WO2009115989A2

WO2009115989A2 - Low-noise mixing unit

Info

Publication number: WO2009115989A2
Application number: PCT/IB2009/051131
Authority: WO
Inventors: Gerben W. De Jong; Manel Collados Asensio
Original assignee: Nxp B.V.
Priority date: 2008-03-19
Filing date: 2009-03-17
Publication date: 2009-09-24
Also published as: WO2009115989A3

Abstract

The present application relates to an apparatus comprising a low-noise mixing unit, and more particularly to a receiver comprising the apparatus. The apparatus comprises a mixing unit configured to frequency down convert an input signal. The mixing unit comprises a number of switching units. The apparatus encompasses a generating unit configured to generate at least one local oscillator signal. The generating unit is configured to drive the number of switching units such that a maximum of one of the switching units is conductive at a certain time.

Description

Low-noise mixing unit

TECHNICAL FIELD

The present application relates to an apparatus comprising a low-noise mixing unit, and more particularly to a receiver comprising the apparatus.

BACKGROUND

In modern wireless communication systems, radio receivers are required for receiving electromagnetic waves sent by remote radio transmitters. Receivers having a Low- Noise Amplifier (LNA) are commonly used. In general, a receiver comprises an antenna provided for receiving and converting electromagnetic waves to a suitable electrical signal. Furthermore, after band-pass filtering provided for suppressing unwanted frequency bands, the received signal is amplified by the LNA. The LNA is an active component and may serve for preparing the signal such that noisier subsequent processing units do not affect the signal unduly. In particular, the filtered signal can be amplified to a suitable power needed for further processing. As a next processing unit, a mixer is arranged comprising a local oscillator for down converting the high-frequency signal received by the antenna to an intermediate- frequency (IF) signal. In turn, this signal can be filtered by suitable filtering units, like low- pass filters, and can be amplified once again by an intermediate-frequency amplifier. Subsequently, an analogue-digital converter can be provided for further processing within the digital domain. For instance, further processing can be performed using a digital signal processor (DSP) or any other suitable digital processor.

Such a process sequence stated above comprises several issues. On the one hand, the LNA is essential due to subsequent noisy mixing unit. However, the linearity specification of the LNA is high due to the fact that the previously arranged band-pass filter allows passing the entire frequency band of interest. To obtain the required high linearity, LNAs are used which dissipate a high amount of energy.

Furthermore, large interfering signals, which are generally included within the interesting frequency band, easily overdrive the LNA and thus, cause gain compression. Hence, the responses can be disturbed, which may yield to the impossibility of correct detection of the information. Another issue occurs since these interfering signals are amplified. In a subsequent processing step, in particular during IF filtering and IF amplifying, the interfering signals also yield to gain compression and spurious responses.

Additionally, in multi-band receivers, the issue arises that multiple LNAs are required, and thus the problems stated above increase.

It is one object of the present application to provide an improved accuracy of a receiver. A further object is to avoid gain compression. Another object is to avoid spurious responses. A further object is to reduce power consumption. Another object is to reduce noise. A further object is to prevent noise folding.

SUMMARY

These and other objects are solved by an apparatus comprising a mixing unit configured to frequency down convert an input signal. The mixing unit comprises a number of switching units. The apparatus encompasses a generating unit configured to generate at least one local oscillator signal. The generating unit is configured to drive the number of switching units such that a maximum of one of the switching units is conductive at a certain time.

The apparatus according to the application can be employed in almost any receiver of modern communication systems. Furthermore, the apparatus may be implemented in the analogue domain. The apparatus comprises a mixing unit. The mixing unit serves for down converting an input signal. In general, an input signal received by the apparatus may be a high frequency signal. For further processing, for example for detecting the sent information, such a signal may be down converted to a suitable intermediate frequency. The intermediate frequency can be predefined depending on further processing requirements. What is more, the mixing unit comprises a number of switching units.

Additionally, for generating at least one local oscillator signal, the apparatus comprises a suitable generating unit. Frequency down converting a signal may be performed by using a local oscillator signal comprising a suitable frequency. The frequency may depend on the frequency of the received signal and the desired intermediate frequency. It is found, according to the present application that noise generation can be significantly reduced by the generating unit for the case the generating unit is configured to drive the number of switching units such that a maximum of one of the switching units is conductive. During operation of the apparatus, merely one switching unit is conductive at a certain time. According to an embodiment of the present application, the number of switching units used for down converting may be four. In case, merely one switching unit is conductive, the noise generation may remain low.

According to prior art, due to the use of an LNA, noise generated by the mixing unit was not a problem. Hence, other properties of the mixing unit have been improved, and thus, more than one switching unit or similar components according to apparatuses of prior art were conductive during operation. According to the present invention, it is found that if a maximum of one switching unit of the whole mixing unit is conductive during operation, the noise generated by the mixing unit and/or the further arranged processing units, like intermediate-frequency (IF) amplifiers or the like, can be reduced such that an originally essential LNA can be omitted.

The power consumption of the present apparatus can be significantly reduced. The noise generated by the mixing unit according to the present application is very low. The use of an LNA can be omitted.

In another embodiment of the present application, the switching units may comprise RF-inputs which may be connected in parallel. Thus, depending on the number of switching units, a number of parallel paths comprising a switching unit may exist, wherein merely one path may be conductive at the same time. The noise generated by the whole mixing unit may be low. More particularly, the generated noise by the whole mixing unit may be reduced such that an LNA can be omitted. According to a further embodiment of the present application, the apparatus may comprise a filtering unit which may be connected to the mixing unit. The filtering unit may be configured to filter the input signal which may be received via an antenna element. Any suitable radiating element, like dipoles, open-waveguides or similar elements can be employed as the antenna element. Furthermore, more than one antenna element can be arranged. The signal received by one antenna element may be forwarded to a filtering unit. This filtering unit may be any suitable band-pass filter for filtering the frequency band of interest. The centre frequency of the band-pass filter may depend on the frequency band of interest. Further frequency bands may be suppressed by using this filter.

The apparatus according to another embodiment may comprise at least one holding element. The holding element may be arranged at the output of at least one switching unit. Each parallel path may comprise one holding element arranged after a switching unit. It is found, according to an embodiment of the present application that a holding element may be used for filtering the down converted and intermediate frequency signal respectively. For instance, the holding element may serve as a low-pass filter. An additionally required IF- filter can be omitted. Noise folding, in particular of RF noise generated by subsequent units, like amplifiers can be prevented. Furthermore, the holding element may provide for a significantly reduced conversion loss. By way of example, the voltage-to-voltage conversion loss may be 1 dB. As a holding element, a capacitive element can be employed according to a further embodiment. A capacitor can be implemented easily and cost-effective, in particular in an integrated form. One terminal of the capacitor can be applied to an output of a switching unit and the other terminal can be grounded. Low-pass filtering can be easily performed. It should be understood that, according to other variants of the present application, the holding element can also be realized by an inductor, or suitable combinations of capacitors, resistors and inductors. Active filter units may also be implemented.

According to a further embodiment, a switching unit may comprise at least one transistor. A transistor can be used advantageously as a switch due to its fast response time. Furthermore, a transistor can be easily driven and implemented. For instance, a field- effect transistor, like a MOS transistor may be used, wherein the transistor may be driven using its gate terminal. Depending on the voltage applied at its gate terminal, the field-effect transistor may be conductive or not conductive. One transistor acting as a switching unit may be advantageous. The use of merely one transistor as a switching unit may be advantageous with respect to reduce noise. The generating unit, according to a further embodiment of the present embodiment, may be employed as at least one local oscillator. A local oscillator may be realized using synthesizing means, like a phase locked loop (PLL) comprising a voltage controlled oscillator (VCO). However, other implementations are also possible. The local oscillator frequency generated by the local oscillator can be determined such that the down converted signal may comprise a desired intermediate frequency. The frequency of the local oscillator signal may depend on the frequency of the received input signal. Furthermore, a local oscillator is especially suitable to drive a switching unit, in particular a transistor. A local oscillator signal may comprise two voltage levels, like 0 V and a certain voltage value. A transistor may be conductive depending on the voltage applied at its gate terminal. The local oscillator signal may comprise a duty cycle. The duty cycle may be determined such that a maximum of one of the switching units may be conductive during operation of the apparatus at a certain time. The duty cycle may define the period of the high level in relation to the period time. Moreover, the duty cycle may depend on the number of used switching units. For instance, if four switching units are employed, the duty cycle should not exceed 25%. If this critical value is met, it can be provided for that the requirement of operating merely one switching unit in a conductive state can be fulfilled. However, defining fewer values for the duty cycle of the local oscillator signal may be possible as well. In addition, the apparatus according to another embodiment of the present application may comprise at least one phase shifting unit which may be connected to the local oscillator. In addition, the phase shifting unit may be configured to drive each switching unit with a signal comprising a phase shift depending on the ratio of 360 degree and the number of switching units. For the case that four switching units are arranged, four local oscillator signals may be generated comprising a phase difference of 90° by using at least one local oscillator and three phase shiftings in a cascaded arrangement. However, further possibilities exist. For instance, a local oscillator signal comprising phase shift of 0° can be generated. Then, the further oscillator signals may comprise a 90°, 180° and 270° phase shift. According to a further embodiment of the present application, the at least one switching unit may comprise an on-resistance during its conductive state. The on-resistance may be smaller than the magnitude of the source impedance. Preferably, the on-resistance may be at least 10 times smaller than the magnitude of the source impedance. For instance, the on-resistance may be at least less than 100 Ohm, preferably less or equal to 10 Ohm. The on-resistance represents the resistance during the conductive state of the switching unit. Noise can be significantly reduced, in particular thermal noise generated by the on-resistance. It is found that, in particular, a transistor used as a switching unit comprising an on- resistance, which is low compared to the magnitude of the source impedance, in particular of the antenna resistance, can be employed.

For amplifying the IF signal, the apparatus may comprise, according to a further embodiment, at least two amplifying units. An amplifying unit may be connected via at least one holding element to a switching unit. As an amplifying unit, a suitable amplifier, in particular an amplifier which dissipates less power can be used. Power consumption of the whole apparatus can be reduced. According to a further embodiment, a differential amplifier can be used as at least one amplifying unit. Such an amplifier may amplify the difference between two received signals. For instance, the amplifier can be supplied by two down converted signals comprising a phase difference of 180°, for the case, four switching units are employed. One amplifying unit may be configured to amplify the inphase component of the intermediate frequency signal and the further amplifier may be configured to amplify the quadrature component of the intermediate frequency signal. According to another variant of the present application, four amplifiers can be arranged in a pseudo differential fashion.

Other amplifiers and another number are also possible. The noise can be reduced, which is generated by a combination of mixing unit and IF amplifier. The generation of noise by more than one IF amplifier due to the fact that more than one switching unit is conductive can be avoided, since merely one switching unit is conductive. Furthermore, an LNA can be omitted.

The amplified signals can be converted to digital signals using analogue/digital converters for further digital processing.

The apparatus may be implemented in CMOS technology and/or bipolar technology and/or BiCMOS technology and/or GaAs and/or discrete device. For instance, the apparatus may be implemented in 90-nm CMOS technology. Such a technology may encompass the advantage that transistors may comprise a low on-resistance. Furthermore, the required space of the apparatus can be low as well as the power consumption of the apparatus may be small. Another aspect of the present application is a receiver comprising an apparatus mentioned above.

The apparatus according to the present application can be used in a broad application field. In particular, any receiver may comprise an apparatus according to the present application. By way of example, multi-band, -mode, -standard radio receivers, software-defined radio (SDR), radio receivers for broadcast, radio receivers for cellular

(GSM, EDGE, UMTS, 4G), connectivity applications (WPAN, Bluetooth, WLAN) and the like can comprise an apparatus according to the present application.

Another aspect of the present application is a method comprising receiving an input signal. The method comprises frequency down converting the received input signal by a number of switching units. The method includes generating at least one local oscillator signal and driving the number of switching units such that a maximum of one of the switching units is conductive at a certain time.

These and other aspects of the present patent application become apparent from and will be elucidated with reference to the following figures. The features of the present application and of its exemplary embodiments as presented above are understood to be disclosed also in all possible combinations with each other.

BRIEF DESCRIPTION OF THE DRAWINGS

In the Figures show: Fig. 1 a first embodiment of the apparatus according to the present application;

Fig. 2 an embodiment of a circuit of a single-ended mixing unit;

Fig. 3 an exemplified local oscillator signal;

Fig. 4 an embodiment of a switching unit; Fig. 5 a simplified linear time-invariant IF model of a single-ended switching mixing unit;

Fig. 6 a second embodiment of the apparatus according to the present application;

Fig. 7 an exemplified diagram of the required single-ended input resistance of an amplifying unit to obtain a power match at the RF-input as a function of the duty cycle of the local oscillator signal;

Fig. 8 an exemplified diagram of the voltage gain from the single-ended RF input to the differential IF output;

Fig. 9 an exemplified diagram of the double-side band noise Figure as function of the IF output frequency;

Fig. 10 an exemplified diagram of the absolute value of the input reflection coefficient.

Like reference numerals in different Figures indicate like elements.

DETAILED DESCRIPTION OF THE DRAWINGS

In the following detailed description of the present application, embodiments of the present application will describe and point out an apparatus comprising IF filter capabilities wherein the apparatus is low noise, reduces power consumption and can be easily implemented. It will be also shown that an LNA may be omitted. Fig. 1 shows a simplified first embodiment of an apparatus according to the present application. A radio signal having a high frequency can be received via an arranged antenna element 4. Any suitable radiating or receiving element, like dipoles, open- waveguides or similar elements can be employed as the antenna element 4. Furthermore, more than one antenna element 4 can be arranged. The antenna element 4 communicates with a filtering unit 2, in particular a band-pass filtering unit. Suitable band-pass filtering units can be used which provide for suppressing unwanted frequency bands. Merely, the frequency band of interest may pass the filtering unit 2 for further processing.

As can be seen from Fig. 1, the filtering unit 2 is connected to a mixing unit 6. The mixing unit 6 comprises several units. More particularly, the mixing unit 6 has four signal paths which inputs are connected in parallel. Each of which comprises one switching unit 8a to 8d and additionally one holding element 10a to 1Od, wherein the holding element 10a to 1Od is arranged at the output of the switching unit 8a to 8d.

The switching units 8a to 8d, for instance suitable transistors, are supplied with local oscillator signals LO, wherein each of the switching units 8a to 8d is supplied with a 90° phase shifted signal LOo, LO90, LOiso and LO270. For lucidity reasons, a local oscillator and required phase shifting units are not shown in Fig. 1. Several implementation possibilities for these components exist. The shown switching units 8a to 8d are configured to down convert the signal filtered by the filtering unit 2. As a local oscillator frequency, any suitable frequency can be created by the local oscillator for down converting a radio frequency to a desired intermediate frequency.

In addition, the holding element 10a to 1Od may be arranged for filtering the signal applied at the output of each switching unit 8a to 8d. By way of example, the holding elements may be provided for low-pass filtering. Holding elements 10a to 1Od may be inductive or capacitive elements or combinations thereof. However, other possibilities for filtering can be also used.

The obtained signals are fed to two arranged amplifying units 12a and 12b, for instance differential IF amplifiers, after passing the filtering units 10a to 1Od, according to the shown embodiment. The amplifying units 12a, 12b are configured to amplify the down converted signal to an appropriate power for further processing. Each amplifying unit 12a and 12b includes two outputs, wherein amplifying unit 12a may comprise the inphase (I) components 1+ and I- meanwhile amplifying unit 12b may comprise the quadrature (Q) components Q+ and Q-. Such an amplifying unit 12a, 12b may comprise transistors and further components suitable for amplification. According to the shown embodiment of the present application, an LNA can be omitted. The noise generated by the apparatus is low, since a maximum of one of the switching units is conductive at a certain time during operation and since their on-resistance is low.

In Fig. 2, an embodiment of a circuit of a single-ended mixing unit is depicted. As can be seen from Fig. 2, a signal voltage source 14 and a source impedance 18 are arranged. These components 14, 18 may represent the antenna element 4 and the filtering unit 2 known from Fig. 1 in a suitable manner.

In series to source impedance 18, one switching unit 8 is arranged. The switching unit 8 is driven by a local oscillator signal LO. In turn, at the output of the switching unit 8, a capacitive element 16, like a capacitor, and a resistance 20 are arranged. Both elements 16, 20 are grounded according to the shown embodiment. The capacitive element 16 may serve as a holding element 10a to 1Od shown in Fig. 1 for low-pass filtering. Additionally, the resistance 20 may represent the single-ended input impedance of an amplifying unit 12a, 12b. Furthermore, Fig. 4 shows a simplified embodiment of a switching unit according to the present application. The depicted switching unit is a transistor implemented in CMOS technology, in particular an NMOS transistor. The shown embodiment comprises two input terminals 22, 24 and one output terminal 26. Input terminal 22 may be the drain terminal, input terminal 24 may be the gate terminal and output terminal 26 may be the source terminal. At input terminal 22, the signal received by an antenna element 4 and filtered by filtering unit 2 may be applied. Meanwhile, at input terminal 24 the local oscillator signal LO can be applied. The output terminal 26 may be connected to a holding element 10a to 1Od. However, according to further variants of the present application, other transistors, for instance implemented in another technology, can be used. Yet another possibility is to interchange the drain terminal 22 with the source terminal 26.

In the following, the functioning of the shown embodiment is elucidated. At input terminal 22, the signal being down converted is applied. The present transistor may be conductive, if a voltage is supplied at the input terminal 24, the gate terminal. The transistor is supplied with a local oscillator signal LO having two levels. In case the level of the local oscillator signal LO is equal to zero, the transistor is blocked. For the other case, the transistor is conductive. It is found, according to the present application that transistors comprising a low on-resistance are especially suitable due to their low noise specifications. For instance, the on-resistance may be less than 100 Ohm, preferably less than 10 Ohm. In particular, the on-resistance may be small compared to the magnitude of the source impedance 18. The use of such switching units 8a to 8d comprising the properties stated above may cause that an LNA can be omitted. Thus, power consumption can be significantly reduced. It should be understood that, according to further variants of the present application, several transistor implementations can be used for the case, the noise generated by the transistor is low. What is more, Fig. 3 shows an exemplified local oscillator signal LO. The depicted signal comprises duty cycle dc and a period T_Lo- Starting from the embodiment shown in Fig. 2 and the local oscillator signal LO shown in Fig. 3, the source voltage Vs represented by signal source 14 may be expressed by equation. V _s = V_s cos((nω_LO + Aω)t + φ₀) ,

wherein V_s represents the amplitude of the RF source voltage Vs, CO₁₀ represents the fundamental LO radian frequency, n represents the LO-harmonic number, Δω represents the frequency difference between the RF frequency and the frequency of the n^th harmonic of the LO signal, t represents the time andφ₀ represents an initial phase.

Furthermore, the output voltage Vo of the circuit shown in Fig. 2 can be easily determined. For the case the capacitive element 16, for instance capacitor C_L, is set that large that the RF voltage and the LO voltage ripple across this capacitor may be negligible, the capacitor C_L can be disregarded resulting in the following equation for the output voltage

,

wherein Rs represents resistance 18 and R_L represents resistance 20. As can be seen from both equations stated above, the gain of the circuit, in particular the conversion gain is given by the factor

According to the present application, it is found that the apparatus, in particular the mixing unit 6, may be low-noise in case a maximum of one of the switching units 8a to 8d is conductive during operation. In this case, only one on-resistance mentioned above may generate noise. One possibility to drive the switching units such that at most one transistor is conductive is to define a suitable duty cycle dc. Thereby, the duty cycle dc of the local oscillator signal driving one switching unit 8a to 8d should not exceed 25%, for the case four switching units 8a to 8d are employed. Generally, the duty cycle dc should be dc ≤ \/M , where M represents the number of switching units. This provides for that a maximum of one of the switching units 8a to 8d is conductive at a certain time and thus, the apparatus and the receiver respectively is low-noise. By way of example, resistance R_L can be set to infinity and the duty cycle dc of a single-ended switching unit to 25%. Following values for conversion gain depicted in table 1 for the respective LO harmonics can be obtained.

Table 1

The values of the conversion transfer obtained for even values will vanish considering the differential component of the IF signal.

For obtaining a more simple time-invariant IF model of an apparatus according to the present application, the IF source voltage or in other words the unloaded IF output voltage can be determined as follows

,. ₇, F, sin(π« dc) . . _x

V_{S IF} = lim V₀ = — -cos(Aωt+φ₀) .

*£→^∞ πndc

Furthermore, the output resistance Ro,mix depends on the duty cycle dc and the

D source resistance Rs. More particularly, it can be shown that R_{0 mix} = — . From these dc equations, the circuit shown in Fig. 5 can be derived. The depicted simplified linear time- invariant IF model of a mixing unit 6 includes an equivalent source voltage 19 and a

D resistance 21, wherein source voltage 19 may be Vs _IF and resistance 21 may be R₀ = — . dc The RF input impedance Z₁(Vi) for an RF frequency equal to that of the n th LO harmonic of the shown mixing unit comprising M parallel switching units 8 can be derived resulting in

As can be seen from equation stated above, impedance Z^n) depends from the source resistance Rs. This is quite remarkable and unusual. By way of example, table 2 shows exemplified values of impedance Z^n) for M=4, dc = 25%, Rs =50 Ohm and R_L =322 Ohm. It should be understood that, according to further variants of the present application, other values for the parameters stated above can be set.

Table 2

An impedance matching at the RF input of the mixing unit 6 may be desirable according to an embodiment of the present application, especially for obtaining a power match. For such an impedance matching, Z₁ can be set equal to Rs. Thus, the above stated equation can be converted to obtain the desired value of R_L depending on Rs. The following equation can be obtained

At this point, one special case should be examined. Resistance R_L may be equal to 0 Ohm, in case the denominator is approximately infinite. Thus, the duty cycle dc must be equal to the factor 1/(2M). For instance, if M is equal four, the duty cycle has to be 12,5%. Such amplifiers comprising this input resistance R_L may be called transimpedance amplifiers (TIA). According to other variants of the application, current amplifiers can also be used.

In Fig. 7, an exemplified diagram of the required single-ended input resistance R_L of an amplifying unit is shown as a function of the duty cycle dc of the local oscillator signal. Thereby, parameter M is set equal to four, n is set equal to one and the value of Rs is set equal to 50 Ohm. Furthermore, impedance Z₁ is equal to resistance Rs. As can be seen from Fig. 7, an increasing duty cycle dc requires an increased value of resistance R_L in order to obtain impedance matching at the RF input. The maximum value of the duty cycle may be 25% due to the use of four switching units and the requirement that at most one of the switching units is conductive at a certain time. According to the above stated value, the maximum value for resistance R_L is given by R_L ~ 322 Ohm. However, in the Fig. 7 shown is merely an exemplified graph. Other values of resistance R_L are possible.

Fig. 6 shows a second simplified embodiment of an apparatus according to the present application. For avoiding repetition, known components are not elucidated once again in depth. Shown apparatus comprises four capacitive elements 28a to 28d, in particular capacitors which may act as a holding element 10a to 1Od. Each of the four capacitive elements 28a to 28d may comprise a connection to an output of a switching unit 8a to 8d, wherein the other terminal of a capacitive element 28a to 28d may be grounded. The capacitive element 28a to 28d may serve as an IF filter, in particular as a low-pass filter. The value of such a capacitive element 28a to 28d can be in the range of 10 pF. It should be understood that, according to other variants of the present application, further holding elements can also be employed as well as other suitable components and capacitors comprising different values. However, a capacitive element 28a to 28d can be used advantageously in an integrated circuit. Such a solution may be cost-effective.

The signal filtered by a capacitive element 28a to 28d may be fed to a single- ended IF amplifier 30a to 3Od. These amplifiers 30a to 3Od may be implemented in a pseudo- differential fashion. Such a single-ended amplifier 30a to 30d may comprise a transistor circuit which may include two or more transistors, for instance one N and one P transistor. Furthermore, the transistor circuit may comprise a feed back path, wherein the feed back path may connect an output of the transistor circuit via a parallel connection of a capacitor and resistor to an input of the transistor circuit. Besides, a second feedback loop being a resistive series feedback loop may be advantageous. By way of example, a dual- loop amplifier can be used to obtain a well-defined input impedance as required by equation

which is graphically represented in Fig. 7 for one particular case.

It should be understood that other amplifiers can be used in a suitable manner according to further embodiments.

Fig. 8 shows an exemplified diagram of the voltage gain from the single-ended RF input to the differential IF output. The depicted curve shows a qualitative run of the voltage gain depending on the IF output frequency. For low frequencies, the curve is constant. It starts to fall from a certain cut-off frequency. For instance, the cut-off frequency may be 400 kHz. For obtaining such a cut-off frequency, the IF amplifier may comprise a feedback loop including a resistance connected in parallel to a capacitor. However, according to other variants of the present application, other components can be employed to obtain a desired cut-off frequency.

In Fig. 9, an exemplified diagram of a double-side band noise Figure as a function of the IF output frequency according to an apparatus of the present application is shown. For low frequencies, the curve falls down until small noise amounts. For instance, values until 1.5 dB can be achieved. Such a value is significantly low, in particular when comparing to receivers of prior art. The cut-off frequency according to the shown embodiment may be 6.1 kHz. After a constant intercept, the curve increases for high frequency values. Thus, the frequency ranges of interest shows significant low noise. However, other values of a cut-off frequency can also be determined according to other variants of the present application. More particularly, it is found according to the present application that the noise contribution apart from the antenna element 4 and filtering unit 2 may comprise a dominating part. The dominating part may be thermal noise generated by the on-resistance of the transistors used as switching units 8a to 8d. Due to the use of transistors comprising low on-resistance and the fact that at most one transistor is conductive at a certain time, the whole apparatus may comprise low noise, especially the mixing unit 6. Another part having less portion of the noise contribution may be the induced gate noise created by input transistors of the IF amplifier. Further thermal noise may also be generated by the resistors arranged within the feedback path of an IF amplifier unit.

What is more, Fig. 10 shows an exemplified diagram of the absolute value of the input reflection coefficient. This parameter points out the improved quality of a receiver according to the present application as well. The values of interest according to the graph shown in Fig. 10 are the range around the local oscillator frequency LO. By way of example, the local oscillator frequency LO may comprise a value of 2 GHz. As can be seen from Fig. 10, the absolute value of the input reflection coefficient is around the local oscillator frequency LO below -10 dB which points out a good impedance matching and thus, the feasibility of the receiver according to the present application. In addition, it is pointed out by the present graph that the apparently IF selectivity may translate to the RF selectivity via the arranged mixing unit. The mixing unit may be regarded as being transparent. An RF bandpass filter, in particular a narrow RF band-pass filter may be automatically formed by the apparatus according to the present application. Interfering signals which are out of channel can be easily suppressed and automatically tracking with the local oscillator frequency is achieved.

Furthermore, it is readily clear for a person skilled in the art that the logical blocks in the schematic block diagrams as well as the flowchart and algorithm steps presented in the above description may at least partially be implemented in electronic hardware and/or computer software, wherein it depends on the functionality of the logical block, flowchart step and algorithm step and on design constraints imposed on the respective devices to which degree a logical block, a flowchart step or algorithm step is implemented in hardware or software. The presented logical blocks, flowchart steps and algorithm steps may for instance be implemented in one or more digital signal processors, application-specific integrated circuits, field programmable gate arrays or other programmable devices. The computer software may be stored in a variety of storage media of electric, magnetic, electromagnetic or optic type and may be read and executed by a processor, such as for instance a microprocessor. To this end, the processor and the storage medium may be coupled to interchange information, or the storage medium may be included in the processor.

Claims

CLAIMS:

1. An apparatus, comprising: a mixing unit configured to frequency down convert an input signal, wherein the mixing unit comprises a number of switching units, and a generating unit configured to generate at least one local oscillator signal, - wherein the generating unit is configured to drive the number of switching units such that a maximum of one of the switching units is conductive at a certain time.

2. The apparatus according to claim 1, wherein the number of switching units is at least four.

3. The apparatus according to claim 1, wherein the switching units comprise RF- inputs which are connected in parallel.

4. The apparatus according to claim 1, further comprising: - a filtering unit connected to the mixing unit, wherein the filtering unit is configured to filter the input signal received via an antenna element.

5. The apparatus according to claim 1, further comprising: - at least one holding element, wherein the holding element is arranged at the output of at least one switching unit.

6. The apparatus according to claim 5, wherein the holding element is configured to filter the output signal of the switching unit.

7. The apparatus according to claim 5, wherein the holding element comprises at least one capacitive element.

8. The apparatus according to claim 1, wherein the switching unit comprises at least one transistor.

9. The apparatus according to claim 1, wherein the local oscillator signal comprises: a duty cycle, wherein the duty cycle is determined such that a maximum of one of the switching units is conductive at a certain time.

10. The apparatus according to claim 1, further comprising: at least one phase shifting unit connected to the local oscillator, wherein the phase shifting unit is configured to drive each switching unit with a local oscillator signal comprising a phase shift depending on the ratio of 360 degree and the number of switching units.

11. The apparatus according to claim 1 , wherein at least one switching unit comprises an on-resistance during its conductive state, wherein the on-resistance is smaller than a magnitude of a source impedance, preferably at least 10 times smaller.

12. The apparatus according to claim 1, further comprising: at least two amplifying units, wherein the amplifying units are connected via at least one holding element to the switching unit, respectively.

13. The apparatus according to claim 12, wherein at least one amplifying unit is a differential amplifier.

14. The apparatus according to claim 1, wherein the apparatus is implemented in at least one of:

A) CMOS technology,

B) bipolar technology,

C) BiCMOS technology, D) GaAs,

E) discrete device.

15. A receiver comprising an apparatus according to claim 1.

16. A method, comprising : receiving an input signal, frequency down converting the received input signal by a number of switching units, - generating at least one local oscillator signal, and driving the number of switching units such that a maximum of one of the switching units is conductive at a certain time.