WO2006134533A1

WO2006134533A1 - A device for receiving a radio frequency signal in a frequency band

Info

Publication number: WO2006134533A1
Application number: PCT/IB2006/051847
Authority: WO
Inventors: Anthony Kerselaers; Felix Elsen; Samuel Becque
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2005-06-14
Filing date: 2006-06-09
Publication date: 2006-12-21

Abstract

Devices (10) comprising antennas (1) for receiving radio frequency signals in frequency bands are provided with circuits (2) for tuning the antennas (1) to the frequency band as well as for, in response to feedback signals, increasing efficiencies of combinations of the tuned antennas (1) and the circuits (2). Such devices (10) are simple and low cost. The antennas (1) are electrical antennas having an electrical length of at most 20% of a wavelength of the frequency band and having an omni directional radiation pattern. Amplifiers (3) amplify antenna signals originating from the antennas (1) via the circuits (2) and supply the feedback signals to the circuits (2). A feedback between the amplifiers (3) and the circuits (2) is a positive feedback and is such that the amplifiers (3) are stable. The circuits (2) comprise coils (21) for tuning the antennas (1) and further coils (22) or capacitors (23,24) for receiving the feedback signals from the amplifiers (3).

Description

A device for receiving a radio frequency signal in a frequency band

The invention relates to a device comprising an antenna for receiving a radio frequency signal in a frequency band, and also relates to a circuit for use in a device, and to a system comprising a device and a further device.

Examples of such a device are wireless communication devices such as baby phones.

A prior art device is known from US 6,055,420, which discloses a device comprising a high Q antenna having a length and width each significantly less than a quarter wavelength within the predetermined frequency range. A controllable reactive element in the form of a varactor is coupled to the antenna and has a variable reactance for tuning the antenna in a high Q resonant circuit to the frequency of a desired signal. An antenna tuning circuit responsive to the frequency to which the antenna is tuned provides an antenna tuning signal to the varactor so that the antenna remains tuned. The antenna and the varactor form a resonant circuit having a Q greater than 100.

The known device is disadvantageous, inter alia, owing to the fact that it is relatively complex and relatively expensive. To realize a tunable high Q antenna having a Q greater than 100, two amplifier stages, first and second varactors, a tuner for indirectly controlling the first varactor coupled to the antenna and for directly controlling the second varactor coupled to a local oscillator, and many feedback loops, one of them comprising a detector and a low pass filter, are necessary.

It is an object of the invention, inter alia, to provide a relatively simple and relatively low cost device.

Further objects of the invention are, inter alia, to provide a circuit for use in a device and to provide a system comprising a device and a further device, with the device being relatively simple and relatively low cost.

The device according to the invention comprises an antenna for receiving a radio frequency signal in a frequency band, and a circuit for tuning the antenna to the frequency band and for, in response to a feedback signal, increasing an efficiency of a combination of the tuned antenna and the circuit. By providing the device according to the invention with the circuit, a relatively simple and relatively low cost device has been created, owing to the fact that the circuit has two functions. A first function is a tuning function for tuning the antenna, and a second function is an efficiency increasing function for, in response to a feedback signal, increasing an efficiency of the combination of the tuned antenna and the circuit. It is to be noted that in US 6,055,420 the feedback signal, as supplied to the varactor coupled to the antenna, has a tuning function only. It is further to be noted that the frequency band may have a size such that it may comprise one frequency signal (or one frequency channel) or more frequency signals (or more frequency channels).

The invention is further advantageous, inter alia, in that the combination of the antenna and the circuit does not need to be a high Q combination, and in that the antenna can be a simple and low cost antenna of a size significantly smaller than a quarter wavelength of a center frequency of the frequency band, which allows the device according to the invention to be of a relatively small size too.

An embodiment of the device according to the invention is defined by the antenna being an electrical antenna having an electrical length of at most 20% of a wavelength of the frequency band. Owing to the fact that the circuit has the efficiency increasing function, the electrical antenna may have an electrical length of at most 20% of a wavelength of (a center frequency of) the frequency band, such as for example 1% to 5% of this wavelength. This allows the device according to the invention to be of an even smaller size.

It is yet further to be noted that US 3,824,473 discloses a radio receiver comprising a magnetic antenna in the form of a ferrite rod aerial coupled to a tuning capacitor. This tuning capacitor has a tuning function only and does not receive any feedback signal. The radio receiver further comprises a transistor circuit comprising four transistors, one of them in a diode configuration, which transistor circuit is fed back to form a Q multiplier. So, in US 3,824,473, there is no circuit for tuning the antenna to the frequency band and for, in response to a feedback signal, increasing an efficiency of a combination of the tuned antenna and the circuit. In US 3,824,473, there is one module comprising an antenna as well as tuning elements that cannot be separated from each other, and there is another module comprising a Q multiplier.

An embodiment of the device according to the invention is defined by the antenna being a non-loop antenna having a substantially omni directional radiation pattern. Such an antenna allows the device according to the invention to be used without an orientation of the device being of too much importance. A substantially omni directional radiation pattern is a non-directional radiation pattern.

An embodiment of the device according to the invention is defined by further comprising - an amplifier for amplifying an antenna signal originating from the antenna via the circuit and for supplying the feedback signal to the circuit.

Such an amplifier for example comprises an input for receiving the antenna signal and a first output for supplying the feedback signal and a second output for supplying the amplified antenna signal and may be of the lowest complexity. An embodiment of the device according to the invention is defined by a feedback between the amplifier and the circuit being a positive feedback, the amplifier being in a stable state. A positive feedback results from the feedback signal being fed back without a sign inversion taking place. To keep the amplifier in a stable state, in other words to prevent that the amplifier gets unstable and starts to oscillate, the amplification of the feedback loop should be smaller than one.

An embodiment of the device according to the invention is defined by the circuit comprising a coil, a first side of the coil being coupled to the antenna and a second side of the coil being coupled to ground. In this case, the antenna comprises a capacitive antenna, and the coil (an inductor) is to be used for tuning the antenna. Of course, in case the antenna comprises an inductive antenna, a capacitor is to be used for tuning the antenna. Generally, in simple and low cost devices, capacitive antennas are to be preferred.

An embodiment of the device according to the invention is defined by the circuit iurther comprising a further coil, a first side of the further coil being coupled to the amplifier and a second side of the further coil being coupled to ground, the coil and the further coil being inductively coupled coils. In this case, the first side of the further coil forms the feedback input of the circuit.

An embodiment of the device according to the invention is defined by the circuit iurther comprising a first capacitor and a second capacitor, a first side of the first capacitor being coupled to the first side of the coil, a second side of the first capacitor being coupled to a first side of the second capacitor and to the amplifier, and a second side of the second capacitor being coupled to ground. In this case, a common point of both capacitors forms the feedback input of the circuit.

An embodiment of the device according to the invention is defined by the amplifier comprising a transistor, a control electrode of the transistor being coupled to the first side of the coil, a first main electrode of the transistor being coupled via a first resistor to ground, and a second main electrode of the transistor being coupled via a second resistor to a voltage supply, the first main electrode supplying the feedback signal via a coupling capacitor, the second main electrode constituting an output of the amplifier. This amplifier is of the lowest complexity. Preferably, the transistor has a high input impedance (FET,

MOSFET). The coupling capacitor provides more feedback at higher frequencies and less feedback at lower frequencies. This results in the increase of the efficiency of the combination of the antenna and the circuit being more constant over the frequency band. An embodiment of the device according to the invention is defined by the circuit iurther comprising a capacitor coupled to the coil in parallel for reducing user-effects and/or surrounding-effects. This capacitor reduces user-effects and/or surrounding-effects.

Embodiments of the circuit according to the invention and of the system according to the invention correspond with the embodiments of the device according to the invention. The invention is based upon an insight, inter alia, that high Q antennas and high Q resonant circuits are to be avoided when reducing complexities and costs, and is based upon a basic idea, inter alia, that a circuit is to be introduced for tuning the antenna to the frequency band and for, in response to a feedback signal, increasing an efficiency of a combination of the tuned antenna and the circuit. The invention solves the problem, inter alia, to provide a relatively simple and relatively low cost device, and is further advantageous, inter alia, in that the combination of the antenna and the circuit does not need to be a high Q combination, and in that the antenna can be a simple and low cost antenna of a size significantly smaller than a quarter wavelength of a center frequency of the frequency band, which allows the device according to the invention to be of a relatively small size too.

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments(s) described hereinafter. In the drawings:

Fig. 1 shows diagrammatically a system according to the invention comprising a device according to the invention with a circuit according to the invention,

Fig. 2 shows diagrammatically a device according to the invention comprising a circuit according to the invention in greater detail,

Fig. 3 shows diagrammatically a device according to the invention comprising a circuit according to the invention in greater detail, and

Fig. 4 shows a transfer response comprising a first curve without feedback and a second curve with feedback.

In Fig. 1, a system 100 according to the invention is shown diagrammatically. The system 100 comprises a (receiving) device 10 according to the invention and a (transmitting) device 11. The device 10 comprises a (receiving) antenna 1 coupled to a circuit 2 according to the invention. The circuit 2 is coupled to an amplifier 3, which is coupled to a first module 4. The device 11 comprises a (transmitting) antenna 5 coupled to a second module 6. The second module 6 is coupled to a third module 7, which is coupled to a fourth module 8.

The device 10 for example comprises a (receiving) baby phone and the device 11 for example comprises a (transmitting) baby phone. The first module 4 for example comprises a demodulator and an amplifier and a loudspeaker. The second module 6 for example comprises a power amplifier, the third module 7 for example comprises a modulator and the fourth module 8 for example comprises a microphone. The device 10 may in addition be provided with the respective modules 6', 7' and 8' all not shown and corresponding with the respective modules 6, 7 and 8, and the device 11 may in addition be provided with the respective modules 2', 3' and 4' all not shown and corresponding with the respective modules 2, 3 and 4. In that case, the antennas 1 and 5 may each be used for receiving as well as for transmitting. Alternatively, the respective devices 10 and 11 may in addition be provided with the respective antennas 5' and 1 ' both not shown and corresponding with the respective antennas 5 and 1.

In Fig. 2, a device 10 according to the invention is shown diagrammatically in greater detail. The device 10 comprises the antenna 1 and the circuit 2 according to the invention and the amplifier 3. The circuit 2 comprises a coil 21. A first side of the coil 21 is coupled to the antenna 1 and a second side of the coil 21 is coupled to ground. The circuit 2 further comprises a further coil 22. A first side of the further coil 22 is coupled via a coupling capacitor 41 to the amplifier 3 and a second side of the further coil 22 is coupled to ground, such that the coils 21 and 22 form inductively coupled coils. The first side of the coil 21 is further coupled via a further coupling capacitor 42 to the amplifier 3. The amplifier 3 comprises a transistor 31. A first main electrode (drain or source) of the transistor 31 is coupled via a first resistor 32 to ground and to the coupling capacitor 41. A second main electrode (source or drain) of the transistor 31 is coupled via a second resistor 33 to a voltage supply and constitutes an output of the amplifier to be coupled to the first module 4. A control electrode (gate) of the transistor 31 is coupled to the further coupling capacitor 42 and is coupled via a third resistor 43 to ground. A capacitor 25 is coupled to the coil 21 in parallel.

The circuit 2 according to the invention has two functions. A first function is a tuning function for tuning the antenna 1, and a second function is an efficiency increasing function for, in response to a feedback signal originating from the first main electrode of the transistor 31, increasing an efficiency of the combination of the tuned antenna 1 and the circuit 2. As a result, the device 10 according to the invention is a relatively simple and relatively low cost device. Further, the combination of the antenna 1 and the circuit 2 does not need to be a high Q combination, and the antenna 1 can be a simple and low cost antenna of a size significantly smaller than a quarter wavelength of a center frequency of the frequency band, which allows the device 10 according to the invention to be of a relatively small size too.

About the increase of the efficiency, the following is to be observed. Especially, but not exclusively, antennas that have a too short electrical length and that need to be tuned with a tuning element such as for example a coil will show a relatively bad efficiency. An antenna of 12 cm length and operating at 40 MHz is equivalent to a capacitance of about 2 pF in series with a resistance of about 1 Ohm. When tuning this antenna with a series coil having a quality factor of about 40 and introducing a loss resistance of about 40 Ohm, the radiating efficiency will be about 2%. By introducing the circuit 2 having a tuning function as well as an efficiency increasing function, the latter in response to a feedback signal originating from the amplifier 3, the radiating efficiency can be increased up to 4%. This is an increase of the efficiency by a factor two and corresponds with a decrease of the loss resistance to 20 Ohm. This is a decrease of the loss resistance by a factor two. Further, an amplitude of a wanted signal and a selectivity will be increased as well, which are further advantages. The antenna 1 may be of any shape. The antenna 1 is preferably an electrical antenna having an electrical length of at most 20% of a wavelength of (a center frequency of) the frequency band, such as for example 1% to 5% of this wavelength. This allows the device 10 according to the invention to be of an even smaller size. The antenna 1 may be a non-loop antenna having a substantially omni directional radiation pattern. Such an antenna 1 allows the device 10 according to the invention to be used without an orientation of the device 10 being of too much importance.

The feedback between the amplifier 3 and the circuit 2 is a positive feedback, such that the amplifier does not get into an unstable state. The coupling capacitor 41 provides more feedback at higher frequencies and less feedback at lower frequencies. This results in the increase of the efficiency of the combination of the antenna 1 and the circuit 2 being more constant over the frequency band. The transistor 31 may have a high input impedance (FET, MOSFET), to reduce its influence on the circuit 2. This capacitor 25 reduces user-effects and/or surrounding-effects. In Fig. 2, it is suggested that the antenna 1 comprises a capacitive antenna, in which case the coil 21 (an inductor) is to be used for tuning the antenna 1. Of course, in case the antenna 1 comprises an inductive antenna, a capacitor is to be used for tuning the antenna 1. Generally, in simple and low cost devices, capacitive antennas are to be preferred.

In Fig. 3, a device 10 according to the invention is shown diagrammatically in greater detail. The device 10 comprises the antenna 1 and the circuit 2 according to the invention and the amplifier 3. The circuit 2 corresponds with the circuit 2 shown in Fig. 2, apart from the fact that, roughly, the further coil 22 (an inductor) has been replaced by a serial circuit of a first capacitor 23 and a second capacitor 24. A first side of the first capacitor 23 is coupled to the first side of the coil 21, a second side of the first capacitor 23 is coupled to a first side of the second capacitor 24 and to the amplifier 3, and a second side of the second capacitor 24 is coupled to ground. In this case, a common point of both capacitors 23 and 24 forms the feedback input of the circuit 2. Of course, the capacitors 23 and 24 on the one hand and the capacitor 25 on the other hand may be combined and/or integrated with each other. In Fig. 4, a transfer response comprising a first curve without feedback and a second curve with feedback is shown. Obviously, an amplitude increase of 10 dB or more can be achieved, which is a great advantage.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb "to comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. The invention may be implemented by means of hardware comprising several distinct elements. In the device claim enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

CLAIMS:

1. A device (10) comprising: an antenna (1) for receiving a radio frequency signal in a frequency band, and a circuit (2) for tuning the antenna (1) to the frequency band and for, in response to a feedback signal, increasing an efficiency of a combination of the tuned antenna (1) and the circuit (2).

2. The device (10) according to claim 1, the antenna (1) being an electrical antenna having an electrical length of at most 20% of a wavelength of the frequency band.

3. The device (10) according to claim 1, the antenna (1) being a non- loop antenna having a substantially omni directional radiation pattern.

4. The device (10) according to claim 1, further comprising: an amplifier (3) for amplifying an antenna signal originating from the antenna (1) via the circuit (2) and for supplying the feedback signal to the circuit (2).

5. The device (10) according to claim 4, a feedback between the amplifier (3) and the circuit (2) being a positive feedback, the amplifier (3) being in a stable state.

6. The device (10) according to claim 4, the circuit (2) comprising a coil (21), a first side of the coil (21) being coupled to the antenna (1) and a second side of the coil (21) being coupled to ground.

7. The device (10) according to claim 6, the circuit (2) further comprising a further coil (22), a first side of the further coil (22) being coupled to the amplifier (3) and a second side of the further coil (22) being coupled to ground, the coil (21) and the further coil (22) being inductively coupled coils.

8. The device (10) according to claim 6, the circuit (2) further comprising a first capacitor (23) and a second capacitor (24), a first side of the first capacitor (23) being coupled to the first side of the coil (21), a second side of the first capacitor (23) being coupled to a first side of the second capacitor (24) and to the amplifier (3), and a second side of the second capacitor (24) being coupled to ground.

9. The device (10) according to claim 6, the amplifier (3) comprising a transistor (31), a control electrode of the transistor (31) being coupled to the first side of the coil, a first main electrode of the transistor (31) being coupled via a first resistor (32) to ground, and a second main electrode of the transistor (31) being coupled via a second resistor (33) to a voltage supply, the first main electrode supplying the feedback signal via a coupling capacitor (41), the second main electrode constituting an output of the amplifier (3).

10. The device (10) according to claim 6, the circuit (2) further comprising a capacitor (25) coupled to the coil (21) in parallel for reducing user-effects and/or surrounding-effects.

11. A circuit (2) for use in a device (10) comprising an antenna (1) for receiving a radio frequency signal in a frequency band, the circuit (2) being designed to tune the antenna (1) to the frequency band and to, in response to the feedback signal, increasing the efficiency of the combination of the tuned antenna (1) and the circuit (2).

12. A system (100) comprising: a device (10) comprising an antenna (1) for receiving a radio frequency signal in a frequency band and comprising a circuit (2) for tuning the antenna (1) to the frequency band and for, in response to a feedback signal, increasing an efficiency of a combination of the tuned antenna (1) and the circuit (2), and a further device (11) comprising a further antenna (5) for transmitting the radio frequency signal in the frequency band to the device (10).