HK1210328B - Signal receiver with a duty-cycle controller - Google Patents

Description

Signal receiver with duty cycle controller

Technical Field

The present invention relates to the field of signal receivers, and more particularly to radio frequency signal receivers. The invention also relates to a corresponding method of suppressing at least one higher harmonic component from an oscillator with a reference resonator in an intermediate signal of a signal receiver.

Background

For the transmission of electromagnetic signals, in particular radio frequency signals, a suitable receiver is required.

A conventional receiver design is an example as shown in fig. 1. The signal receiver 1 comprises an antenna 12 for receiving electromagnetic signals. The antenna 12 is connected to a Low Noise Amplifier (LNA)14 adapted to amplify signals received by the antenna 12. The LNA is further coupled to the mixer 16 to mix the amplified and received signal with an oscillator signal 50 provided by a local oscillator or crystal oscillator 20. The output of the mixer 16 is connected to a bandpass filter 18 to filter the intermediate signal of the mixer 16 and the down-converted signal 40. In fig. 1, the spectrum of the intermediate signal 40 is shown as amplitude or power (P) over frequency (f).

In general, the local oscillator or crystal oscillator 20 uses a signal generator 24 having a reference resonator, such as a quartz resonator, to provide a well-defined and fairly stable reference frequency signal. For example, for a typical quartz resonator application, the signal generator 24 with the reference resonator 24 operates at a reference frequency of 26 MHz. In general, the local oscillator 20 further comprises a PLL unit, not shown in fig. 1, connected between the signal generator 24 and the mixer 16, in order to provide a high frequency oscillating signal. In the case of radio-frequency transmission, the signal receiver 1 can operate, for example, in the bluetooth range, and can therefore operate in the 2.4GHz band using a signal generator 24 with a quartz-based reference resonator 22 operating at 26 MHz. Then the 93 th, 94 th and 95 th higher harmonics in the frequency of the oscillator 20 based on the reference resonator 22 may coincide with the frequency band 41 of the band-pass filter 18.

Although these higher harmonic components 42 of the reference resonator based oscillator may be suppressed and attenuated, the electromagnetic signal received by the antenna, in particular the intermediate signal 40 received after down-conversion by the mixer 16) may still be subject to severe interference. This problem becomes even more pronounced when the signal receiver is implemented in a separate integrated circuit.

Another example of a conventional FSK radio frequency signal receiver with a high sensitivity demodulator is described in U.S. patent application serial No. 2012/0164966 a 1.

U.S. patent application serial No. 2012/0046005 a1 describes a wireless communication device with oscillator duty cycle control. This device requires simultaneous operation of the transmit path and the receive path tuned for adjusting the duty cycle, which is a disadvantage.

Disclosure of Invention

It is therefore an object of the present invention to provide an improved signal receiver, in particular for receiving radio frequency signals, in which the influence of higher harmonics from an oscillator with a reference resonator can be effectively reduced or even completely compensated. The solution should be rather space and cost efficient. Furthermore, the solution does not affect the conventional operation and function of the signal receiver and its various components.

A first aspect of the invention relates to a signal receiver, in particular a radio frequency signal receiver. The signal receiver includes an antenna for receiving electromagnetic signals. Furthermore, the signal receiver comprises at least one Low Noise Amplifier (LNA) for amplifying the signal received by the antenna. Thus, the antenna may be directly interconnected with the low noise amplifier. Furthermore, the signal receiver comprises at least one crystal or MEMS oscillator comprising a signal generator with a reference resonator to generate an oscillating signal with a predetermined duty cycle.

In general, a MEMS or crystal oscillator is an electronic oscillating circuit that uses an oscillating crystal of piezoelectric material or the mechanical resonance of an oscillating MEMS to create an electrical signal with a fairly precise frequency. The reference resonator may be a MEMS resonator or comprise a quartz crystal, as is often used in electronic applications. The MEMS or crystal oscillator, i.e. the electronic oscillating circuit, is adapted to generate an oscillating signal having a predetermined duty cycle. The duty cycle defines the ratio of the on-time to the off-time of the oscillating signal. In general, the oscillating signal is characterized by a square pulse train, but may also be characterized by any other suitable waveform that can be processed by the mixer.

In addition, the signal receiver further comprises a mixer for mixing the amplified and received signal obtained from the LNA with an oscillating signal obtained from a MEMS or crystal oscillator to generate an intermediate, i.e. down-converted, signal. Furthermore, the crystal or MEMS oscillator may comprise a PLL unit at the output to provide an oscillating signal for the mixer. The mixer can also be implemented in a number of different ways. For example, the mixers may include or be implemented as subsampling mixers, gilbert cells, or as passive (passive) mixers, to name a few. The mixer receives an oscillating signal from a local oscillator or from a MEMS or crystal oscillator and an amplified signal from an LNA as an input signal. The mixer then provides and generates at an output an intermediate signal which is further processed by a band-pass filter adapted to filter the intermediate signal provided by the mixer.

The signal receiver further comprises a duty cycle controller coupled to the MEMS or crystal oscillator and further coupled to an output of the band pass filter. By connecting the duty cycle controller to the output of the band pass filter, the spectrum of the filtered intermediate signal obtained at the output of the band pass filter can be analyzed using the duty cycle controller. The duty cycle controller is operable to modify a duty cycle of the oscillating signal in response to the spectral analysis. This modification may be implemented by coupling the duty cycle controller to the MEMS or crystal oscillator.

By modifying the duty cycle of the oscillator signal, higher harmonics in the filtered intermediate signal from the frequency of the oscillator having the signal generator with a reference resonator may be attenuated or modified. Thus, by selectively modifying the waveform of the oscillating signal, its fourier spectrum and the output of the mixer are modified. In this way, certain higher harmonics, which coincide with a frequency band of interest in the band pass filter, can be attenuated in the filtered intermediate signal without modifying the frequency of the oscillating signal. Thus, the modification of the duty cycle of the oscillation signal does not affect the signal mixing at all, nor the normal behavior and operation of the signal receiver at all. The modification of the duty cycle of the oscillating signal is mainly reflected in a variation of the fourier spectrum of the oscillating signal and results in a variation of the amplitude distribution of higher harmonics in the intermediate signal from the frequency of the oscillator with the reference resonator.

According to a further embodiment, the duty cycle controller is adapted to minimize an amplitude of at least one higher harmonic component in the filtered intermediate signal from the oscillator with the reference resonator. For example, if the frequency band of interest in the signal receiver is a 2.4GHz frequency band, the duty cycle controller may be operable to adjust or modify the duty cycle of the oscillating signal of the MEMS or crystal oscillator by: the manner is such that higher harmonics of a reference frequency, e.g. 26MHz, from the MEMS or crystal oscillator are attenuated or suppressed at least in the filtered intermediate signal. For example, the 93, 94 and 95 th harmonics of the 26MHz reference frequency, which are equal to 2.418GHz, 2.444GHz and 2.470GHz, may be effectively attenuated or suppressed in the filtered intermediate signal. In this way, the influence of higher harmonics from the oscillator with the reference resonator downstream of the mixer can be compensated for or even completely suppressed.

According to a further embodiment, the duty cycle controller comprises a measuring unit to measure the amplitude of selected harmonic components of the filtered intermediate signal. The measurement unit is typically connected and coupled to the output of the band pass filter. Since the regulation loop provided by the duty cycle controller, the MEMS or crystal oscillator and the mixer and/or the band pass filter may be operated to suppress selected harmonics of the reference frequency from the oscillator with the reference resonator, the measurement unit may be particularly adapted to measure the amplitude, i.e. the amplitude of the predefined harmonic component of the filtered intermediate signal coinciding with the higher harmonics from the oscillator with the reference resonator.

For example, the measuring unit may be adapted to exclusively detect and perceive the amplitude or the amplitude of the 93 th, 94 th or 95 th higher harmonic of the reference frequency. The other frequency components of the filtered intermediate signal are no longer of interest to the duty cycle controller. The above frequencies and specific higher harmonics are only examples and should not be construed as limiting the scope of the invention to a specific radio frequency band. In general, a signal receiver with its own duty cycle controller may operate for a large number of different higher harmonics and reference frequencies of the MMES or crystal oscillator.

According to a further embodiment, the duty cycle controller comprises a control unit coupled to the measurement unit and operable to generate a duty cycle correction signal in response to the measured magnitude of the selected harmonic component. Typically, the control unit is operable to compare the measured amplitude or amplitude of the actually selected harmonic with a predetermined value or a variable value, which may be dynamically determined during operation of the signal receiver. The duty cycle correction signal generated by the control unit is used to increase or decrease the duty cycle, i.e. the relation between the on-time and the off-time of the oscillating signal of the MEMS or crystal oscillator.

Additionally or alternatively, it is also conceivable that the control unit is operable to modify not only the duty cycle but also a combination of the duty cycle of the oscillation signal and the waveform of the oscillation signal. In this way, the higher harmonic content in the filtered intermediate signal can be modified in a number of different ways.

In a further embodiment, the signal receiver further comprises a duty cycle modifier coupled to the duty cycle controller or even integrated into the duty cycle controller. The duty cycle corrector is particularly operable to increase or decrease the duty cycle of the oscillating signal in response to the duty cycle correction signal retrieved from the control unit. In other words, the duty cycle corrector is operable to process the duty cycle correction signal generated by the control unit of the duty cycle controller. The duty cycle modifier is operable to increase the duty cycle, for example by a predefined discrete step, if the duty cycle modification signal indicates an increase of the duty cycle.

If the duty cycle correction signal indicates a decrease in the duty cycle, the duty cycle corrector will correspondingly decrease the duty cycle of the oscillating signal. The duty cycle modifier may also be implemented in or may belong to the MEMS or crystal oscillator. Thus, the duty cycle modifier may be an integrated or separate component of an electronic oscillating circuit of the MEMS or crystal oscillator operable to generate an oscillating signal based on a reference frequency from the oscillator with the reference resonator.

In a further embodiment, the duty cycle controller further comprises a memory or some storage space to at least temporarily store the measured amplitudes of the selected harmonic components of the filtered intermediate signal. By means of the memory, the previously measured amplitudes of the selected harmonic components of the output of the band pass filter can be stored in order to provide a comparison with the actually measured amplitudes of the same harmonic components. In this way, the effect of the modified duty cycle can be directly monitored and evaluated. It is even conceivable that the memory comprises different memory locations allowing to store a time series of the successively measured amplitudes of the harmonic components. It is even conceivable that the entire series of multiple harmonic components is stored in the memory.

Additionally or alternatively, the memory may also be used to store some sort of look-up table in which the respective amplitudes of the selected harmonic components are directly assigned to the predefined duty cycle. In this way, the control unit may be adapted to select the predefined duty cycle in response to a measurement of a specific amplitude of the selected harmonic component.

In a further embodiment, the control unit is operable to compare the actually measured amplitudes of the harmonic components with previously stored amplitudes of harmonic components of the filtered intermediate signal, thereby generating the duty cycle correction signal. It is therefore conceivable that the control unit permanently performs a comparison of the actually measured harmonic components with a previously stored single harmonic component or sequence of harmonic components.

The control unit may perform the comparison continuously. The control unit may be operable to modify the duty cycle by small but discrete steps and monitor the results of this modification. If the initial modification results in the desired suppression of harmonic content in the filtered intermediate signal, the duty cycle will be modified repeatedly in the same manner at subsequent stages. This regulation loop may continue until the control unit detects an increase in the amplitude of the harmonic components of the filtered intermediate signal. The duty cycle controller will then return to the previously selected duty cycle. It is conceivable that the control unit permanently performs operations by means of such a signal comparison control loop.

According to a further embodiment, the duty cycle controller may be at least temporarily deactivated, in particular when the antenna receives an electromagnetic signal to be further processed by the signal receiver. According to this implementation, the modification of the duty cycle of the oscillation signal only occurs if: i.e. the signal receiver is idle or the signal receiver does not actually receive any electromagnetic signal. By selectively deactivating or activating the duty cycle controller, the power consumption of the signal receiver may be reduced. Furthermore, the performance of the signal receiver for processing the received electromagnetic signal will not be affected or disturbed by the duty cycle regulation loop, which is typically implemented by the duty cycle controller, the MEMS or crystal oscillator, the mixer and the band pass filter.

Another aspect of the invention also relates to a method of suppressing at least one higher harmonic component from an oscillator with a reference resonator in a filtered intermediate signal of a signal receiver as described above. The method comprises the following steps:

-receiving the electromagnetic signal by means of an antenna,

-amplifying the received signal by means of at least one low noise amplifier,

-generating an oscillating signal by at least one MEMS or crystal oscillator comprising a reference resonator,

-mixing the amplified and received signal with the oscillating signal to generate an intermediate signal,

-filtering the intermediate signal,

-analyzing the frequency spectrum of said filtered intermediate signal, and

-modifying, by a duty cycle controller coupled to a MEMS or crystal oscillator and further coupled to an output of a band pass filter, a duty cycle of the oscillating signal in response to the spectral analysis of the filtered intermediate signal in order to minimize an amplitude of at least one higher harmonic component of the filtered intermediate signal from the MEMS or crystal oscillator with the reference resonator, the duty cycle being defined by a ratio of an on-time and an off-time of the oscillating signal.

By modifying the duty cycle of the oscillating signal generated by the MEMS or crystal oscillator, the distribution of higher harmonic components in the intermediate signal can be modified to effectively suppress the effect of the selected higher harmonic components in the frequency band of interest of the band pass filter.

Generally, the method of suppressing at least one higher harmonic component is performed by a signal receiver as described above. In this respect, any of the features and advantages described above in relation to the signal receiver apply equally to the method of suppressing the at least one higher harmonic component of the MEMS or crystal oscillator with the reference oscillator, and vice versa.

According to a further embodiment, the duty cycle of the oscillating signal is modified by the duty cycle controller to minimize an amplitude of at least one higher harmonic component of the MEMS or crystal oscillator with the reference resonator in the filtered intermediate signal. Here, the duty cycle controller is typically part of a control loop operable to find and identify an optimum duty cycle to minimise selected higher harmonic content of or in the filtered intermediate signal. The duty cycle controller may be implemented and operated in a number of different ways.

In a further embodiment, the duty cycle of the oscillating signal is increased or decreased by predetermined discrete steps until the amplitude of selected higher harmonic components of the MEMS or crystal oscillator with the reference resonator reaches a minimum in the filtered intermediate signal. The higher harmonic components of interest may be predetermined or may be selected by various settings of the signal receiver. The selection of the higher harmonic components to be suppressed may be based on the selected frequency band and the bandwidth of the band pass filter.

Generally, the above method will suppress such harmonic components: they come from the higher harmonic components of MEMS or crystal oscillators having a reference resonator falling within the frequency band of the band-pass filter, which are not only individual higher harmonic components but all higher harmonic components. In order to find the minimum amplitude of the at least one selected higher harmonic component or the selected higher harmonic components, the duty cycle controller may by default make at least a slight but stepped modification of the given duty cycle. Thereafter, the effect of the initial duty cycle modification is monitored in the filtered intermediate signal. If the amplitude of the selected higher harmonic component increases, the duty cycle is corrected in the opposite direction in a subsequent phase.

For example, the duty cycle may be decreased or increased until the amplitude of selected higher harmonic components of the MEMS or crystal oscillator with the reference resonator reaches a minimum. The optimum duty cycle can be obtained by implementing a regulation loop using standard digital signal processing units.

According to a further embodiment, the amplitude of the higher harmonic component in the filtered intermediate signal is measured and at least temporarily stored in a memory. The memory is typically located in the duty cycle controller. Since the duty cycle controller is clocked, the memory may comprise a shift register adapted to store at least one or some of the continuously measured amplitudes taken at subsequent clock times. By using a memory the actually measured amplitude of the selected higher harmonic component can be compared with a previously stored amplitude.

The comparison then indicates whether the duty cycle has been previously modified by reducing the selected higher harmonic content. If the comparison reveals that the amplitude of the higher harmonic component is decreasing, the duty cycle will be modified in the same direction until a minimum value is reached. If the comparison reveals an increase in the amplitude of the higher harmonic component, the direction of the duty cycle modification will change or invert.

In this way and according to a further embodiment, the duty cycle of the oscillating signal is reduced or increased according to a comparison between an actually measured amplitude of higher harmonic components in the filtered intermediate signal and a previously stored amplitude.

According to a further embodiment, the duty cycle of the oscillating signal is kept constant at all times when the antenna actually receives an electromagnetic signal for further processing by the signal receiver. In this way, the duty cycle controller may be at least temporarily deactivated in order to save energy and avoid any interference to the signal processing of the signal receiver due to the duty cycle modification of the oscillating signal.

Drawings

Various embodiments of the present invention will be described hereinafter with reference to the accompanying drawings, in which:

figure 1 shows a conventional signal receiver known in the prior art,

fig. 2 shows a schematic block diagram of a signal receiver according to the invention, an

Fig. 3 shows a flow chart of a method that can be performed by the signal receiver according to fig. 2 to suppress at least one higher harmonic component of a MEMS or crystal oscillator of the signal receiver.

Detailed Description

Fig. 2 shows a schematic block diagram of a signal receiver 10 according to the present invention. A signal receiver 10, typically implemented to receive radio frequency signals, includes an antenna 12 operable at a carrier frequency, e.g., about 2.4 GHz. Signals received by antenna 12 are amplified by a Low Noise Amplifier (LNA) 14. The amplified signal is frequency converted, in particular down converted in the mixer 16 by means of an oscillating signal 50 provided via a MEMS or crystal oscillator 20. The output of the mixer 16 is coupled to an input of a band pass filter 18, whereby a frequency band 41 of interest in the intermediate signal 40 can be selected for further processing in the signal receiver 10.

The MEMS or crystal oscillator 20 includes a signal generator 24 that is locked to the reference resonator 22. The reference resonator 22 may be a MEMS resonator or implemented by a quartz crystal, or similar resonator for the signal generator 24 to provide a constant and very stable reference frequency, typically in the range of several MHz, for example 26 MHz. The signal generator 24 is operable to generate a signal having a generally sinusoidal waveform. The signal generator 24 is further connected to a duty cycle modifier 26, which will be described below, which generates an oscillation signal 50 based on the signal from the signal generator 24, the oscillation signal 50 for example having a rectangular waveform characterized by an interleaved sequence of on-times 51 and off-times 52. The length relationship of the on-time 51 and the off-time 52 determines the duty cycle of the oscillating signal 50. Furthermore, the crystal or MEMS oscillator 20 may comprise a PLL unit, not shown in fig. 2, which is located between the duty cycle multiplier 26 and the mixer 16 to provide the oscillating signal with a high order frequency for the mixer.

In general, higher harmonics of the frequency of the oscillation signal 50 depend heavily on the duty ratio and waveform of the oscillation signal 50. Thus, if the selected frequency band 41 of the filtered intermediate signal 40 coincides with at least one higher harmonic component of the frequency from the MEMS or crystal oscillator 20 with the reference resonator 22, the corresponding component 42 appears to be a disturbance in the frequency band of interest of the filtered intermediate signal 40.

In order to suppress the contribution and distribution of selected higher harmonic components from the MEMS or crystal oscillator with reference resonator in the filtered intermediate signal 40, the receiver 10 further comprises a duty cycle controller 30 coupled to the output section 19 of the band pass filter 18. The duty cycle controller 30 is operable to measure and analyze the frequency spectrum of the filtered intermediate signal 40. The duty cycle controller 30 is particularly adapted to determine and measure the amplitude or magnitude of selected higher harmonic components 42 from the frequency of the MEMS or crystal oscillator 20 with the reference resonator 22.

To this end, the duty cycle controller 30 comprises a measuring unit 32 to determine and measure the amplitude or amplitude of selected higher harmonic components 42 of the filtered intermediate signal 40. Based on this measurement, the control unit 34 of the duty cycle controller 30 is operable to modify the duty cycle of the oscillating signal 50 from the MEMS or crystal oscillator 20. To this end, the control unit 34 of the duty cycle controller 30 is coupled or connected to the duty cycle modifier 26, which is implemented in the MEMS or crystal oscillator 20 and receives the signal from the signal generator 24.

The duty cycle modifier 26 is operable to modify the duty cycle of the oscillating signal 50 when receiving the individual duty cycle modification signal from the control unit 34. The modification of the duty cycle may be performed by discrete steps. For example, a given or default duty cycle of the oscillating signal 50 may be increased by discrete steps. Then, in a next phase, the duty cycle controller 30, in particular the measurement unit 32 thereof, is operable to monitor the filtered intermediate signal 40 for any changes in the amplitude or magnitude of selected harmonic components 42.

If the amplitude or amplitude of the higher harmonic component 42 decreases, the duty cycle will be further modified in the same direction until a minimum value of the amplitude or amplitude of the harmonic component 42 is reached. If the initial increase in duty cycle results in an increase in the amplitude or amplitude of the higher harmonic components 42 in the filtered intermediate signal 40, the duty cycle will be reduced in subsequent stages and eventually in successive stages until a minimum value of the amplitude or amplitude of the higher harmonic components 42 in the filtered intermediate signal 40 is reached.

In order to perform a comparison between the actually measured and the previously present amplitudes or amplitudes of the higher harmonic components 42 of the filtered intermediate signal 40, the duty cycle controller 30 comprises a memory 36. The memory 36 may be implemented in a variety of different ways. It may comprise a shift register allowing to temporarily store the actually measured amplitudes or magnitudes of the selected higher harmonic components 42 of the filtered centre signal 40 and finally the previously measured amplitudes or magnitudes of the selected higher harmonic components 42 of the plurality of filtered centre signals 40. Thus, the memory 36 allows a comparison to be performed between the actual measured harmonic components 42 and the previously stored harmonic components in order to determine whether the actual or previous duty cycle modification resulted in further suppression and reduction of selected harmonic components 42 in the filtered intermediate signal 40.

In this manner, the duty cycle controller 30, the MEMS or crystal oscillator 20, the mixer 16, and the band pass filter 18 form a regulation loop. In addition to this, it is also conceivable to implement other control mechanisms for suppressing the higher harmonic components 42 of the filtered intermediate signal 40. It is generally contemplated that the memory 36 includes and provides a look-up table by which a predetermined duty cycle may be selected in response to an actual and quantitative measurement of the amplitude or other characteristic of the higher harmonic components 42 of the filtered intermediate signal 40.

A method of suppressing the higher harmonic components 42 in the filtered intermediate signal 40 is exemplarily shown in the flow chart in fig. 3. In a first step 100, at least one selected spectral component, i.e. the single or multiple higher harmonic components 42 of the filtered intermediate signal 40 coinciding with the frequency band of interest 41, is actually measured and analyzed, typically by the measuring unit 32 of the duty cycle controller 30. In a next step 102, the amplitude of the actually measured and analyzed spectral components 42 is stored in the memory 36. Subsequently, at step 104, the duty cycle of the oscillating signal 50 generated by the crystal oscillator 20 is modified, typically in a predetermined manner and/or in discrete steps.

Subsequently, in step 106, the spectral content, i.e. the amplitude or the amplitude, of the selected higher harmonic content 42 of the filtered intermediate signal 40 is measured again. The measured amplitude or magnitude is then repeatedly stored in the memory 36 in a following step 108. The actual stored amplitude or magnitude is then compared with the amplitude or magnitude of one or more previously stored individual spectral components, i.e. with the previously measured higher harmonic components 42 of the filtered intermediate signal 40.

After performing this comparison at step 110, the method jumps back to step 104 and modifies the duty cycle of the oscillating signal 50 again. The modification depends on the output of the comparison. If the comparison reveals that the suppression of selected higher harmonic components 42 is improved, the duty cycle will be changed in the previous way. If the suppression of the selected higher harmonic component 42 becomes worse, the duty cycle will be changed and corrected in the opposite way.

Even if not specifically shown, the method may specifically operate to disable the duty cycle modification function, particularly when the antenna 12 is actually receiving an electromagnetic signal to be further processed by the signal receiver 10. The present invention is therefore directed to performing the duty cycle modification procedure described above exclusively only when the signal receiver is in a standby mode or when the signal receiver is substantially idle.

Further, it is noted that the illustration of the various components of the duty cycle controller 30 and the various components of the MEMS or crystal oscillator 20 merely reflect the operability and functionality of the duty cycle controller and MEMS or crystal oscillator. For example, the measurement unit 32, the control unit 34 and the memory 36 of the duty cycle controller may be implemented in a single and common integrated circuit. The same is true for the internal structure of the MEMS or crystal oscillator 20.

Claims (12)

1. A signal receiver (10) comprising:

an antenna (12) for receiving electromagnetic signals,

-at least one low noise amplifier (14) for amplifying signals received by the antenna (12),

-at least one MEMS or crystal oscillator (20) comprising a reference resonator (22) to generate an oscillating signal (50) having a predetermined duty cycle,

a mixer (16) for mixing the received and amplified signal with the oscillating signal (50) to generate an intermediate signal (40),

-a band-pass filter (18) filtering the intermediate signal (40), an

-a duty cycle controller (30) coupled to the MEMS or crystal oscillator (20) and further coupled to an output (19) of the band pass filter (18) to analyze the spectrum of the filtered intermediate signal (40) and to modify the duty cycle of the oscillating signal (50) in response to the spectral analysis of the filtered intermediate signal (40) in order to minimize the amplitude of at least one higher harmonic component (42) of the filtered intermediate signal (40) from the MEMS or crystal oscillator (20) with the reference resonator (22), the duty cycle being defined by the ratio of the on-time and the off-time of the oscillating signal.

2. The signal receiver (10) of claim 1, wherein the duty cycle controller (30) comprises a measurement unit (32) to measure the amplitude of selected harmonic components (42) of the filtered intermediate signal (40).

3. The signal receiver (10) of claim 2, wherein the duty cycle controller (30) comprises a control unit (34), the control unit (34) being coupled to the measurement unit (32) and operable to generate a duty cycle correction signal in response to the measured magnitude of the selected harmonic component (42).

4. The signal receiver (10) of claim 3, further comprising a duty cycle modifier (26) coupled or integrated to the duty cycle controller (30), wherein the duty cycle modifier (26) is operable to increase or decrease the duty cycle of the oscillating signal (50) in response to the duty cycle modification signal obtained from the control unit (34).

5. The signal receiver (10) of claim 3, wherein the duty cycle controller (30) includes a memory (36) to at least temporarily store the measured amplitudes of the harmonic components (42).

6. The signal receiver (10) of claim 5, wherein the control unit (34) is operable to compare the actually measured amplitude of the harmonic component (42) with the previously stored amplitude of the harmonic component (42) to generate the duty cycle correction signal.

7. The signal receiver (10) of claim 1, wherein the duty cycle controller (30) is at least temporarily deactivatable when the antenna (12) is receiving electromagnetic signals.

8. A method of suppressing at least one higher harmonic component (42) in a filtered intermediate signal (40) of a signal receiver (10) according to claim 1, the at least one higher harmonic component (42) coming from the MEMS or crystal oscillator (20) with the reference resonator (22), the method comprising the steps of:

-receiving an electromagnetic signal by an antenna (12),

-amplifying the received signal by means of at least one low noise amplifier (14),

-generating an oscillating signal (50) by at least one MEMS or crystal oscillator (20) comprising a reference resonator (22),

-mixing the amplified and received signal with the oscillating signal (50) to generate an intermediate signal (40),

-filtering the intermediate signal (40) by means of a band-pass filter (18),

-analyzing the frequency spectrum of the filtered intermediate signal (40), and

-modifying the duty cycle of the oscillating signal (50) in response to a spectral analysis of the filtered intermediate signal (40) by a duty cycle controller (30) coupled to the MEMS or crystal oscillator (20) and to an output (19) of the band-pass filter (18) in order to minimize the amplitude of at least one higher harmonic component (42) of the filtered intermediate signal (40) from the MEMS or crystal oscillator (20) with the reference resonator (22), the duty cycle being defined by the ratio of the on-time and the off-time of the oscillating signal.

9. The method of claim 8, wherein the duty cycle of the oscillating signal (50) is increased or decreased by predetermined discrete steps until the amplitude of selected higher harmonic components (42) from the MEMS or crystal oscillator (20) with the reference resonator (22) reaches a minimum in the filtered intermediate signal (40).

10. The method of claim 8, wherein the amplitude of the higher harmonic components (42) in the filtered intermediate signal (40) is measured and at least temporarily stored in a memory (36).

11. The method according to claim 10, wherein the duty cycle of the oscillating signal (50) is increased or decreased depending on a comparison between an actually measured amplitude (42) of the higher harmonic component (42) in the filtered intermediate signal (40) and a previously stored amplitude (42).

12. The method of claim 8, wherein the duty cycle of the oscillating signal (50) remains constant throughout the time the antenna (12) receives an electromagnetic signal.