RECEIVER INPUT CIRCUIT

Abstract: An input circuit of for example (a tuner of) a radio receiver has to receive an input signal, for example, from an antenna and having filter means coupled to the input. Such an input circuit further comprises an amplifier and an output stage for supplying an output signal to a mixer circuit. To improve the operation of such an input circuit an inductive feedback is proposed, resulting in a flat frequency response.

Description: RECEIVER INPUT CIRCUIT The invention relates to an input circuit as described in the preamble of claim 1. The invention further relates to a tuner and to a radio receiver comprising such an input circuit. Input circuits are known and are for example used in (mobile) radio receivers, television receivers etc. Input circuits, for example AM input circuits, must have a great sensibility to radio frequencies of 150 kHz and higher, the long wave signals. Further such an input circuit has to block frequencies below 150 kHz. To achieve this, high pass filter means are necessary. One of the known solutions is to realise a filter consisting of capacitors and resistors coupled to the input of the input circuit. A disadvantage of this solution is that the resistors supply thermal noise to the received signal. A second known solution is the use of a filter comprising an inductance. This solution is preferred because of less supplied noise. The value of the inductance has to be such that the cut off frequency is below the lowest signal frequency that has to be received, for example 150 kHz for an AM input circuit. As a consequence of the capacitive character of the source (the antenna) and the high input impedance of the input an unwanted resonance from the inductance results with a resonance frequency just below 150 kHz. This resonance can result in an instability and further leads to an unpredictable great sensitivity around this resonance frequency. Another reason to use an inductance is that when an active antenna is used requiring power supply via the signal cable a high impedance is necessary for the circuit element that forms the connection between the power supply and the signal cable. For this circuit element also an inductance is used. This inductance has further, when used with a passive antenna, a filtering function with the problems as mentioned above. Further disadvantages of the known input circuits are poor signal handling for 50 Hz or 60 Hz signals from power lines, poor signal handling for tune transmitting signals (such as DCF77 at 77 5 kHz and RUGBY at 60 kHz), navigation systems (having frequencies in the region of 100 kHz), in the region of 20 kHz from submarine communication base stations, and instability when components are added for supply of an active antenna An object of the invention is to provide an input circuit that has not the drawbacks of the known circuits and further having an excellent noise-limited sensitivity To this end a first aspect of the invention provides an input circuit as claimed in claim 1 A second aspect of the invention provides a tuner as clauned in claim 5 A third aspect of the mvention provides a radio receiver as claimed in claim 6 By optimizing the noise matchmg and the power matching of the (AM) receiver mdependently of each other, it is possible to cancel the above mentioned resonance and to mamtain the great sensitivity, meanwhile having a large attenuation for the disturbing low frequencies by choosmg the cut off frequency optimally This uncouplmg is achieved by lowering the input impedance by the inductive feedback loop and at the same time not changing the noise matching, resulting m an attenuation of the unwanted resonance without a loss of sensitivity of the received signals An input circuit according to the invention can be used as an AM input circuit but also as an FM input circuit Especially by an AM input circuit the linearity is improved and a very good attenuation of low frequency signals is obtained As a consequence of the inductive character of the feedback loop the feedback signal decreases with higher frequencies An embodiment according to the invention comprises the features of claim 2 By using a transformer as inductive feedback means coupled w ith a pπman w inding between the input of the amplifier and the RF ground terminal and with a secondan winding coupled with an output of the amplifier, the frequencx response can be optimized without affecting the noise-limited sensitivity The so called RF ground terminal is a decoupled point which is free of RF signals but not necessarily ' real ' ground (zero) An embodiment accordmg to the mvention comprises the features of claim 3 By dividing the primary winding mto a first and a second pan and couplmg the filter means to the connection point the operation of the input circuit is further improved The mvention and additional features which may optionally be used to implement the mvention to advantage will be apparent from and elucidated with reference to the examples described hereinafter and shown in the figures Herein shows Figure 1 a block schematic example of a radio receiver, Figure 2 block schematic an example of an input circuit according to the invention, and Figure 3 an example of an input circuit accordmg to the invention in more detail Same elements are indicated with corresponding reference signs throughout the different figures Figure 1 shows a block schematic example of a radio receiver RR compnsmg an input circuit INPC having an input ll for receiving an input signal from, for example, an antenna ANT, and an output for supplying an output signal to a mixer MIX The operation of the mixer is well known in the art, which mixer is coupled to a processing unit PROC The processing unit normally has the following functions channel filtering, demodulation and audio processing, as is common practice to the man in the art The processing unit PROC is finally coupled to a loudspeaker LS (or more loudspeakers) for reproducing the received signal Figure 2 shows a block schematic example of an input circuit INPC2 naving an input ι2 for receiving an input signal from an antenna (see figure 1) The input ι2 is coupled to filter means FLM2 for partial filtering the input signal The filter means are coupled to an amplifier AMP2 for amplifying the partially filtered signal An output of the amplifier is coupled to an output stage OS2 for supplying the filtered and amplified signal at an output o2 to the mixer (see figure 1). The output stage OS2 also performs a partial filtering operation (see figure 3). The filter means FLM2 take care of a roughly filtering to attenuate the signals outside the wanted signal range. The output stage OS2 takes care of a further partial filtering of the signal. An (other) output of the amplifier AMP2 is further coupled to feedback means FBM2 for feeding back a signal to the mput of the amplifier. Figure 3 shows m more detail an example of an mput circuit INPC3 accordmg to the invention comprising an input i3, filter means FLM3, feed back means FBM3, an amplifier AMP3 and an output stage OS3 with an output o3 The operation of the input circuit is as follows. At the input i3 the input circuit receives an input signal from an antenna (see figure 1). This input signal is first partially filtered in filter means FLM3, which filter means comprise a series arrangement of a capacitor Cf and a parallel arrangement of an inductance Lf and a resistor Rf . The elements of the filter means are chosen such that the cut off frequency is well below the lowest frequency to be received. The capacitor Cf attenuates the lower frequencies such as the 50 or 60 Hz signals, the 16 kHz signals etc. The filter means comprise an inductance Lf to attenuate the so called VHF signals such as television and FM signals etc The filter means FLM3 are coupled to feed back means FBM3 formed in this example by a transformer T, having a first and a second primary winding prl and pr2, and a secondary winding sec. The filter means FLM3 are coupled to the connection point of the first and second primary winding. The other end of the second primary winding pr2 is coupled to a so called RF ground terminal R G, whereas the other end of the first primary winding prl is coupled to the amplifier AMP3 The amplifier AMP3 is in this example implemented as a Field Effect Transistor FET The gate g is coupled to the first primary winding prl , whereas the drain d is coupled to the output stage OS3 The source s of the FET is coupled to the secondary w inding sec of the transformer T, for feeding back part of the amplified signal. The winding directions are mdicated in the figure. The first and second primary windings prl and pr2 operate as a so called auto-transformer. The second primary winding pr2 is coupled to the RF ground terminal RFG. This RF ground terminal is a decoupled point which is free of RF signals but not necessary ' real" ground (zero). From experiments it appeared that a winding ratio of 1 1.33 for the transformer part formed by the first and second primary windings prl and pr2 respectively performed well in combination with the FET. In that case the secondary winding sec can be chosen to a ratio of 60 : 1 to the primary (so prl : pr2 : sec = 1 : 1.33 : 0.015.). However it will be clear to the man in the art that the winding ratios are not essential to the invention concerned. The output stage OS3 comprises an parallel arrangement of a resistor Ro, an inductance Lol and a capacitor Col between the drain of the FET and a supply voltage Vos. The connection point between the parallel arrangement and the drain d is coupled via a series arrangement of a second capacitor Co2 and a second inductance Lo2 to the output o3. Herewith also a partially filtering is achieved m the output stage. As described above (see description of figure 2) the partial filtering in the output stage concerns the further partial filtering of the signal(s). As mentioned above the input circuit can be used as an AM input circuit but also as a FM input circuit. When used as an AM input circuit the mput circuit must have a great sensibility for signals above 150 kHz. In the above description the idea of the invention has been described on the basis of an detail example. The man skilled in the an will be well aware of a lot of different solutions that fall within the scope of the mvention concerned For example is it possible to use one primary w inding, so not a divided winding with a tap (prl and pr2), whereby the filter means FLM are coupled (directly) to the input of the amplifier. Also the amplifier is not necessaπh limited to one single active element The invention provides an input circuit and a ner and a receiver having such an input circuit which is able to attenuate the unwanted signals received at the input of the input circuit, and which further provides a flat frequency response. This is achieved by using inductive feedback means.

Claims: Claims 1. Input circuit for a radio receiver comprising an input for receiving an input signal from an antenna, filter means coupled between the mput and an amplifier which amplifier is coupled to an output stage, which ouφut stage is arranged for supplying an output signal, via an output, to a mixer circuit characterized in that the input circuit comprises inductive feedback means coupled between the amplifier and the filter means. 2. Input circuit as claimed in claim 1, characterized in that the inductive feedback means comprise a transformer having a secondary winding coupled to an ouφut of the amplifier and a primary winding coupled between an input of the amplifier and an RF ground terminal. 3. Input circuit as claimed m claim 2, characterized in that the primary winding of the transformer having a first and a second pan, whereby the connection point of the first and the second pan is coupled to the filter mean**, and the other end of the first pan is coupled to the amplifier. 4 Input circuit as claimed in claim 1 , characterized in that the filter means comprise a capacitor in series with a parallel arrangement of a resistor and an inductor. 5. Tuner comprising an input circuit as claimed in claim 1. 6. Radio receiver comprising an input circuit as claimed in claim 1.

Inventors: SESSINK FRANCISCUS JOSEPHUS AL, SESSINK, FRANCISCUS, JOSEPHUS, ALFONSUS, MARIA
Applicants: MANNESMANN VDO AG, SESSINK FRANCISCUS JOSEPHUS AL, SESSINK, FRANCISCUS, JOSEPHUS, ALFONSUS, MARIA
Application year: 1999
Main classification: H04B1
IPC codes: H03F1, H04B1