Patent Application
20110243277
This patent document relates to systems and methods of frequency offset measurement for adjustment of an FSK receiver to the carrier frequency of an FSK transmitter.
Communication systems based on magnetic induction technology involve short range wireless communication by coupling a tight magnetic field between a transmitter device and a receiver device. A transmitter coil in the transmitter device modulates a magnetic field, which is detected and measured by a receiver coil in the receiver device. Magnetic induction communication technology is based on the quasi-static magnetic component of the field generated by a coil through which is flowing a sinusoidal current. Through the well-known transformer principle, when a second coil (at the receiver side) is introduced within the field generated by the transmitter, the magnetic flux passing through the receiver coil induces a modulated current in the winding, resulting in the ability to transfer information. Near field magnetic induction communication systems are designed to contain magnetic field localized around the communication system, with little radiations into free space. Wireless communication via near field magnetic induction technology can occur when the devices are separated by less than a small distance (typically about 1.5 meters), making such communications secure from long range eavesdropping.
Standard modulation schemes used in typical RF communications can be used in near-field magnetic induction systems, including frequency-shift keying (FSK). In FSK modulation, information is transmitted through discrete frequency changes of a sinusoidal carrier wave. This involves modulating digital data onto the carrier wave to thereby encode the information present in the data into variations of the carrier's instantaneous frequency between one of two frequencies. In magnetic induction and other communication systems using FSK modulation, the local oscillator frequency of the receiver is aligned to the carrier frequency of the transmitter in order to properly demodulate the transmitted signal.
The present invention is directed to overcoming the above-mentioned challenges and others related to the types of applications discussed above and in other applications. These and other aspects of the present invention are exemplified in a number of illustrated implementations and applications, some of which are shown in the figures and characterized in the claims section that follows.
According to an example embodiment of the present invention, a method is provided for matching a local oscillator frequency of an RF receiver to the carrier frequency of received FSK-modulated bit streams on a carrier frequency. The method includes detecting zero-crossings for at least one combination of I and Q signal pairs of the FSK-modulated bit stream, and determining positive occurrences of each zero-crossing of the at least one combination of I and Q signal pairs in a first direction, and negative occurrences for each zero-crossing of the at least one combination of I and Q signal pairs in a second direction opposite the first direction. Over a plurality of positive and negative occurrences, the total time elapsed between consecutive positive occurrences and the total time elapsed between consecutive negative occurrences are measured and accumulated along with the total positive occurrences and the total negative occurrences. A positive occurrence ratio of the accumulated total time elapsed between consecutive positive occurrences to the total positive occurrences is compared to a negative occurrence ratio of the accumulated total time elapsed between consecutive negative occurrences to the total negative occurrences, and in response to the comparison, the local oscillator frequency of the RF receiver is adjusted.
According to another example embodiment of the present invention, an RF receiver is provided for use in receiving an RF signal including FSK-modulated bit streams on a carrier frequency. The RF receiver includes an LC antenna circuit having a local oscillator tuned at a central frequency for receiving the RF signal, and a demodulator arranged to demodulate the FSK-modulated bit streams of the received RF signal. The demodulator includes a zero-crossing detector having logic circuitry arranged to convert I and Q signal representations of the received RF signal into positive pulses representing zero-crossing in a first direction of at least one combination of I and Q signal pairs, and negative pulses representing zero-crossing in a second direction opposite the first direction of the at least one combination of I and Q signal pairs. The demodulator further includes a frequency control module having circuitry to adjust the central frequency of the local oscillator based on comparing a positive pulse ratio of time elapsed between consecutive positive pulses to the number of positive pulses and a negative pulse ratio of time elapsed between consecutive negative pulses to the number of negative pulses.
According to another example embodiment of the present invention, a method is provided for matching a local oscillator frequency of an RF receiver to the carrier frequency of received FSK-modulated bit streams on a carrier frequency. The method includes detecting zero-crossings for at least one combination of I and Q signal pairs of an FSK-modulated bit stream, and determining high occurrences of each zero-crossing of the at least one combination of I and Q signal pairs according to a first crossing vector, and low occurrences for each zero-crossing of the at least one combination of I and Q signal pairs according to a second crossing vector different from the first crossing vector. Over a plurality of high and low occurrences, the total time elapsed between consecutive high occurrences is accumulated, along with the total time elapsed between consecutive low occurrences, the total high occurrences, and the total low occurrences. A high occurrence ratio is developed based on the accumulated total time elapsed between consecutive high occurrences and the total high occurrences, and a low occurrence ratio is developed based on the accumulated total time elapsed between consecutive low occurrences and the total low occurrences. The local oscillator frequency of the RF receiver is adjusted based on the developed high occurrence ratio and low occurrence ratio.
The above summary is not intended to describe each illustrated embodiment or every implementation of the present invention.
The invention may be more completely understood in consideration of the following detailed description of various embodiments of the invention in connection with the accompanying drawings, in which:
While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the scope of the invention, including that defined by the claims.
The present invention is applicable to a variety of applications, such as near field magnetic induction communication applications, that utilize FSK modulation for sending signals on a carrier frequency, and particularly where the local frequency of the receiver may be adjusted to lock on to the carrier frequency. Examples include systems that use zero-IF FSK receivers, and particularly those systems in which constraints such as component size, cost, and power consumption advise against the inclusion of a crystal capable of providing a highly accurate frequency reference. As a particular example, certain embodiments of the present invention may be suitable for implementation in an ultra low power, magnetic induction based, FSK audio/data link system. Applications of such devices include hearing aids, headsets, microphones, implanted medical devices, and so forth, including those that communicate via Bluetooth or any other suitable near-communications protocol. While the present invention is not necessarily limited to such applications, an appreciation of various aspects of the invention is gained through a discussion of examples in such an environment. Moreover, while aspects of the present invention may be conveniently illustrated in the context of a zero-IF receiver, it will be appreciated that embodiments of the present invention are also applicable to low-IF receivers, as indicated in the descriptions that follow.
According to an example embodiment of the present invention, an RF receiver receives FSK-modulated bit streams on a carrier frequency. The local oscillator frequency of the RF receiver is matched to the carrier frequency by comparing the accumulated spread between the higher tone of the FSK signal and the frequency of the local oscillator to the accumulated spread of the lower tone of the FSK signal and the frequency of the local oscillator, and adjusting accordingly. In certain embodiments, this involves detecting zero-crossings for I and Q signal pairs of the received FSK-modulated bit stream, and determining high occurrences of each zero-crossing of the I and Q signal pairs according to a crossing vector (direction or rate), and low occurrences for each zero-crossing of the I and Q signal pairs according to a different crossing vector (opposite direction or different rate). For example, positive occurrences of each zero-crossing of the I and Q signal pairs are determined based on crossings in a first direction, and negative occurrences for each zero-crossing of the I and Q signal pairs are determined based on crossings in a second, opposite direction. Over a plurality of positive and negative occurrences, the total time elapsed between consecutive positive occurrences is measured and accumulated along with the total time elapsed between consecutive negative occurrences. The total time elapsed between consecutive positive occurrences is divided by the total positive occurrences to develop a positive occurrence ratio. Similarly, a negative occurrence ratio is developed. The positive and negative occurrence ratios are then compared, and the result of the comparison is used to adjust the local oscillator frequency of the RF receiver.
Because aspects of the present invention involve comparing ratios developed between the accumulated positive occurrence times and the number of positive occurrences, and between the accumulated negative occurrence times and the number of negative occurrences, implementations of embodiments of the present invention need not rely upon having a balanced bit stream. As such, certain embodiments of systems and methods of the present invention may be utilized with bit streams having an unequal number of 1's and 0's, allowing implementation of systems that are independent of data features of the transmitted bit stream.
In accordance with certain embodiments, the described methods of measuring the frequency offset between the carrier frequency of FSK-modulated bit streams received by a receiver and the local oscillator frequency of the receiver, and of adjusting the frequency of the local oscillator accordingly are implemented in the demodulator circuit of a receiver device. Such receiver devices include a processing unit having a zero-crossing detector and an automatic frequency control loop (AFC) circuit for measuring the frequency offset and adjusting the receiver to lock on to the carrier frequency.
Receivers in accordance with certain embodiments of the present invention include low power on-chip oscillator as frequency reference, for example an RC type oscillator. While the accuracy of crystal oscillators can reach values of ±20 ppm or better, the accuracy of on-chip oscillators is more typically limited to a ±1% or more. As such, frequency offset measurement and control in accordance with the present invention may be used to adjust the on-chip local oscillator of the receiver to match the carrier frequency even without the use of a highly accurate frequency reference.
Various aspects of the present invention can be implemented using a variety of hardware and software solutions. As generally discussed herein, functions of the AFC can be realized through software stored on a storage medium. When the software is executed by a processing circuit, such as a micro-processor, one or more aspects of the invention are carried out or facilitated. Alternatively, functions of the AFC could be implemented as a logic circuit. For instance, functional software code can be transferred to a programmable logic array. In another instance, an integrated logic circuit could be designed to carry out one or more aspects of the present invention.
As schematically shown in
The transmitter 110 is shown schematically to include a modulator block 112 to generate the modulated data signal, a mixer 114 to shift the data signal around the carrier frequency, and a coil driver module 116, including the coil, which constitutes an LC circuit tuned at Fc. The receiver 120 is also shown in schematic form to have a front end that includes an LC circuit 122, with the coil, that is tuned at the central frequency used for communications. The receiver 120 also includes amplification stages 124 to preamplify the received signal, a down-mixer module 126 to downconvert the signal to baseband, and a demodulator 128 to process the signal.
As indicated by the arrow, the demodulator 128 includes a frequency detection and processing circuit 130 for measuring and adjusting for frequency offset, and for processing the received signals accordingly. This includes a zero-crossing detector (ZCD) 132 that functions to detect occurrences of the signal phase crossing a phase reference, including the direction (clockwise or anticlockwise) of the crossing. The function of the ZCD 132 is described in more detail in discussions that follow. The output of the ZCD 132 feeds into the automatic frequency control loop (AFC) circuit 134, which measures the frequency offset and adjusts the local oscillator of the receiver accordingly. The output of the ZCD 132, along with the adjustments made by the AFC 134, are used by the bit determination circuit 136 in final demodulation of the transmitted signal.
As described, in certain embodiments the AFC 134 measures and accumulates time elapsed between consecutive forward direction zero-crossings, and measures and accumulates time elapsed between consecutive backward direction zero-crossings. The AFC 134 further compares the ratio of the accumulated forward (positive) times to the total forward zero-crossings detected to the ratio of the accumulated backward (negative) times to the total backward zero-crossings detected. Adjustment of the local oscillator of the receiver 120 is made based on the results of comparing these ratios.
Detection of zero-crossings involves the use of pairs of I and Q signals, the generation of which are briefly described in the following discussion. As indicated above, the RF signal captured by the antenna of a receiver is downconverted to baseband, typically after being preamplified. The downconversion is performed by multiplying the RF signal by in-phase and quadrature-phase local oscillator signals, namely cos(ωt) which is used to generate the I(t) signal, and −sin(ωt) which is used to generate the Q(t) signal. The I(t) and Q(t) signals are filtered and combined together using analog circuits to provide I and Q signal pairs. For illustration, eight I and Q signal pairs are discussed, although it will be recognized that any even number of I and Q signal pairs can be used (e.g., 2, 4, 8, 16, 32). A zero-crossing on these eight phase axis signals corresponds to the crossing of a phase reference. The eight phase axis signals are defined as follows:
C0=Q
C22=cos(22.5)×Q−sin(22.5)×I
C45=cos(45)×Q−sin(45)×I
C67=sin(22.5)×Q−cos(22.5)×I
C90=−I
C112=−sin(22.5)×Q−cos(22.5)×I
C135=−cos(45)×I−sin(45)×Q
C157=−cos(22.5)×Q−sin(22.5)×I.
These phase axis signals are used to generate separate positive and negative pulse signals, which in turn are used to drive the AFC processing as well as the signal decoding. The AFC processing involves exploiting the occurrences of positive and negative pulses (the sign of the pulse indicating a frequency increase or decrease) to derive a bit value and frequency offset. The output of the post-processing is a bit signal with associated clock, and AFC indicators.
The separated pulse signals 222H and 222L are supplied to post-processing portion 220 for frequency offset measurement and adjustment by AFC module 234, and for bit stream demodulation. In an example embodiment, the AFC module takes as inputs the positive and negative pulses from the processing of C0 and C90 signals (in that case only zero-crossing of the I signal (−C90) and the Q signal (C0) are exploited). It will be appreciated that the pulses from processing any or all comparator combinations of signal pairs (C0, C22, C45, C67, C90, C112, C135, and C157) may be used. It will also be appreciated that the comparators may combine the signal pairs in any other suitable manner.
The AFC module 234 measures the time (for example, measured by the number of clock cycles) between two consecutive pulse occurrences having the same polarity. If there is a positive frequency offset (meaning the distance between the carrier frequency and high tone of the FSK signal is lower than the distance between the carrier frequency and the low tone of the FSK signal), the signal is turning slower for the high tone than for the low tone in the I/Q signal plan. This means a longer time elapses (or a higher number of clock cycles) between two zero-crossings for high tones than for low tones. In certain embodiments, the AFC module 234 includes registers (not shown) for accumulating and storing the time between consecutive positive pulses (referred to as the highfreq_dist register), the time between consecutive negative pulses (referred to as the lowfreq_dist register), the number of positive pulses (referred to as the highfreq_meascount register), and the number of negative pulses (referred to as the lowfreq_meascount register). The AFC module 234 may be enabled by the issuance of an enable parameter by software control, which signals the AFC registers to begin accumulating. The AFC registers are reset once they are read for frequency offset measurement and adjustment. AFC measurements may be interrupted by resetting the enable parameter (for example due to detecting that a full pack has been received, that a threshold number of measurements have been made, and so forth), or due to an overflow of one of the registers.
Referring again to
The phase offsets are filtered to derive a correction (in phase) to apply to the local clock. Such filtering may include, for example, calculating the phase offset average on the last phase offset, the last two phase offsets, or the last four phase offsets, applying a gain (e.g., 1, 0.5 or 0.25) to the calculated average, and applying a programmable threshold (e.g., up to 31) to the final value. The loop filter parameters are separately programmable before and after frame synchronization.
Finally the bit values are determined by comparing the number of positive and negative pulses along a bit time. If the number of positive pulses exceeds the number of negative pulses within bit boundaries, the demodulated bit is ‘1’, and otherwise the demodulated bit is ‘0’.
As described, frequency offset detection involves measuring the distance between the higher tone of the FSK signal and the frequency of the local oscillator of the receiver (running at carrier frequency), as well as the distance between the lower tone of the FSK signal and the frequency of the local oscillator. If the local oscillator is sufficiently aligned with the carrier frequency of the received signal, these distances will be equal, and there will be no frequency offset. Misalignment results in a measurable frequency offset which indicates how much to adjust the local oscillator. To derive a frequency offset, occurrences of both high and low tones in the FSK signal (positive and negative pulses) are used, although the signal need not be balanced, as discussed.
In accordance with certain embodiments, the frequency offset may be measured and characterized in terms of the ratio of the tone distances with carrier, which can be deduced from the measurements accumulated in the AFC registers identified above, and which can be expressed as a frequency ratio in accordance with the following formula:
where: highfreq_dist is the accumulated time between positive pulses, which represents the accumulated distances between the carrier frequency and the higher FSK tone over several measurements; lowfreq_dist is the accumulated time between negative pulses, which represents the accumulated distances between the carrier frequency and the lower FSK tone over several measurements; highfreq_meascount is the accumulated number of positive pulses, which represents the number of accumulated measurements of the higher FSK tone; and lowfreq_meascount is the accumulated number of negative pulses, which represents the number of accumulated measurements of the lower FSK tone.
Note that in practice, the frequency ratio need not be explicitly computed. Instead, the high and low ratios of accumulated time to accumulated counts can be compared with each other, with the comparison driving the local oscillator tuning (e.g., in a binary mode to step-wise increase/decrease the local oscillator frequency).
The local oscillator is matched to the carrier frequency of the transmitter when the frequency ratio is equal to, or sufficiently close to, 1. In certain embodiments, the frequency ratio is in a range of about 0.8 to 1.2 before any adjustment and signal demodulation is performed. Because the frequency detector operates within a limited range, any signal outside the range will be attenuated by the baseband filters of the radio, which may result in a convergence to an incorrect ratio, and thus an incorrect frequency adjustment if adjustments are made based on frequency ratio measurements outside of the range.
The frequency offset may be also be expressed as a frequency difference, in which the ratio of accumulated time between negative pulses to the number of negative pulses is subtracted from the ratio of accumulated time between positive pulses to the number of positive pulses, according to the formula:
A frequency difference value of 0 indicates frequency alignment. Because the difference is developed between the high ratio and the low ratio, a balanced bit stream is not required.
As mentioned, the AFC module measurements can be triggered by the processor controlling the hardware of the receiver. Measurements are interrupted by the processor based on criteria such as reaching a threshold or receiving a complete data packet, or on an overflow of one of the registers. In certain embodiments, a minimum of 100 measurements are made before calculating a frequency offset (e.g., frequency ratio), and the resulting local oscillator adjustments are made after completing a full measurement cycle. In other embodiments, local oscillator adjustments are made continuously as new measurements are made and folded into the frequency offset calculation. Frequency offset measurements can be susceptible to channel noise, mixer phase noise and distortions caused by the transceiver's (analog) signal processing. As such, to achieve a certain accuracy and to help avoid unwanted oscillation of the AFC, the frequency offset measurements may be processed, for example by averaging.
The operation of the AFC module in accordance with certain embodiments of the present invention is described in the example pseudo-code set forth in Table 1. In the pseudo-code, the pulses are encoded as +1 and −1 (no pulse is encoded as a ‘0’ value) in the “pulses” vector. Each value of “pulses” corresponds to a sample. The parameters tuned are the sampling frequency of the AFC module as well as the phase resolution.
As discussed, the frequency offset measurement techniques in accordance with the present invention are applicable to a radio transceiver using FSK modulation, and in which the carrier frequency of the receiver can be tuned. These techniques may be readily incorporated into a zero-crossing detector of the receiver. Systems and methods of the present invention do not rely on a balanced modulating bit stream having an equal number of zeros and ones. As such, the present invention may be implemented independent of the transmitted bit stream. While aspects of the present invention have been illustrated in the context of a zero-IF receiver (i.e., downmixing of RF signal down to baseband), it will be appreciated that embodiments of the present invention are applicable to low-IF receivers.
For low-IF systems, there are no negative occurrences, meaning that the signal is always turning one direction in the I/Q plan. As such, rather than detecting zero-crossings by the direction of the trajectory on the circle in the I/Q plan (i.e., through positive and negative occurrences), the system sorts zero-crossings by the rate of zero-crossing in the same direction. The ratio between the averaged time between two occurrences of one rate, and the averaged time between two occurrences of the other rate will converge to 1 for large intermediate frequency. The ratio is equal to the absolute value of [(Fif+δf)/(Fif−δf)] with perfect synchronization, where Fif is the Low-IF downmixing frequency and δf is the frequency offset of the modulation.
As indicated, the pulses are always of the same sign in low-IF receivers, and as such the time between two phase crossings can be averaged. It should be noted that for low-IF receivers, the measurement may be sensitive to the balance between the number of 0's and 1's transmitted. As indicated in the following equation, the frequency offset measurement may include the deviation of the average frequency compared to the low-IF frequency.
where AvNbCCBetweenZC is the average number of clock cycles between 2 zero-crossings. This value of the frequency average offset should be zero in case of perfect frequency alignment.
The processing of the times for low-IF receivers can be approached by partitioning the processing into two categories: those higher than a defined threshold, to be accumulated in “highfreq_dist” register, and those lower than the defined threshold, to be accumulated in “lowfreq_dist” register. As an example, the threshold can be the value of AvNbCCBetweenZC. The frequency offset measurement can then be expressed as:
In addition to the above, the various processing approaches described herein can be implemented using a variety of devices and methods, including general purpose processors implementing specialized software, digital signal processors, programmable logic arrays, discrete logic components and fully-programmable and semi-programmable circuits such as PLAs (programmable logic arrays). For example, the above algorithms are executed on a microprocessor in connection with certain embodiments, and may be implemented as part of one or more of the devices shown in the figures.
The various embodiments described above and shown in the figures are provided by way of illustration only and should not be construed to limit the invention. Based on the above discussion and illustrations, it will be recognized that the circuits described herein may be manufactured using standard processes and techniques. Those skilled in the art will readily recognize that various modifications and changes may be made to the present invention without strictly following the exemplary embodiments and applications illustrated and described herein. Such modifications and changes do not depart from the true scope of the present invention.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/IB2009/055018 | 11/12/2009 | WO | 00 | 5/18/2011 |
| Number | Date | Country | |
|---|---|---|---|
| 61116773 | Nov 2008 | US |
