Patent Application
20070002972
The present invention relates to processing radio frequency (RF) signals, and more particularly to canceling unwanted noise in such signals.
Typical wireless communications systems, including cellular telephones, radios, and other wireless systems communicate data at high frequencies, i.e., at radio frequency (RF). Radio frequency signals are electrical signals conveying useful information having a frequency from about 3 kilohertz (kHz) to thousands of gigahertz (GHz), regardless of the medium through which such signals are conveyed. Thus an RF signal may be transmitted through air, free space, coaxial cable, fiber optic cable, etc. To process RF signals receive circuitry of a receiver, for example, generally converts the received RF signals to one or more lower frequencies, including an intermediate frequency (IF) and a baseband frequency. As an example, in a radio tuner, a frequency corresponding to a desired radio channel is tuned by mixing an incoming RF signal spectrum with a frequency generated in a local oscillator (LO) to obtain signal information of the desired channel. In various implementations, such a LO may be a voltage controlled oscillator or a numerically controlled oscillator (NCO), such as a digitally controlled oscillator (DCO).
In practice, a controlled oscillator can have its frequency controlled by changing capacitance values of one or more capacitors coupled to an oscillator element, such as a resonant tank. By varying the capacitance, the frequency generated by the controlled oscillator may be correspondingly varied. This capacitance may be controlled via digital control signals, e.g., a digital control word. Various implementations may include automatic frequency control (AFC) circuitry and/or algorithms to perform frequency control. Such circuitry and algorithms thus change the value of the digital signals that in turn change capacitance values, in turn adjusting the NCO frequency.
While such frequency adjustments are needed to tune to a desired channel, e.g., a radio station, a cellular frequency channel or the like, such frequency changes can have adverse effects on other parts of a receiver. For example, instead of adjusting a frequency continuously or smoothly, a NCO is discretely controlled, often leading to a step change in the digital control signals, which in turn leads to a corresponding frequency step change. Such a step change can induce noise in different receiver circuitry. For example, the step change can lead to generation of an undesired impulse in demodulation circuitry. Furthermore, the step change may appear as step change on the output of the demodulation circuitry, resulting in a step change in output audio signals, for example. Such step changes and impulse generation can undesirably affect receiver performance.
Accordingly, a need exists to reduce or eliminate such undesired effects of a frequency change.
In one aspect, the present invention includes a method for determining when a step change occurs to a control value for a numerically controlled oscillator (NCO) providing a mixing signal and mixing the mixing signal with an input radio frequency (RF) signal to obtain an intermediate frequency (IF) signal. The IF signal may then be demodulated into a demodulated signal and a correction value generated based on the step change. The correction value may then be applied to the demodulated signal. In this way, noise caused by the step change may be reduced or removed. The demodulated signal may be filtered prior to applying the correction value, in some embodiments.
Yet another aspect resides in a method for determining when a step change occurs to a control value for an NCO, and repeating at least one prior demodulated sample generated from an input RF signal when the step change occurs. In this manner, an impulse generated in a demodulator upon the step change may be cancelled. Different manners of repeating the sample(s) may be implemented, such as outputting at least one delayed demodulator sample, e.g., via a delay stage, to an output signal path upon occurrence of the step change. After the impulse has cleared, a current demodulated sample may be output to the output signal path.
In one embodiment, an apparatus may include a digital signal processor (DSP) to generate a demodulated signal based on an IF signal obtained from an incoming RF signal mixed with a controlled oscillator frequency signal and a cancellation circuit to cancel a portion of the demodulated signal caused by a change in the controlled oscillator frequency signal. The cancellation circuit may be adapted to insert a step change correction into the demodulated signal and/or to repeat at least one demodulated sample to avoid the portion of the demodulated signal. In some embodiments, the cancellation circuit may be implemented as part of the DSP.
Applications for cancellations in accordance with an embodiment of the present invention are numerous. As one example, an integrated terrestrial audio broadcast receiver may implement the methods. The receiver may be used in a portable device having an integrated terrestrial audio broadcast receiver. The portable device, which may be a digital media player, such as an MP3 player, can include the ability to receive a wide variety of audio broadcasts, including AM spectrum and FM spectrum signals.
In various embodiments, adverse consequences of a frequency step change occurring in receiver circuitry may be reduced or eliminated using different cancellation methods and apparatus. For example, in a receiver that implements a NCO, a discrete frequency change can lead to a frequency step change in the resulting processed signals, as well as generation of an undesired impulse spike in demodulator circuitry of the receiver. Accordingly, embodiments of the present invention may cancel these undesired elements based upon knowledge of a relative size and timing of the frequency change.
In some embodiments, circuitry and/or algorithms may be implemented to reduce or remove a frequency step change occurring on a change to control signals for an NCO. As discussed, this step change in frequency arising from a change in numerical control produces a step change in the demodulation output. As an example, an incoming FM signal is downconverted to an IF signal of the form IF=Ac cos(ωift+k∫s(z)dz), where s(z) is the desired signal, Ac is the value of the front-end gain, and k is the value of frequency deviation. FM demodulation recovers the derivative of the angle, i.e., FMout=ωIF+k s(t).
However, ωIF is the result of a mixing/low pass filtering (LPF) operation. Ideally, ωIF=k2(ωFMsignal±ωLO), where k2 is the gain of the filtering operation. When the NCO control signal N is changed, a nearly instantaneous change in ωLO to ωLO+Δ occurs, where Δ is the frequency change corresponding to the value of N. Accordingly, the IF signal also adjusts to ωIF+Δ(or ωIF−Δ depending on which component the mixer passes). Hence, the audio output is also changed. In the case of an FM signal, FMout, the new value corresponds to FMout=k2 (wIF+Δ+k s(t)), and there is thus a nearly instantaneous change in the output of size k2Δ.
To compensate for this instantaneous change, a post-correction may be implemented after demodulation is performed. In one form, the post-correction is a scaled, delayed (by the group delay of demodulation) step. Thus, the correction may correspond to −k2Δ for a change to N that produces a frequency change of Δ. This correction value may be applied to the audio signal generated by the demodulation circuitry, delayed to match the k2Δ step from the demodulator.
Referring now to
As discussed above, such a frequency step change can lead to an undesired step change in an audio output signal. Specifically, the frequency step change causes a change in the LO output, in turn causing a nearly instantaneous frequency change to an intermediate frequency (IF) signal generated by mixing the LO frequency with an incoming RF signal. This changed IF signal is received by demodulation circuitry which receives it and generates a demodulated signal therefrom (block 30). The demodulated signal may include a undesired step function. While the scope of the present invention is not so limited, in various embodiments the demodulation circuitry may be implemented in digital signal processor (DSP) circuitry that performs demodulation as well as other processing on incoming signals such as filtering, decoding, and the like.
Still referring to
Next, the correction signal may be combined with the demodulated signal to generate an output signal (block 50). Specifically, the correction signal may be combined with the demodulated signal that includes the step change to cancel the undesired step change. In such manner, the output signal (e.g., a FM output signal) is generated that corrects for (i.e., cancels) the undesired step change.
In different embodiments, various manners of compensating for the step change may be implemented. For example, step change cancellation may be implemented in specific hardware, or may be implemented in software and/or firmware, or a combination thereof. For example, in one embodiment step change cancellation may be implemented in a DSP.
Referring now to
As shown in
As discussed, a demodulated signal may include a step function when NCO 70 is controlled to a different frequency. To effect step cancellation in accordance with an embodiment of the present invention, NCO 70 may provide its control signal, N, to a correction unit 85. While shown in
In some embodiments, post-correction may occur before filtering the demodulated output. By performing post-correction before filtering, the correction signal may be more easily generated and applied to the demodulator output. Then, if desired, filtering of the corrected demodulated signal may occur. In some implementations a calibration method may be applied to the NCO to determine the exact values of frequency changes caused by a given change to a digital control signal. For example, a calibration method may determine differences between differently sized and/or implemented capacitor banks that are used to generate the NCO frequency. Such calibration methods may more closely track changes on a per-device basis. In some implementations a calibration method such as that disclosed in co-pending and commonly owned U.S. Provisional Patent Application No. 60/695,320 entitled “Methods And Apparatus To Generate Small Frequency Changes”, which was filed on Jun. 30, 2005, and which is hereby incorporated by reference in its entirety, may be performed.
Still further in some embodiments, a slow decay process may be placed on the applied correction to avoid accumulating a (partially unfounded) DC component to the NCO correction, while keeping the artifacts from the decay out of the desired bandwidth of the audio output. For example, an exponential decay with a time constant on the order of between approximately 0.1 second and 1 second has most of its energy below 30 Hz, which would be suitable for audio signals.
In addition to the step change described above, a change to the numerical control N can produce a nearly instantaneous change in the slope of the demodulator input. This near discontinuity in slope is ideally differentiated by the demodulator, producing a large undesired spike in the demodulator output. Referring now to
An additional complication exists for phase-locked loop (PLL) demodulators. For large spikes, the PLL will wrap, since the phase tracking is modulo 2π. Hence, the magnitude of the spike is not linear to the magnitude of the frequency step. This makes such an impulse spike difficult to predict and cancel. Another complication is that the discontinuity in slope depends on the characteristics of the signal at the time of the discontinuity, and so the magnitude and sign of the discontinuity changes with time. In particular, for many cases the physical processes that determine the signal and those that initiate a change in NCO settings are distinct and uncorrelated. Hence, characteristics of the resulting distortion may be difficult to predict.
While it may be difficult to predict the size of an impulse spike, in at least certain implementations, it is known when the spike will occur. For example, for systems with automatic frequency control, it is known when frequency changes are applied (since the system determines when to apply them). Based on this knowledge, the timing of the correction may be controlled. In some embodiments, previous data may be repeated to overwrite the period of the predicted spike. For slow moving signals (e.g., audio signals), the repetition of data may be far more accurate than the spike. Referring now to
Referring now to
Because it is difficult to determine an appropriate correction value to apply to remove an impulse generated during demodulation occurring as a result of a step change, in some embodiments a previous demodulated signal may be used in place of the impulse-affected signal. For example, at least one demodulated sample may be repeated as the output signal (block 130). In some implementations, a single repeated sample may be sufficient to avoid the spike. In one implementation, the repeated signal may be a delayed version of a prior audio signal that is inserted into the processing path to replace a portion of a demodulated signal that includes the undesired impulse. In other embodiments, other substitution schemes may be implemented.
For example, the last value generated before the spike (i.e., a last good sample) and a first value after the spike concludes (i.e., a first good sample) may be combined to effect a smoother response period. For example, in one embodiment these two values may be linearly interpolated to obtain a smoother response period. Still further, in some embodiments additional filtering may obtain a set of interpolated values that maintain the understood bandwidth of the expected signal.
In yet other embodiments, various manners of implementing impulse cancellation may be provided. For example, a delay path may be coupled to a processing path to store demodulated samples and later provide them to the processing path when an impulse event occurs. When it is determined that the impulse has passed, normal operation may continue. Specifically when the impulse ends, the output signal may again be provided from the output of the demodulator (block 140).
Referring now to
During normal operation, demodulated samples from demodulator 175 are provided as the audio output signals, as a switch S1 is normally set to allow the demodulated samples along the audio output line. However, when a frequency step change is occurring, a signal is provided to switch S1 from NCO 170. This signal may indicate a step change is occurring and accordingly, switch S1 acts to couple the output of delay unit 185 to the audio output line. In some embodiments, delay unit 185 may be configured to perform interpolations and/or filtering of delayed signals, such as a last clean sample prior to an impulse and a first clean sample after the impulse. Regardless of the type of correction signal, it may be switched onto the audio output line in place of the demodulated impulse spike. In such manner, an impulse spike which is generated by demodulator 175 upon a frequency step change is not sent along the audio output line. While shown as being controlled by a signal from NCO 170, it is to be understood that in other embodiments switch S1 may be controlled from other circuitry that is used to control a frequency step change in NCO 70, for example AFC circuitry, or other circuitry that may be separate from NCO 170. At a predetermined time after the step change, or as otherwise controlled, switch S1 may be set back to provide audio output from demodulator 175.
Referring now to
Looking back to the embodiment 1000 in
In some embodiments of the invention, processor 1050 and components of the RF and IF processing chain may be integrated on the same semiconductor die (i.e., substrate) and thus may be part of the same semiconductor package or integrated circuit (IC). In other embodiments of the invention, processor 1050 may be part of the same semiconductor package as the components of the RF/IF chain but located on a separate die. In still other embodiments of the invention, processor 1050 and RF/IF chain components may be located in different semiconductor packages. Thus, many variations are possible and are within the scope of the appended claims.
Still referring to
Low-IF conversion circuitry 1060 receives the in-phase (I) and quadrature (Q) signals 1160 and outputs real and imaginary digital signals, as represented by signals 1200. The low-IF conversion circuitry 1060 preferably includes band-pass or low-pass analog-to-digital converter (ADC) circuitry that converts the low-IF input signals to the digital domain. And the low-IF conversion circuitry 1060 provides, in part, analog-to-digital conversion, signal gain and signal filtering functions. Further digital filtering and digital processing circuitry with the digital signal processing (DSP) circuitry 1080 is then used to further tune and extract the signal information from the digital signals 1200. The DSP circuitry 1080 then produces baseband digital output signals 1220. When the input signals relate to FM broadcasts, this digital processing provided by the DSP circuitry 1080 can include, for example, FM demodulation and stereo decoding. Furthermore, the DSP circuitry 1080 can perform impulse cancellation in accordance with the embodiments described herein. Digital output signals 1220 can be left (L) and right (R) digital audio output signals 1220 that represent the content of the FM broadcast channel being tuned, as depicted in the embodiment 1000 of
It is further noted that the architecture of the present invention can be utilized for receiving signals in a wide variety of signal bands, including AM audio broadcasts, FM audio broadcasts, television audio broadcasts, weather channels, television signals, satellite radio signals, global positioning signals (GPS), and other desired broadcasts, among many other signal types.
In some embodiments receiver 1000 may be implemented in a portable device. While different implementations are possible, it is noted that a portable device may preferably be a small portable device. For example, the portable device could be a cellular phone, an MP3 player, a PC card for a portable computer, a USB connected device or any other small portable device having an integrated receiver.
While the present invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.
This application claims priority to U.S. Provisional Patent Application No. 60/695,335 filed on Jun. 30, 2005 in the name of Dana Taipale entitled CANCELLATION OF UNDESIRED PORTIONS OF AUDIO SIGNALS.
| Number | Date | Country | |
|---|---|---|---|
| 60695335 | Jun 2005 | US |
