Patent Application
20070057737
This invention relates in general to communication transmitters and more specifically to compensation for phase locked loop (PLL) modulation distortion in such transmitters.
The use of phase locked loops in transceivers is known. They are normally used together with a crystal oscillator to realize a signal with a frequency that may be varied in accordance with the radio frequency of different communication channels and still maintain the relatively stable frequency drift over time and temperature associated with the crystal oscillator. In systems where information is transferred by the radio frequency signal in the form of phase modulation of the radio frequency signal, a phase locked loop may also be used to induce this modulation on the radio frequency signal.
However when the phase locked loop is directly modulated various problems may be encountered. For example the characteristics of a phase locked loop and the ability to directly modulate the phase locked loop will depend on the loop bandwidth. Other characteristics of a phase locked loop will also depend on the loop bandwidth. For example, the time to switch from one frequency to another (sometimes referred to as lock time) will depend on the phase locked loop bandwidth. Furthermore, the loop bandwidth as well as other loop parameters will influence phase noise levels, etc.
One of the results of these tradeoffs is that a practical phase locked loop may not have a flat amplitude or linear phase response for a modulation signal. Other variables, for example the tuning sensitivity of a voltage controlled oscillator associated with the phase locked loop, may vary in practical loops. All of these factors contribute to modulation distortion and the need to compensate for this distortion. One known technique is using a filter to shape a modulation signal prior to modulating a loop, however this often does not provide sufficient compensation for many systems. Other approaches use an adaptive filter but these require training times and some systems may not have the time or processing resources for the training.
The accompanying figures, where like reference numerals refer to identical or functionally similar elements throughout the separate views and which together with the detailed description below are incorporated in and form part of the specification, serve to further illustrate various embodiments and to explain various principles and advantages all in accordance with the present invention.
In overview, the present disclosure concerns communications equipment such as transmitters used in equipment and devices or units that are utilized to provide services for users thereof. More particularly various inventive concepts and principles embodied in apparatus and methods for providing or effecting compensation for phase locked loop modulation distortion for transmitters within communication equipment or units, where the phase locked loop systems operate to provide a radio frequency signal with modulation to transmitter amplifiers and the like are discussed and described. The systems and transmitters of particular interest are those being developed and deployed that use various forms of phase or complex modulation, i.e., modulation with a phase component, of a radio frequency signal to transfer information. In such transmitters a phase locked loop can be used to apply the modulation to the radio frequency carrier or signal and known problems with modulation distortion by the phase locked loop or synthesizer often need to be addressed. In some systems or equipment, e.g., frequency hopped systems utilized in Instrumentation, Scientific, and Medical (ISM) band or Family Radio Service (FRS) band, it is important that the compensation be efficient in terms of time or processing resources and the concepts and principles described herein may be particularly useful in these situations.
As further discussed below various inventive principles and combinations thereof are advantageously employed to select a filter response that corresponds, for example, to one or more of a tuning voltage associated with a phase locked loop or other environmental variables, e.g., power supply voltage or temperature and then predistort a modulation signal in accordance with the filter response and provide the resultant distorted signal to a modulation input, e.g., a feedback divider or loop divider input and thereby facilitate modulation and effect compensation for any modulation distortion resulting from the phase locked loop. This inventive compensation system and techniques can be particularly advantageously utilized in transmitters where carrier frequencies change often or where very little time or processing resources are available when a carrier frequency does change, thereby alleviating various problems associated with known compensation systems and facilitating lower cost higher performance transmitters, provided these principles or equivalents thereof are utilized.
The instant disclosure is provided to further explain in an enabling fashion, the best modes of making and using various embodiments in accordance with the present invention. The disclosure is further offered to enhance an understanding and appreciation for the inventive principles and advantages thereof, rather than to limit in any manner the invention. The invention is defined solely by the appended claims including any amendments made during the pendency of this application and all equivalents of those claims as issued.
It is further understood that the use of relational terms, if any, such as first and second, top and bottom, and the like are used solely to distinguish one from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions.
Much of the inventive functionality and many of the inventive principles are best implemented with or in integrated circuits (ICs) using hardware or hardware and software or firmware instructions, such as custom or semi-custom ICs, e.g., application specific ICs, that include a processing or digital signal processing function. It is expected that one of ordinary skill, notwithstanding possibly significant effort and many design choices motivated by, for example, available time, current technology, and economic considerations, when guided by the concepts and principles disclosed herein will be readily capable of generating such instructions and ICs with minimal experimentation. Therefore, in the interest of brevity and minimization of any risk of obscuring the principles and concepts according to the invention, further discussion of such software and ICs, if any, will be limited to the essentials with respect to the principles and concepts of the preferred embodiments
Referring to
As will be further described below, the output of the pulse shaping filter 105 is a modulation signal and is applied to a filter 107, i.e., a predistortion or compensation filter, where the response of this filter is selected from a plurality of filter responses. The output of the filter is a distorted signal or predistorted modulation signal that is coupled to a transmit frequency synthesizer or phase locked loop (PLL) and used facilitate modulation of the PLL or specifically voltage controlled oscillator (VCO) that is part of the PLL and to effect compensation for modulation distortion resulting from the PLL characteristics or variations within components of the PLL. The output 110 of the PLL 109 includes a radio frequency signal that can be centered, e.g., at the carrier frequency, and includes modulation where any modulation distortion (amplitude or group delay) has been reduced or compensated for by the distorted signal. This radio frequency signal will be processed by various additional known transmitter functions, e.g., transmit amplifiers, etc. 111 and applied to an antenna 113.
Also shown in
Referring to
Generally the operation of a PLL is known and the literature is replete with references for those who are interested or desire a more detailed review. Essentially the phase detector outputs a signal corresponding to a phase difference between the reference signal and the signal at the output of the feedback divider. This signal is filtered by the loop filter and applied to the VCO. The frequency of the radio frequency signal from the VCO is divided down by the feedback divider according to the divider modulus or divisor. Due to the feedback loop, any difference between the frequency of the signals at the inputs to the phase detector will be eliminated since the phase difference would otherwise grow without bound. The frequency of the signal at the output of the VCO will be equal to the reference frequency multiplied by the divisor or divider modulus of the feedback divider. Many PLLs today use so called fractional (frac) N feedback dividers and these operate as is known to effectively implement a non integer, e.g., 10,000.5, divisor in the feedback divider. Generally a frac N divider switches between different divisors and the proportion of time the divider is operating with one divisor versus other divisors determines the effective divisor.
Frac N dividers may also be used to frequency or phase modulate the radio frequency signal by essentially changing the effective divider modulus in accordance with the modulating signal. Those or ordinary skill will realize that frequency components of the modulation signal as reflected at the output of the feedback divider and thus phase detector will modulate the VCO output signal frequency only to the extent these frequency components are allowed to pass through the loop filter. In many practical systems the loop filter bandwidth will be on the order of the modulation signal bandwidth and thus the loop filter magnitude and phase response can be a significant contributing factor to modulation distortion.
Another source of modulation distortion is a VCO. As the desired channel or carrier frequency changes, the tuning voltage required to drive the VCO to provide this signal frequency also changes. Typical VCOs will also experience a change in tuning voltage sensitivity (normally expressed in MHz/volt units) as a function of the tuning voltage. For example in one embodiment, as the tuning voltage varies from a minimum to a maximum, e.g., 0.5. volts to 2.0 volts, the tuning sensitivity of the VCO more than doubles, e.g., 6 MHz/volt to 14 MHz/volt. When a modulation signal is applied and a certain change in frequency is expected, this change in tuning sensitivity means that the change in frequency will vary widely depending on what channel or carrier frequency is being provided by the VCO. Other reasons for or causes of modulation distortion, such as changes in environmental variables (temperature, supply voltage, etc.) or loop gains, etc. may also need to be taken into consideration. Compensation systems for mitigation of modulation distortion can be used to reduce the amount of the modulation distortion.
One or more embodiments of modulation distortion compensation system for use with the PLL is shown in
The modulation distortion compensation system also comprises a filter 224, operationally analogous to the filter 107, that is configured to distort a modulation signal, e.g., transmit data symbols from 103 as shaped by the shaping filter 105, in accordance with the filter response and provide a distorted signal to the modulation input 211. The distorted signal or predistorted modulation signal can be used by the feedback divider in the phase locked loop to facilitate modulation of the VCO and effect compensation for or reduce or mitigate modulation distortion resulting from the phase locked loop, whether due to the dynamics of the PLL or variations in its constituent components.
The filter in certain embodiments includes a digital filter with programmable coefficients and the compensation function is configured to select the filter response by providing coefficients, e.g., predetermined coefficients, for the digital filter that correspond to the filter response. In other embodiments, such as those shown in
In one or more embodiments, the compensation function further comprises one or more environmental measuring functions 125, such as a temperature measuring function 127 configured to provide a temperature signal or power supply voltage measuring function 129 configured to provide a voltage signal. The filter response in these instances will be selected in accordance with the tuning voltage signal and one or more of the temperature signal and the voltage signal.
The temperature measuring function 127 in some embodiments further comprises an analog to digital converter 225 with an input for receiving a temperature indication, e.g., from a device or sensor or the like (PN junction varies about 2.2 milli volts per degree Celsius) and an output for providing a digital temperature indication. The digital temperature indication is provided to a temperature converter 227 for converting the digital temperature indication to the temperature signal, where the temperature signal comprises in some embodiments a digital temperature signal. Note the temperature converter may be as simple as a register for storing a digital word, e.g., an 8 bit word, but can also represent any normalization or calibration processes that may be desired. The output of the converter 227 is shown coupled to a rounding operation 229 where the digital word can be rounded off as needed, e.g., the 8 bit word can be converted to a 4 bit word corresponding to a 4 bit rounded version of the 5 most significant bits. The resultant temperature signal is coupled to the look up table 221 and can be used in conjunction with the tuning voltage signal to select a frequency response.
Similarly, the power supply voltage measuring function further comprises an analog to digital converter 231 with an input for receiving a power supply voltage indication and an output for providing a digital voltage indication. The digital voltage indication is provided to a voltage converter 233 for converting the digital voltage indication to the voltage signal. Again this can be a register for storing a digital word and may include normalization and calibration processes. Thus the voltage signal comprises a digital voltage signal and may also be sent through a rounding operation 235 where, e.g., an 8 bit word is rounded to a 3 bit voltage signal and the resultant voltage signal is coupled to the look up table 221 and can be used in conjunction with the tuning voltage signal, temperature signal, etc. to select a frequency response.
Given the tuning voltage signal, temperature signal, or voltage signal or a particular combination of these signals, as an address to the look up table, access to a particular location in the look up table 221 can provide one or more parameters associated with a desired filter response. For example, predetermined filter coefficients can be stored at that location and provided at output 223. These predetermined filter coefficients can be used to configure the predistortion filter 224. Given that an IIR filter is one embodiment of a suitable filter and possibly as many as 8 coefficients (4 numerator and 4 denominator coefficients) each 32 bits long may be needed to implement an IIR filter that provides a proper response, the look up table can become relatively large, if each of the possible locations available with a 10 bit address contains these 8 coefficients. However using this approach allows a large number of predetermined filter responses to be made available if that is required in order to sufficiently compensate for the modulation distortion. Similarly, if an FIR filter architecture is used, the number of taps required can be approximately 40 taps where each tap may require approximately 16 bits to define.
In an alternative embodiment, access to a particular location in the look up table can provide an indication of one set predetermined filter coefficients out of a plurality or multitude of such sets with the indication provided at output 223. The indication would be used to select the one set from various such sets and program or configure the filter 224. For example, if a small number (relative to the 10 bit address range of the look up table) of predetermined filter responses is deemed sufficient to provide compensation, each location in the look up table can be populated with an indication for one of the small number of filter responses, i.e., 000, 001, 010, 011, . . . , where each of these 3 bit words indicates a particular set of predetermined filter coefficients. The indication would be used to select via a multiplexer or the like (not specifically shown in
A look up table 303 is coupled to various signals, e.g., a tuning voltage signal, temperature signal, and supply voltage signal that are used to form an address that allows access to a particular location in the look up table. In this look up table an indication, e.g., 3 bit word or the like, much as described above for a particular predetermined filter response and thus filter is stored at each location in the table. This indication is provided at output 304 to a multiplexer 305. The multiplexer then couples the output of one filter (filter1307, . . . filterN 311) out of a multitude of filters 309, where the one filter corresponds to the indication from the look up table, to the modulation input 211. In situations where the compensation system must be modified and begin acting quickly the approach of
As a brief review
The compensation function in certain embodiments further comprises an analog to digital converter arranged to be coupled to the tuning voltage signal and configured to convert the tuning voltage signal to a digital tuning voltage signal; and a look up table, the digital tuning voltage signal used to facilitate access to the lookup table to select one or more parameters associated with the filter response. The compensation function can include an environmental measuring function configured to provide an environmental signal, where the filter response is selected in accordance with the environmental signal and the tuning voltage signal.
The environmental measuring function can include a temperature measuring function or supply voltage measuring function each configured to provide, respectively a temperature signal or voltage signal. The filter response is then selected in accordance with the temperature signal, the voltage signal and the tuning voltage signal. The temperature measuring function and supply voltage measuring function each comprise either a shared or an individual analog to digital converter with an input for receiving a temperature indication or supply voltage indication and an output for providing a digital temperature or digital voltage indication; and a temperature or voltage converter for converting the digital temperature or voltage indication to the temperature or voltage signal, respectively.
The filter for the modulation distortion compensation system in various embodiments further comprises a digital filter with programmable coefficients and the compensation function is configured to select the filter response by providing coefficients for the digital filter that correspond to the filter response. In one embodiment, the modulation distortion compensation system further comprises a multiplexer coupled to the compensation function and the filter includes a plurality or multitude of filters each with a unique filter response corresponding to one of the plurality of filter responses. The compensation function is configured to select the filter response by providing a select signal to the multiplexer.
Referring to
Referring to
Note that the three different situations (max, typical, min) are plotted for each of the magnitude and group delay. The maximum situation, corresponding to curves 507, 508, represents a simulated situation where all factors (loop gain, tuning sensitivity, temperature, supply voltage, etc) assume a more or less worst case maximum situation. The typical situation, corresponding to curves 509, 510, represents a simulated situation where all factors (loop gain, tuning voltage sensitivity, temperature, supply voltage, etc) assume a more or less typical situation. The minimum situation, corresponding to curves 511, 512, represents a simulated situation where all factors (loop gain, tuning sensitivity, temperature, supply voltage, etc) assume a more or less worst case minimum situation. If there was no modulation distortion, the magnitude curves would be flat at least across the bandwidth of interest, e.g., a channel bandwidth, and the group delay curve would be a straight line with a given slope.
Referring to
Referring to
Referring to
There are various techniques available to generate the filter coefficients, i.e., digital predistortion filter coefficients. For example, an IIR filter can be designed to perform the inverse response of the closed loop PLL system or an FIR filter can be designed using adaptive equalization techniques (such as Recursive Least Squares methods) to equalize for the magnitude and group delay distortion of the PLL over the modulated signal bandwidth. The important thing to remember is that these filter coefficients should be predetermined or pre-calculated and stored in memory in order to have a multitude of filter responses available, i.e. multiple predetermined filter responses are available. This gives the advantage of multiple filters in terms of EVM improvement as compared to a single compensation filter and also avoids the system overhead and timing delays associated with training procedures and the like when an adaptive filter is used.
Thus a method of facilitating compensation for modulation distortion in a phase locked loop, and corresponding compensation systems and apparatus have been described and disclosed where the method includes providing a tuning voltage from the phase locked loop, selecting, responsive to the tuning voltage, a filter response from a plurality of filter responses; and filtering a modulation signal in accordance with the filter response to provide a distorted signal, such that the distorted signal is suitable for modulation and compensation of a radio frequency signal provided by the phase locked loop.
In various embodiments, this includes converting the tuning voltage to a digital tuning voltage signal and using the digital tuning voltage signal to facilitate access to a memory to select one or more parameters associated with the filter response. Other embodiments additionally include providing one or more environmental signals associated with corresponding environmental conditions and selecting the filter response in accordance with the tuning voltage signal and the one or more environmental signals.
A modulation distortion compensation system for a PLL used, for example, in a transmitter, is arranged and constructed to mitigate or compensate for modulation distortion via a predistorted signal. The system uses pre-determined or pre-calculated filter parameters or coefficients to provide a selected one of a plurality of filter responses and a tuning voltage or environmental signals to guide the selection of the appropriate one of the filter responses. The selected filter response is used to predistort the modulation signal and thus compensate for modulation distortion of the PLL as has been discussed and described.
This disclosure is intended to explain how to fashion and use various embodiments in accordance with the invention rather than to limit the true, intended, and fair scope and spirit thereof. The foregoing description is not intended to be exhaustive or to limit the invention to the precise form disclosed. Modifications or variations are possible in light of the above teachings. The embodiment(s) was chosen and described to provide the best illustration of the principles of the invention and its practical application, and to enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the invention as determined by the appended claims, as may be amended during the pendency of this application for patent, and all equivalents thereof, when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.
