JPS6241442Y2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to an FM receiving device that is effective for use in a vehicle or for portable use. The present invention relates to a circuit system for an FM receiver suitable for such an environment where the electric field strength of FM waves changes significantly, and in particular improves the degree of AM suppression.

Generally, in the FM transmission system, FM waves are easily subjected to amplitude modulation or phase modulation due to interference waves such as fading and noise being mixed therein. To deal with the amplitude modulation caused by such interference waves, the gain of the intermediate frequency amplifier circuit is set sufficiently large, and multiple stages of limiter circuits are used to increase the degree of AM suppression and improve the SN ratio and distortion. Ta. However, in the past, the limiter level of the limiter circuit of the FM receiver was set to a constant value, so when an FM wave with a weak electric field strength arrives at the FM receiver, even if the gain is sufficiently large in the intermediate frequency amplification stage, Even if the FM wave reaches the limiter level, it is injected into the FM detection circuit without being suppressed. Then, the AM component mixed into the FM component is not removed and is derived as an audio output.

In particular, recent quadrature detectors constructed from transistor differential amplifier circuits have an excellent ability to be sensitive to AM components. Generally speaking, it is considered natural that when the input level of the FM wave electric field strength in the front end is 20 to 30 dBμ, the degree of AM suppression decreases and the S/N ratio worsens. This becomes a serious problem in environments where the electric field strength fluctuates frequently, such as in equipment.

The present invention was devised in view of the above-mentioned problems, and its purpose is to provide a new circuit system for an FM receiver.

The following is the first part regarding the FM receiver according to the present invention.
This will be explained based on the diagram.

FIG. 1 is a block diagram of an FM receiver according to the present invention.

In FIG. 1, 1 is a front end, 2 is an intermediate frequency amplifier, 3 is a limiter circuit, and 4 is an FM detection circuit, and a normal FM receiver consists of these circuits. The present invention has a circuit system in which the output from the intermediate frequency amplifier 2 is introduced into the limiter level control circuit 5, and the limiter level of the limiter circuit 3 is controlled by the limiter level control circuit 5. A compensation circuit 6 is provided which compensates for the detection efficiency of the FM detection circuit 4 using the output of the intermediate frequency amplifier 2.

The limiter level control circuit 5 is a gain control circuit that derives the output from each stage of the intermediate frequency amplification stage, detects the electric field strength of the FM carrier wave, and has the function of varying the limiter level of the limiter circuit according to the electric field strength. be. To give an example of deriving the output from an intermediate frequency amplification stage, the electric field strength of the FM carrier wave is derived by using a current mirror circuit to derive the driving current of a signal meter circuit included in a normal FM receiver. , the limiter level can be easily controlled by its output.

When the electric field strength of the FM wave decreases to below the limiter level, the AM component mixed into the FM wave is injected into the FM detection circuit 4 without being suppressed. In such a weak electric field, the limiter level is controlled to be lowered by the output from the intermediate frequency amplification stage according to the electric field strength. At this time, by switching several predetermined limiter levels depending on the electric field strength, a sufficient limiter can be applied when the electric field is weak. It is also possible to continuously vary the limiter level depending on the electric field strength of the FM carrier wave.

Furthermore, by lowering the limiter level, the FM
Since the detection efficiency decreases, the output from the intermediate frequency amplifier 2 is injected into the compensation circuit 6, similar to the limiter level control circuit, and an output having a potential opposite to the input level is supplied to the FM detection circuit 4. The detection efficiency is prevented from decreasing by acting to increase the gain.

The outline of the present invention will be explained below based on FIG. 2 showing one embodiment.

In FIG. 2, a differential amplifier circuit is used as the limiter circuit 3, and the circuit consists of a differential pair of transistors Q ₁ , Q ₂ and a transistor Q ₃ , and the transistor Q ₃ is a limiter level control circuit 5. controlled by. 8 is a terminal into which the output of the FM wave from the intermediate frequency amplifier 2 is injected, and 9 is an output terminal.

The limiter level control circuit 5 derives the drive current of the strength meter drive circuit according to the electric field strength of the FM wave from the intermediate frequency amplifier 2 via the current mirror circuit, and converts the current output to a desired level. The current is supplied to the limiter circuit 3 by a current mirror circuit consisting of transistor Q ₃ and transistor Q ₄ . The current mirror current flowing through transistor _Q3 is the transistor differential pair _Q1 and
It is the sum of the collector current of _Q2 . Therefore, by controlling the collector current of transistor Q ₃ by the limiter level control circuit, the collector current of transistor differential pair Q ₁ and Q ₂ is controlled, and the collector current of transistor Q 3 is controlled by the output from the signal meter drive current. You can get the Tsuta Limit level.

Also, like the limiter level control circuit 5, the compensation circuit 6 obtains an output corresponding to the electric field strength of the FM wave from the intermediate frequency amplifier circuit 2, and inverts the output with respect to the output voltage via an inverter circuit or the like. This potential is supplied to the FM detection circuit 4, and the gain of the FM detection circuit 4 is controlled to be increased, thereby preventing a reduction in detection efficiency.

As described above, the FM receiver of the present invention can apply a limiter even in a weak electric field by varying the limiter level according to the electric field strength of the FM wave, and the circuit system of the FM receiver has a good signal-to-noise ratio. can be provided. As in one embodiment of the present invention, the limiter level can be controlled by a simple method.

In the present invention, a differential amplifier circuit is used as the limiter circuit, but it is clear that the limiter level of the FM receiver can also be varied using a diode or the like.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of an FM receiver according to the present invention, and FIG. 2 is a circuit diagram showing an embodiment of the FM receiver according to the present invention.

Claims (1)

[Scope of utility model registration request] A circuit system that varies the limiter level of the limiter circuit by the output of the intermediate frequency amplification circuit according to the electric field strength of the FM carrier wave, and a circuit system that controls the gain of the FM detection circuit, and the output from the intermediate frequency amplification circuit. An FM receiving device characterized in that control is performed to lower the limiter level of the limiter circuit and increase the gain of the FM detection circuit as the level decreases.