JPS596013Y2 - search - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to a search tuner that can set a constant search sensitivity regardless of the frequency arrangement of broadcasting stations, radio field strength, and search speed.

A search stop detection method for a conventional AM (amplitude modulation) search tuner is generally performed using a circuit as shown in FIG.

In other words, the carriers amplified by the first intermediate frequency amplification circuit IF1 and the second intermediate frequency amplification circuit F2 are extracted, one of which is detected by the AM detection circuit DET1 and output as audio, and the other is level-adjusted by the semi-fixed resistor R1. and the second AM via the third intermediate frequency amplification circuit IF3.
The detection circuit DET2 detects the wave and outputs it as a search stop signal.

However, in such a conventional circuit, feedback is provided from the detection circuit DET1 to the intermediate frequency amplifier circuits IF1 and IF2 by a circuit including a resistor R and a capacitor C as AGC (automatic gain control). Due to the time constant exerted by C, the gain of the search stop circuit may not be constant and the search stop may not operate stably depending on the arrangement of stations (radio wave frequencies of broadcasting stations) or electric field strength.

That is, in the conventional system, as shown in FIG. 3, in the graph of electric field strength E and reception frequency f, station S1
When a synchronization point is reached at station S2, which is far enough away from station Sl, when the synchronization point is reached at station S2, which is far enough away from station Sl, if there is enough time to reach station S2 from station S1 compared to the time constant of the AGC circuit, the AGC is sufficiently disconnected. The carrier of station S2 is detected in a state where the gain of the research stop is maximum.

However, when the search moves to station S3, which is very close to station S2, the carrier of station S3 is detected while AGC is applied due to the AGC time constant, and the gain of the search stop circuit is very small. There is a drawback that it is impossible to stop at station S3.

Further, although the above drawbacks can be eliminated by slowing down the search speed, it still causes the drawback that it takes time to select a channel.

This invention was developed in consideration of this point, and during the search, A
The present invention aims to provide a search tuner that solves the above-mentioned problems by separating the GC.

Next, embodiments of the present invention will be described in detail with reference to the drawings.

In FIG. 2, the same parts as in FIG. 1 are designated by the same reference numerals, and the description thereof will be omitted.

In this embodiment, the output point a of the time constant circuit consisting of the resistor R and the capacitor C of the AGC circuit of the AM detection circuit DET1 is connected to the AGC input circuit of the intermediate frequency amplifier circuit 1 and F2 by switching the switch SW. By disconnecting the AGC feedback loop, the AGC feedback loop is opened, and at the same time, a voltage that can be set by the semi-fixed resistor vR2 is applied to the AGC input circuits of the intermediate frequency amplifier circuits IF1 and 1F2.
In this state, the search sensitivity is always kept constant.

On the other hand, the carrier output branched from the intermediate frequency amplification circuit F2 is amplified by the third intermediate frequency amplification circuit IF3, and sent from the second AM detection circuit DET2 as a search stop signal to the controller COT configured by the digital large integrated circuit. is input.

Upon receiving the search stop signal, the controller COT stops the search operation of the tuner (not shown in its entirety) and operates the switch SW to close the AGC feedback loop.

When the receiver is in search operation, the AGC feedback loop is opened by the switch SW, and a settable constant voltage is applied to the AGC voltage terminals of the intermediate frequency amplifier circuits IF1 and ■F2 to constantly maintain the search sensitivity of the station. It is kept constant regardless of the arrangement or electric field strength.

In the above, the search sensitivity setting may be adjusted by using the semi-fixed resistor VR2 shown in Figure 2, or by inserting a semi-fixed resistor into the output of the intermediate frequency amplifier circuit F2 as in the conventional circuit. You can.

Also, during a search, it is effective to use an audio mute output with a general search width in a digital tuner as a control voltage for opening the AGC feedback loop.

As described above, the search tuner of the present invention is configured to always maintain a constant search sensitivity during the search operation, so it is independent of the irregularly arranged broadcast station frequencies, electric field strength, and search speed. A certain search stop sensitivity can be obtained and the search operation can be performed reliably.

In particular, if applied to a digital tuner using a controller configured with a large integrated circuit, the search tuner of the present invention can be controlled very easily.

[Brief explanation of drawings]

Figure 1 is a block circuit diagram showing a conventional search tuner.
FIG. 2 is a block circuit diagram of a search tuner according to an embodiment of the present invention, and FIG. 3 is a graph showing an arrangement of electric field strength and station frequency for explaining the operation of the search tuner. IF1, ■F2, IF3...Intermediate frequency amplifier circuit, DET2...AM detection circuit, COT...
・・Controller, SW・・・・Switch, ■R2
・・・・・・Variable resistance.

Claims (1)

[Scope of utility model registration request] A switching circuit is provided that opens a feedback loop of an automatic gain control circuit of a tuner during a search operation and applies a constant voltage to an automatic gain control voltage terminal of the tuner, and closes the feedback loop during a search stop, A search tuner in which the switching circuit is controlled by a mute output of a controller operated by a search stop signal from an intermediate frequency amplifier circuit.