System and method for performing RF filtering

A method of filtering and a RF filtering circuit comprising a LO adapted to generate in-phase and quadrature LO signals; a quadrature passive mixer operatively connected to the LO; a filtering impedance operatively connected to the quadrature passive mixer, wherein the voltage at an input node of the quadrature passive mixer comprises the voltage across the filtering impedance up-converted to a frequency of a LO signal received by the quadrature passive mixer. Preferably, the voltage across the filtering impedance comprises a frequency of an input signal of the quadrature passive mixer down-converted by a frequency of the in-phase and quadrature LO signals and filtered by the filtering impedance.

BACKGROUND

1. Technical Field

The embodiments herein generally relate to radio frequency (RF) technologies, and, more particularly, to filtering undesirable RF signals in a RF network using filters.

2. Description of the Related Art

The spectrum input to RF devices typically includes a large number of undesired signals in addition to the desired band of interest. Such interferences can be very large, possibly causing intermodulation distortion, desensitization, cross-band modulation, and oscillator pulling, among other undesirable effects. Most typical RF receivers require a band-limiting filter at their input to eliminate or reduce such interferences. These filters typically require very high selectivity (that is, a very narrow passband relative to the filter center frequency). In certain wide-band applications, these filters must move to track the desired channel. Typically, such “tracking” filters must be very carefully tuned or they may unintentionally attenuate the desired signal.

There are generally two conventional approaches to RF filtering. In applications where tracking is not required, an off-chip resonator such as a surface acoustic wave (SAW) filter is employed. The benefit of these filters is excellent selectivity and accurate passband location. However, the disadvantages are twofold. First, these filters generally have approximately 2 dB loss in their passband. This translates to an additional 2 dB of noise figure (NF) and thus directly affects the minimum possible sensitivity of the system. Second, these filters invariably add significant cost to the bill of material (BOM) and generally increases the circuit board area. For tracking applications, a tuning element such as a p-type intrinsic, n-type diode (PIN diode) is used to tune the resonance of a tank or similar resonant circuit. While this approach manages to provide a tunable filtering, it generally suffers from poor stopband attenuation and less passband frequency accuracy than SAW filters. Furthermore, these filters are off-chip, and again impact BOM costs. Even in the case of a tracking filter or an active notch filter, factory calibration/tuning is generally required (i.e., leading to more cost and complexity of implementation).

Accordingly, there remains a need for a high Q factor (high-Q) filter which does not require calibration and which can track the local oscillator (LO) signal and achieves large stopband attenuation.

SUMMARY

In view of the foregoing, an embodiment herein provides a RF filtering circuit comprising a LO adapted to generate in-phase and quadrature LO signals; a quadrature passive mixer operatively connected to the LO; a filtering impedance operatively connected to the quadrature passive mixer, wherein the voltage at an input node of the quadrature passive mixer comprises the voltage across the filtering impedance up-converted to a frequency of a LO signal received by the quadrature passive mixer. Preferably, the voltage across the filtering impedance comprises a frequency of an input signal of the quadrature passive mixer down-converted by a frequency of the in-phase and quadrature LO signals and filtered by the filtering impedance. Moreover, the filtering impedance preferably comprises a resistor in parallel with a capacitor. Furthermore, the filtering impedance preferably comprises a first component comprising a first resistor in parallel with a first capacitor; and a second component comprising an active impedance, wherein the first component is in parallel with the second component.

The quadrature passive mixer may comprise a plurality of metal oxide semiconductor field effect transistor (MOSFET) switches driven by the in-phase and quadrature LO signals, wherein each of the MOSFET switches are preferably connected in parallel to one another, wherein each of the MOSFET switches comprises a gate, a drain, and a source, wherein the drain of each of the MOSFET switches are operatively tied to one another for receiving a RF signal, wherein the source of each of the MOSFET switches are operatively connected to a respective the filtering impedance, and wherein the gate of each of the MOSFET switches are operatively connected to the LO for receiving the a LO signal for turning on a respective MOSFET switch.

Another embodiment provides a wireless network system comprising an antenna; a LO adapted to generate in-phase and quadrature LO signals; a quadrature passive mixer operatively connected to each of the antenna and the LO; and a filtering impedance operatively connected to the quadrature passive mixer, wherein the voltage at an input node of the quadrature passive mixer comprises the voltage across the filtering impedance up-converted to a frequency of a LO signal received by the quadrature passive mixer. Preferably, the voltage across the filtering impedance comprises a frequency of an input signal of the quadrature passive mixer down-converted by a frequency of the in-phase and quadrature LO signals and filtered by the filtering impedance. Furthermore, the filtering impedance preferably comprises a resistor in parallel with a capacitor. Additionally, the filtering impedance preferably comprises a first component comprising a first resistor in parallel with a first capacitor; and a second component comprising an active impedance, wherein the first component is in parallel with the second component. The quadrature passive mixer may comprise a plurality of MOSFET switches driven by the in-phase and quadrature LO signals.

Preferably each of the MOSFET switches are connected in parallel to one another, wherein each of the MOSFET switches comprises a gate, a drain, and a source, wherein the drain of each of the MOSFET switches are operatively tied to one another for receiving a RF signal, wherein the source of each of the MOSFET switches are operatively connected to a respective the filtering impedance, and wherein the gate of each of the MOSFET switches are operatively connected to the LO for receiving the a LO signal for turning on a respective MOSFET switch. The wireless network system may further comprise a pair of low noise amplifiers (LNAs) connected to the filtering impedance, wherein the pair of LNAs preferably comprise an in-phase channel low intermediate frequency (IF) LNA and a quadrature channel IF LNA.

Another embodiment provides a method of filtering signals in a RF wireless network, wherein the method comprises transmitting a RF signal; generating in-phase and quadrature LO signals; providing a quadrature passive mixer adapted to receive the RF signal and the LO signals; and operatively connecting a filtering impedance to the quadrature passive mixer, wherein the voltage at an input node of the quadrature passive mixer comprises the voltage across the filtering impedance up-converted to a frequency of a LO signal received by the quadrature passive mixer. Preferably, the voltage across the filtering impedance comprises a frequency of an input signal of the quadrature passive mixer down-converted by a frequency of the in-phase and quadrature LO signals and filtered by the filtering impedance.

The method may further comprise configuring the filtering impedance to filter the RF signal, wherein the filtering impedance is configured to comprise a resistor in parallel with a capacitor. Additionally, the method may further comprise configuring the filtering impedance to filter the RF signal, wherein the filtering impedance is configured to comprise a first component comprising a first resistor in parallel with a first capacitor; and a second component comprising an active impedance, wherein the first component is in parallel with the second component. Moreover, the method may further comprise configuring the quadrature passive mixer to comprise a plurality of MOSFET switches driven by the in-phase and quadrature LO signals. Additionally, the method may further comprise configuring each of the MOSFET switches to be connected in parallel to one another, wherein each of the MOSFET switches is configured to comprise a gate, a drain, and a source, wherein the drain of each of the MOSFET switches are operatively tied to one another for receiving a RF signal, wherein the source of each of the MOSFET switches are operatively connected to a respective the filtering impedance, and wherein the gate of each of the MOSFET switches are operatively connected to the LO for receiving the LO signal for turning on a respective MOSFET switch.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

As mentioned, there remains a need for a high-Q filter which does not require calibration and which can track the LO signal and achieves large stopband attenuation. The embodiments herein achieve this by providing a system and method of performing high level RF filtering by eliminating the need for a SAW filter, which improves the overall NF level (a 2 dB improvement in the NF level) and requires fewer external components, thus reducing BOM costs. Referring now to the drawings, and more particularly toFIGS. 1 through 8, where similar reference characters denote corresponding features consistently throughout the figures, there are shown preferred embodiments.

FIG. 1illustrates a system diagram of wireless system comprising an antenna100adapted to receive and transmit signals to a matching network105. In one embodiment, the matching network105may be formed on an integrated circuit chip. The matching network105typically transforms the LNA input impedance108(modeled as a resistor110and capacitor120) to match the impedance of the antenna100.

FIG. 2illustrates a filtering circuit in accordance with an embodiment herein, which comprises a quadrature passive mixer200in series with a filtering impedance220. This circuit can be integrated on a chip, and in the preferred embodiment is placed as part of the matching network105to the antenna100ofFIG. 1. Here the RF input signal is taken as a current, represented by source230. The input current is mixed down (shifted down in frequency) by a pair of mixers200driven by quadrature LO signals210and211. Once the signal is mixed down, it is filtered and converted to a voltage by impedance220. Finally, the voltage at impedance220is mixed back up to RF by the mixers200, and defines the voltage at201. This entire sequence of events is equivalent to the input signal being filtered at RF by the impedance220upconverted and centered around the LO frequency.

The circuit provided by the embodiments herein takes advantage of a non-obvious property of current-mode passive mixers as depicted inFIG. 2: the voltage at node201is simply the voltage across the impedances220up-converted to the local oscillator frequency, ωLO. In a preferred embodiment, the frequency, ωLO, is set equal to the frequency of the channel desired to be received. The voltage across impedance220in this case is the frequency of the entire input signal230down-converted by the frequency of signals210and211and filtered by the impedances220.

FIG. 3illustrates an alternative embodiment, wherein an antenna300adapted to transmit RF signals to a quadrature passive mixer310, which is coupled to a filtering impedance mechanism320. The filtering impedance mechanism320is operatively connected to a plurality of LNAs330,340. The first LNA330comprises an in-phase (I) channel low intermediate frequency (IF) LNA, and the second LNA340comprises a quadrature (Q) channel IF LNA. Each of the LNAs330,340output amplified I and Q signals, respectively. The mixers310downconvert the signal received at the antenna300. The (current mode) mixer outputs are then simultaneously converted to voltage and filtered by the impedances320. The voltages are then applied to amplifiers330and340, which effectively act as low noise amplifiers at Baseband instead of RF.

The impedances220can be embodied as a simple RC filter comprising resistor400in parallel with capacitor410as shown inFIG. 4(A), or a more complicated, higher order impedance such as the one depicted inFIG. 4(B), which further includes the resistor and capacitor shown in series430.

The quadrature passive mixer200ofFIG. 2is preferably implemented as mechanism500inFIG. 5. InFIG. 5, four metal oxide semiconductor field effect transistor (MOSFET) switches510-513are driven by four quadrature LO phases. This occurs because the quadrature voltage waveforms are applied to the gates of the MOSFET switches510-513. The drains of the respective MOSFET switches510-513are tied together to node520, and the respective sources531-534of the MOSFET switches510-513are taken out to four identical load impedances220ofFIG. 2.

The quadrature waveforms driving the gates of the MOSFET switches510-513ofFIG. 5are represented inFIG. 6, with waveforms600-630corresponding with MOSFET switches510-513, respectively. As shown inFIG. 6, there are four non-overlapping phases with frequency ωLOand an amplitude sufficiently large enough to fully switch the MOSFET switches510-513on or off. These four non-overlapping phases shown inFIG. 6correspond with the four non-overlapping signals720ofFIG. 7and may be generated from overlapping quadrature signals700using a logic synthesizer710.

FIG. 8illustrates a flow diagram of a method of filtering signals in a RF wireless network according to an embodiment herein, wherein the method comprises transmitting (801) a RF signal; generating (803) in-phase and quadrature LO signals; providing (805) a quadrature passive mixer adapted to receive the RF signal and the LO signals; and operatively connecting (807) a filtering impedance to the quadrature passive mixer, wherein the voltage at an input node of the quadrature passive mixer comprises the voltage across the filtering impedance up-converted to a frequency of a LO signal received by the quadrature passive mixer. Preferably, the voltage across the filtering impedance comprises a frequency of an input signal of the quadrature passive mixer down-converted by a frequency of the in-phase and quadrature LO signals and filtered by the filtering impedance.

The method may further comprise configuring the filtering impedance to filter the RF signal, wherein the filtering impedance is configured to comprise a resistor in parallel with a capacitor. Additionally, the method may further comprise configuring the filtering impedance to filter the RF signal, wherein the filtering impedance is configured to comprise a first component comprising a first resistor in parallel with a first capacitor; and a second component comprising an active impedance, wherein the first component is in parallel with the second component. Moreover, the method may further comprise configuring the quadrature passive mixer to comprise a plurality of MOSFET switches driven by the in-phase and quadrature LO signals. Additionally, the method may further comprise configuring each of the MOSFET switches to be connected in parallel to one another, wherein each of the MOSFET switches is configured to comprise a gate, a drain, and a source, wherein the drain of each of the MOSFET switches are operatively tied to one another for receiving a RF signal, wherein the source of each of the MOSFET switches are operatively connected to a respective the filtering impedance, and wherein the gate of each of the MOSFET switches are operatively connected to the LO for receiving the LO signal for turning on a respective MOSFET switch.

The embodiments provide a method for performing filtering at RF frequencies (i.e., in the range of hundreds of MHz to hundreds of GHz). The embodiments can be used to eliminate off-chip SAW filters while allowing for channel selection. Elimination of the SAW filter has a significant design and performance impact: less external components (lower BOM costs) and 2 dB improvement in the NF. The embodiments herein are applicable to both static and tracking filters. They combine the accuracy and attenuation benefits of the SAW filter approach without degrading NF and allow for tuning.

Furthermore, the embodiments can be used anywhere frequency selectivity is needed at high frequencies. In one example, the embodiments herein may be used to replace an off-chip SAW filter as the band select filter in front of an RF receiver. The embodiments herein may be part of a larger receiver, but need not be. For example, the embodiments could be used as a stand alone circuit.

The embodiments herein are applicable to both static and tracking filters. Furthermore, as mentioned, the embodiments combine the accuracy and attenuation benefits of the SAW filter approach without degrading NF and allowing for tuning. Generally, in one embodiment a circuit is provided comprising a switch in series with a filtering network. The overall network can be integrated on chip, and in the preferred embodiment is placed as part of the matching network to an antenna in a wireless system.