This invention generally relates to communication systems, and more specifically to using transferred-impedance filtering in RF receivers.
The RF receivers must tolerate high blocking signals while maintaining their own performance. This requires filtering for RF-signals prior to a LNA (low noise amplifier) and in many systems also after the LNA. This is especially true in code division multiple access systems (e.g., CDMA2000 and WCDMA) where a transmitter usually sends its high-level signal while a receiver receives a very low-level signal.
At the present time, filtering is done mainly with SAW (surface acoustic wave) or BAW (bulk acoustic wave) filters or resonators. These components are expensive, impossible to integrate with a standard CMOS or BiCMOS process and also require large areas of PWBs (printed wiring boards). Such filters also decrease the possibility for modularity and also increase the number of I/O's (inputs/outputs) in RFIC's (radio frequency integrated circuits) thus increasing their complexity.
The object of the present invention is to provide a novel method for using transferred-impedance filtering in RF (radio frequency) receivers, wherein said filtering can be done with MOS-switches transferring impedance of a regular RC or RCL circuit to RF frequency filtering inside an RFIC (radio frequency integrated circuit).
According to a first aspect of the invention, a method for transferred-impedance filtering in a receiver, comprises the steps of: receiving a radio frequency signal and converting it to an electrical domain; amplifying the radio frequency signal in the electrical domain using an amplifier containing a resistance R, thus generating an amplified RF signal; and filtering the amplified RF signal using a transferred-impedance filter containing at least one capacitor C and having a pass band with a center frequency indicated by a reference frequency, wherein −3 dB frequencies of the pass band are given by the reference frequency plus a corner frequency which depends on the resistor R and the at least one capacitor C and by the reference frequency minus a further corner frequency which also depends on the resistor R and the at least one capacitor C.
According further to the first aspect of the invention, the corner frequency and the further corner frequency may be equal and may be given by FRC=1/π2RC. Further, the transferred-impedance filter may also perform a down conversion mixing function such that a low frequency baseband signal may be an output signal of the transferred-impedance filter.
Further according to the first aspect of the invention, the reference frequency may be a local oscillator frequency FLO provided to the transferred-impedance filter.
Still further according to the first aspect of the invention, the filtering may be performed using two transferred-impedance filters in inphase and quadrature branches, respectively, wherein each of two local oscillator signals having the frequency FLO but π/2 apart in a phase domain may be provided to only one of the two transferred-impedance filters.
According further to the first aspect of the invention, the parasitic capacitances of the transferred-impedance filter are compensated by an inductor in the amplifier such that an absolute value of a reactive component of an amplifier output impedance (for the amplified RF signal) is close to zero and negligible compared to a resistive component of said output impedance.
According still further to the first aspect of the invention, the at least one inductor L may be added in series with the at least one capacitor C and the reference frequency may be given by FLO−FLC or FLO+FLC, the FLO being a local oscillator frequency provided to the transferred-impedance filter and the FLC being an LC resonant frequency given by FLC=½π√{square root over (LC)}.
According further still to the first aspect of the invention, the at least one inductor L may be added in parallel with the at least one capacitor C and the reference frequency may be given by FLO−FLC or FLO+FLC, the FLO being a local oscillator frequency provided to the transferred-impedance filter and the FLC being an LC resonant frequency given by FLC=½π√{square root over (LC)}. Still further, the corner frequency and the further corner frequency further may depend on the at least one inductor L.
According yet further still to the first aspect of the invention, the receiver may be a part of a mobile terminal, mobile phone or a mobile communication device.
Yet still further according to the first aspect of the invention, the receiver may be a radio frequency (RF) receiver.
According to a second aspect of the invention, a receiver for transferred-impedance filtering, comprises: an antenna, for receiving a radio frequency signal and converting it to an electrical domain; an amplifier containing a resistance R, for amplifying the radio frequency signal in the electrical domain, thus generating an amplified RF signal; and at least one transferred-impedance filter, for filtering the amplified RF signal, the transferred-impedance filter containing at least one capacitor C and having a pass band with a center frequency indicated by a reference frequency, wherein −3 dB frequencies of the pass band are given by the reference frequency plus a corner frequency which depends on the resistor R and the at least one capacitor C and by the reference frequency minus a further corner frequency which also depends on the resistor R and the at least one capacitor C.
According further to the second aspect of the invention, the parasitic capacitances of the transferred-impedance filter are compensated by an inductor in the amplifier such that an absolute value of a reactive component of an amplifier output impedance (for the amplified RF signal) is close to zero and negligible compared to a resistive component of said output impedance.
Further according to the second aspect of the invention, the reference frequency may be a local oscillator frequency FLO provided to the transferred-impedance filter.
Further still according to the second aspect of the invention, the transferred-impedance filter may also perform a down conversion mixing function such that a low frequency baseband signal may be an output signal of the transferred-impedance filter.
According further to the second aspect of the invention, the parasitic capacitances of the amplifier may be compensated by an inductor such that an absolute value of a reactive component of the amplified RF signal may be close to zero and negligible compared to a resistive component of the amplified RF signal.
According still further to the second aspect of the invention, the at least one inductor L may be added in series with the at least one capacitor C and the reference frequency may be given by FLO−FLC or FLO+FLC, the FLO being a local oscillator frequency provided to the transferred-impedance filter and the FLC being an LC resonant frequency given by FLC=1/π2LC.
According further still to the second aspect of the invention, the at least one inductor L may be added in parallel with the at least one capacitor C and the reference frequency may be given by FLO−FLC or FLO+FLC, the FLO being a local oscillator frequency provided to the transferred-impedance filter and the FLC being an LC resonant frequency given by FLC=½π√{square root over (LC)}. Further, the corner frequency and the further corner frequency further may depend on the at least one inductor L.
According yet further still to the second aspect of the invention, the receiver may be a part of a mobile terminal, mobile phone or a mobile communication device.
Yet still further according to the second aspect of the invention, the receiver may be a radio frequency (RF) receiver.
According to a third aspect of the invention, an a communication device, comprises: a receiver, for transferred-impedance filtering, the receiver comprises: an antenna, for receiving a radio frequency signal and converting it to an electrical domain; an amplifier containing a resistance R, for amplifying the radio frequency signal in the electrical domain, thus generating an amplified RF signal; and at least one transferred-impedance filter, for filtering the amplified RF signal, the transferred-impedance filter containing at least one capacitor C and having a pass band with a center frequency indicated by a reference frequency, wherein ba-3 dB frequencies of the pass band are given by the reference frequency plus a corner frequency which depends on the resistor R and the at least one capacitor C and by the reference frequency minus a further corner frequency which also depends on the resistor R and the at least one capacitor C.
The advantages of the present invention include (but are not limited to):
For a better understanding of the nature and objects of the present invention, reference is made to the following detailed description taken in conjunction with the following drawings, in which:
a and 2b are block diagrams of a front end of an RF receiver, according to the present invention;
a and 3b are block diagrams of a front end of an RF receiver showing inphase and quadrature branches, according to the present invention;
a, 5b and 5c are simplified schematics of a transferred-impedance filter, according to the present invention.
The present invention provides a method for using transferred-impedance filtering in RF (radio frequency) receivers (e.g., inside of a mobile communication deice), wherein said filtering can be done with MOS-switches transferring impedance of a regular RC or RCL circuit to RF frequency filtering inside an RFIC (radio frequency integrated circuit).
a and 2b show examples among others of block diagrams of a front end of RF receivers 10a and 10b, respectively, according to the present invention. Compared to the prior art processing shown in
a corresponds to a case where the low noise amplifier (or in general just an amplifier) 14 is connected in parallel with the transferred-impedance filter 20 using an amplified RF signal 22, and the output RF signal 24 is provided to the mixer 18 for a normal further processing. In this scenario a LO (local oscillator) signal 34 with a frequency FLO can be provided to both the transferred-impedance filter 20 and to the mixer 18 by a local oscillator 30.
b demonstrates a further improvement of the present invention wherein the transferred-impedance filter 20 fulfills a function of the mixer 18 and is shown as a filter-mixer module 20a, so its output signal 26 is the same as the output signal of the mixer 18. A more detailed description for implementing blocks 14, 20 and 20a presented in
a and 3b show examples among others of block diagrams of front end receivers 10a and 10b, respectively, showing inphase and quadrature branches, according to the present invention. In
Furthermore, mixers 28i and 28q, in response to output RF signals 24i and 24q, provide output signals 26i and 26q to analog-to-digital converters 28i and 28q, respectively, for further processing.
a, 5b and 5c are examples among others of a simplified schematic of the filter-mixer module 20a (or similarly of modules 20a–i and 20a–q), according to the present invention utilizing MOSFETs (metal-oxide-semiconductor field-effect transistors) 44.
In one embodiment of the present invention shown in
If the incoming RF signal (e.g., amplified RF signals 22, 22i or 22q) differs from the frequency of the LO signal 34, then the capacitors C 42 will be charged with a signal which frequency is the difference of the RF and LO signals. The driving impedance is the impedance of the LNA output, which is the resistor R 38. Therefore we get impedance filtering at the frequency FLO+FRC, where FLO is the LO-signal frequency and FRC is the corner frequency of the resistor R 38 and the capacitor C 42 (i.e., ½πRC).
This means that we get a band pass filter at the LNA 14 output with pass band corner frequencies (also called −3 dB frequencies or half-power frequencies) FLO+FRC and FLO−FRC, respectively. This band pass filter then follows the LO-signal and there is enough attenuation for adjacent channels, blockers and for a transmitter (connected to the antenna 11 but not shown in
The shape of this filter is also very steep, since the attenuation increases as a function of the RC constant corresponding to low frequencies. This is easier to explain with an example. If the LO frequency is 2 GHz and an RC time constant is equivalent to 2 MHz, then the signal of frequency 2.002 GHz attenuates 3 dB. If we had a standard RC−3 dB point at that frequency, 20 dm attenuation would be reached at the frequency of 20.002 GHz (i.e., one decade away). With the filter mixer module 20a(similarly for modules 20a–i or 20a–q), the 20 dB attenuation will be reached at 2.022 GHz (i.e., one decade away from the RC frequency 2 MHz). Thus the low frequency (defined by the RC constant) is transferred to the RF frequencies. This is a significant improvement over the possible prior art solutions.
It is noted that other impedances can be transferred to higher frequency filtering using the methodology described in the present invention. The capacitors 42 in
According to the present invention, as shown in
Moreover, according to the present invention, as shown in
Inductors 46 or 48 can be generated, e.g., from capacitors with operational amplifiers (which imitate inductors) or by making a second (or higher) order filter by generating an impedance with a magnitude degrading as a second order filter response thus providing a low area, high performance filter systems.
There are a lot of variations of the present invention. It is noted that, according to the present invention, NMOS switches, typically used in schematics presented in
It is to be understood that the above-described arrangements are only illustrative of the application of the principles of the present invention. Numerous modifications and alternative arrangements may be devised by those skilled in the art without departing from the scope of the present invention, and the appended claims are intended to cover such modifications and arrangements.
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