the capacitor 16. That is, a filter capacitor 16 is installed at the output section of the automatic gain adjustment amplifier 12 via a resistor 18 and a diode 22.

With this configuration, the diode 22 constitutes a floating type quick charging circuit. When an AGC voltage is generated at the output terminal 14 of the automatic gain adjustment amplifier 12 by the AM detection output, if the AGC voltage is higher than the terminal voltage of the capacitor 16 between the output terminal 14 and the terminal 20,
A potential difference occurs.

When this potential difference exceeds the forward voltage drop VF of the diode 22, the diode 22 becomes conductive, and a charging current flows through the diode 22 to the capacitor 16.

Then, charging of the capacitor 16 progresses, and the terminal 14.20
When the potential difference between them becomes less than the forward voltage drop vF of the diode 22, the diode 22 becomes non-conductive and the capacitor 16 is charged via the resistor 18.

If the capacitor 16 is charged through the diode 22 in this way, the charging speed is determined by the dynamic resistance of the diode and the capacitor capacity, so the speed at which the voltage of the capacitor 16 reaches the AGC voltage required in a steady state becomes faster. The response of automatic gain adjustment can be improved.

In addition, due to this improvement in charging speed, the dynamic resistance of diodes is in the range of several ohms to several tens of ohms, so
The capacitance of capacitor 16 can be increased relative to conventional values. Then, when the AGC voltage is in a steady state, the diode 22 becomes non-conductive, so the time constant is set by the resistor 18 and capacitor 16 from the automatic gain adjustment amplifier 12, and as a result, the signal change frequency component is A smoothing effect can be obtained, and distortion of the signal waveform can be prevented.

In addition, in the embodiment shown in FIG.
are connected in series. In this way, the current flowing through the diode 22 can be limited, so that destruction of the diode 22 can be prevented.

FIG. 5 shows the rise characteristics of the AGC voltage of the automatic gain adjustment circuit of the present invention. Characteristic A is when the AGC voltage in the steady state is high, and characteristic B is when it is low.Characteristics A and B have greatly improved speeds compared to the conventional characteristic C.

In other words, in conventional quick charging circuits, when the AGC voltages are different, it is necessary to set a voltage value for quick charging for each AGC voltage value, but with this automatic gain adjustment circuit, The capacitor 16 is quickly charged to a voltage value slightly lower than the AGC voltage, that is, the voltage value is lower than the steady AGC voltage by the forward drop voltage VF of the diode 22, and after that, the resistor 24 and the capacitor are charged. A time constant of 16 provides its control action. This automatic gain adjustment circuit uses various AGC
The voltage value for rapid charging is automatically set according to the voltage, and the charging speed can be significantly improved. In addition,
As a result of such a control operation, the diode 22 becomes non-conductive, so that the capacitance of the capacitor 16 can be set to a large value.

As explained above, according to the present invention, the capacitor is
In addition to being able to charge rapidly in response to the generation of the GC voltage, the filter time constant can be set large, so signal modulation components in the AGC voltage can be sufficiently smoothed and removed, reducing the distortion factor. can be lowered.

[Brief explanation of drawings]

Figure 1 is a block diagram showing a general automatic gain adjustment circuit.
FIG. 2 is an explanatory diagram showing the rise characteristics of a conventional AGC voltage, FIG. 3 is a circuit diagram showing an embodiment of the automatic gain adjustment circuit of the present invention, and FIG. 4 is another embodiment of the automatic gain adjustment circuit of the present invention. Circuit diagram showing an example, Figure 5 shows AGC in a capacitor
FIG. 3 is an explanatory diagram showing voltage rise characteristics. 12...Automatic gain adjustment circuit, 16--Capacitor constituting filter, 22...Diode, 24...
・Resistance as a current limiting element. Figure 1 Figure 2 1.5 hours Figure 3 Figure 4 Time

Claims (1)

[Scope of Claims] (An automatic gain adjustment circuit characterized in that a filter capacitor is installed via a diode at the output part of an automatic gain adjustment amplifier that forms a gain control voltage from the 11AM detection output. (2) The above. 2. The automatic gain adjustment circuit according to claim 1, wherein a current limiting element is connected in series to the diode.