JPH0545089B2 - - Google Patents

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention automatically detects a signal in response to a sudden change in received input.
This invention relates to an AM receiver with a function to improve the responsiveness of AGC (automatic gain control).

[Conventional technology]

In AM receivers, the gain of the 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, and 3
is a detection stage, 4 is a primary filter that smoothes a part of the detection output (audio output), 5 is an AGC amplifier,
6 is a secondary filter that obtains the final AGC voltage _V2 . Primary filter 4 consists of resistor R ₁ and capacitor C ₁
The secondary filter 6 is composed of a resistor _R2 and a capacitor _C2 .

Figure 5 is an operating waveform diagram, where V ₁ is the primary filter 4.
The output is DC amplified by the AGC amplifier 5. The secondary filter 6 is for further smoothing V ₁ , but under normal reception conditions there is not much difference in voltage between V ₁ and V ₂ . On the other hand, when the receiving input suddenly changes, the primary voltage V ₁ changes suddenly, but the change in the secondary voltage V ₂ is suppressed slowly. This is the primary voltage
This is because if a rapidly changing voltage such as V ₁ is used as the AGC voltage, there will be an inconvenience that low frequency components of the AM modulation components will not be 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, 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 a change in the electric field, and since the change is rapid, conventionally the transistor _Q1 is sometimes turned off to stop the function of the secondary filter 6. In this way, the time constant of the AGC loop becomes small, so that the broadcast content of the temporarily stopped station can be quickly heard at an appropriate level.

[Problem that the invention seeks to solve]

However, in order to do this, port 7 that outputs the control signal to the microcomputer side
This is disadvantageous because it requires Furthermore, since the time constant of the AGC loop remains large in response to natural electric field fluctuations, it has the disadvantage of not being able to 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 diagram for explaining the basic operation of the present invention, where 4 is a primary filter, 5 is an AGC amplifier, and 6 is a diagram for explaining the basic operation of the present invention.
8 is a secondary filter, and 8 is a differential voltage detection circuit. This differential voltage detection circuit 8 detects the primary voltage V ₁ and the secondary voltage
The purpose is to detect the difference ΔV from V ₂ ′, but when the difference voltage ΔV increases, the primary voltage V ₁ changes suddenly, so in order to detect the latter, the capacitor C ₀ and the resistor R A differentiating circuit is used to differentiate the primary voltage V ₁ with respect to ₀ . 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 it is turned ON, it shorts both ends of the resistor _R2 and makes the secondary filter 6 ineffective (through).

[Effect]

FIG. 2 is an operating waveform diagram, and a is a differential waveform of the primary voltage V ₁ shown in FIG. 5. This differential output normally has a low level, but when there is a sudden input change, the level exceeds the thresholds +L and -L. Peak P corresponds to the rising edge of V ₁ and bottom B corresponds to the falling edge of V 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 essentially limited to the primary filter 4, and the secondary voltage V ₂ is turned on as shown by the broken line. ′ responsiveness increases. The solid line V ₂ shows the secondary voltage for reference 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 replaced by a secondary filter 6.
and the switch 10 is a diode D ₁ ,
This has been achieved with _D2 . When the forward voltage of diodes D ₁ and D ₂ is V _F and the forward resistance is R _D , if the difference ΔV between the primary voltage V ₁ and the secondary voltage V ₂ ' exceeds V _F , the diode D ₁ or D ₂ conducts. Therefore, this forward voltage V _F corresponds to the threshold ±L of the switch drive circuit 9. For example, if V ₁ rises rapidly and becomes V ₁ −V _F > V ₂ ', diode D ₁ becomes conductive and charges capacitor C ₂ with time constant C ₂ · _RD . Generally, R _D is several tens of Ω, so if R ₂ is selected to be several KΩ, the time constant of the secondary filter 6 will be dominated by R _D ·C ₂ . Furthermore, since R _D ·C ₂ is much smaller than the time constant C ₁ ·R ₁ of the primary filter 4, the time constant of the entire AGC loop is approximately determined by C ₁ ·R ₁ .

The above case corresponds to the peak P in Figure 2a, but conversely, the case corresponds to the bottom B.
Since V ₁ suddenly drops to V ₂ ′−V _F >V ₁ , diode D ₂ becomes conductive. As a result, capacitor C ₂
The discharge resistance for is switched from R ₂ to R _D of diode D ₂ , resulting in a small time constant (fast response) as in the case described above.

In this circuit, the switch switching threshold (±L in Figure 2) is determined by the forward voltage V _F of the diodes D ₁ and D ₂ , so the number of stages is adjusted to
Although you can select _VF , you cannot make detailed level settings. Therefore, the AGC 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 response of the AM receiver's AGC is normally slowed down to fully reproduce the low range, and only when the input voltage suddenly changes, it is temporarily increased to maintain the appropriate level. can be quickly realized. Furthermore, since the responsive 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. In addition, by adjusting the time constants of both filters and the gains of both AGC amplifiers, the response itself and the response switching sensitivity can be adjusted freely and easily.

[Brief explanation of the drawing]

Figure 1 is a basic configuration diagram of the present invention, Figure 2 is its operating waveform diagram, Figure 3 is a circuit diagram showing an embodiment of the present invention, Figure 4 is a configuration diagram of a conventional AM receiver, and Figure 5 is a diagram of the basic configuration of the present invention. The figure is an operation waveform diagram. In the figure, 2 is a tuned amplification stage including a variable gain amplifier;
3 is the detection stage, 4 is the primary filter, 5 and 5' are AGC
amplifier, 6 is a secondary filter, 8 is a differential voltage detection circuit, 9 is a switch drive circuit, 10 is a switch,
D ₁ and D ₂ are diodes, and R ₂ is a resistor.

Claims (1)

[Claims] 1. In an AM receiver that creates an AGC voltage by sequentially smoothing the AM detection output using a primary filter and a secondary filter, a first
an AGC amplifier; the secondary filter provided after the first AGC amplifier; and the second filter provided after the secondary filter.
an AGC amplifier, and an output voltage V1 of the first AGC amplifier and an output voltage V2' of the secondary filter provided between the first AGC amplifier and the second AGC amplifier.
an AM receiver comprising: a switch that significantly reduces the time constant of the secondary filter when the difference between the two filters exceeds a certain value;