Abstract:
Satellite set-top boxes (STB) are increasingly being designed with multiple tuners, making them capable of receiving more than one program at a time. In addition, satellite STBs are increasingly being designed with multiple inputs, to permit reception of additional channels that will not fit within the conventional satellite intermediate frequency (IF) band (950-2150 MHz). Often, the STB must route these multiple inputs to the multiple tuners with some form of switching function, to allow each tuner to receive all channel bands. Accordingly, the invention includes an RFIC with two RF inputs and three RF outputs, and a crossbar switch that can route any input to any output. The two inputs are amplified by low-noise amplifier stages.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of U.S. Provisional Application No. 60/860,225 filed Nov. 21, 2006, which is incorporated herein by reference in its entirety. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention generally relates to a low noise amplifier with multiple inputs and multiple outputs. 
   2. Background Art 
   Satellite set-top boxes (STB) are increasingly being designed with multiple tuners, making them capable of receiving more than one program at a time. In addition, satellite STBs are increasingly being designed with multiple inputs, to permit reception of additional channels that will not fit within the conventional satellite intermediate frequency (IF) band (950-2150 MHz). Often, the STB must route these multiple inputs to the multiple tuners with some form of switching function, to allow each tuner to receive all channel bands. 
   At present, satellite STBs use complex front ends designed with discrete transistors, diodes and filters to perform these functions. These discrete front ends have limited performance and require large amounts of area on the STB printed circuit boards (PCBs). This is because complex circuits that would improve the performance, such as automatic gain control (AGC) and differential amplifiers are prohibitively large and expensive when implemented with standard discrete components. 
   In addition, discrete RF design is a difficult and time-consuming process. Given the short life cycles of consumer electronic products, a lengthy and error-prone design process may be unacceptable. 
   What is necessary is a radio-frequency integrated circuit (RFIC) that incorporates splitting, switching, AGC, and filtering functions for multi-input/multi-tuner satellite STBs. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES 
     The accompanying drawings illustrate the present invention and, together with the description, further serve to explain the principles of the invention and to enable one skilled in the pertinent art to make and use the invention. 
       FIG. 1  is a block diagram of a multi-input multi-output LNA according to embodiments of the present invention. 
       FIG. 2  illustrates a RF shielding package according to embodiments of the present invention. 
       FIG. 3  illustrates a high isolation T-switch for use in the crossbar switch  104 . 
       FIGS. 4A-4D  illustrate various input/output configurations for the LNA. 
       FIG. 5  illustrates a switchable notch filter according to embodiments of the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   This specification discloses one or more embodiments that incorporate the features of this invention. The embodiment(s) described, and references in the specification to “one embodiment”, “an embodiment”, “an example embodiment”, etc., indicate that the embodiment(s) described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is understood that it is within the knowledge of one skilled in the art to effect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described. An embodiment of the present invention is now described. While specific methods and configurations are discussed, it should be understood that this is done for illustration purposes only. A person skilled in the art will recognize that other configurations and procedures may be used without departing from the spirit and scope of the invention. 
   Overview 
   One or more embodiments of the present invention provide an amplifying circuit that includes a first amplifying stage, a second amplifying stage, and a crossbar switch. The first amplifying stage has multiple inputs. The second amplifying stage has multiple outputs. The crossbar switch is configured to direct information or electrical signals from any of the multiple inputs to any of the multiple outputs. The first amplifying stage comprises a first variable gain amplifier (VGA) having an output coupled to the crossbar switch and a second variable gain amplifier having an output coupled to the crossbar switch. Each of the VGAs serves as an input to the amplifying circuit. 
   The amplifying circuit also includes a buffer amplifier coupled in parallel to the first VGA. In this way, the amplifying circuit may receive a pre-amplified signal from an external source, i.e., another amplifying circuit, and bypass first VGA via the buffer amplifier. This configuration helps avoid signals to be over amplified which cause the amplifier circuit to saturate. 
   Electrical signals could also be transferred from one of the multiple inputs to one of the multiple outputs. This is accomplished using a daisy chain which directs electrical signals from one of the inputs to one of the outputs via a second buffer amplifier. The output of the second buffer amplifier is coupled to the crossbar, which can switch information or electrical signals to any of the multiple outputs of the amplifying circuit. 
   The amplifying circuit also includes power detectors configured to adjust the output power level of the first and second VGAs based on their respective previous output. In this way, the amplifying circuit may achieve a desired level of signal amplification. 
   Exemplary Amplifying Circuit Embodiment(s) 
     FIG. 1  shows a block diagram of a radio frequency integrated circuit (RFIC)  100  according to an embodiment of this invention. RFIC  100  includes a first amplifying stage or input amplifying stage  102 , a second amplifying stage or output amplifying stage  104 , a crossbar switch  106 , and an optional controller  125 . Stage  102  has two inputs. Each input is an input to a variable gain amplifier (VGA) or a low noise amplifier (LNA)  108   a - b , which amplifies received data signals and outputs amplified signals to crossbar switch  106 . In an example, the RFIC  100  processes a signal in a range between 250 MHz and 2150 MHz. Processing by the RFIC  100  is not limited to this frequency range. 
   Crossbar switch  106  can route any input to any output. RFIC  100  also includes a daisy chain bypass  120  that couples together any input to any output. The invention is not limited to the number of inputs and outputs shown, as any number inputs could be routed to any number of outputs. In an embodiment, controller  125  is configured to instruct crossbar switch  106  to route signal from anyone of the inputs to anyone of the outputs. In this way, RFIC  100  may be programmed to route data signal in various ways. 
   In the embodiment where VGAs are used, each of the VGAs ( 108   a  or  108   b ) is controlled by an automatic gain control (AGC) loop. In this embodiment, the AGC loop adjusts the input amplifier gain to maintain the total power of all of the output signals constant. The AGC loop includes power detectors  110   a - b  to detect the respective output power of input amplifiers  108   a - b  and control the gain of amplifiers  108   a - b . If the AGC set point is chosen appropriately, this approach will optimally balance noise and distortion arising from each of the input amplifiers. This is in contrast to an AGC loop which operates to maintain only the desired signal power at some set level. Such a loop will set the gain very high when the desired signal is weak; possibly producing to much distortion if the unwanted signals are strong. Vice-versa, when the desired signal is strong but most other signals are weak, it will set the gain too low, possibly compromising signal-to-noise ratio (SNR). If no AGC loop is used, the dynamic range of the RF components in the STB must be higher, usually leading to higher costs and power dissipation. 
   A feature of this embodiment is a circuit  121  which measures the gain control voltage of the AGC loop. This value is then used in combination with other information to obtain a RSSI (received signal strength indication) function. 
   RFIC  100  also includes two buffer amplifiers  112  and  114 . Buffer amplifier  112  is coupled in parallel to VGA  108   a . Buffer amplifiers  112  and  114  are used to drive controlled-impedance outputs at the desired power level. Other embodiments might have more inputs and/or outputs, or have less than full crossbar switches. As shown in  FIG. 1 , the LNA is a two stage amplifier with crossbar switch  106  between the input amplifier stage  102  and the output amplifier stage  104 . Any input can be coupled to any output via crossbar switch  106 . For example, output signals from VGA  108   a  can be switched to output node  116   a ,  116   b , or  116   c  via node  118   a ,  118   b , or  118   c , respectively. Similarly, output signals from VGA  108   b  can be switched to output node  116   a , node  116   b , or node  116   c  via node  118   d ,  118   e , or  118   f , respectively. Output amplifiers  122   a - c  are coupled to common output node  116   a . Output amplifiers  112   d - f  are coupled to common output node  116   b . Similarly, output amplifiers  122   g - h  are coupled to common output node  116   c.    
   As shown in  FIG. 1 , output node  116   c  is coupled to daisy chain  120  which provides input signals to buffer amplifier  114 . Alternatively, daisy chain  120  receives a signal at the input of buffer amplifier  114  and outputs the received signal at node  116   c . As mentioned, daisy chain  120  is a bi-directional medium, meaning signal may be transferred to or from node  116   c  and an input node of buffer amplifier  114 . Signals from output node  116   c  may be already amplified by VGA  108   a  or  108   b , accordingly these pre-amplified signals are forwarded to output node  116   a  or  116   b  via buffer amplifier  114  to avoid over amplification which may cause output amplifier  122   c ,  122   f , or  122   i  to saturate. In this embodiment, buffer amplifiers  112  and  114  are unity gain buffer amplifier. 
   Although not shown, RFIC  100  may include a switching controller coupled to crossbar switch  106 . Switching controller may receive switching inputs from an external source. Switching controller main responsibility is to provide instructions to crossbar switch  106  on where to direct an input RF signal. For example, switching controller may inform crossbar switch  106  to gate input RF signals from VGA  108   a  to output node  116   b  or  116   c.    
   An important requirement for multi-input STBs is that the multiple inputs do not interfere with each other. This means that there must be a high isolation between the different inputs and outputs. High isolation and low noise can be achieved with a combination of circuit and package design techniques. 
   One technique is the use of differential RF input signals. Differential signals have several advantages over single-ended input signal such as higher operating frequency, higher signal to noise ratios, and less sensitivity to noises. Unlike single-ended signals which need a reference signal, differential signals are referenced to each other, thus allowing a differential circuit to operate at a higher frequency by eliminating the need of timing the single-ended signal with respect to the reference signal. Differential signals are less susceptible to noises because any external noises that enter the system will be found on both differential signals, thus creating common mode signals. In a differential signals system, common mode signals cancel each other out and have little effect on the original signal. 
     FIG. 2  illustrates an IC package  200  according to an embodiment of the present invention. IC package  200  includes an integrated circuit or die  202  and a circuit board  204 . Differential signals  210   a  and  210   b  are fed into the circuit board  204  at input terminals  206 . Die  202  receives differential signals  210   a  and  210   b  at input terminals  208 . Input terminals  206  includes ground pads  207   a  and  207   d , an inverting input pad  207   b , and a non-inverting input pad  207   c . Input pads  207   b - c  are placed between and close to ground pads  207   a  and  207   d . In this way, stray electrical noises are induced to couple onto ground pads  207   a  and  207   d  instead of input pads  207   b - c , thus shielding input pads  207   b - c  from external noises. Similarly, transmission lines  211   b - c  are also shielded by transmission lines  211   a  and  211   d.    
   As shown in  FIG. 2 , input terminals  208  includes shield pads  212   a  and  212   d , an inverting input pad  212   b , and a non-inverting input pad  212   c . Shield pads  212   a  and  212   d  are tied together by a common transmission line  214  and are grounded via ground pads  207   a  and  207   d . In this way, input pads  212   b - c  are effectively shielded from noises that are common to both shield pads  212   a  and  212   d . Further, transmission line  214  is placed such that it surrounds input pads  212   b - c . This helps attract external noises such as stray electrical couplings away from input pads  212   b - c.    
   Another technique is to place the inputs and outputs that must be isolated from each other on different sides of the IC package. This reduces unwanted coupling both by increasing the distance between signal lines and (when the signals are on adjacent sides) because of the lower mutual inductance for lines that are oriented at 90 degrees to each other, compared to parallel lines. 
   Isolation must also be considered in the design of crossbar switch  106 . In a switch, off isolation is a measure of how well the switch isolate the output from any input signal during “off” or break mode. Generally, the off isolation of a switch is frequency dependent. At very high frequency, isolation degrades as more signals from the input couple into the output. Thus it is essential to use high-isolation switch in designing crossbar switch  106 . Thus, whenever appropriate, hi-isolation T-Switch is used at every switching junction. 
     FIG. 3  illustrates an exemplary T-Switch  300  used in crossbar switch  106 . T-Switch  300  is generally constructed of three n-channel MOSFETs (metal-oxide semiconductor field-effect transistor). T-Switch  300  provides high isolation by coupling a transistor  302  to ground. When T-Switch  300  is in off mode, transistor  302  is on. In this way, signals that bleed through the input are shunted to ground. It should be noted that other type of isolation switches could also be used in designing and fabricating crossbar switch  106 . 
   To further reduce noises and interferences, RFIC  100  utilizes frequency filters to filter out any harmonics of the input RF signals. In general, RF tuners are susceptible to interference from RF signals at multiples (e.g. twice) of the desired frequency. This is due to the harmonic response of the tuner mixer. Specifically, an interfering RF signals can be received at 2× the desired RF input signal when using direct conversion. The interfering RF signal can mix with the 2nd harmonic of the local oscillator, so as to be down-converted directly to baseband, thereby interfering with the preferred down-converted baseband signal. To reduce this susceptibility, switched filters may be inserted in the output signal path. The switched filters operate to remove the interfering RF signal that occurs at 2× the local oscillator frequency, which is also 2× p the desired RF frequency for direct conversion. An embodiment of such a switched filter is shown in  FIG. 5 . This is a switched LC notch filter, with the notch centered at about 2 GHz. This reduces the level of unwanted double-frequency signal reaching the tuner when the desired RF signal is near 1 GHz. The switched filer may also reduce a total power input to the tuner. 
   Filters might also be included for other purposes, such as to reduce low-frequency signals that produce unwanted second-order distortion. 
     FIG. 4A-4D  illustrates some of the possible applications of this IC in a STB. As shown, by having three outputs, it is possible to cascade two or more ICs and drive more than three tuners. This also permits more than two inputs per STB. 
     FIGS. 4A-B  illustrate exemplary implementations of RFIC  100  in integrated circuits  400  and  410  for use in a STB. As shown in  FIG. 4A , IC  400  has two inputs  402   a - b  and three outputs  404   a - c . Output  404   b  is a daisy output, which may be routed to a tuner or to another RFIC. An example of such implementation is shown in  FIG. 4C . Similar to IC  400 , IC  410  has three outputs  414   a - c , but with only a single input, as shown in  FIG. 4B . 
     FIG. 4C  illustrates an exemplary implementation of RFIC  100  in a cascade configuration  420  that has two ICs interconnected by the daisy output of one of the ICs. Configuration  420  includes two ICs  100   a - b . Each IC is similar to IC  100 . As shown, daisy output  423  of IC  100   a  is coupled to an input  425  of IC  100   b.    
   In an embodiment, input  425  is coupled to a RFIC similar to RFIC  100  that is part of IC  100   b . More specifically, input  425  is coupled to a buffer amplifier similar to buffer amplifier  114 . In this way, RF signals from daisy output  423  will not be over-amplified which may lead to saturation. Alternatively, input  425  may be coupled to buffer amplifier  112 . As shown, configuration  420  yields 5 outputs for one input. 
     FIG. 4D  illustrates a cascade configuration  430  similar to configuration  420  according to an embodiment of the present invention, but with multiple inputs. Configuration  430  includes two ICs  432   a - b . Each of the ICs  432   a - b  is similar to IC  400 . As shown, configuration  430  has four inputs and four outputs. 
   It should be understood that the configurations above are not limited to the number of inputs and outputs shown, as any number inputs could be routed to any number of outputs, and that more than two ICs could be used in a configuration. 
   This invention describes a satellite STB front end that can drive multiple tuners from multiple sources. It incorporates a crossbar switch, so that any tuner can be driven from any input. It may incorporate a daisy-chain output, to permit cascading multiple ICs. This allows the STB to include more inputs and/or more tuners. It may incorporate AGC loops, which reduce the dynamic range requirements of the STB RF circuits and therefore their cost and complexity. It may incorporate switched filters to reduce the susceptibility of the STB to unwanted signals. 
   CONCLUSION 
   Example embodiments of the methods, systems, and components of the present invention have been described herein. As noted elsewhere, these example embodiments have been described for illustrative purposes only, and are not limiting. Other embodiments are possible and are covered by the invention. Such other embodiments will be apparent to persons skilled in the relevant art(s) based on the teachings contained herein. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.