JPS6363208A - Am receiver - Google Patents

Abstract

PURPOSE:To quickly carry out reproduction at an appropriate level without affecting an external constitution and to cope with the fluctuation of an input by normally slowing down AGC's responsiveness so as to sufficiently reproduce a low frequency and temporarily speeding up the responsiveness only if an input voltage suddenly changes. CONSTITUTION:A difference voltage detection circuit 8 doubles as a secondary filter 6, and a switch 10 is formed with diodes D1 and D2. It is assumed that where the normal direction voltage of the diodes D1 and D2 and their normal direction resistance are VF and RD, respectively, the difference V between a 1st order voltage V1 and a 2nd order voltage V2 exceeds the voltage VF. Then the diode D1 or D2 is conducted. If the voltage V1 suddenly rises to attain V1 - VF>V2', the diode D1 is conducted, and a capacitor C2 is charged by a time constant C2.RD. Since, normally the normal direction resistance RD is several tens of OMEGA, RD.C2 becomes dominant over the time constant of a 2nd order filter 6 if a resistance R2 is set several KOMEGA. The time constant RD.C2 is far smaller than the time constant C1.R1 of a 1st order filter 4. Conversely, if the AGC corresponds to a bottom part B, the diode D2 is conducted to attain a low time constant(quick responsiveness) similarily.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention automatically performs AGC (
The present invention relates to an AM receiver having a function of improving responsiveness of automatic gain control.

[Conventional technology]

In an AM receiver, the gain of a variable gain amplifier is controlled by negative feedback in order to keep the output voltage constant even if the input voltage changes. Figure 4 is a schematic configuration diagram of an AM receiver with this type of AGC function, where 1 is a receiving antenna, 2 is a tuned amplification stage including a variable gain amplifier, 3 is a detection stage, and 4 is a detection output (audio output). 1st order filter that smoothes a part, 5 is AGC
The amplifier 6 is a secondary filter that obtains the final AGC voltage v2. The primary filter 4 consists of a resistor R1 and a capacitor C1.
The secondary filter 6 is composed of a resistor R2 and a capacitor C2.

FIG. 5 is an operating waveform diagram, where vl is the output of the primary filter 4 which is DC amplified by the AGC amplifier 5. The secondary filter 6 is for further smoothing Vl, but under normal reception conditions there is not much difference in voltage between Vl and V2. On the other hand, when the receiving input suddenly changes, the primary voltage (1) changes suddenly, but the change in the secondary voltage (2) is suppressed slowly. This is because if a rapidly changing voltage such as the primary voltage (1) is used as the AGC voltage, a problem arises in that low frequency components of the AM modulation components are not reproduced.

Therefore, in order to improve the bass reproduction function, the larger the overall time constant of the primary and secondary filters 4 and 6 is, the better. However, in reality, there are sudden changes in the electric field, and electronically tuned radio receivers may suddenly change the tuning point due to the search function. In the latter case, it is equivalent to artificially causing an electric field fluctuation, and since the change is rapid, conventionally the function of the secondary filter 6 may be stopped by turning off the transistor Q1. In this way, the time constant of the AGC loop becomes small, so that the broadcast contents of the next stopped station can be heard promptly at an appropriate level.

[Problem that the invention seeks to solve]

However, doing so requires a port 7 for outputting control signals to the microcomputer, which is disadvantageous. In addition, AGC can handle natural electric field fluctuations.
Since the loop time constant remains large, it has the disadvantage that it cannot follow sudden fluctuations.

The present invention attempts to eliminate the above-mentioned drawbacks by incorporating a circuit that internally detects a sudden change in the AM detection output and disables the function of the secondary filter at that time. .

[Means for solving problems]

FIG. 1 is a basic configuration diagram of the present invention, where 4 is a first-order filter, 5
is an AGC amplifier, 6 is a secondary filter, and 8 is a differential voltage detection circuit. This differential voltage detection circuit 8 detects the difference ΔV between the primary voltage (1) and the secondary voltage V2', but when the differential voltage (ΔV) increases, the primary voltage (1) changes suddenly. Here, in order to detect the latter, the capacitor Co
and resistance R.

A differentiating circuit is used to differentiate the primary voltage (1).

However, this configuration can also be modified as in the embodiments described later. Reference numeral 9 denotes a circuit that compares the output of the differential circuit 8 with a certain threshold level ±L and turns on the switch 10 when the differential output is large. The switch 10 is normally in an off state and enables the secondary filter 6, but
When turned on, both ends of the resistor R2 are shorted and the secondary filter 6
Disable (through).

[Effect]

Figure 2 is an operating waveform diagram, (al is the differential waveform of the primary voltage ■1 shown in Figure 5. This differential output is normally low in level, but when there is a sudden input change, the threshold +L,
-The level will exceed L. Beak P corresponds to the rising edge of VI, and bottom B corresponds to the falling edge of ■1. Since the switch drive circuit 9 temporarily turns on the switch 10 in response to the peak P and bottom B, the on period is substantially limited to the primary filter 4, and the secondary voltage V2' is reduced as shown by the broken line. responsiveness will be increased. The solid line (2) shows, for reference, the secondary voltage when the secondary filter 6 is always enabled.

In this way, it is possible to quickly respond to sudden electric field fluctuations caused by natural phenomena as well as artificial cases such as searches, and moreover, it is usually possible to sufficiently reproduce up to the low frequency range with a large time constant.

〔Example〕

FIG. 3 is a circuit diagram showing one embodiment of the present invention.

In this example, the differential voltage detection circuit 8 is also used as the secondary filter 6,
And switch 0 is connected to diode D1. This was realized in D2. When the forward voltage of diodes D1 and D2 is vP and the forward resistance is RD, the primary voltage V+ and the secondary voltage V2'
When the difference ΔV exceeds vF, the diode D1 or D
2 is conductive. Therefore, this forward voltage vF corresponds to the threshold ±L of the switch drive circuit 9. For example, if ■1 suddenly becomes VI VF >V2', diode D+ becomes conductive and capacitor C2 changes to a time constant of 02・RD.
Charge it with Generally, RD is several tens of Ω, so if R2 is selected to be several Ω, the time constant of the secondary filter 6 is RD−
02 becomes dominant. Moreover, since RD-C2 is much smaller than the time constant 01·R+ of the primary filter 4, AG
The time constant of the entire C loop is approximately determined by C+ - R+.

The above case is shown in Figure 2 (corresponding to the peak P of al, but conversely, when it corresponds to the bottom B, Vl suddenly drops and V2'VP>Vl, so the diode D
2 is conductive. As a result, the discharge resistance for the capacitor C2 is switched from R2 to RD of the diode D2, resulting in a small time constant (fast response) as in the case described above.

In this circuit, the switching threshold (±L in Figure 2) is determined by the forward voltage ■F of the diodes Dl and D2, so although it is possible to select n or ■F by adjusting the number of stages, it is difficult to Level cannot be set. Therefore, A.G.
C amplifier 5' is added and the threshold is optimized by adjusting the gains of both amplifiers 5.5'.

〔Effect of the invention〕

As described above, according to the present invention, the AGC of the AM receiver
Normally, the response is kept slow (low range) to fully reproduce the low range, and only when the input voltage suddenly changes, it is possible to temporarily speed up the response to quickly achieve the appropriate level of reproduction.Moreover, the response Since the switching circuit does not require an external switching signal, it has the advantage of not affecting the external configuration and being able to respond to input fluctuations caused by either human or natural phenomena.

[Brief explanation of drawings]

FIG. 1 is a basic configuration diagram of the present invention, FIG. 2 is an operational waveform diagram thereof, FIG. 3 is a circuit diagram showing an embodiment of the present invention, FIG. 4 is a configuration diagram of a conventional AM receiver, and FIG. The figure shows its operating waveform diagram. In the figure, 2 is a tuned amplification stage including a variable gain amplifier, 3 is a detection stage, 4 is a primary filter, 5 and 5' are AGC amplifiers, 6 is a secondary filter, 8 is a differential voltage detection circuit, and 9 is a switch drive In the circuit, 10 is a switch, D+D2 is a diode, and R2 is a resistor.

Claims (3)

[Claims] (1) In an AM receiver that creates an AGC voltage by sequentially smoothing the AM detection output with a primary and secondary filter, the output voltage (V_1) of the primary filter (4) and the output voltage (V_1) of the secondary filter (6) A differential voltage detection output circuit (8) that detects the difference (ΔV) from the output voltage (V_2') and a time constant of the secondary filter (6) that is significantly reduced when the output of the circuit exceeds a certain value. An AM receiver comprising a switch (10). (2) The AM receiver according to claim 1, wherein the differential voltage detection circuit (8) is a differentiating circuit. (3) The differential voltage detection circuit (8) is configured to also function as a secondary filter (6), and the switch (10) is connected in parallel with the resistor (R_2) of the secondary filter (6) with opposite polarity. AM receiver according to claim 1, characterized in that it consists of a pair of diodes (D_1, D_2).