JPS6246354Y2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to an AM radio receiver that can perform reception with less interference and noise in both medium and strong electric fields as well as weak electric fields.

Conventionally, as is well known, an envelope detection circuit or a synchronous detection circuit is used in the demodulation circuit of an AM radio receiver that receives amplitude-modulated radio waves.

AM radio receivers that use envelope detection circuits have the advantage of being able to receive signals even in weak electric fields, but they have the disadvantage of being susceptible to interference and noise, and having a poor S/N (signal/noise) ratio. .

AM radio receivers that use a synchronous detection circuit are
On the other hand, it has the advantage that it is less susceptible to interference signals and noise and can receive good reception, but on the other hand, as the antenna input becomes weaker, synchronization may be lost and large noise may be generated. Another drawback is that if the synchronization is lost, interference will occur between the input signal and the voltage controlled oscillator, producing an unpleasant beat sound. To eliminate this drawback, a muting circuit was installed to prevent noise and beat sounds from being output. However, when receiving radio signals while driving in a place far away from the broadcast station where the electric field strength changes drastically, the muting circuit often activates, causing the playback sound to drop out intermittently. The problem was that it became difficult to hear.

This invention eliminates the above drawbacks and is an AM radio receiver that is equipped with both an envelope detection circuit and a synchronous detection circuit, automatically switches depending on the strength of the electric field, and can perform good reception in both weak and strong electric fields. It provides:

The AM radio receiver of the present invention includes an envelope detection circuit and a synchronous detection circuit that are connected in parallel to an intermediate frequency amplification circuit, and one of the outputs of the envelope detection circuit and synchronous detection circuit is connected to a low frequency amplification circuit. A switching circuit, an automatic gain control voltage detection circuit that receives the output of the envelope detection circuit and generates an automatic gain control voltage, and compares a built-in set voltage with the automatic gain control voltage to determine the automatic gain control voltage. A command is given to the switching circuit so that when the voltage is higher than a set voltage, the output of the synchronous detection circuit is connected to the low frequency amplifier, and when the voltage is lower than the set voltage, the output of the envelope detection circuit is connected to the low frequency amplifier. and a voltage comparison circuit for simultaneously stopping oscillation of the voltage controlled oscillator in the synchronous detection circuit.

Embodiments of the present invention will be described with reference to the drawings.

The figure is a block diagram of one embodiment of the present invention.

The AM radio receiver of this embodiment has antenna 1.
a high-frequency amplifier circuit 2 that amplifies the amplitude-modulated radio waves input from the antenna 1; a local oscillator 4; and a frequency mixing circuit that mixes the outputs from the high-frequency amplifier circuit 2 and the local oscillator 4 to generate an intermediate frequency. 3, an intermediate frequency amplification circuit 5 that amplifies the generated intermediate frequency, and an envelope detection circuit 7 and a synchronous detection circuit 6 connected in parallel to this intermediate frequency amplifier circuit 5.
, a switching circuit 9 that connects either the output of the envelope detection circuit 7 or the synchronous detection circuit 6 to the low frequency amplifier circuit 11, and an automatic gain control (hereinafter referred to as AGC) voltage in response to the output of the envelope detection circuit 7. occurs. The AGC voltage detection circuit 8 compares the built-in set voltage and the AGC voltage, and when the AGC voltage is higher than the set voltage, the output of the synchronous detection circuit 6 is connected to the low frequency amplification circuit 11, and when it is lower, it is connected to the envelope. A voltage comparator circuit 10 that instructs the switching circuit 9 to operate so that the output of the detection circuit 7 is connected to the low frequency amplifier 11, and at the same time stops the oscillation of the voltage controlled oscillator in the synchronous detection circuit 6; circuit 11
and a speaker 12 that converts the output of the device into audio.

Next, the operation of this embodiment will be explained.

The output of the intermediate frequency amplifier circuit 5 is applied to a synchronous detection circuit 6 and an envelope detection circuit 7 to perform AM detection, respectively, and the respective detection outputs are input to a switching circuit 9. Normally, the switching circuit 9 outputs the signal from the synchronous detection circuit 6, which is amplified by the low frequency amplification circuit 11 and audible as a sound from the speaker 12. On the other hand, the output of the envelope detection circuit 7 is also input to the AGC voltage detection circuit 8, and a low-pass filter produces an AGC voltage from which audio signal components are removed.
This voltage is a DC voltage approximately proportional to the antenna input in a weak electric field until the AGC circuit operates. Compare this voltage with the set voltage of the voltage comparator circuit 10, and if the AGC voltage becomes lower than the set voltage, that is, if the antenna input becomes weak or the tuning shifts, the output of the voltage comparator circuit 10 will be inverted. , the switching circuit 9 is switched from the output of the synchronous detection circuit 6 to the output of the envelope detection circuit 7, and at the same time, the oscillation of the voltage controlled oscillator in the synchronous detection circuit 6 is stopped in order to eliminate the beat sound. This prevents the sound from being emitted due to overheating, making tuning operations easier.

As explained in detail above, the AM radio receiver of the present invention uses a synchronous detection circuit to achieve good reception with less interference and noise in medium to strong electric fields, and switches to an envelope detection circuit in weak electric fields. ,
Unlike conventional AM radio receivers that use only synchronous detection circuits, this system has the advantage of providing excellent reception without interruptions in sound or beat sounds caused by out-of-synchronization.

[Brief explanation of the drawing]

The figure is a block diagram of one embodiment of the present invention. 1...Antenna, 2...High frequency amplification circuit, 3...
...Frequency mixing circuit, 4...Local oscillation circuit, 5...
Intermediate frequency amplifier circuit, 6... Synchronous detection circuit, 7...
Envelope detection circuit, 8...AGC voltage detection circuit, 9
...Switching circuit, 10...Voltage comparison circuit, 11...
Low frequency amplifier circuit, 12...speaker.

Claims (1)

[Scope of utility model registration request] an envelope detection circuit and a synchronous detection circuit connected in parallel to the intermediate frequency amplification circuit; a switching circuit that connects one of the outputs of the envelope detection circuit and the synchronous detection circuit to a low frequency amplification circuit; and the envelope detection circuit. an automatic gain control voltage detection circuit that generates an automatic gain control voltage in response to the output of the synchronous detection circuit; The output of the synchronous detection circuit is connected to the low frequency amplifier, and when the output of the envelope detection circuit is low, the output of the envelope detection circuit is connected to the low frequency amplifier. An AM radio receiver comprising: a voltage comparison circuit for stopping oscillation of a controlled oscillator.