Patent Application
20130044842
A portion of the disclosure of this patent document contains material, which is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever.
This invention relates to automatic gain control (AGC) for radio frequency (RF) receivers. Particularly, the present invention is related to improved gain setting for AGC in RF front ends by taking into consideration both the in-band signal strength and the out-of-band interference power in the gain computation.
In a RF communication system that supports multiple users for communications, such as a cellular mobile communication system, the frequency band can be partitioned into a number of frequency channels and a user is assigned a channel for making communications. The RF receiver of a user is therefore required to process an in-band signal in the presence of out-of-band interference. The in-band signal, which is the signal intended for reception by this user, also referred to as a desired user, is a signal that the receiver desires to receive, demodulate, decode and process. The out-of-band interference is the sum of signal(s) originated from or intended to be received by users other than the desired user. Notice that the in-band signal is transmitted over the frequency channel assigned to the desired user while out-of-band signals for other users are transmitted on other frequency channels.
Although the RF receiver is only required to decode the signal on the frequency channel assigned to the desired user, there are practical reasons that the RF receiver is capable to tune to other frequency channels. First, the RF receiver is provided to the desired user by a supplier or a manufacturer. Mass production of RF receivers, thereby reducing production cost, is not possible if different RF receiver models are used for different frequency channels. A common practice is that a RF receiver model is manufactured to be able to tune to different frequency channels, and the desired user personalizes the RF receiver he or she acquires from the supplier. Second, the RF communication system may dynamically assign the frequency channels to the users. Dynamic channel assignment is commonly used in cellular mobile communication systems, for capacity optimization and provision of services with different quality requirements. In the presence of dynamic channel assignment, the frequency channel of the desired user varies from time to time. A receiver capable of tuning to different frequency channels is therefore required.
For a RF receiver capable of tuning to different channels over the frequency band of interest, the RF signal is first captured by an antenna with some RF filtering such that the received signal covers a bandwidth of at least the whole frequency band. The received signal is amplified by a low-noise amplifier (LNA), wherein the LNA contains some filtering arrangement in order to confine the amplified signal to have a bandwidth substantially covering the frequency band of interest. The amplified signal is then processed by a mixer, which is a nonlinear device functioning as a multiplier for multiplying the amplified signal and the signal from a local oscillator. The purpose of the mixer is to move the in-band signal (i.e., the desired-user signal) from a channel assigned to the desired user to a frequency segment with a certain fixed carrier frequency, commonly known in the art as an intermediate frequency. The mixer output is then filtered with a channel selection filter (CSF), which is a narrowband filter centered at the intermediate frequency. This filter is to extract the in-band signal in the presence of the out-of-band interference. The filter output is the desired-user signal for further signal processing.
An important component in a RF receiver is an AGC unit. Its main function is to adjust the gain of the LNA in order to prevent it from saturation in case the received signal is overly strong. Optionally, the AGC unit may also be used to adjust the gains of the mixer and of the CSF to prevent them from saturation. Saturation of the LNA/mixer/CSF introduces significant nonlinear distortion to their output signals, greatly degrading the receiver performance. The AGC unit computes the gain of the LNA/mixer/CSF based on one or more signal power levels observed along the signal-processing chain for the received signal. For instance, the gain can be computed based on the received signal strength measured at the LNA input, or the signal strength of the LNA output, or the strength of the mixer output, or the signal power at the CSF output. Algorithms to compute the gain can be found in references such as Smith, J., Modern Communication Circuits, 2nd edition, McGraw-Hill, 1998 (referred to as Smith hereinafter), the disclosure of which is incorporated by reference herein in its entirety. Notice that the signals at the LNA input, the LNA output and the mixer output are wideband signals containing both the in-band signal and the out-of-band interference, whereas the CSF output contains the in-band signal only. Also notice that the in-band signal and the out-of-band interference are mutually uncorrelated, so that the out-of-band interference power cannot be estimated based on knowledge of the in-band signal strength. In the art, there are several methods of using the signal strength information in the realization of an AGC unit.
In U.S. Pat. No. 6,324,230, a realization of an AGC unit is disclosed, in which the gain of the LNA is computed based on the signal strength of the CSF output only. The AGC unit only keeps track of the strength of the in-band signal. In case the out-of-band interference is strong, the gain can be incorrectly determined so that saturation of the LNA and/or the mixer occurs. This condition leads to significant distortion of the desired-user signal, thereby significantly degrading the receiver performance.
In the disclosure of Huang, Y. T., A 1.2V 67 mW 4 mm2 mobile ISDB-T tuner in 0.13 μm CMOS,” Proceedings of International Solid-State Circuits Conference (ISSCC), 2009, pages 124-125, the gain of the LNA is computed based on the signal strength of the mixer output. This signal contains both the in-band signal and the out-of-band interference. Computation of the gain based on such information ensures that saturation of the LNA is prevented. However, the AGC unit does not have knowledge of the in-band signal in the gain computation. The gain can be underestimated so that the desired-user signal has a lower than adequate signal strength. It results in a reduced signal-to-noise ratio, thereby degrading the receiver performance.
In U.S. Pat. No. 7,668,517, the AGC unit disclosed therein computes the gains for the LNA, the mixer and the CSF based on the signal strength of the LNA output. Similar to the disclosure in the previous reference, knowledge of the in-band signal is not available in the gain computation. Therefore, the gains can be underestimated and the desired-user signal strength can be lower than adequate, thereby degrading the receiver performance.
In U.S. Pat. No. 6,670,901, the AGC unit disclosed therein computes the gains for the LNA, the mixer and the CSF based on the signal strength at the LNA input, the mixer output signal strength and the signal power of the CSF output. With knowledge of both the in-band signal strength and the out-of-band interference power, the gains can be properly set to prevent saturation of the LNA/mixer/CSF as well as to produce a desired-user signal that has adequate power. The main disadvantage of this realization, however, is that a dedicated RF wideband power detector is required, thereby consuming additional power in operating the receiver and requiring additional die area in integrated circuits (ICs) used for such receiver.
There is a need for an improved AGC unit that fulfills the following requirements. First, the gain(s) are computed based on knowledge of both the in-band signal strength and the out-of-band interference power. Second, a RF wideband power detector is not required in the realization of the AGC unit.
The presently claimed invention includes a plurality of RF front ends each of which employs an improved AGC unit that fulfills the aforementioned two requirements. A RF front end, as defined herein, is a sub-system of a RF receiver that resides between the antenna and the first intermediate frequency stage. The improved AGC unit computes the gain of the LNA and that of the mixer based on knowledge of the in-band signal strength and the out-of-band interference power, wherein this knowledge is derived from the signal-power levels observed at the mixer output and at the CSF output. As a result, the gains can be correctly computed for preventing saturation of the LNA and the mixer. In addition, the desired-user signal, which is the CSF output, can be obtained with adequate signal power, thus maintaining a good signal-to-noise ratio, which is essential for good receiver performance. Another advantage of the presently claimed invention is that no RF wideband power detector is required. Elimination of the RF wideband power detector saves battery power in portable applications such as in mobile phones, and also reduces the required die size in the design of receiver ICs.
Embodiments of the invention are described in more detail hereinafter with reference to the drawings, in which
a shows a block diagram of an embodiment of a RF front end that employs an improved AGC unit;
b shows a block diagram of an embodiment of the improved AGC unit used in the RF front end;
In the following description, system and apparatus of RF front ends with AGC and the like are set forth as preferred examples. It will be apparent to those skilled in the art that modifications, including additions and/or substitutions may be made without departing from the scope and spirit of the invention. Specific details may be omitted so as not to obscure the invention; however, the disclosure is written to enable one skilled in the art to practice the teachings herein without undue experimentation.
Referring to
For an AGC in the RF front end, the AGC unit 15 computes a desired gain for the LNA 11 and a desired conversion gain for the mixer 12 based on a mixer output 16 and a CSF output 17 as inputs. The desired gain for the LNA 11 is then communicated from the AGC unit 15 to the LNA 11 through the first gain-setting signal 18, in order to set the gain of the LNA 11. The desired conversion gain for the mixer 12 is also communicated from the AGC unit 15 to the mixer via the second gain-setting signal 19 for setting the conversion gain of the mixer 12. One distinguishing feature of the present disclosure is that the desired gain for the LNA 11 and the desired conversion gain for the mixer 12 are computed based on the in-band signal strength and the out-of-band interference power, both of which are in turn computed based on the mixer output 16 and the CSF output 17. Another distinguishing feature is that the AGC unit 15 does not require knowledge of the RF signal strength at the LNA input, eliminating the need for a RF wideband power detector. Computation of the in-band signal strength and the out-of-band interference power is elaborated as follows.
Since the CSF output 17 contains only the in-band signal component, the in-band signal strength can be computed from the CSF output 17 by a mathematical procedure realized in an electronic circuit in the AGC unit 15. One example of such procedure is squaring the amplitude of the CSF output 17 and averaging the squared values over a certain period of time, the average value being the in-band signal strength. The out-of-band interference power can also be computed from the signal power of the mixer output 16 and the signal power of the CSF output 17. Note that the signal power of the CSF output 17 is the in-band signal strength. Since the mixer output 16 comprises the in-band signal component and the out-of-band interference as mentioned in the above, and since this signal component and this interference are mutually uncorrelated, it follows that, if averaged over a sufficiently long time, the signal power of the mixer output 16 is, or statistically converges to, the sum of the in-band signal strength and the out-of-band interference power.
The signal power of the mixer output 16 can be computed by a mathematical procedure realized in an electronic circuit in the AGC unit 15. One example of such procedure is by squaring the amplitude of the mixer output 16 and taking an average value of the squared values over a certain period of time, the resultant average value being the signal power of the mixer output. In the case where the CSF 14 is providing neither a gain nor an attenuation to the in-band signal component in the CSF output 17, the in-band signal strength calculated from the CSF output 17 can be subtracted from the signal power of the mixer output 16 to obtain the out-of-band interference power. Otherwise, the in-band signal strength calculated from the CSF output 17 is required to be weighted accordingly to remove the gain/attenuation effect introduced by the CSF 14 before performing the subtraction. Based on the in-band signal strength and the out-of-band interference power, the desired gain for the LNA 11 and the desired conversion gain for the mixer 12 can be computed accordingly. Algorithms and electronic circuits for computing these two gains can be found in Smith.
Accordingly to various embodiments, the AGC unit 15 can be realized by a structure shown in
A simple implementation of the first computation means 20 can be achieved if the CSF 14 has an additional feature of a unity-gain filter providing a unity gain to the in-band signal component that passes the filter.
The second power detector 32 accepts a CSF output as its input and estimates the signal power of the CSF output. An exemplary estimation procedure is by squaring the amplitude of the CSF output and averaging the squared values over a certain period of time, the average value being the signal power of the CSF output. The subtractor 33 is used to subtract the output of the first power detector 31 from the output of the second power detector 32, yielding an out-of-band interference power at the output of the subtractor 33. The output of the second power detector 32 indicates the in-band signal strength.
The antenna 40, LNA 41, mixer 42, local oscillator 43, CSF 44, and AGC unit 46 perform substantially similar functions as their counterparts 10, 11, 12, 13, 14, and 15, respectively, of the aforementioned disclosed RF front end embodiment as shown in
The AGC unit 46 can also be realized in a structure substantially similar to an AGC unit 15 shown in
The foregoing description of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many modifications and variations will be apparent to the practitioner skilled in the art.
The embodiments were chosen and described in order to best explain the principles of the invention and its practical application, thereby enabling others skilled in the art to understand the invention for various embodiments and with various modifications that are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalence.
