Abstract:
A method of tuning a receiver includes: providing a tunable Low Noise Amplifier (LNA) having a plurality of LNAs and a plurality of LC loads coupled to the plurality of LNAs; selecting a desired LNA and a LC load corresponding to the desired LNA; isolating the desired LNA from the selected LC load; generating a local oscillation (LO) signal; routing the LO signal to the output of the tunable LNA to generate a filtered LO signal; determining a first signal power level of the filtered LO signal; changing the capacitance in the selected LC load and determining a second signal power level of the filtered LO signal; comparing the first filtered signal power level to the second filtered signal power level; and according to the comparison result, determining whether or not to adjust the capacitance in the selected LC load.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This divisional application claims the benefit of co-pending U.S. patent application Ser. No. 11/623,309, filed on Jan. 15, 2007 and incorporated herein by reference. 
    
    
     BACKGROUND 
     The present invention relates to the tuning of integrated LC filters, and more particularly to the tuning of integrated LC filters in broadband television receivers. 
     Modern receiver systems utilize broadband technology to receive a wide range of frequencies. In order to prevent in-band blockers from interfering with the receiver operation, a receiver requires very high linearity at its front end. In broadband television receivers, a conventional method of achieving high linearity in the presence of in-band blockers is to utilize Automatic Gain Control (AGC). A desired signal together with the in-band blocker are applied to a conventional receiver and amplified utilizing a broadband Low Noise Amplifier (LNA) and a Variable Gain Amplifier (VGA). A Wide Band Power Detector, coupled to the output of the VGA, converts the desired signal and blocker power to a DC voltage, which is then input to the base-band part of the receiver. As the DC voltage is directly proportional to the input signal power, the base-band is able to detect the presence of strong blocker(s) if the corresponding base-band Radio Signal Strength Indicator (RSSI) level is not proportional to the WBPD DC level. When this occurs, the base-band will send a signal to the front end to decrease the gain of the VGA. As the gain decreases, however, the receiver noise figure will also increase. Therefore, the use of AGC is limited. 
     Many receivers therefore utilize tracking filters to amplify only the desired input signal, thus reducing the need for such high linearity. In a direct conversion receiver the tracking filter can track the internal Local Oscillation (LO) signal generated by a phase locked loop (PLL). The frequency response of a filter refers to the characteristic(s) of the filter that conditions the input/internal signal to the filter. The filter will show frequency responses based on certain circuit parameters. 
     In a broadband receiver, a filter is required that has many frequency responses, i.e. a tunable filter. Prior art receivers utilize radio frequency (RF) tracking filters, which are equivalent to band-pass filters. U.S. Pat. No. 6,285,865, which is included here for reference, discloses a receiver with such a tunable filter. The disclosed integrated chip comprises: a first adjustable on-chip filter having a first plurality of selectable capacitors that determine its center frequency; a second adjustable on-chip filter having a second plurality of selectable capacitors that determine its center frequency; means for selecting a number of the first plurality of capacitors to adjust the first filter to a desired center frequency; and means for transferring the selection of the first plurality of capacitors to the second plurality of capacitors to adjust the second filter to a center frequency proportional to the desired frequency. The receiver described needs to calibrate a dummy tunable filter first before transferring the results from the dummy filter to the other tunable filter in the main signal path. The need to duplicate and tune another tunable filter is partly due to the fact the tuning of the filter in the main signal path will be affected by interferences coming from the antenna. This causes the die area to increase unnecessarily. 
     Recently, other receivers incorporating integrated tracking filters have been developed. U.S. Pat. No. 7,127,217 is included as an illustration, and shown in  FIG. 4 . In this prior art, the entire receiving path including the down-mixers must be configured to receive the filtered signal from the tunable filter. In addition, the calibration signal will radiate through the antenna if an additional antenna switch is not present. 
     In both prior arts, the calibration of these tunable filters must either be done in the factory to prevent radiation through the antenna, or outside the signal path. A receiver comprising an integrated tracking filter that minimizes interference to the receiver and prevents the radiation of calibration signal into the air during the filter calibration process is needed. If the entire receiver path is not required during the tuning process, receiver power consumption may be reduced even further. 
     SUMMARY 
     It is therefore an objective of the present invention to introduce a method of tuning a receiver having an integrated tracking filter. 
     According to one aspect of the present invention, an exemplary method includes the following steps: providing a tunable Low Noise Amplifier (LNA) having a plurality of LNAs and a plurality of LC loads coupled to the plurality of LNAs; selecting a desired LNA and a LC load corresponding to the desired LNA; isolating the desired LNA from the selected LC load; generating a local oscillation (LO) signal; routing the LO signal to the selected LC load; determining a first filtered signal power level of the LO signal with the WBPD; changing the capacitance in the selected LC load; determining a second filtered signal power level of the LO signal; comparing the first filtered signal power level to the second filtered signal power level; and according to the comparison result, determining whether to adjust the capacitance in the selected LC load. 
     According to another aspect of the present invention, an exemplary method includes the following steps: providing a tunable Low Noise Amplifier (LNA) having an LNA and a filter selected during a calibration mode, wherein an input signal of the tunable LNA is passed to the selected filter through the selected LNA during a normal mode; and during the calibration mode, generating a local oscillation (LO) signal, generating a filter LO signal by routing the LO signal to the selected filter, and tuning the selected filter according to the filter LO signal. 
     These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram of an exemplary embodiment of a receiver comprising an integrated tracking filter. 
         FIG. 2  is a diagram of the integrated tracking filter 
         FIG. 3  is a flowchart detailing the tuning method of the receiver in  FIG. 1 . 
         FIG. 4  is a diagram of a conventional receiver. 
     
    
    
     DETAILED DESCRIPTION 
     Certain terms are used throughout the description and following claims to refer to particular components. As one skilled in the art will appreciate, electronic equipment manufacturers may refer to a component by different names. This document does not intend to distinguish between components that differ in name but not function. In the following description and in the claims, the terms “include” and “comprise” are used in an open-ended fashion, and thus should be interpreted to mean “include, but not limited to . . . ”. Also, the term “couple” is intended to mean either an indirect or direct electrical connection. Accordingly, if one device is coupled to another device, that connection may be through a direct electrical connection, or through an indirect electrical connection via other devices and connections. 
     The disclosed receiver utilizes an integrated filter for tracking to an LO signal, wherein the filter includes a plurality of LC banks integrated in a Low Noise Amplifier (LNA). The filter works by tuning the LC banks to the LO signal in a calibration mode. 
     Please refer to  FIG. 1 .  FIG. 1  is a diagram of an exemplary embodiment of the disclosed receiver  200 . In this embodiment the receiver is a digital direct conversion receiver, however, the disclosed invention and method are also applicable to other receivers. The receiver  200  comprises an antenna  202  coupled to a tunable Low Noise Amplifier (LNA)  204 . The output of the tunable LNA  204  is coupled to a power detecting circuit  206 , comprising a Variable Gain Amplifier (VGA)  207  coupled to a Wide Band Power Detector (WBPD)  209 . The output of the VGA  207  is also coupled to a plurality of down mixers  221 ,  223 , which down convert received signals to base-band signals. A phase locked loop (PLL)  212  in the receiver  200  is coupled to buffer  217 , which is in turn coupled to the output of the tunable LNA  204 . A first switch unit  214  is coupled to the input of the tunable LNA  204 . A controller  220  is coupled to the first switch unit  214 , the tunable LNA  204 , and the power detecting circuit  206 . 
     The tunable LNA  204  in  FIG. 1  is made tunable by integrating a plurality of LC loads into the amplifier. Please refer to  FIG. 2 .  FIG. 2  is a diagram of the tunable LNA  204 . The tunable LNA  204  includes a plurality of band LNAs  232 ,  234 ,  236  coupled to a plurality of first switches  242 ,  244 ,  246  respectively. The plurality of first switches  242 ,  244 ,  246  is further coupled to LC loads  252 ,  254 ,  256  comprising the integrated filter. Each LC load  252 ,  254 ,  256  is coupled to a buffer  262 ,  264 ,  266  respectively, and coupled to the LO signal by means of a plurality of second switches  272 ,  274 ,  276 . 
     During calibration mode, an LNA (e.g.  232 ) is selected from the plurality of LNAs  232 ,  234 ,  236  and a corresponding LC load (e.g.  252 ) is selected from the plurality of LC loads  252 ,  254 ,  256  according to the frequency of the desired signal. 
     Next, an LO signal is generated according to the frequency of the desired signal and routed to the tunable LNA  204  by the buffer  217 . The first switch unit  214  is for shorting the tunable LNA  204  input during tuning. The controller  220  controls the first switch unit  214  to open or close and the buffer  217  to turn on or off. The LNAs  232 ,  234 ,  236 , down-mixers  221 ,  223  and the base-band circuits coupled to the output of these down-mixers are turned off during this mode. In addition, the buffer (e.g.  262 ) corresponding to the selected LNA is turned-on. The other buffers  264 ,  266  will be turned off. 
     All the first switches  242 ,  244 ,  246  are opened during calibration. This further has the function of isolating the LC load from the antenna  202 . The operation of the plurality of second switches is controlled by the controller  220 . The LO signal will then be passed to one of the selected LC loads  252 ,  254 ,  256  and the filtered LO signal power level measured with the WBPD. Then capacitance of the LC load is changed, and the filtered LO signal power level measured again. If the second filtered signal power level is higher than the first filtered signal power level, the capacitance in the LC load has been changed correctly and the filter is being tuned in the correct direction. This process will continue until the filter is centered. 
     During normal operation, the selected LNA (e.g.  232 ), the corresponding first switch (e.g.  242 ) is closed to couple the selected LNA to the tuned LC load (e.g.  252 ), the corresponding buffer (e.g.  262 ) is turned on and the first switch unit  214  is open, enabling input signals to be passed to the tunable LNA  204 . 
     In one embodiment, during the calibration mode, all capacitances in the selected LC load are switched in. The filtered LO signal is passed through the VGA  207  and the signal power then determined by the WBPD  209 . A unit capacitance is then removed from the LC load, and the signal power of the filtered LO signal determined again. If the first signal power level is lower than the second signal power level then the calibrating procedure must be repeated once more, by removing more unit capacitances from the selected LC load. Once the filter is centered then the tuning phase can be exited. 
     Please refer to  FIG. 3 .  FIG. 3  is a flowchart detailing an exemplary embodiment of the calibration mode of the disclosed receiver. The steps are as follows: 
     Step  400 : Open the first switch unit  214 ; 
     Step  402 : Select an appropriate LNA from the plurality of LNAs  232 ,  234 ,  236  (e.g.  232 ) along with a corresponding first switch from the plurality of first switches  242 ,  244 ,  246  (e.g.  242 ), a LC load from the plurality of LC loads  252 ,  254 ,  256  (e.g.  252 ), a buffer from the plurality of buffers  262 ,  264 ,  266  (e.g.  262 ) and a second switch from the plurality of second switches  272 ,  274 ,  276  (e.g.  272 ) according to desired input signal frequency; 
     Step  404 : Power off all un-selected LNAs ( 234 ,  236 ), buffers ( 264 ,  266 ) and down-mixers  221 ,  223 ; 
     Step  406 : PLL  212  lock to generate LO signal according to desired input signal frequency; 
     Step  408 : Open first switches  242 ,  244 ,  246  in LNA  204  and close selected second switch (e.g.  272 ); 
     Step  410 : Power on buffer  217  to route LO signal to LNA  204 ; 
     Step  412 : Switch in all capacitance in selected LC load (e.g.  252 ); 
     Step  414 : Detect signal power level (x) of filtered LO signal at VGA  207  output utilizing WBPD  209 ; 
     Step  416 : Remove one unit capacitance from LC load (e.g.  252 ); 
     Step  418 : Detect signal power level (y) of filtered LO signal at VGA  207  output utilizing WBPD  209 ; 
     Step  420 : Is x&gt;y? If yes go to Step  424 , if no go to Step  422 ; 
     Step  422 : Remove a next unit capacitance from LC load (e.g.  252 ), set x=y and go back to Step  418 ; 
     Step  424 : Power off buffer  217 ; 
     Step  426 : Open first switch unit  214  and selected second switch  272 , close selected first switch (e.g.  242 ); 
     Step  428 : Power on selected LNA (e.g.  232 ). 
     Please note that the above method is for a single pole LC, however, the disclosed method and apparatus can also be applied to a tracking filter having more than two LC poles. In this case, steps  418 ˜ 422  will be repeated for each pole until all poles are centered, and the method will then proceed to step  424 . Both embodiments fall within the scope of the present invention. 
     By implementing the tracking filter after the tunable LNA  204 , the tracking filter can be isolated from front-end interference and the calibration signal (LO signal) will not radiate into the air via the antenna  202 . Implementation of first switches  242 ,  244 ,  246  in the tracking filter, and the first switch unit  214  at the tunable LNA  204  input further ensure these benefits. Furthermore, by integrating the tracking filter in the tunable LNA  204 , the filter can be tuned directly in the signal path. 
     Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.