The present invention relates generally to a system and method for wireless communications, and more particularly to a system and method for corner frequency compensation in a wireless receiver.
Analog baseband filters (ABF) are widely used in wireless receivers to help eliminate or attenuate unwanted out-of-band interferers/blockers, which may negatively impact the overall performance of the wireless receivers. In general, an ABF may pass desired signals that are in a band of interest while blocking or attenuating undesired signals that are outside the band of interest.
An ABF, e.g., a low-pass filter, a band-pass filter, a high-pass filter, and so forth, may be designed so that the band of interest lies within the ABF's pass band, while the ABF's stop band encompasses frequencies outside of the band of interest. For example, if the ABF is a low-pass filter, then the ABF's corner frequency fc) or cut-off frequency may be set so that it is above the band of interest. Thereby, the ABF will pass signals at frequencies lower than the corner frequency while attenuating signals at frequencies higher than the corner frequency.
a is a diagram illustrating a magnitude response 100 of an ABF. Magnitude response 100 displays a signal gain (output signal magnitude/input signal magnitude) of the ABF as a function of frequency. As shown in
of the magnitude response at DC, shown as span 105.
However, during the operation of the wireless receiver, filtering characteristics of the ABF may change. The changes may be due to factors such as variations in operating conditions of the wireless receiver, such as temperature, supply voltage, and so forth. The changes in the operating conditions of the wireless receiver may change the corner frequency of the ABF. Therefore, if the corner frequency is increased, then some out-of-band signals that were previously eliminated (or attenuated) may be allowed to pass, while if the corner frequency is decreased, the some of the desired signals may be eliminated (or attenuated). In either case, the performance of the wireless receiver is degraded.
b is a diagram illustrating a magnitude response 120 of an ABF wherein the corner frequency of the ABF has increased. The changed corner frequency is shown as fc and the original corner frequency is shown as fc. Since the corner frequency of the ABF has increased, the magnitude response of the ABF at the original corner frequency is higher (shown as span 125). Therefore, the magnitude response of the ABF at the original corner frequency is now greater than
times that of the magnitude response at DC.
c is a diagram illustrating a magnitude response 140 of an ABF wherein the corner frequency of the ABF has decreased. The changed corner frequency is shown as f″c and the original corner frequency is shown as fc. Since the corner frequency of the ABF has decreased, the magnitude response of the ABF at the original corner frequency is lower (shown as span 145). Therefore, the magnitude response of the ABF at the original corner frequency is now smaller than
times that of the magnitude response at DC.
A prior art technique used to determine a corner frequency of an ABF involves measuring a magnitude response of the ABF at DC and at a desired corner frequency. As discussed previously, if the desired corner frequency is substantially equal to the actual corner frequency of the ABF, then the magnitude response of the ABF at the desired corner frequency (GEC) is
of the magnitude response at DC. Therefore, if
is smaller than or equal to a pre-determined error margin, then the actual corner frequency is equal (or about equal) to the desired corner frequency. However, if
then the actual corner frequency is greater than the desired corner frequency. While, if
then the actual corner frequency is smaller than the desired corner frequency. In either case, the actual corner frequency may be adjusted by tuning resistive (R), capacitive (C), inductive (L), and/or other parameters of the ABF and the measurement repeated until the actual corner frequency of the ABF has been tuned or compensated to the desired corner frequency.
These and other problems are generally solved or circumvented, and technical advantages are generally achieved, by embodiments of a system and a method for corner frequency compensation in a wireless receiver.
In accordance with an embodiment, a method for computing a corner frequency of a filter is provided. The method includes injecting a signal into an input of the filter, measuring a phase response of the filter at the frequency, and computing a corner frequency of the filter using the phase response. The signal has value substantially only at a frequency outside of a passband of the filter.
In accordance with another embodiment, a method for adjusting a corner frequency of a filter is provided. The method includes computing the corner frequency of the filter, wherein the computing uses a measured phase response of the filter at a frequency outside of a passband of the filter, determining if the computed corner frequency is different from a desired corner frequency by more or less than a threshold. The method also includes adjusting parameters of the filter to alter the corner frequency, and repeating the computing and the determining if the computed corner frequency differs from the desired corner frequency by more than the threshold. The method further includes leaving the parameters of the filter unchanged if the computed corner frequency differs from the desired corner frequency by less than the threshold.
In accordance with another embodiment, a receiver is provided. The receiver includes a signal path coupled to a signal input and to a baseband unit, the signal path includes a mixer that demodulates a signal provided by the signal input and changes the signal to a baseband signal, a filter coupled to the mixer, an analog to digital converter, and a decimation filter coupled to the analog to digital converter. The filter attenuates undesired signals outside of a frequency band of interest, the analog to digital converter digitizes the filtered baseband signal, and the decimation filter reduces a number of samples produced by the analog to digital converter. The receiver also includes a signal generator coupled to the filter, a demodulator coupled to the decimation filter and to the signal generator, a value estimator coupled to the demodulator, and a processor coupled to the value estimator. The signal generator generates an out-of-band signal having substantially value only at a frequency outside of a passband of the filter, the demodulator demodulates the reduced digital sample stream produced by the decimation filter with the out-of-band signal, the value estimator extracts a direct current (DC) component from the demodulated digital sample stream and selects a value of the DC component. The processor computes a phase response of the filter from the value provided by the value estimator.
An advantage of an embodiment is that out-of-band signals are used in the compensation of the corner frequency. Since the out-of-band signals will be subsequently eliminated in the wireless receiver, the out-of-band signals do not negatively impact the performance of the wireless receiver by introducing any distortion to desired in-band signals.
A further advantage of an embodiment is that the phase response is used in the compensation of the corner frequency. The use of phase response may make the compensation less sensitive to noise, numerical error, and so forth, which may be present in the process of estimating the corner frequency. Therefore, the compensation of the corner frequency may be performed with greater accuracy.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the embodiments that follow may be better understood. Additional features and advantages of the embodiments will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiments disclosed may be readily utilized as a basis for modifying or designing other structures or processes for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.
For a more complete understanding of the embodiments, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:
a is a plot illustrating a magnitude response of a filter;
b is a plot illustrating a magnitude response of a filter wherein the corner frequency of the filter has increased;
c is a plot illustrating a magnitude response of a filter wherein the corner frequency of the filter has decreased;
a is a diagram of a high-level view of a portion of a wireless transceiver;
b is a diagram of a portion of a receiver;
c is a diagram of a demodulation unit and a direct current (DC) estimation unit;
a is a flow diagram of a computing of a corner frequency of a filter using out-of-band signals;
b is a flow diagram of a computing of a corner frequency of a filter using out-of-band signals, wherein a receiver containing the filter has components with non-zero phase shifts; and
The making and using of the embodiments are discussed in detail below. It should be appreciated, however, that the present invention provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use the invention, and do not limit the scope of the invention.
The embodiments will be described in a specific context, namely a wireless Cartesian receiver utilizing an analog baseband filter to help reduce or eliminate unwanted out-of-band interferers. The invention may also be applied, however, to other forms of wireless receivers, such as non-Cartesian wireless receivers. Furthermore, the invention may also be applied to other communications devices desiring to use analog filters to help reduce or eliminate interferers that may be present outside of a desired frequency band. The invention is especially important for a full duplex communication system such as the third-generation cellular communication system based on WCDMA. In this kind of system, the receiver continuously receives signals from the air. An advantage of the invention is that out-of-band signals are used in the compensation of the corner frequency. Since the out-of-band signals will be subsequently eliminated in the wireless receiver, the out-of-band signals do not negatively impact the performance of the wireless receiver by introducing any distortion to desired in-band signals.
a shows a diagram that illustrates a high-level view of a portion of a wireless transceiver 200. Wireless transceiver 200 includes a transmitter 205, a receiver 210, a front-end module (FEM) 215, and an antenna 220. FEM 215 may include a power amplifier, antenna switches, duplexer, diplexer, SAW filters, and so forth. Transmitter 205 may be used to provide signal processing necessary to transmit information from a baseband unit over the air using antenna 220, while the receiver may be used to provide signal processing to provide information received over the air via antenna 220 to the baseband unit.
Generally, in a transmitter (TX), a digital signal from a digital baseband unit may be processed (for example, filtering, digital-to-analog conversion, etc.) and then modulated onto an RF carrier signal. The RF signal containing the modulated digital signal may then be amplified and radiated through an antenna. This modulation (or up-conversion) in a transmitter may require the use of a local oscillator (LO) or an RF frequency synthesizer (for example, a phase-locked loop). Phase modulation may also be performed at the LO when a polar architecture is adopted for the transmitter. Generally, in a receiver (RX), a received RF signal may be amplified by a low-noise amplifier (LNA) and then down-converted by mixers to an analog baseband signal. There may also be filters between the LNA and the mixers. The analog baseband signal may then be filtered by analog baseband filters and may be further amplified. The baseband signal may then be digitized by an ADC. The down-conversion in the mixers generally requires the use of a local oscillator (LO). The transmitter and the receiver may share a common LO or have separate transmit (TX) LO and receive (RX) LO.
b is a diagram illustrating a portion of receiver 210. The portion of receiver 210 shown in
As shown in
A received signal, such as one received by antenna 220 and processed by FEM 215, may be provided to receiver 210. Once at receiver 210, the received signal may be amplified by amplifiers, such as a low-noise amplifier and/or a transconductance amplifier. After amplification, the amplified received signal may be provided to the both the first signal path and the second signal path.
In the first signal path, the amplified received signal may be down-converted to an analog baseband signal by mixer 240. Mixer 240 may multiply the amplified received signal with an I-phase output of a local oscillator (LO). In the second signal path, the amplified received signal may similarly be down-converted by another mixer that multiplies it with a Q-phase output of the LO. After conversion to the analog baseband signal by mixer 240, an ABF 242 may be used to filter the analog baseband signal to eliminate or attenuate out-of-band signals. ABF 242 may be a first-order, a second-order, a third-order, or higher low-pass filter, a high-pass filter, a band-pass filter, and so on. The order and type of ABF 242 may be dependent on factors such as the nature of the in-band signal, the nature of the out-of-band signals, the closeness of the out-of-band signals to the in-band signal, and so forth. An ADC 244 may then digitize the output of ABF 242. A decimation filter (RCF) 246 may then be used to reduce the number of digitized samples of the output of ADC 244. The digitized and decimated baseband signal may then be provided to a baseband unit for further processing.
Receiver 210 also includes a first signal generator (SIG GEN I) 248 that may be used to generate an out-of-band signal for use in performing corner frequency compensation. In general, a single signal generator may be used to generate the various out-of-band signals to be used in corner frequency compensation. However, logically, a different signal generator may be used to generate each of the various out-of-band signals. Therefore, since the out-of-band signal used in the first signal path may be different than an out-of-band signal used in the second signal path, a second signal generator (SIG GEN Q) 249 may be used to generate the out-of-band signal for the second signal path.
The out-of-band signal generated by SIG GEN I 248 may be a cosine wave at a desired frequency and the out-of-band signal generated by SIG GEN Q 249 may be a sine wave at the same desired frequency. SIG GEN I 248 (and SIG GEN Q 249) may be implemented as a memory containing a look-up table containing entries descriptive of the out-of-band signal to be generated, for example. Alternatively, an oscillator along with a delay may be used to generate the out-of-band signals needed for the first signal path and the second signal path. Although the discussion focuses on sine and cosine out-of-band signals, other signal forms may be used as the out-of-band signals. Therefore, the discussion of sine and cosine out-of-band signals should not be construed as being limiting to either the scope or the spirit of the embodiments.
The out-of-band signal generated by SIG GEN I 248 may be converted into an analog signal by a DAC 250 and then combined (added, for example) with the analog baseband signal produced by mixer 240. The out-of-band signal in combination with the analog baseband signal may then be filtered by ABF 242, digitized by ADC 244, and decimated by RCF 246. In addition to being provided to the baseband unit, the output of RCF 246 may also be provided to a demodulation unit 252, which may demodulate the output of RCF 246 with the out-of-band signals, as generated by SIG GEN I 248 and SIG GEN Q 249. Additionally, a DC estimation unit 254 may be used to detect a maximum and a minimum value of the output of demodulation unit 252. Collectively, demodulation unit 252 and DC estimation unit 254 may be used to measure a phase response of ABF 242. The maximum and the minimum values of the output of demodulation unit 252 may be provided to a processor, such as a script processor, that may make use of the maximum and the minimum values to compute the corner frequency of ABF 242 and make any adjustments to ABF 242 as needed.
Like demodulation unit 252 and DC estimation unit 254, a demodulation unit 258 and a DC estimation unit 260 may be used to measure a phase response of an ABF in the second signal path. Maximum and minimum values of the output of demodulation unit 258 may also be provided to the processor, where the corner frequency of the ABF in the second signal path may be computed and any adjustments to the ABF may be performed if needed.
Although shown in
c is a diagram illustrating a detailed view of demodulation unit 252 and DC estimation unit 254. Although
Demodulation unit 252 comprises a first multiplier 270 and a second multiplier 271. First multiplier 270 may be used to demodulate the output of RCF 246 with the out-of-band signal produced by SIG GEN I 248, while second multiplier 271 may be used to demodulate the output of RCF 246 with the out-of-band signal produced by SIG GEN Q 249.
DC estimation unit 254 includes a first filter 274 and a second filter 275, preferably low-pass filters, to remove unwanted high-frequency components from outputs of first multiplier 270 and second multiplier 271, respectively. First filter 274 and second filter 275 may be used to produce DC signals from outputs of first multiplier 270 and second multiplier 271. Filter characteristics of first filter 274 and second filter 275 may be similar and should be set so that as much of non-DC signals present in the outputs of first multiplier 270 and second multiplier 271 as possible are eliminated (or attenuated).
After filtering, a first min/max detector 278 and a second min/max filter 279 may be used to select values (minimum and maximum output values) from the outputs of first filter 274 and second filter 275. First min/max detector 278 and second min/max detector 279 may be implemented comparators and memories, with the comparators used to compare a current output of a filter (either first filter 274 or second filter 275) with minimum and maximum values stored in the memories. Outputs from first min/max detector 278 and second detector 279 may be provided to a processor that may be used to compute the corner frequency of ABF 242 and make necessary adjustments if needed.
a is a flow diagram illustrating a flow chart 300 in the computing of a corner frequency of a filter using out-of-band signals. Flow chart 300 may be descriptive of operations taking place in a wireless receiver performing corner frequency compensation. The computing of the corner frequency of a filter, such as an ABF, may be performed periodically. Alternatively, the computing of the corner frequency may occur when a specified event occurs. For example, the computing of the corner frequency may occur prior to receiving a transmission, when performance of the wireless transceiver reaches a threshold, when measured performance metrics reach a threshold, when a request to perform corner frequency compensation is received, a detected change in operating conditions, a detected change in supply voltage, and so forth.
The computing of the corner frequency may begin with an injection of out-of-band signals (block 305). Referencing
After the out-of-band signals have been injected, a phase response of the ABFs of wireless receiver 200, such as ABF 242 and a corresponding ABF of the second signal path, may be measured (block 310). In order to simplify measurement of the phase response, the following assumptions are made: 1) a phase shift introduced by ABF 242 is φI; 2) a phase shift introduced by ADC 244 is zero (typically true if fIF is not significantly larger than the corner frequency of ABF 242); 3) a phase shift of RCF 246 is known and is zero (also typically true if fIF is not significantly larger than the corner frequency of ABF 242); 4) a phase shift of DAC 250 is zero. Similar assumptions are also made for the second signal path.
If the phase shift due to ADC 244, RCF 246, and/or DAC 250 is non-zero, then the phase shift may be measured using a technique similar to what is described below for measuring the phase shift for use in computing the corner frequency, but with adjustments made to ABF 242 so that the corner frequency of ABF 242 is at its maximum value. By maximizing the corner frequency of ABF 242, the phase shift introduced by ABF 242 may be negligible. The measured phase shift due to ADC 244, RCF 246, and/or DAC 250 may then be removed (for example, subtracted) from a measured phase shift due to ABF 242 and ADC 244, RCF 246, and/or DAC 250. This may then yield the phase shift due to ABF 242 alone.
Denote the output of RCF 246 as d1 cos(2πfIFnT+θI). From θI, θI may be computed as being equal to either θI or θI minus the phase shift due to DAC 250, RCF 246, and/or ADC 224 (as discussed above).
Referencing
and an output of multiplier 271 is expressible as:
After first filter 274 and second filter 275 remove high frequency terms from the above, they become DC signals expressible as:
LI and LQ may then be provided to first min/max detector 278 and second min/max detector 279 and the phase of the output of RCF 246 may be solved,
As such, the phase response of ABF 242 φI may be measured at frequency fIF.
Similarly, the measuring of the phase response of the ABF in the second signal path, φQ, at frequency fIF with the output of SIG GEN Q 249 as d0 sin(2πfIFnT) may also be performed. Let an output of an RCF in the second signal path be expressible as d2 sin(2πfIFnT+θQ). Then, an output of a multiplier in an I signal path, similar to multiplier 270, is expressible as:
and an output of a multiplier in a Q signal path, similar to multiplier 271, is expressible as:
After filters remove high frequency terms, the remaining DC signals expressible as:
{tilde over (L)}I and {tilde over (L)}Q may then be measured by min/max detectors and the phase of the output of the RCF in the second signal path, θQ, may be solved,
As such, the phase response of the ABF in the second signal path, φQ, may be measured at frequency fIF.
Referencing back to
The computing of the corner frequency may be dependent on the order of the ABF. For example, if the ABF is a first order low-pass filter with a transfer function expressible as:
where fc is the corner frequency of the ABF and G0 is the magnitude response at DC, then the phase response of the ABF is expressible as:
Therefore, if the measured phase response at fI is equal to φ1, then the corner frequency may be computed as:
The estimated term in the phase response measurement is tan(φ1), not φ1). This may imply that in calculating fc, mathematical functions, such as tan and tan−1, do not have to be used. Since the mathematical functions, such as tan and tan−1, tend to be sensitive to noise, numerical errors, and so forth, the estimation of tan(φ1) and the computing of the corner frequency may be less affected by noise, numerical errors, and so on.
If the ABF is a second-order low-pass filter with a transfer function expressible as:
Then the phase response of the ABF is expressible as:
where ωn is the natural frequency.
Assuming that the phase response at two out-of-band frequencies have been measured (φ1 and φ2 at f1 and f2, respectively), then:
where ω1=2πf1 and ω2=2πf2. From the above two equations, two unknowns Q and ωn (filter parameters) need to be solved. Once solved, the corner frequency may then be computed from Q and ωn.
To solve for the two unknowns Q and ωn, rewrite the equations for φ1 and φ2 above as:
It then follows that
At the corner frequency of the second-order ABF,
where ωc=2πfc. This implies that:
then
The above equation yields a solution
Furthermore, as discussed above, the avoidance of using math functions, such as tan and tan−1, may make the proposed approach less sensitive to the noise and numerical errors in estimating the corner frequency.
In general, for an n-th order ABF, measurements of the phase response of the ABF may be made at n different out-of-band frequencies. The n phase responses may then be used to solve for n different unknowns, which in turn, may be used to compute the corner frequency. In order to help ensure that the assumptions made above remain valid, the n different out-of-band frequencies should be relatively close to the corner frequency, however, they should be far enough apart so that they are not substantially a single frequency.
The corner frequency compensation may begin with an injection of out-of-band signals (block 405). In general, the number of out-of-band signals injected may be dependent on the ABF's order. For an n-th order ABF used in a Cartesian wireless receiver, n pairs of out-of-band signals may be injected. Each pair of out-of-band signals comprises an out-of-band signal for an I-phase signal path and a Q-phase signal path. For example, a pair of out-of-band signals may include a cosine wave and a sine wave, with the cosine wave and the sine wave having the same frequency.
After injecting a pair of out-of-band signal at a frequency, measurements of the ABF's phase response at the frequency of the out-of-band signals may be made (block 410). Generally, if multiple pairs of out-of-band signals are to be used, a pair of out-of-band signals may be injected one at a time and the measurement of the ABF's phase response made before another pair is injected.
If multiple pairs of out-of-band signals are to be injected, the injecting (block 405) and the measuring (block 410) may be repeated for each pair of out-of-band signals. Once all of the pairs of out-of-band signals have been injected and the phase response of the ABF has been measured, then the corner frequency of the ABF may be computed using the measured phase response (block 415).
The computed corner frequency of the ABF may then be compared with a desired corner frequency of the ABF (block 420). If the computed corner frequency and the desired corner frequency are equal or differ by less than a threshold, then the corner frequency does not need compensation and may terminate. However, if the computed corner frequency and the desired corner frequency differ by more than the threshold, then corner frequency compensation is needed. The corner frequency compensation may be performed by adjusting capacitor, resistor, or inductor values in the ABF (block 425). The capacitor, resistor, or inductor values may be adjusted by switching in and/or out different capacitors/resistors/inductors in the ABF, digitally adjusting capacitor/resistor/inductor values based on a control word, and so forth.
Since in a Cartesian wireless receiver there are two signal paths (the I-phase and Q-phase signal paths), any adjustments should be made to both signal paths at substantially the same time. By making the adjustments to both signal paths at the same time, potential mismatches in the two signal paths may be minimized. Additionally, the injection of the out-of-band signals should also occur at substantially the same time, again to minimize potential mismatches in the two signal paths.
After adjusting the ABF (block 425), the corner frequency compensation may be repeated to determine if the adjustments are sufficient. The corner frequency compensation may be repeated until the computed corner frequency differs from the desired corner frequency by less than the threshold.
In addition to using the computed corner frequency to compare and then adjust the ABF, other relevant parameters of the transfer function of the ABF (such as Q and ωn) may also be used to compare with their desired values to guide and assist in the tuning of the ABF.
Although the embodiments and their advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.