Unlocking Attenuator for Automatic Tuning Receiver

1. Field of the Invention

This invention relates to an automatic tuning receiver, and more particularly to a receiver having a tuning means which generates search muting signals during the sweeping operation and wherein the sensitivity for stopping the sweeping operation can be controlled by attenuating the antenna inputs by means of the search muting signals, the attenuating mode being released after the sweeping operation stops.

2. Description of the Prior Art

In an automatic tuning receiver arranged to stop the sweeping operation by detecting electric field intensities of received signals, there is usually provided a switch for switching to a low (LO) sensitivity setting for stopping sweeping operation solely in response to signals having electric field intensities higher than a predetermined value and a high high (DX) sensitivity setting for stopping the sweeping operation by detecting signals having electric field intensities lower than the above predetermined value. The switching action between the LO and DX stations has generally been performed in response if required to attenuate antenna inputs or not.

However, in such conventional switching system, antenna inputs in the low sensitivity setting remain in the attenuated condition not only during the sweeping operation but also after the sweeping operation ceases, thus causing a deterioration of signal-to-noise ratio due to noises generated inside the receiver.

It is therefore an object of the invention to solve the above problem in the conventional tuning receiver, and more specifically to cause an automatic tuning receiver to cancel the attenuation of antenna input signals in the low sensitivity setting after the sweeping operation stops.

In accordance with the present invention, there is provided an automatic tuning receiver which comprises:

an AM radio receiver having an automatic tuning means;

a first attenuating means connected to said AM radio receiver for attenuating antenna input signals;

a sensitivity switching device connected to said attenuating means and said automatic tuning means; and wherein

said attenuating means is controlled arranged to vary its attenuating ratio by search muting signals from said automatic tuning means in response to the switching action of said sensitivity switching device.

FIG. 1 shows a circuit diagram of an embodiment according to the present invention; and

FIG. 2 shows a circuit diagram of a further embodiment according to the present invention.

The present invention will now be described in detail referring to the preferred embodiments illustrated in the accompanying drawings.

In FIG. 1, an antenna ANT is connected to an FM receiver FMT through a diode D (a p-i-n diode) interposed in series therebetween. The antenna ANT is grounded through an AM noise preventing coil L.sub.1, an antenna connecting capacitor C.sub.2 and an AM antenna loading coil L.sub.2, all connected in series. The junction of the AM antenna connecting capacitor C.sub.2 and the AM antenna loading coil L.sub.2 is also connected both to an AM receiver AMT including a conventional automatic tuning means S, and to an antenna input attenuating means ATT. The antenna input attenuating means ATT comprises an impedance element Z having a capacitor or a resistor and a transistor Q.sub.2 having a collector terminal connected to impedance element Z and an emitter terminal connected to the ground. The base terminal of the transistor Q.sub.2 is connected to a search muting signal output terminal O of the automatic tuning means S and to a DX terminal of a sensitivity switching device SW. This part of the circuit is, (as is referred to in the claims) a means responsive to a control signal (at the terminal O) for operating an attenuating means, (the impedance Z and the transistor Q2) to, a signal attenuating condition when the movable contact T is on the LO terminal.) The sensitivity switching device SW is of a type having one movable contact and two stationary contacts. A common terminal E of the sensitivity switching device SW is grounded and a switching plate T always contacting terminal E is movable into contact with either a DX terminal or a LO terminal. The junction of the FM antenna connecting capacitor C.sub.1 and the Diode D is connected to the emitter terminal of a transistor Q.sub.1 through an FM antenna loading coil L.sub.3 interposed in series. The collector terminal of the transistor Q.sub.1 is connected to a power source B as well as to a base terminal of the same transistor Q.sub.1 through a resistor R.sub.1. The base terminal is further connected to the LO terminal of the sensitivity switching device SW.

The function of the device as constructed above according to the present invention will now be described.

(a) Under DX setting:

When the switching plate T of the sensitivity switching device SW is in contact with the DX terminal (as shown by a dashed line in the drawings), the transistor Q.sub.2 is brought to an off condition with its base terminal grounded, and accordingly, AM antenna input signals conducted through the AM noise preventing coil L.sub.1 and the AM antenna connecting capacitor C.sub.2 are applied to the AM receiver AMT without being attenuated.

At the same time, the transistor Q.sub.1 is put in an on-condition with its base terminal being provided with a bias by the power source B through the resistor R.sub.1, and accordingly, the diode D is supplied with forwardly directed voltage from the power source B through the FM antenna loading coil L.sub.3 to be in a low impedance condition. Therefore, FM antenna input signals conducted through the FM antenna connecting capacitor C.sub.1 are applied to the FM receiver FMT without being attenuated.

(b) Under LO setting:

When the switching plate T of the sensitivity switching device SW contacts the LO terminal (as shown by a solid line in the drawings), the transistor Q.sub.2, with its base terminal now released from the ground, is subject to control by search muting signals from the automatic tuning means S, so as to be switched into an off state or an on state.

Search muting signals of +5 V, for example, are put out only during the sweeping operation of the automatic tuning means S, but are not put out after the sweeping operation stops. Therefore, during the sweeping operation, the transistor Q.sub.2 is held in its on condition to attenuate AM antenna input signals through the impedance element Z. On the other hand, after the sweeping operation stops the transistor Q.sub.2 is brought in its off condition so as thereafter not to annenuate AM antenna input signals. The attenuation ratio can be adjusted by varying the value of the impedance element.

At the same time, the transistor Q.sub.1 is cut off with its base terminal grounded, and accordingly, the diode D is not supplied with voltage from the power source B and thus is in a high impedance condition, thus attenuating FM antenna input signals.

As apparent from the above description, according to the present invention, since the antenna input attenuating means is controlled by search muting signals from the automatic tuning means when in the LO setting, antenna input signals are attenuated so as to lower the sensitivity for stopping the sweeping operation, but they are not attenuated after the sweeping operation stops. Therefore, there is no substantial deterioration of signal-to-noise ratio due to internal noises in the receiver during normal reception, thus securing a preferable receiving condition.

There is provided an FM receiver in addition to an AM receiver in the above-referred embodiment. However, the present invention can naturally be employed in the case where only an AM receiver is employed, as shown in FIG. 2.