WO2009115996A1

WO2009115996A1 - Apparatus comprising a broadcast receiver circuit and an antenna and a tuning circuit

Info

Publication number: WO2009115996A1
Application number: PCT/IB2009/051144
Authority: WO
Inventors: Anthony Kerselaers
Original assignee: Nxp B.V.
Priority date: 2008-03-21
Filing date: 2009-03-18
Publication date: 2009-09-24

Abstract

The invention provides an apparatus comprising a broadcast receiver circuit based on CMOS technology, an embedded antenna for receiving broadcast signals and a tuning circuit coupled between the antenna and the receiver circuit, which tuning circuit comprises a signal line and a ground line coupled to ground, in between of which an inductor and a plurality of switchable capacitors are coupled, and which tuning circuit further comprises an amplifier with an output to the receiver circuit, wherein the antenna and the amplifier are coupled to the signal line.

Description

APPARATUS COMPRISING A BROADCAST RECEIVER CIRCUIT, AN ANTENNA AND A TUNING CIRCUIT

FIELD OF THE INVENTION

The invention relates to an apparatus comprising a broadcast receiver circuit, an antenna for receiving broadcast signals and a matching circuit coupled between the antenna and the receiver circuit.

BACKGROUND OF THE INVENTION

In audio devices, radio reception in the AM frequency range is normally achieved by providing an internal ferrite antenna. Such ferrite antennas are configured for a predetermined frequency band in the radio spectrum. The ferrite antenna provides the functions of both an antenna receiving the magnetic part of the electromagnetic wave, and a resonant circuit with an additional capacitance. The first stage of a subsequent antenna amplifier may then have a wide band configuration, while selectivity is obtained in the second amplifier stage.

Such ferrite antennas may be disadvantageous in that internally generated electromagnetic fields (e.g. from a central processing unit (CPU), a microcomputer (μC) or the like) are picked up as noise. Additionally, the physical dimensions of the internal ferrite antenna are a key factor for sensitivity of AM reception. Increased miniaturization of audio devices with radio receivers requires smaller ferrites, which leads to undesirable decreases in sensitivity of radio reception.

By putting the AM antenna outside the housing, sensitivity can be increased and additional area can be made available on a printed circuit board (PCB) by removing the ferrite. This freed area can be used for other circuit elements.

One placing option for external radio antennas may be a headset of the audio device, including a wire to a handset. This is suggested for example in the US 2005/0285799 Al which discloses a headset loop antenna implemented by loop sections which include inductors and wherein conductors to the earplugs contain ferrite beads. Each conductor in the loop section forms a matching element in that it matches to the desired reception frequency of the loop antenna. The loop antenna segments are coupled to one another and to a conductive antenna lead section at a Y- type coupler. Alternatively, the two antenna loop segments may be joined directly at a plug of the headset.

However, to emulate the presence of a ferrite antenna and/or to tune the loop antenna to the desired frequency range, proper selection of the inductors within the antenna loop configuration is crucial for matching the antenna to the receiver circuit and frequency range.

Another option for an external antenna placement resides in the use of the headset wire. For such headset wire, a reception curve is deducible. However, this curve has been measured as a wire attached to the roof in free space. In practice, the input impedance continuously changes, as the wire is in contact with a human body. This decreases the antenna performance. The decrease in antenna performance is due to reflection. Since both antenna and the radio are designed at 75 Ohm input impedance, coupling with the body generates reflection, leading to a radiating antenna and decreasing power transmitted to the radio. The decreased transmission of power is problematic, as many radio channels operate at lower field strength than is supposed on the basis of the standardisation for radio channels.

A further option is known from WO2005/006579. This document proposes the use of tunable loop (e.g. inductive) antenna. The loop antenna is tuned by means of a varicap diode, that is driven from the broadcast receiver circuit. A matching circuit is present between the antenna and the receiver, in order to convert the input impedance to 200 Ω. However, the shown solution is discrete and large and there is no viable manner of size reduction. Additionally, the driving of the tunable antenna is complex and it makes use of a control signal which is not intended for driving an antenna.

Thus, in short, it is a problem of the use of the headset wire as antenna that the antenna is sensitive to changes with the environment and thus prone to deviations/shifts in the frequency band.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide an apparatus with an antenna and a broadcast receiver circuit, wherein the reception of signals is less sensitive to changes with the environment and thus less prone to deviations/shifts in the frequency band. In a first aspect of the present invention, an apparatus comprising a broadcast receiver circuit, an antenna and tuning circuit is provided, wherein the receiver circuit is based on CMOS technology, the antenna is embedded, and the tuning circuit comprises a signal line and a ground line coupled to ground, in between of which an inductor and a plurality of switchable capacitors are coupled, and which tuning circuit further comprises an amplifier with an output to the receiver circuit, wherein the antenna and the amplifier are coupled to the signal line.

In the system of the invention, outcoupling of the amplifier and therewith the receiver circuit occurs at the signal line. This measure is based on the insight that an amplifier based on CMOS technology can be designed with a relatively high input impedance, which effectively reduces noise. Additionally, the use of switchable capacitors allows tuning to achieve appropriate resonance with the embedded antenna. Thus, in short, the tuning circuit has been improved such that use of an embedded antenna is enabled.

With the term 'embedded antenna' reference is made in the context of the present application to an antenna that is assembled within a housing of the apparatus. Specifically, the embedded antenna is small compared to wavelengths corresponding to the broadcast band. Though an embedded antenna is present at a short distance from other electronic components and thus sensitive to distortion, the use of an antenna with a small size compared to the wavelength turns out to be viable. Suitably, the ratio between wavelength and antenna size is more than 10 and preferably more than 50. As a consequence, the embedded antenna of the present invention can be considered, with respect to its electrical operation, as part of the antenna tuning circuit.

Preferably, the embedded antenna is a capacitive antenna. An inductive antenna is certainly not excluded, but a capacitive antenna has the further benefit of small size. In case of a capacitive antenna, it is advantageous that the antenna is coupled inductively to the tuning circuit.

In one embodiment, the each switchable capacitor comprises in series a capacitor and a switch. Particularly, the switch has an input impedance between 1 and 10 kΩ. This combination allows use within or in combination with a CMOS circuit with a supply voltage of less than 2.0V, particularly less than 1.5 V. The plurality of capacitors form a bank designed on the basis of binary logic principles (8-4-2), allowing a quick tuning. The smallest capacitor may be very small (1 pF) and the largest may be large, allowing a bigger range. A combination of a switch with high input impedance and a capacitor is needed, instead of a varicap, due to the low supply voltage of certain CMOS circuits, particularly CMOS circuits made in advanced process nodes comprising transistors with channel lengths of 90 nm, 65 nm or less. This low voltage, particularly 1.2 V or lower does not allow driving of the varicaps and hence, no variation of the capacitance. Though it is principally not exclude to convert the supply voltage to a higher level within the circuit, this is not beneficial for applications in which the surface area of the circuit is critical, such as in mobile phones and other portable equipment. In order to provide a switch with a sufficiently large input impedance, use can be made of a series connection of a first and a second transistor, to which if needed even a third transistor may be added.

Preferably, the capacitors are integrated into a single component. The integration of the capacitors within an IC (or a system- in-package) ensures that the mutual capacitance ratio of the capacitors is appropriate and is not affected by manufacturing tolerance and assembly spread. It has the additional advantage that the number of I/Os is kept limited.

Such single component is preferably the integrated circuit (IC) of the receiver. In that case, it is suitable to apply transistors as capacitors in known manner. However, it is not excluded that fringe capacitors are applied. These fringe capacitors also meet the requirement of minimal use of surface area. Several layouts for fringe capacitors have been proposed to maximize the capacitance density. In an alternative embodiment, use is made of a separate passive IC comprising several capacitors. This is particularly appropriate in the event that bigger capacitances are desired for the capacitor bank. The use of such passive IC simultaneously allows integration of capacitors and inductors needed for other portions of the receiver circuit, including decoupling capacitors and capacitors for the phase locked loop (PLL). Implementations of such capacitors suitable for passive ICs include trench capacitors and ferroelectric capacitors.

In a further embodiment, an ESD protection is provided for the tuning circuit. One suitable implementation is that the ESD protection forms the minimum capacitance of the switchable capacitor bank. In order to maintain sufficient tuning range, the capacitance of the ESD protection is therefore kept as small as possible. For instance, it is possible to apply a series connection of two ESD diodes with a capacitance of a couple of pF or less.

The amplifier of the present invention suitably provides an amplification of 10-30 dB. With such amplification, the input signal to the broadcast receiver circuit has a strength comparable to the strength of signals received through an external antenna. Suitably, the receiver circuit is provided with an output to an external antenna and comprises a switch for choosing between the external antenna and the embedded antenna. Such external antenna may be integrated in the wire to the headset. Though communication between a mobile phone and a headset may be established wirelessly, for instance by wireless communication with the Bluetooth protocol, it is desired to offer a user the choice to use a wired headset. An external antenna is suitably coupled to the receiver circuit over a balun.

Preferably, the amplifier has a high ohmic input. Amplifiers with such input effectively inhibit noise. Particularly, the input impedance is at least 1 kΩ and has a capacitance of less than 5 pF. A cascode topology is preferred for the amplifier.

In a further embodiment, the system comprises a transmit circuit coupled to the signal line for wireless transmission of audio signals through the antenna, wherein the tuning circuit is designed to operate as a filter for suppressing of harmonics in a transmitted signal. Though one conventionally merely receives broadcast signals, recent standardisation also allows transmission within the frequency bands used for broadcast. This particularly relates to audio to be transmitted at low power levels as frequency modulated (FM) signals in the FM frequency range. Object hereof is the transmission of music and other audio from a portable apparatus to an integrated audio system coupled to larger speakers, including car radio and home audio systems.

The integration of such transmit signal to the signal line of the tuning circuit turns out beneficial since the tuning circuit acts as a filter for suppressing of (higher) harmonics. As known to the skilled person in RF signal transmission, higher harmonics may lead to interference and loss of audio quality. Such filtering is enabled in that the tuning circuit need not to form a load to the transmit circuit. As a consequence thereof, the Q-factor of the tuning circuit is not or only to a limited extent affected by the presence of the transmit circuit. In order to achieve this use, it will be understood that a receive/transmit switch will prevent the flow of amplified transmit signals into the receiver circuit.

Most suitably, the transmit circuit is coupled to the signal line with a capacitive coupling. This capacitive coupling allows transmission while the circuit is tuned for reception of a specific channel (e.g. a specific setting of the capacitor bank). The capacitance of this capacitive coupling preferably has a value that is in the same range to the equivalent capacitance of the embedded antenna (for instance appr. 5 pF). Additionally, such capacitive coupling turns out most beneficial to keep the Q-factor of the tuning circuit as high as possible. Amplification of the transmit signal can be achieved appropriately with a cascode stage power amplifier circuits. This architecture is very suitable for integration into CMOS circuits, as is known per se from wo-a 2003/001661.

In a further embodiment, the tuning circuit is inductively coupled to the embedded antenna. Addition of an inductor reduces the magnitude of reactance of the antenna. This leads to a higher voltage level on the signal line, which enables an improvement in the carrier-to-noise ratio (CNR) of up to 6 dB. Such improvement can be achieved both for reception and transmission. It is to be observed for clarity that the term 'CNR' has to be distinguished from the term 'signal to noise ratio'. The CNR defines the ratio of the level of a signal in the broadcast band at a certain frequency and the noise at the same frequency, prior to demodulation. The signal to noise ratio relates to the demodulated signal, i.e. after demodulation in the receiver circuit. While the CNR relates to noise at the same frequency (for instance 100 MHz), the signal to noise ratio relates to a signal at a specified frequency (for instance 10 KHz) with the noise in a certain frequency domain (for instance 0 to 20 KHz).

Preferably the reactance of the combination of inductor and antenna are in the range of-jlOO to -jlOOO Ohm, more preferably in the range of-j300 to -j800 Ohm. The maximum of this range is set by the carrier-to-noise ratio, as explained above. The minimum is set by the needed tuning of the antenna at higher frequencies.

By switching over to mono instead of stereo, the input sensitivity of the amplification can be further improved. The peak in the reception curve may be shifted with a demodulator (operating principle is here an extended threshold). In an alternative application, the antenna is used for video. This requires extension of the bandwidth of the antenna and of the tuning circuit up to 12 MHz.

In order to combine audio and video, which is most beneficial, the inductor of the tuning circuit is to be made switchable between a first and second inductance. In order to achieve the higher bandwidth for video applications, the inductance need to be reduced. An additional measure for achieving higher bandwidth resides in the application of a bandwidth setting circuit as part of the receiver circuit. Such bandwidth setting circuit suitably operates on the basis of a switchable resistor, such as currently available in an automatic gain control (AGC) circuit.

In further aspects of the invention, methods for receiving signals and for transmitting signals are provided, using the system of the invention.

In another aspect of the invention, an integrated circuit is provided comprising both the receiver circuit and the switchable capacitors of the tuning circuit.

In still another aspect of the invention, a system is provided with an embedded antenna, a receiver circuit, a transmit circuit and a tuning circuit in between of both antenna and receiver circuit and antenna and transmit circuit. According to this aspect a measurement circuit is present to measure a voltage of a signal to be transmitted to the antenna. Such measurement circuit is preferably coupled back in a feedback loop to the transmit circuit, and/or any power management unit or processor coupled thereto. The benefit thereof is that the voltage of the signal can be kept within the limits, which include both limits sent by governmental agencies and standard bodies, as well as limits for appropriate operation. It turns out that such measurement circuit may be further used as for tuning the tuning circuit. Such tuning circuit preferably comprises switchable capacitors, although other embodiments are not excluded. This system is preferably implemented with a CMOS-based integrated circuit, but alternative implementations are not excluded. Further features hereof are mentioned in the figure description.

In again another aspect of the invention, a method of calibrating the tuning circuit in the system of the invention is provided. Such method comprises the steps of: providing a signal of a first frequency at a first intensity to be received by the receiver circuit, while the plurality of switchable capacitors is switched in a first state; measuring signal strength of the signal in the receiver circuit; resetting the plurality of switchable capacitors in a further state, and - measuring signal strength again, and optionally repeating this, and storing that state of the first and further states that is received with optimum signal strength into a memory together with the first frequency.

It will be clear that usually this method will be repeated for a plurality of frequencies within the frequency band. It is not excluded that this plurality does not include all frequencies, but just a subset. The term Optimum signal strength' is usually the highest signal strength as measured, but there may be further boundary conditions as a consequence of which another state is nevertheless preferable in a certain situation. It is further observed for clarity that the plurality of switchable capacitors usually has more than two states. In case of four switchable capacitors with each an on and an off state, that amounts to a maximum of 16 states. There is no need that signal strength of all states are measured.

Preferably, this method is applied with a system including a transmit circuit. Then the transmit circuit can provide the signal of the first frequency at a first intensity. Suitably, in such case, the system has a antenna/receive switches between both the transmit circuit and the tuning circuit, as well as the filter circuit and the amplifier. Then both switches will be closed (switched on) to enable this calibration mode.

As will be further explained in the figure description, the calibration method can be split up in a master calibration with a further autocalibration protocol. This is particularly possible with the combination of receiver and transmit circuits, although any other configuration for providing a low frequency test signal as mentioned below is not excluded.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. In the following drawings: Fig. 1 is a block schematic of the system;

Fig. 2 is a schematic of the system in a first embodiment; Fig. 3 is a schematic of the system in a second embodiment; Fig. 4 is a graph showing the audio signal to noise ratio as a function of the external field for the invention and for a reference, and

Fig. 5 shows a process flow for the calibration method.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will now be described. The system according to the present invention comprises an embedded antenna, an tuning circuit and a receiver circuit. The tuning circuit and the receiver circuit are preferably integrated into a single integrated circuit, usually excluding specific components such as inductors. The system may be used, for example, to receive broadcast stations like FM radio, DVB-H or others and to transmit an mp3 audio signal originating in the handheld product to an available broadcast FM radio, such as a car radio.

Fig. 1 shows an block schematic of the system of the invention in a first embodiment. The system of this embodiment includes receiver functionality but does not include transmitter functionality. It comprises an embedded antenna 30, a switched capacitors circuit 10, an amplifier 20 and a broadcast receiver circuit 50. The switched capacitors circuit 10 and the amplifier 20 together constitute the tuning circuit 100 for the embedded antenna 30. The combination of the embedded antenna 30 and the tuning circuit 100 demonstrates satisfactory reception performance for the broadcast band of interest. In the present example, this broadcast band of interest is an audio broadcast band, in particularly the FM-radio band in the range of 76 to 108 MHz. Shown further is a transmit circuit 40, which is optional.

The embedded antenna 30 and its tuning circuit 100 is aimed at replacement of an external antenna. A conventional antenna solution for the same broadcast band uses a wire connected to a user's headset as external antenna. Fig. 1 shows this external antenna 60 too, and a balun 61 for adequate signal transformation. This has several advantages; first a user of the apparatus may choose which antenna it prefers. Secondly, performance of the antenna is very sensitive to environmental conditions and distortion of nearby signals. It may turn out that the embedded antenna has under certain use conditions a distinctly better performance than the external antenna, or vice versa, and this could even be different for a certain subrange in the broadcast band, for a certain type of signals to be received, or for a specific broadcast band to be chosen, in case several broadcast bands are available (eg. DVB-H, etc in addition to FM). The availability of both an external and an embedded antenna allows that the receiver circuit 50 can optimize reception performance by either choosing one antenna or by enabling better error correction on the basis of comparison of signals received through the embedded antenna 30 and the external antenna 60.

As further indicated in Fig.l, it is most suitable that the tuning circuit 100, with the exception of any inductors 11, is integrated with the broadcast receiver circuit 50, and the optional transmit circuit 40 into a single component (i.e. integrated circuit). The other elements are then added as discrete components. For the inductor suitably a small size SMD is preferred, with an inductance of 150 to 220 nH and a Q- factor of 50 at 100 MHz. However, it is not excluded that passive components of the tuning circuit are left outside the integrated circuit, or are for instance integrated in the package of the integrated circuit. Such integrated circuit is then a CMOS integrated circuit, preferably of an advanced proces node, such as those based on transistors with minimum channel lengths of 90 nm, 65 nm, 45 nm or even less.

Fig. 2 is a schematic showing the first embodiment of the system, and particularly the tuning circuit 100 of the invention in more detail. The portion at the right hand side of the line A-A designates the portion of the system that is suitably integrated into a single integrated circuit. The tuning circuit 100 comprises a signal line 19 and a ground line 18, between which an inductor 11 and a plurality of switched capacitors 13-16 are present. A bank of four switched capacitors, with a overall tuning range of 1-20 pF appears suitable for tuning the antenna appropriately within the broadcast band of interest. Nevertheless, it is not excluded that the bank comprises more or less than four switched capacitors. Each switched capacitor 13-16 of this example comprises a capacitor and a switch 13S-16S. The switch 13S-16S suitably comprises more than one transistor in series, so as to withstand the voltage difference between signal line 19 and ground line 18. Requirements for the transmit mode specify that at least 1 V RMS ought to be present on the embedded antenna 30. Such 1 V RMS corresponds to 2.8 V peak-to-peak. Standard transistors with a supply voltage of less than 2 V will not withstand such voltage, e.g. leakage will occur. A series connection of more than one transistor may indeed withstand such peak voltage. Additionally, the transistors may be embodied with thicker gate oxide so as to have per se a higher voltage stability. The capacitor bank may further include capacitors or diodes that are not switched, for various reasons. These have not been indicated in the Figure.

The amplifier 20 is coupled to the signal line 19, instead of to the ground line 18. This is enabled in that the amplifier 20 has an high ohmic input. Suitably, it has an input impedance higher than 1 kΩ, and more preferably higher than 4 kΩ. Preferably, the amplifier has an optimal noise performance for a source resistance of more than 1 kΩ, preferably in the order of 2 kΩ. The amplifier has a low noise figure of preferably less than 5 dB and more preferably less than 2 dB. A switch 42 is present between the signal line 19 and the amplifier 20. This switch 42 acts as a receive/transmit switch and ensures that signals amplified in the transmit circuit 40 will not enter the amplifier 20. In order to prevent leakage of amplified signals in case the switch is open (no transmission), it is suitable that this switch 42 is embodied as a series connection of a plurality of transistors.

The transmit circuit 40 is also coupled to the signal line 19. Preferably, the coupling is a capacitive coupling by means of a capacitor 41. A further switch is provided between the capacitor 41 and the transmit circuit 40, in order to prevent that the transmit circuit would deteriorate the tuning in receive mode. Suitably, the capacitor 41 has a value corresponding to the capacitance of the antenna, for instance in the range of 1-5 pF, and preferably around 2-3 pF. That ensures that the current delivered by the transmit circuit 40 equals or at least largely equals the current through the antenna 30.

The transmit circuit can make use of the same tuning circuit as the receive circuit, as the current through the antenna is limited to comply with the maximum value of 50 nW ERP as imposed by law. The resulting current is then suitably in the range of 0.5 to 4.0 mA. In order to ensure that the actual transmitted power does not go beyond that maximum value, it is proposed to add a measurement circuit - this is not indicated in the Figure. Such measurement circuit is not merely relevant so as to comply with said restrictions, but a high current will have a negative impact on the battery life and may indeed be not beneficial to human health. While cellular transmission uses higher output power, there is an important difference: the cellular transmission is limited to short periods basically corresponding to telephone calls; the transmission in the FM radio band may however continue over a longer period of time. An unforeseen increase of the current of the output power easily leads thereto that the system battery is empty earlier than expected or desired. A measurement circuit will have a feedback to the transmit circuit and/or a power management unit coupled thereto, so as to reduce the current.

In a further embodiment, a calibration signal for frequency calibration is included in the transmitter. Such calibration signal is subsequently received again by the the embedded antenna. On the basis thereof, the needed capacitor setting can be determined.

The antenna is preferably a capacitive antenna. In this example, the antenna is a conducting strip of any shape and can be made of for example of adhesive conductive tape. The antenna 30 comprises a feeding point can be on any place on the conducting strip. The feeding point position gives only a slight variation in performance. Each of the dimensions - length, height and thickness - is below 5% of the wavelength. The thickness (T) can be below 0.1 % of the wavelength. Such a construction can be attached on the plastic housing of a final product almost without consuming any volume. Openings in the conductive strip can be used to pass temporary connections to further parts inside the final product. The antenna is embedded, e.g. it is or may be embedded in the system usually formed by a portable apparatus.

The embedded antenna 30 is suitably assembled at a bottom side of a portable apparatus such as a mobile phone. The bottom side herein referred to as the side which is usually at the bottom when holding the mobile phone near one's ear so as to pursue a telephone conversation. Alternatively, it could be any other side that is available and wherein interference with other antennas is relatively small. One alternative appropriate side appears the top side opposite to the bottom side, in case of a substantially block shaped equipment. This turns out to be an advantageous position to minimize the handeffect of the user.

Preferably, the antenna is capacitive in the FM frequency band. The reactance is heavily dependent on the structure and surroundings. For such small antenna, the radiation resistance and induced voltage are also relative constant over the FM band. The antenna feed can be at the centre of the foil or in the corner. A corner feed increases slightly the induced voltage. The embedding of the antenna within a portable apparatus turns out suitable for reception. First, the sensitivity to environmental changes limited in compared to an external antenna. Moreover, the portable apparatus and any body in contact therewith turn out to have a positive effect on the antenna operation: the antenna radiation resistance is increased by those bodies of larger size as the cellular phone itself. While such bodies (including the apparatus) give only a slight increase in the capacitance value of the antenna, the induced voltage of the capacitive, embedded antenna is increased in this way.

Fig. 3 shows the system in a second embodiment. In this embodiment, an additional inductor 12 is added in the signal line 19 between the embedded antenna 30 and the bank of switched capacitors 13-16. Therewith, the tuning circuit 100 is inductively coupled to the embedded antenna 30. Addition of the additional inductor 12 reduces the magnitude of reactance of the antenna 30. This leads to a higher voltage level on the signal line, which enables an improvement in the carrier-to-noise ratio of up to 6 dB. Such improvement can be achieved both for reception and transmission.

Preferably the reactance of the combination of additional inductor 12 and the antenna 30 are in the range of -j 100 to — jlOOO Ohm, more preferably in the range of -j300 to -j800 Ohm. The maximum of this range is set by the signal-to-noise ratio, as explained above. The minimum is set by the needed tuning of the antenna 30 at the higher end of the broadcast frequency band.

Fig. 4 is a graph of the receiving performance of the system of the invention, in comparison with a prior art system. The same receiver circuit, a NXP FM radio circuit with product name TEA5760, was used for the comparison. The performance is expressed as the signal to noise ratio (in dB) as a function of the externally applied field strength E. The left hand curve in red indicates the performance when the receiver circuit is used in combination with an external antenna, in particular a headphone wire of a mobile phone. This curve can be considered as a reference performance as it is a commercially available solution. The dot on the right end of the figure shows the minimum level of field strength for good reception as defined by the ITU. The right hand curve in blue indicates the performance with a prototype of the embedded antenna 30 and the tunable circuit 100 of this invention. The audio signal to noise ratio is better then 35 dB for a received electrical field strength E of more than 200 μV/m. The performance is clearly better than required according to the ITU minimum levels. This demonstrates that use of an embedded antenna is indeed realistic. The performance for the embedded antenna 30 and the tuning circuit 100 is likely to be further improved.

Fig. 5 shows a flow diagram of a frequency calibration method of the tuning circuit 100 in the apparatus of the invention. The frequency calibration method enables rapid frequency tuning. According thereto, the states of the bank of switchable capacitors 13-16 are measured to provide a link between state and frequency. This calibration method can be applied in advance of operation of the apparatus, but also during operation in order to achieve recalibration. The result of the frequency calibration is programmed into a memory that will be available as part of the receiver circuit 50 or as a separate chip or as part of another chip, such as known to a skilled person. The frequency calibration method can be advantageously applied in a system provided with both a broadcast receiver circuit 50 and a transmit circuit 40. Most advantageously is its application in case the receiver circuit 50, the transmit circuit 40 and the tuning circuit 100 with exception of the inductor(s) 11,12 are integrated into a single integrated circuit. In that case, all switched capacitors 13-16 are made in the same technology such that they have the same spread and tolerance and that there is a known ratio of between the individual switchable capacitors. In the following, it is assumed that use is made of the embodiment in which the switchable capacitors 13-16 are implemented with capacitors and switches 13S-16S. Evidently, this is merely a preferred embodiment and this is not essential for the calibration method, which can thus also be applied with other implementations of switchable capacitors 13-16.

In a first step of the method, indicated with 201, all transmit/receive switches 42, 43 are closed such that both transmission and reception are enabled.

In a second step of the method, indicated with 202, the transmit circuit 40 will provide a signal of a first intensity (voltage) at a first frequency fl. During this operation, the transmit circuit 40, and particularly any power amplifier therein, will be operated at such a low voltage that the amplifier 20 and the receiver circuit 50 will not receive any signal in a strength that will or might bring damage to either of them. Alternative implementations for ensuring that the transmit circuit 40 only has a low voltage output can be applied as well. In the further embodiment discussed above that a measurement circuit for the output power of the transmit circuit 40 is available, this measurement circuit is advantageously applied to ensure limitation of the output power to the maximum for the amplifier 20 and/or receiver circuit 50.

In a third step of the method, indicated with 203, the receiver circuit 50 will measure the received signal strength. This signal strength is suitably measured on the basis of the field strength.

In a fourth step of the method, indicated with 204, the switches 13S- 16S of the bank of switchable capacitors 13-16 will be switched off and on consecutively. During the switching, the signal from the transmit circuit 40 may continue or may be switched off. The receiver circuit 50 will measure the signal strength at each state and in this manner find a maximum signal strength.

In a fifth step of the method, indicated with 205, the settings of the switches 13S-16S of the bank of switchable capacitors 13-16 with maximum signal strength will be stored into the memory in combination with the frequency and optionally any further parameters of the measurement. It appears appropriate that the stored data will be in the form of a look-up table.

In a sixth step of the method, indicated with 206, the calibration will be restarted for a different frequency, or if the bank has been calibrated for all relevant frequencies, the method will be finalized.

Suitably, the first step 201 also includes the closing of all the switches 13S-16S of the switchable capacitor bank. This is a most efficient version of the method, as the closing (switching on) of all switches results in the lowest frequency. Thereafter, one may search a higher frequency by use of a so-called binary sweep to find an optimum value.

In order to facilitate the calibration step, a master calibration may be carried out for a certain class or group of tuning circuits i.e. integrated circuits. This master calibration can then be programmed into the memory prior to the calibration, such that the individual tuning circuits merely need fine-tuning. A class of products could even be a product type defined for a specific application design. Alternatively, it could comprise all products of a single semiconductor wafer or of the same manufacturing batch.

The fine-tuning is preferably carried out with an autocalibration protocol. Such autocalibration suitably comprises following steps: transmission of a lowfrequent signal by the transmit circuit determination of the capacitance (absolute value) of each setting of the bank of switchable capacitors adaptation of the table in the memory, if needed on the basis of the determined capacitances.

If no transmit circuit is available, the method can nevertheless be applied, but then a calibration signal of known frequency and strength is to be applied externally. Evidently, it is then even more favoured to use a master calibration, such that merely fine-tuning is needed. It is not excluded that the system will contain means for the determination of the capacitance of each setting of the bank of capacitors (including for instance means for transmission of a low- frequent signal). This alternative implementation could be applied as well, in case that the transmit circuit turns out not-suitable for the provision of signals of sufficiently low strengths, or in case that there is merely a single receive/transmit switch 42 such that the transmit circuit 40 and the amplifier 20 and receive circuit 50 cannot be used simultaneously.

Claims

CLAIMS:

1. Apparatus, comprising a broadcast receiver circuit based on CMOS technology, an embedded antenna for receiving broadcast signals and a tuning circuit coupled between the antenna and the receiver circuit, which tuning circuit comprises a signal line and a ground line coupled to ground, in between of which an inductor and a plurality of switchable capacitors are coupled, and which tuning circuit further comprises an amplifier with an output to the receiver circuit, wherein the antenna and the amplifier are coupled to the signal line.

2. The apparatus as claimed in claim 1, wherein each switchable capacitor comprises in series a capacitor and a switch having an input impedance between 1 and 10 kΩ.

3. The apparatus as claimed in claim 2, wherein the switch comprises a series connection of a first and a second field effect transistor.

4. The apparatus as claimed in claim 1, wherein the tuning circuit comprises an ESD protection, which is coupled between the signal line and the ground line and comprises a series connection of two ESD diodes acting as a minimum capacitance of the tuning circuit.

5. The apparatus as claimed in claim 1, further comprising a transmit circuit coupled to the signal line for wireless transmission of audio signals through the antenna, wherein the tuning circuit is designed to operate as a filter for suppressing of harmonics in a transmitted signal.

6. The apparatus as claimed in claim 5, wherein the transmit circuit is capacitively coupled to the signal line.

7. The apparatus as claimed in claim 6, wherein the capacitive coupling has a capacitance which substantially corresponds to a capacitance of the antenna.

8. The apparatus as claimed in claim 5, wherein a measurement circuit is coupled to the signal line for measuring the voltage in transmit mode.

9. The apparatus as claimed in any of the previous claims, wherein the embedded antenna is inductively coupled to the tuning circuit.

10. The apparatus as claimed in claim 1, wherein the inductor in the tuning circuit is switchable for switching between an audio mode and a video mode.

11. The apparatus as claimed in claim 1 or 10, wherein the receiver circuit comprises a bandwidth setting circuit comprising a switchable resistor.

12. A method of receiving a signal in a system as claimed in any of the previous claims, comprising the steps of: setting the switchable capacitors in accordance with a frequency to be received, so as that the tuning circuit resonates at the chosen frequency, and receiving signals in the chosen frequency band, wherein the resonating tuning circuit amplies the signal received by the antenna.

13. A method of transmitting signals in a system as claimed in any of the claims 5-7, comprising the steps of: switching the system to a transmit mode such that transmitted signals cannot enter the receiver circuit transmitting the signals through the tuning circuit and the antenna to generate an effective radiated power of 1 uW or less.

14. An integrated circuit package for use in the apparatus as claimed in any of the claims 1-11 comprising the broadcast receiver circuit and the switchable capacitors of the tuning circuit.

15. An integrated circuit as claimed in claim 14, wherein the supply voltage of the integrated circuit is less than 1.5 V.

16. An apparatus comprising a broadcast receiver circuit, an embedded antenna for receiving broadcast signals and a tuning circuit coupled between the antenna and the receiver circuit, further comprising a transmit circuit coupled to the tuning circuit for wireless transmission of audio signals through the antenna, wherein the tuning circuit is designed to operate as a filter for suppressing of harmonics in a transmitted signal.

17. An apparatus as claimed in claim 16, further comprising a measurement circuit coupled to the tuning circuit for measuring a voltage of the transmitted signal in transmit mode.

18. A method of calibrating the tuning circuit in the apparatus as claimed in any of the claims 1 to 11, 16 and 17, comprising the steps of: providing a signal of a first frequency at a first intensity to be received by the receiver circuit, while the plurality of switchable capacitors is switched in a first state; measuring signal strength of the signal in the receiver circuit; resetting the plurality of switchable capacitors in a further state, and -measuring signal strength again, and optionally repeating this, and storing that state of the first and further states that is received with optimum signal strength into a memory together with the first frequency.

19. A method as claimed in claim 18, wherein the apparatus as claimed in claim as claimed in claim 5 or 16 is present and the signal of the first frequency is provided by the transmit circuit.