Abstract:
During tuner manufacture, the unique power parameters as a function of frequency and channel are stored on a memory unique to each tuner. When the tuner is subsequently incorporated into another device, such as a modem, the stored power parameters are made available to the subsequent device, thereby overriding the need for subsequent calibration tests to be run. The stored parameters aid in remote testing of the communication device and also allow for individual channel by channel setting of TOP levels.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a divisional of, commonly assigned, U.S. patent application Ser. No. 09/949,008 entitled “SYSTEM AND METHOD FOR COMMUNICATING STORED POWER MEASUREMENT PARAMETERS BETWEEN DEVICES IN A COMMUNICATIONS SYSTEM,” filed Sep. 7, 2001, the disclosure of which is hereby incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     This invention relates to tuners and more particularly to tuners where the unique power levels for each tuner is derived during manufacture of the tuner and stored on the tuner for use by devices which incorporate the tuner. 
     BACKGROUND OF THE INVENTION 
     A tuner is a frequency translation device which translates incoming RF signals from one frequency to a typically lower frequency. The information content of the received RE signal is normally modulated is some fashion on a carrier wave and the tuner serves to de-modulate the RE carrier in order to extract the original data stream. Tuners are used in practically every wireless transmitting/receiving device, particularly in the consumer product market. 
     For example, TV tuners are found in set top boxes, televisions, VCRs, as well as cable modems and other wide bandwidth access devices. These tuners take in a particular high frequency signal, translate and filter the signal, to a lower frequency signal, which is typically a fixed frequency for a particular application. The output of the tuner at the second frequency (called an intermediate frequency) is then available for further processing. 
     For most consumer applications, the price of the tuner is one of the driving motivations. The price drives the acceptance for use in the consumer market and tuner manufacturers must balance the price/performance model. With price in mind, there are many different topologies to achieve the functionality required by the tuner. Certain shortcuts or compromises must be made in terms of the hardware realization of the tuner in order to achieve competitive prices/performances. 
     In the field of cable modems, there are two basic broad categories of tuners in current use. One of those is what&#39;s commonly referred to as a double conversion tuner, which has two frequency translations. Between these two frequency translations that is a fixed, (typically higher) frequency where the filtering is achieved. The input of these devices is typically very wide band. 
     The other approach is what&#39;s commonly called a single conversion tuner. Single conversion tuners have tracking filters at the input that track the frequency of the signal, thereby reducing the total signal power on the input of the tuner. Single conversion tuners have one frequency translation direct from the incoming RF to a fixed frequency where filtering occurs. Traditionally, single conversion tuners are used in terrestrial devices, such as televisions. VCR receivers, etc. Through economies of scale due to the large number of these tuners, they tend to be the lower cost alternatives. 
     The single conversion tuner, however, because it does have tracking filters, requires a different architecture than does the double conversion tuner. One of these architectures is that the entire frequency band, which is very wide covering roughly from 50 to almost 900 megahertz, must be split into several bands in order to achieve the necessary tracking filter functionality. Since tuners are used as a “from end” to other devices it is important to calibrate the gain of the tuner to subsequent circuitry for proper performance. Thus, it is necessary to know the “typical” gain for a tuner to achieve proper overall system operation. 
     One drawback of single conversion tuners is that the gain of the tuner, that is the gain from the input signal to the output signal of the tuner as a whole, varies significantly over frequency, temperature and other aspects. The gain varies not only within the tuner across frequencies, (mainly because different circuits handle different bands) but also the characteristic and absolute gain of a particular frequency varies from tuner to tuner. Current tuners are typically set to provide a gain variation across all receive frequencies of a maximum of 8 db. As discussed, this presents a problem in that the gain is not repeatable from tuner to tuner nor within bands of the same tuner. Therefore, to extrapolate what a given gain will be at a given frequency of the tuner is difficult without actually measuring the gain of that device. Therefore, when tuners are selected at random and when a given frequency is selected it is difficult to know what the gain of that tuner will be, except that it will lie within the specified 8 db of gain range. 
     Due to the switching between bands of a tuner, there is discontinuity in gain between the higher end of one band and the lower end of the next band. One method of working around this problem is to have the demodulator which follows the tuner device extract the digital information from the frequency translated signal by a closed loop system. This means that although the absolute gain of the tuner over frequency may vary up to 8 db, the closed loop function of the demodulator will control the system such that as far as the demodulator input sees, the power is constant. These applications are acceptable for approximately 95% of current users. 
     The new DOCSIS Specification (DOCSIS1.1) has a number of modifications, many of which deal with software modifications that are irrelevant to the tuner functionality. However, there is one addition or modification to this specification which does have significant impact on the tuner, and that is a requirement allowing cable companies or others, to communicate with the modem end user device and poll this device to determine the incoming signal level to that modem. This power measurement capability will allow the cable company to troubleshoot their network since they could, in theory, find the power level at each end user on their cable plant, and in such a fashion, determine information regarding the quality of their signal, as well as any problems or interruptions in that signal. 
     Specifications for this ability to measure the power are in terms of an absolute gain variation. In other words, it is desired to determine the input power level within the variation of, for example, plus or minus 2 db. In addition, it is desired to be able to step change the power levels to within a certain specified resolution. For example, for a step increase of 1 db of power, the end user device should be able to report a modification in gain of 1 db plus or minus ½ db. These requirements are contrary to the gain variation of current tuners, since, as discussed above, tuners typically have a gain variation of 8 db. 
     In order to achieve accurate downstream power measurement the cable operator must perform (or have performed by the modem vendor) a very detailed, multi-point power calibration for each tuner/demodulator combination. Because of the fact that the measurement of power from the operator can be performed at any time and at any input power frequency, the closed loop gain variation controlled by the demodulator and which controls the gain of the tuner may be at any arbitrary setting. This requires that the end unit cable modem. be able to report the input power level within the specified accuracy regardless of the input power, as long as that is within the specified range. 
     Due to the gain variations across frequency, and also the non-linear behavior of the gain control AGC function, it is necessary to derive a matrix of data gain versus frequency and gain versus AGC control voltage, and store this in non-volatile memory existing within the end user&#39;s device. Currently, the end user measures the input power through reading of a register within the demodulator that is mounted on their cable modem. One drawback of this approach is that the reading of this downstream power measurement is very slow—the order of three minutes per device. Each device must be separately calibrated and the data stored therein. Once the data is stored, a simple table lookup procedure based upon frequency can be used to provide correction for power measurement determinations. 
     In typical RF receivers, there are two variable gain stages. The first is the RF amplifier, sometimes referred to as a Low Noise Amplifier, and the second is the IF (intermediate frequency) amplifier. To maximize the signal-to-noise-ratio (SNR), it is desirable for the RF amplifier to operate at maximum gain. However, if the input signal becomes large, it may overload the circuits downstream of the RF amp. Thus, there is some input signal level where it becomes necessary to roll off the gain of the RF amp. This point is called the TOP. In general, engineers defining receiver AGC characteristics call for the RF amp to operate at full gain for weaker signals and account for increasing signal strength by first reducing the gain of the IF amp. Once the IF amp is operating at a minimum gain setting, then the gain of the RF amp is reduced. Because of the variation of gain versus frequency and AGC setting in the RF amp one TOP setting for all channels does not result in the best overall performance. 
     One solution is to reduce the necessity of performing multi-point calibration by reducing the amount of allowed gain variation across the frequency range in the tuner itself. However, due to the fact that single conversion tuners are typically manually adjusted through the alignment of coils within the tuner and the fact that multiple operators align tuners at a given time, and given the number of degrees of freedom available within a tuner in order to achieve correct tracking and alignment, it is not feasible within a high production environment to achieve the necessary tolerance. 
     BRIEF SUMMARY OF THE INVENTION 
     These problems, as well as other, have been solved by performing the calibration while the tuner is being built and then providing the calibration data to the customer (tuner user) for each tuner. The logistics of providing the data to the user can be significant in terms of the number of points of gain versus frequency is solved by loading the data on, for example an electrical programmable read only memory (EEPROM) device on the tuner. In this way, each tuner would have all of the information necessary to make the proper identification. Since the calibration information is generated on test machines specific to tuners, the data can be generated concurrently with the alignment testing, thereby achieving lower costs. 
     In essence, the invention uses a non-volatile memory on the tuner to record the tuner&#39;s gain versus frequency and AGC setting during final test. This information is then used by a cable modern manufacturer, or more generally a communications receiver manufacturer, to account for the substantial gain variations from tuner to tuner as well as the variations at different frequencies and AGC settings within a given tuner when computing received power. The only information available to the modern regarding received power is the gain settings of the tuner and IF stage, and so to compute the received power, the modern must have accurate information regarding the gain. 
     We have solved the TOP control problem by using the data pertaining to gain as a function of frequency which is stored on the tuner to also control the TOP setting on a channel by channel basis. Thus optimizing the signal to noise ratio (SNR) of TOP received signals. 
     The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawing, in which: 
         FIGS. 1 and 2  show prior art graphs of various tuners with respect to different frequencies; 
         FIG. 3  shows a typical device into which a tuner can be incorporated; 
         FIG. 4  shows one embodiment of a tuner; and 
         FIG. 5  shows a flow chart of the steps of the method of one embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows the problem that must be overcome with respect to the use of different tuners across four different bands.  FIG. 1 , shows the gain variation of a single conversion tuner currently being produced. The wide frequency range from 58 to 860 megahertz is shown broken into three sub bands VHF-L (45 MHz-162 MHz); VHF-H (163 MHz-468 MHz; and UHF (469 MHz-860 MHz). On the plot of  FIG. 1  we have four different curves indicating the gain variation or the gain versus frequency of four different tuners in order to show the variation between tuners and the variation between bands. Within band  1 , extending from 50 MHz to 158 MHz, we have four gain curves,  101 - 1 ,  102 - 1 ,  103 - 1  and  104 - 1 . Each line shows a different tuner of a similar type showing the gain variation over frequency for that tuner. 
     Several facts can be seen from this plot. Within a given band, the shape of the gain variation may change dramatically. Also the gain for a particular tuner for a given frequency may be significantly different than other tuners at the same frequency.  FIG. 1  highlights the problem of gain variation between single conversion tuners over frequency. 
     Also, note that in  FIG. 1  the gain variation versus frequency is for one given gain setting of the tuner. Thus the AGC control function of the tuner is set at a constant level and the gain variation seen across frequency is due simply to the gain variation within the internal tracking filters and other internal circuitry of each tuner. 
       FIG. 2 , shows gain variation in another dimension, namely at one particular frequency. The frequency is fixed and we are looking at the gain versus the AGC control voltage. This voltage, as it is reduced, causes the gain of the tuner to reduce as well. There are six different curves on this figure. These six curves are at different frequencies as shown on the legend and shows the variation of gain versus voltage versus frequency. From the plot in  FIG. 2 , several things can be seen. First of all, gain varies with voltage, as would be expected with an AGC function. But this variation is also very non-linear and is frequency dependent due to the gain variation characteristics of the components being used in the tuner. This non-linear behavior then requires a multi-point calibration to replicate this curve—simply using two or three point linear interpolations cannot achieve the required resolution. 
       FIG. 4  is a depiction of the invention showing the block structure of a single conversion tuner currently being used, plus the additional onboard memory device, in this embodiment, EEPROM  300 , in which the calibration data which is obtained through the calibration process is stored. This information can later be accessed by the customer from within the end user device, such as a modem. EEPROM  300  is accessed through serial bus  26  which is also used to control the tuning function of the tuner via the PLL  72 . This means that there are no additional inputs or access points to the tuner required in order to obtain this functionality, thereby also allowing a pin for pin compatibility with existing solutions. 
       FIG. 3  shows the block diagram of a complete cable modem access device, one part of which is tuners with memory device. Also shown is bus control  26  feature going from processor subsystem  22 , to tuner  12 . The processor software is written by the customer for the end user device to control the end user device, and also to provide for tuner functionality or programming of the tuner. In a similar way, the customer can provide additional software features, transmitted via the same bus to access the information stored on memory device  300 . One can either access that information directly from the memory device for immediate processing, or download the complete information stored in that device to subsystem memory  24 . 
     Continuing in  FIG. 3 , cable modern  10  which includes tuner  12  will now by described in greater detail. Cable modem  10  is coupled to a network (not shown) by cable  11 , typically, a coaxial cable, which is connected to an input/output (“I/O”) port (not shown in  FIG. 3 ) of tuner  12 . For example, the I/O port may be a conventional F-type RF connector adapted to receive a feed within the CATV-frequency band which as discussed above extends from a 54 MHz to 858 MHz. In addition to a traditional CATV system site which generates a CATV signal onto cable  11  for distribution to plural destinations, one of which is tuner  12 , it is contemplated that various other systems for receiving or transmitting video, audio and/or other types of data signals, may also reside on the network. The power measurement signal requests comes form the network and the response to such request is fed back over the network. Tuner  12  is a single conversion tuner, but could be any type of tuner. 
     The tuner  12  is bi-directional in that an incoming signal received from the network via the cable  11  is directed downstream to the computer system while an incoming signal received from the computer system is directed upstream to the network. In operation, an input signal to cable modem  10 , for example, an RF signal, preferably a CATV signal which extends from 54 MHz to 858 MHz, generated by a device residing on the network, is received by tuner  12 . Tuner  12  converts the RF signal to differential IF signals (IF 1  and IF 2 ) at 43.75 MHz and outputs the first and second IF signals to a saw filter  14  where the signal is filtered and limited to a 6 MHz bandwidth. The resultant signal is passed to demodulator  16 , where an analog-to-digital (or “A/D”) conversion of the signal, followed by a QAM 64/256 demodulation, viterbi decoding and forward error correction of the input signal is performed. The digital signal produced thereby is then transmitted to media access controller (or “MAC”)  18  which controls the protocol and administration layer of cable modem  10 . 
     MAC  18 , which may be implemented in either hardware or a combination of hardware and software, assigns frequencies and data rates for upstream transmission and allocates time slots for upstream transmission. From MAC  18 , data continues on to interface  19  for a computer system (not shown). For example, interface  19  may be a peripheral connection interface (or “PCI”) bus of the computer system. Preferably, the PCI bus should include a slot in which cable modem tuner  10 , implemented on a card, is inserted. Unlike most cards, however, cable modem  10  should be enclosed in a shielded unit to prevent the computer system from interfering with proper operation of tuner  12 . 
     In addition to being part of the downstream path from the CATV signal source to the computer system, MAC  18  is also in the upstream path which originates at the computer system and extends to the network. Digital data originating a the computer system, for example, at the processor of memory subsystems thereof, is placed on the PCI bus or other interface  19  for transfer to MAC  18 . From MAC  18 , the digital data is passed on to the modulator  20 . There, the digital data is modulated onto a selected frequency and converted into an analog signal. From modulator  20 , the signal is transmitted to tuner  12  where it enters at a first input port of a front end duplex filter stage  32  which, as will be more fully described below, directs the analog signal out an input/output (or “I/O”) port and onto the cable  11  for transmission to a specified device residing on the network. 
     Finally, cable modem  10  also includes processor subsystem  22 , for example, a microcontroller, and memory subsystem  24 . Processor subsystem  22  is coupled to tuner  12  by an IIC bus  26  while memory subsystem  24  is coupled to processor subsystem  22  by local bus  28 . Via IIC bus  26 , processor subsystem  22  controls band selection and tuning for tuner  12 , for example, using information residing in memory subsystem  24 . 
     Referring next to  FIG. 4 , tuner  12  will now be described in greater detail. As will be more fully described below, tuner  12  is comprised of a front end comprised of front end duplex filter stage  32  and front end decoupler stage  34  and back end comprised of single conversion tuner stage  36 . Front end duplex filter stage  32  has first input/output (or “I/O”), line coupled to cable  11  by an I/O port  13 , a second, input, line coupled to upstream modulator  20  of cable modem  10  by input port  15  and a third, output, line coupled to front end decoupler stage  34  of tuner  12 . The front end duplex filter stage  32  enables tuner  12  to handle bi-directional transmissions. Specifically, front end duplex filter stage  32  is configured such that an incoming CATV signal originating on the network will propagate along the downstream path to the output line of front end duplex filter stage  32 . CATV signal will, however, blocked from propagating along the upstream path along the second input line. Conversely, a signal will, however, be blocked from propagating along the upstream path along the second input line. Conversely, a signal originating at the computer will propagate along the upstream path and onto cable  11  while being blocked from traveling along the downstream path to the output line of front end duplex filter stage  32 . 
     Front end decoupler stage  34  has an input line coupled to the output line of front end duplex filter stage  32  and the output line coupled to single conversion tuner stage  36 . In turn, single conversion tuner stage  36  has an input line coupled to the output line of front end decoupler stage  34  and first and second output lines IF 1  and IF 2  coupled to saw filter  14  of cable modem  10 . From multiple CATV signals, each at a discrete frequency within the 54 to 858 MHz frequency band, entering the single conversion tuner, the CATV signal operating at a selected frequency is converted into an IF signal and passed to saw filter  14  ( FIG. 3 ) while the CATV signals at the remaining frequencies are rejected. 
     Front end decoupler stage  34  decouples the front end of tuner  12 , i.e., front end duplex filter stage  32  and front end decoupler stage  34 , from back end of the tuner  12  i.e., single conversion tuner stage  36 , coupled to the output line of front end decoupler stage  34 . As a result, an incoming CATV signal comprised of a plurality of RF signals passes through front end decoupler stage  34  and enters single conversion tuner stage  36  where a selected RF signal is converted into the IF signal while the remaining ones of the plurality of RF signals are rejected but the ones of the plurality of RF signals, as well as return signals produced by single conversion tuner stage  36 , are blocked from passing back through front end decoupler stage  34 . Without front end decoupler stage  34 , both the rejected ones of the plurality of RF signals and the return signals produced by the single conversion tuner stage  36  would pass through front end duplex filter stage  32  and back onto cable  11  where, since cable  11  is typically coupled to multiple tuners, the return signal would create interference with the CATV signal being received by other tuners. 
     The configuration of each of front end duplex filter stage  32 , front end decoupler stage  34  and single conversion tuner stage  36  will now be described in greater detail. As previously mentioned, front end duplex filter stage  32  has an I/O port coupled to cable  11 , an input port  15  coupled to the upstream modulator  20  of cable modem  10  and an output coupled to an input of front end decoupler stage  34 . Front end duplex filter stage  32  further includes node  17  coupled to I/O port  13 , low bandpass filer  52  positioned between node  17  and input port  13  and a high bandpass filter  54  positioned between node  17  and the output line. Cable modem  10  is bi-directional in that the front end duplex filter stage  32  of tuner  12  enables both the transmission of signals from a signal source on the network to the PC and from the PC to the network. Specifically, the frequency band for the input signal received from a signal source on the network is the CATV frequency band—54 MHz to 858 MHz. Low bandpass filer  52  is selected such that only signals below the CATV frequency band can pass. For example, low bandpass filter  52  may have a pass band having a lower limit at 5 MHz and an upper limit at 42 MHz. As a result, low bandpass filter  52  blocks the input signal to cable modem  10  which originates at a signal source on the network from propagating to upstream modulation circuit  20 . High bandpass filter  54 , on the other hand, is selected to have a pass band having lower and upper limits which correspondence to the lower and upper limits of the CATV frequency band. Accordingly, high bandpass filter  54  passes the input signal to cable modem  10  which originates on the network to front end decoupler stage  34 . Conversely, the frequency band for the input signal to cable modem  10  which originates at the computer extends from 5 MHz to 42 MHz. As a result, low bandpass filter  52  passes the input signal to I/O port  13  while high bandpass filer  54  blocks the input signal from propagating to front end decoupler stage  34 . 
     Front end decoupler stage  34  is comprised of an automatic gain control (or “AGC”) circuit  56  having an input coupled to the output of front end duplex filter state  32  and an output coupled to an input of broadband amplifier  58 . In turn, the output of broadband amplifier  58  is input single conversion tuner stage  36 . The input signal to tuner  12  which is passed by high bandpass filter  54  is fed to AGC circuit  56  where, in response to an AGC level signal generated by demodulator circuit  16 , the input signal is sufficiently attenuated to avoid an overload within single conversion tuner stage  36  when the input signal is later passed thereto. 
     In order for broadband amplifier  58  to carry the input signal to single conversion tuner stage  36 , broadband amplifier  58  must have a source impedance matching that of cable  11 . Otherwise, the input signal would be rejected by the broad band amplifier  58 . Accordingly, broadband amplifier  58  is a low gain, high power amplifier which matches the source impedance of cable  11 , typically 75 ohms, and which includes pass bands centered at the frequency of each of the 110 channels within the 54 MHz to 858 MHz CATV frequency band for which a signal is carried. Thus, unlike conventional single conversion tuners, all channels within the frequency band are passed to single conversion tuner stage  36 . Broadband amplifiers such as broadband amplifier  54  are unidirectional. Accordingly, broadband amplifier  58  decouples single conversion tuner stage  36  from the front end of tuner  12  such that the channels passed to single conversion tuner stage  36  but later rejected thereby, as well as any return signals generated by single conversion tuner stage  36  itself, are rejected by broad band amplifier  58 , thereby preventing these signals from passing back through front end decoupler stage  38  and into the front end of tuner  12 . 
     Single conversion tuner stage  36  is comprised of first, second and third tuner circuits, each for a different frequency range. The first (or UHF) tuner circuit selects channels within the 469 MHz to 858 MHz frequency range and is comprised of a first adjustable bandpass filter  60 - 1 , AGC amplifier circuit  62 - 1 , second adjustable bandpass filter  64 - 1 , mixer  66 - 1  and local oscillator (or “LO”)  68 - 1 . The second (or VHF high) tuner circuit selects channels within the 163 MHz to 468 MHz frequency range and is comprised of a first adjustable bandpass filter  60 - 2 , AGC amplifier circuit  62 - 2 , second adjustable bandpass filter  64 - 2 , mixer  68 - 2  and local oscillator  68 - 2 . Finally, the third (or VHF low) tuner circuit selects channels within the 54 MHz to 162 MHz frequency range and is comprised of first adjustable filter  60 - 3 , AGC amplifier circuit  62 - 3 , second adjustable bandpass filter  64 - 3 , mixer  66 - 3  and local oscillator  68 - 3 . 
     Apart from the frequency of the channel selected thereby, each of the first, second and third tuner circuits operate identically. Accordingly, only one of the three tuner circuits need be described in greater detail. Thus, by way of example, the operation of the first tuner circuit shall now be described. As previously set forth, the input signal entering the single conversion tuner stage  36  includes all of the 110 channels contained within the CATV frequency range. Processor subsystem  22  selects a channel to be selected by adjustable bandpass filer  60 - 1 , typically of the varactor tuning type, for conversion to an IF signal and issues an appropriate instruction to phase locked loop (or “PLL”) circuit  72  to lock to the frequency of the selected channel. In response, PLL circuit  72  adjusts the bandpass for adjustable bandpass filer  60 - 1  such that the selected channel passes to AGC amplifier circuit  62 - 1  where the signal for the selected channel is amplified with sufficient gain to eliminate any contribution of noise from successive stages of the single conversion tuner. The signal is then passed on to second adjustable bandpass filter  64 - 1 , again, typically of the varactor tuning type, for a second filtering of the, now amplified, signal, again at the frequency of the selected channel. 
     From second adjustable bandpass filter  64 - 1 , the selected signal propagates to mixer circuit  66 - 1  for conversion from an RF signal to an IF signal. Specifically, under control of PLL circuit  72 , local oscillator  68 - 1  generates a signal having a frequency of the received signal plus the IF of 43.75 MHz. When transmitted to mixer  66 - 1  together with the received signal, the frequency of the selected signal, the IF signal of 43.75 MHz, is generated. The IF signal is then passed by mixer  66 - 1  to IF stage  70  where a differential amplifier converts the IF signal to a differential IF signal which is passed to saw filter  14  and along the downstream path through the cable modern  10  in the manner previously described. 
     The strength of the signal generated by local oscillator  68 - 1  is often of great concern as a possible source of a return signal which, when added to the selected signal, would pass back through second adjustable bandpass filters  64 - 1 , AGC amplifier circuit  62 - 1  and first adjustable bandpass filter  60 - 1  of single conversion tuner stage  36  and the front end of tuner  12  and onto the network where it could potentially interfere with the CATV signal being received by other tuners residing on the network. However, broadband filter  58  sufficiently decouples any return signal generated by local oscillator  68 - 1  which passes back through single conversion tuner stage  36  to comply with the &lt;−55 dBmV limitation on return signal specified by MCNS standards. Further, the isolation of the return path input and the IF output is at least 85 dB to ensure no cross-interference from the presence of an upstream signal. Finally, the pass band flatness within a 6 MHz band is, at most, 2.5 dB. 
     Turning now to  FIG. 5 , there is shown system  500  where the tuner is built at Step  501 . Following production of the tuner, the gain variation must be accounted for in the alignment process. This is done at Step  502 . In Step  502  the tuner is automatically calibrated, taking into account measured gain variation over frequency and AGC voltage. This gain data is then stored in a tabular format within the EEPROM (or within some other non-volatile memory device) at Step  503  located in the tuner. Typical calibration equipment would include an RF signal source and a power or spectrum measuring device which would accurately detect the output power from the unit, thereby allowing a comparison of input versus output power, which would treble the absolute gain of the tuner. 
     One data format for storage is shown below. 
     
       
         
               
               
               
               
             
               
             
               
               
               
               
             
               
             
               
               
               
               
               
             
               
               
               
               
             
               
             
               
               
               
               
             
           
               
                   
               
               
                 Byte Address 
                   
                   
                   
               
               
                 [hex] 
                 Byte Value 
                 Symbol 
                 Remark 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Fixed Production Data 
               
             
          
           
               
                 00 
                 t.b.d. 
                   
                   
               
               
                 01 
                 t.b.d. 
                   
                   
               
               
                 . . . 
                   
                   
                   
               
               
                 0F 
                 Data Check Sum 
                   
                 calculation t.b.d. 
               
             
          
           
               
                 Fixed Tuner Data 
               
             
          
           
               
                 10 
                 Lowest Frequency  
                 VHF-L 
                 FVHF 
                 2 bytes 
               
               
                 11 
                 Takeover Frequency  
                 VHF-L to VHF-H 
                 TOF1 
                 2 bytes 
               
               
                 12 
                 Takeover Frequency  
                 VHF-H to UHF 
                 TOF2 
                 2 bytes 
               
               
                 13 
                 Highest Frequency  
                 UHF 
                 FUHF 
                 2 bytes 
               
             
          
           
               
                 14 
                 Nominal Gain Value 
                 GNOM 
                 1 byte 
               
               
                 15 
                 Number of Frequencies for RF Gain 
                 N 
                 1 byte 
               
               
                 16 
                 Number of Frequencies for RF AGC 
                 L 
                 1 byte 
               
               
                 17 
                 Number of Voltages for RF AGC 
                 K 
                 1 byte 
               
               
                 18 
                 Number of Voltages for AGC 
                 M 
                 1 byte 
               
               
                 19 
                 Reserved 
                   
                   
               
               
                 . . . 
                   
                   
                   
               
             
          
           
               
                 Dynamic Data 
               
             
          
           
               
                 19 + 1 
                 Frequency 1 for RF Gain 
                 FG1 
                   
               
               
                 19 + i 
                 Frequency i for RF Gain 
                 FGi 
                   
               
               
                 . . . 
                   
                   
                   
               
               
                 19 + N 
                 Frequency N for RF Gain 
                 FGN 
                   
               
               
                 19 + N + 1 
                 RF Gain at Frequency 1 
                 G1 
                   
               
               
                 . . . 
                   
                   
                   
               
               
                 19 + N + N 
                 RF Gain at Frequency N 
                 GN 
                   
               
               
                 19 + 2 * N + 1 
                 Frequency for RF AGC 1 
                 FA1 
                   
               
               
                 . . . 
                   
                   
                   
               
               
                 19 + 2 * N + L 
                 Frequency for RF AGC L 
                 FAL 
                   
               
               
                 19 + 2 * N + L + 1 
                 Voltage for RF AGC 1 
                 VA1 
                   
               
               
                 . . . 
                   
                   
                   
               
               
                 19 + 2 * N + L + K 
                 Voltage. for RF AGC K 
                 VAK 
                   
               
               
                 (c1) + 1 
                 RF Gain Reduction at FA1 and VA I 
                 A11 
                 (c1) = (19 + 2 * N + L + K) 
               
               
                 . . . 
                   
                   
                   
               
               
                 (c1) + K 
                 RF Gain Reduction at FA1 and VAK 
                 A1K 
                   
               
               
                 (c1) + K + 1 
                 RF Gain Reduction at FA2 and VA1 
                 A21 
                   
               
               
                 . . . 
                   
                   
                   
               
               
                 (c1) + (L − 1) * K + K 
                 RF Gain Reduction at FAL and VAK 
                 ALK 
                   
               
               
                 (c2) + 1 
                 Voltage for IF AGC 1 
                 VIF1 
                 (c2) = (c1) + (L − 1) * K + K 
               
               
                 . . . 
                   
                   
                   
               
               
                 (c2) + M 
                 Voltage for IF AGC M 
                 VIFM 
                   
               
               
                 (c3) + 1 
                 IF Gain Reduction at VIF1 
                 B1 
                 (c3) = (c2) + M 
               
               
                 . . . 
                   
                   
                   
               
               
                 (c3) + M 
                 IF Gain Reduction at VIF1 
                 BM 
               
               
                   
               
             
          
         
       
     
     The customer, perhaps a company building a modem or other device, would then receive the tuner for mounting. STEP  504 , into the customer&#39;s device. The tuner then would include the non-volatile memory with the calibration data stored in it. In order to account for any gain variations within the customer&#39;s device, which includes components or circuitry external and additional to the tuner, the customer would ideally perform a single or multi-point power calibration, STEP  505 , themselves in order to account for any additional gain variation within their entire system. However, since the tuner variation has already been accounted for in the calibration step performed during the tuner manufacture, the chances are that most of the gain variation in the customer&#39;s device will not be frequency dependent. Therefore, one or a low number of point calibration could be used. 
     Following this simple calibration at the customer&#39;s site, the data stored in the memory of the tuner is then read, STEP  506 . This data could be maintained in the tuner or it can be shifted to the customer&#39;s device. In some situations, the customer&#39;s data could be stored in the tuner&#39;s memory. The customer could adjust the simple calibration done in their own factory by an offset, as a function of frequency using the tuner&#39;s stored data. By accounting for these two calibrations, one performed by the tuner manufacturer and a simple one performed at the customer&#39;s site, one can then calculate within the required amount of accuracy, the received power coming into the device. Of course the data format which would be stored in the tuner and the calibration that the customer uses at their site could be in many different versions, fashions or configurations, and therefore the calculation done at the end to determine the complete power gain in the tuner could take many forms. 
     It should be noted also that the information stored in the tuner can be generic for many different end users, or could be tailored to a specific user&#39;s requirement to match a specific format, if so required. Thus, a tuner for one application may have one type of information stored therein, and a tuner that is destined to be used for another group of devices could have a different format of data. Also, it is possible that a master tuner data memory can be created and maintained and different users then would use different sections of that data as they deem appropriate. 
     In addition, the concepts of this invention can be used to control the “take-over-point” (TOP). TOP refers to the input level where one AGC stops (i.e. RE gain control) and another takes over (i.e. IF gain control). Typically, one IF AGC voltage (at one selected frequency) is defined as the TOP. So long as the IF AGC voltage is above this level the RE AGC will be at its maximum (no gain reduction). Using this invention one could, if desired, set different TOP values for different frequencies. Or one could tailor the TOP for each modem based on data stored in the tuner. 
     Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.