JPS61123318A - Signal meter circuit - Google Patents

Abstract

PURPOSE:To obtain a signal meter output with high linearity by applying an amplified voltage of a change voltage in an AGC voltage of an AM reception circuit to a DC voltage obtained through the reception and detection of an AM broadcast wave and applying the resulting voltage to a signal meter. CONSTITUTION:When no antenna input signal exists, a voltage at a signal meter output terminal 17 is zero. When the antenna input signal is inputted next, the increment of a collector current of a transistor (TR)9 is fed to a terminal 17 through a diode 14 of current mirrors 12, 15. In setting a power supply 22 to turn of a TR18 with an antenna input level of A or below, a current flows to a collector of the TR18 through the diode 14 in response a minute increment of an AGC voltage at an output terminal 24 at a level of the antenna input A or over. Thus, a current is added to a resistor 16 by the current mirrors 12, 15, resulting that the output characteristic at the terminal 17 changes from (a) to (b).

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a signal meter circuit for an AM receiver having an AGC circuit.

BACKGROUND OF THE INVENTION A conventional signal meter circuit is constructed as shown in FIG. That is, the 7AM broadcast wave received by the antenna 1 is frequency-converted by the high frequency amplification/mixer 2, amplified by the intermediate frequency amplifier 3, detected by the wave detector 4, and output as a low frequency signal. On the other hand, obtained by AGC detector 5, 7=j
The AGC voltage was applied to the high frequency amplifier/mixer 2 and the intermediate frequency amplifier 3. Then, this AGC voltage is directly applied to the signal meter drive circuit 6 and amplified.
The output shown in Figure 4 is obtained, and this is used to drive a signal meter (not shown).

Problems to be Solved by the Invention However, in the conventional configuration as described above, even if the antenna input level is higher than the input level at which the AGC operation starts, the output of the signal meter circuit will increase by a small amount due to the AGC operation. However, as shown in FIG. 4, the output curve of the signal meter does not change linearly depending on the antenna input level.

There is a drawback that the fluctuation of this precision meter is also inaccurate.

In view of the above problems, the present invention provides, regardless of the operation of AGC,
It is an object of the present invention to provide a signal meter circuit in which a signal meter output curve is linear with respect to an antenna input level.

Means for Solving the Problems In order to solve the above problems, the present invention operates AGC using the voltage obtained by detecting the output of the intermediate frequency amplifier and the AGC voltage that is the output of the AGC detector. Level (A in Figure 4)
A slight increase in the AGC voltage at an input level higher than (point level) is amplified and added to the specific voltage, and the result is used as an output for the signal meter.

Operation The present invention has the above-described configuration, so that when the signal meter output is the same as the conventional one and the level at which the AGC operates (the level at point A in Fig. 4) is exceeded, the total amplification ratio voltage is reduced by the slight increase in the AGC voltage. A signal meter output with good linearity is obtained with respect to the antenna input level.

Embodiment FIG. 1 is a diagram showing an example of a signal meter circuit according to the present invention. In FIG. 1, 3 is an intermediate frequency amplifier; 6 is an intermediate frequency amplifier;
is an AGC detector, and 23 is a power supply terminal. The output of the intermediate frequency amplifier 3 is connected to the base of a transistor 9, its emitter is connected to the constant current source 1o and one side of a resistor 24, the other end of the resistor 24 is grounded by a capacitor 25, and the collector is It is connected to the collector of transistor 12, the anode of diode 14, the base of transistor 15, and the collector of transistor 18. transistor 1
20 base is the anode of diode 13 and constant current source 1
Connected to 1. The constant power source 11 supplies the same current as the constant current source 1o.

The collector of the transistor 16 is grounded through a resistor 1.6, and the upper end of the resistor 16 is connected to the signal meter output terminal 1.
It is 7. The output terminal 24 of the AGC detector 5 is connected to the base of the transistor 18.
The emitter of is connected to a resistor 19. One end of the resistor 19 is connected to the emitter of the transistor 21 and the resistor 20, and one end of the resistor 20 is grounded. The bases of the two transistors are biased by a power supply 22.

The entire operation of the i-signal meter circuit constructed as described above will be explained with reference to FIGS. 1 and 2.

First, the operation of fif and the part 7 in FIG. 1 will be explained. As mentioned above, the constant current source 10 and the constant current source 11 flow the same current, and the diode 13 and the transistor 12
It constitutes a current mirror.

When there is no antenna input signal, the collector current of transistor 9 is equal to the current of constant current sources 11 and 10, so
No current flows through the second current mirror composed of the diode 14 and the transistor 15, so that the voltage at the signal meter output terminal 17 is zero.

Next, when the antenna input signal is input, the signal amplified by the intermediate frequency amplifier 3 is input to the base of the transistor 9, and the increase in the collector current of the transistor 9 due to this signal is the second The signal is supplied to the signal meter output terminal 17 through the current mirrors 14 and 15 of the diodes 14. At this time, the gain of the signal meter output voltage obtained at the output terminal 17 is determined by the resistor 24 if the impedance of the capacitor 26 is sufficiently small at the intermediate frequency.
, 18 is R24t R16, then R16/R24 is obtained. However, in Figure 2,
Assuming that the AGC operates at the antenna input level A, the output amplitude of the intermediate frequency amplifier increases only slightly at levels above , so the output curve of the voltage at the signal meter output terminal 17 is as follows. It will look like a in Figure 2.

Next, the operation of the section 8 in FIG. 1 will be explained. The transistors 18 and 21 constitute a differential amplifier, and if the power supply 22 is set so that the transistor 18 is turned off when the antenna input level is below A, the AGC voltage at the output terminal 24 will be turned off when the antenna input level is above A. Depending on the minute increment,
A current is supplied to the collector of transistor 18 through diode 14 . Friendly second current mirror 1
4 and 16 add current to the resistor 16, so that the output curve at the signal meter output terminal 17 becomes from a to b in Fig. 2. The signal meter output curve shown in Figure 2 shows the resistances 19, 20. By operating the antenna, the slope can be changed, and the signal meter output can be set to an output curve with good linearity with respect to the antenna input level, regardless of the AGC operation start level.

Effects of the Invention As described above, according to the present invention, a signal meter output with good linearity with respect to the input signal level can be obtained with an extremely simple circuit configuration.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram of a signal meter circuit according to an embodiment of the present invention, Fig. 2 is a characteristic diagram showing signal meter output according to an embodiment of the present invention, and Fig. 3 is an AM receiver including a conventional signal meter circuit. The block diagram of the device, FIG. 4, is an output characteristic diagram of a conventional signal meter drive circuit. 1... Antenna, 3... Intermediate frequency amplifier, 5... A G C detector, 17...
・Signal meter output terminal. Name of agent: Patent attorney Toshio Nakao and 1 other person 2nd
Figure 8 P level 7 to antenna Figure 3 Figure 4 8 antenna included 1 Calpel

Claims (1)

[Claims] A feature is that a voltage amplified by the change in the AGC voltage of the AM receiving circuit is added to the DC voltage obtained by receiving and detecting AM broadcast waves, and the voltage after this addition is applied to the signal meter. signal meter circuit.