GB2140235A

GB2140235A - Circuits for reducing noise in electromagnetic wave receivers

Info

Publication number: GB2140235A
Application number: GB08411875A
Authority: GB
Inventors: Ettore Mastrojeni
Original assignee: ATES Componenti Elettronici SpA; SGS ATES Componenti Elettronici SpA
Current assignee: STMicroelectronics SRL
Priority date: 1983-05-19
Filing date: 1984-05-10
Publication date: 1984-11-21
Also published as: DE3418037A1; NL8401564A; FR2546350A1; IT1212744B; IT8321173A0; GB8411875D0

Abstract

The circuit comprises a low-pass filter 10 having an adjustable pass band located between the demodulator 3 and the audio-frequency amplifier 4 of the receiver. A voltage which is dependent on the level of the high frequency signal tuned by the receiver adjusts the pass band of the filter such that the upper cut-off frequency is maximum when the tuned signal is of a sufficiently high level and decreases gradually to a minimum value when the level of the signal is lower than a predetermined value. <IMAGE>

Description

SPECIFICATION Improvements in or relating to circuits for reducing noise in electromagnetic wave receivers The present invention relates to electromagnetic wave receivers and, more particularly to a circuit for attenuating noise in a receiver of this type.

In electromagnetic wave receivers, such as radio and television sets, an automatic gain control circuit is in practice disconnected if there is no signal or if there is a low signal level, as a result of which the noise picked up by the aerial and the noise of the receiver itself are amplified to the maximum extend when they reach the demodulator and lead to the emission of disturbing background noise from the loudspeaker once they have been demodulated. In order to remedy this drawback, which is particularly pronounced whilst searching for stations or in the case of weak stations, many noise suppressor or attenuator circuits have been proposed either for insertion in the high frequency stage of the receiver, or in the low frequency stage or, in the case of stereo receivers, in the decoder.

Some of these circuits act, if there is no signal, by blocking the output of an amplifier stage, for example by creating a low impedance towards earth. However this solution is not acceptable in many cases, for example when the mean intensity of the electrical field associated with the signal is not constant, as in the case of car radios, since the repeated variations of the electrical field around the threshold level of the circuit itself, which variations are typical of this application, lead to a series of connections and disconnections which are very disturbing for the listener.

Other known circuits, although using a gradual action criterion for the attenuation of the noise in the absence of a signal, are very complex.

According to the invention, there is provided a circuit for reducing noise in an electromagnetic wave receiver, comprising a high frequency signal tuning stage, a field detector stage arranged to supply at its output terminal, a variable voltage as a continuing function of the level of the high frequency signal tuned by the tuning stage, a demodulator stage arranged to obtain audio frequency signals from the high frequency signals supplied as output from the tuning stage, an amplifier stage for the audio frequency signals, and a low-pass filter having an adjustable upper cut-off frequency located between the demodulator stage and the audio frequency amplifier stage and having a control terminal connected to the output terminal of the field detector stage, the filter being dimensioned such that its upper cut-off frequency is continuously variable between a maximum value and a minimum value as a function of the voltage at its control terminal and passage from the maximum value towards the minimum value begins when the high frequency signal tuned by the tuner has a level which is lower than a predetermined level.

It is thus possible to provide a circuit for the attenuation of noise in an electromagnetic wave receiver which is structurally simple and acts in a gradual manner.

The invention will be further described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 shows a diagram, partly in block form and partly in circuit form, of a radio receiver including an embodiment of the invention; and Fig. 2 is a graph illustrating the operation of the embodiment of Fig. 1.

The circuit of Fig. 1 includes a high frequency amplifier and converter stage 1 an input terminal of which receives the signals picked up by an aerial 2, a demodulator stage 3 connected to the high frequency stage in order to obtain an audio frequency signal from a modulated high frequency signal coming from the stage 1, and an audio frequency amplifier stage 4 whose output is connected to a loudspeaker 5. The high frequency stage 1 is also connected to a field detector stage 6 whose output is connected, by a resistor 7, to a voltmeter 8 and to a capacitor 9 which have a further terminal connected to earth. The field detector stage 6 provides at its output a signal whose level is dependent on the field strength of the received signal and is generally provided in the form of a field strength meter driver or a tuning meter driver as shown in the drawing.A low-pass filter 10 having a variable pass band which has a control terminal 11 connected to the output of the field detector stage 6 by a resistor 1 2 is located between the output of the demodulator stage 3 and the input of the amplifier stage 4. The filter 10 comprises a resistor 1 3 whose terminals are connected to the input and output terminals of the filter itself, a depletion type field effect transistor 14 having its gate electrode connected directly to earth, a capacitor 1 5 which connects the drain electrode of the transistor 14 to the output terminal of the filter and a capacitor 1 6 which connects the source electrode of the transistor 14 to earth.The source electrode which forms the control terminal 11 of the filter 10 is also connected to earth via a resistor 1 7.

During operation, when the high frequency stage 1 is tuned to a frequency corresponding to the frequency of a signal picked up by the aerial 2 having a level which is higher than a predetermined level, the output of the field detector stage 6 is at a maximum voltage, for example +2 volts. The voltmeter 8 indicates this maximum and therefore that tuning to a station reaching the aerial has taken place with a sufficient intensity. The signal is amplified and converted in frequency in the stage 1, has the high frequency carrier removed in the demodulator 3, and is supplied as an audio frequency signal to the amplifier stage 4 in order to be converted into an acoustic signal by the loudspeaker 5.The audio frequency signal, before being amplified in the stage 4, passes through the filter 1 0. The source electrode of the field effect transistor 1 4 has a positive voltage determined by the divider formed by the resistors 12 and 1 7.

The resistances of these resistors are selected such that the gate-source voltage of the transistor 14 is sufficiently negative to keep the transistor cut off, as a result of which there is a maximum resistance between the source and drain. In these conditions, the filter 10 has a cut-off frequency fT1, for example 20,000 Hz, as shown in Fig. 2, in which the curve plotted in bold-face shows the pass band of the filter 10 in adequate signal conditions.

When the stage 1 is tuned to a signal having a level which is just below the predetermined level, the output of the field detector stage 6 is at a voltage which is no longer sufficient to establish a voltage which is more negative than the threshold voltage of the transistor itself between the gate and source electrodes of the transistor 14.

Consequently, the transistor 1 4 begins to conduct and its equivalent resistance is lower than the resistance in the case described above. The pass band of the filter 10 is therefore brought down from the high frequency area to lower frequencies, as a result of which the disturbing effect of noise and interference, which is essentially due, as is known, to the higher frequency components, is reduced.

When the stage 1 is tuned to a signal having a level which is even lower or to a frequency which does not correspond to any signal, as is the case during a search for a station, the output of the field detector stage 6 is at its lowest voltage, and in practice at earth potential, as a result of which the voltage drop between the gate and source electrodes of the transistor 1 4 is in practice zero and the transistor is conductive, i.e. it has a very low resistance between the source and drain.

Consequently, the pass band of the filter is brought further down from the high frequency area to lower frequencies. In a practical application, as shown by the curve in broken lines in Fig. 2, the pass band is limited to a cut-off frequency FT2 of approximately 2,000 Hz. In this case there is a maximum reduction of the noise and interference of the receiver itself.

It can be seen that the signal level at which the pass band of the filter 10 begins to be modified is determined by the ratio between the values of the resistors 1 2 and 17, as well as by the threshold voltage of the transistor 14, as a result of which it may be advisable for one of the resistors to be of the adjustable type.

It can readily be seen that the attenuator acts in a very gradual manner, does not cancel out the output signal, and does not, therefore, cause disturbing effects when weak signals are being received or when searching for a station. In addition, it oniy requires a few components and is of particular advantage when used with a receiver with a field indicator as, in this case, the low-pass filter control voltage is already available.

Claims

1. A circuit for reducing noise in an electromagnetic wave receiver, comprising a high frequency signal tuning stage, a field detector stage arranged to supply at its output terminal, a variable voltage as a continuing function of the level of the high frequency signal tuned by the tuning stage, a demodulator stage arranged to obtain audio frequency signals from the high frequency signals supplied as output from the tuning stage, an amplifier stage for the audio frequency signals, and a low-pass filter having an adjustable upper cut-off frequency located between the demodulator stage and the audio frequency amplifier stage and having a control terminal connected to the output terminal of the field detector stage, the filter being dimensioned such that its upper cut-off frequency is continuously variable between a maximum value and a minimum value as a function of the voltage at its control terminal and passage from the maximum value towards the minimum value begins when the high frequency signal tuned by the tuner has a level which is lower than a predetermined level.

2. A circuit as claimed in claim 1, in which the low-pass filter includes a field effect transistor and the control terminal is a terminal of the field effect transistor.

3. A circuit substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.