Calibration of modular system using an embedded calibration signal generator

Abstract

A user obtains a set of modules, inserts them into slots of a chassis, and interconnects the modules to form a modular instrument. A signal path extends through the modules. To support calibration of the signal path, a first of the modules (or the chassis or a calibration module) includes a calibration signal generator. A computer directs the first module to apply the calibration signal from the generator to the signal path, and measures the power (or amplitude) of the output of the signal path. The computer reads a factory-measured value A of the calibration signal amplitude from a memory of the first module (or the chassis or the calibration module). The value A and the measured output power of the signal path are used to determine a gain of the signal path. The system compensates for that gain when the signal path is used to measure live operational signals.

We claim: 
     
         1 . An electronic module comprising:
 a first input port for receiving an input signal;   a calibration signal generator configured to generate a calibration signal so that a frequency and amplitude of the calibration signal are stable with respect to time and with respect to changes in temperature;   a switching device configured to selectively pass either the input signal or the calibration signal as a selected signal to an internal signal path within the electronic module, wherein the switching device selectively passes either the input signal or the calibration signal based on a selection input;   first circuitry configured to receive the selected signal from the internal signal path and operate on the selected signal in order to generate an output signal;   an output port for outputting the output signal;   memory that stores measured values associated with the calibration signal, wherein the measured values include a measured value of the frequency of the calibration signal and a measured value of the amplitude of the calibration signal;   control logic configured to access the memory and to provide the measured values to a computer external to the module in response to a request received from the computer;   wherein the module is constructed as a single package.   
     
     
         2 . The module of  claim 1 , wherein the calibration signal generator is configured so that a nominal frequency of the calibration signal is programmable, wherein the control logic is configured to receive a frequency control instruction from the computer and to direct the calibration signal generator to generate the calibration signal according to the nominal frequency. 
     
     
         3 . The module of  claim 1 , wherein the measured values have been determined by measurement of the calibration signal using measurement equipment external to the module. 
     
     
         4 . The module of  claim 1 , wherein the measured values have been determined by measurement of the calibration signal at a facility that manufactures the module. 
     
     
         5 . The module of  claim 1 , wherein the input signal is a modulated carrier signal. 
     
     
         6 . The module of  claim 1 , wherein the first circuitry includes frequency downconversion circuitry, wherein the output signal is a frequency down-converted version of the selected signal. 
     
     
         7 . The module of  claim 1  further comprising:
 a second input port configured to receive a local oscillator signal from a local oscillator source that is external to the module, wherein the first circuitry includes a mixer and a filter, wherein the mixer is configured to mix the selected signal with the local oscillator signal to obtain a mixed signal, wherein the filter is configured to bandpass filter the mixed signal to obtain the output signal. 
 
     
     
         8 . The module of  claim 1 , wherein the control logic is further configured to control the selection input of the switching device in response to a selection control instruction received from the computer. 
     
     
         9 . The module of  claim 1  further comprising:
 a calibration signal port coupled to receive the calibration signal from the calibration signal generator, wherein the calibration signal port is configured to output the calibration signal. 
 
     
     
         10 . The module of  claim 1 , wherein the module is configured for inserting in a chassis along with one or more other modules, wherein the module and the one or more other modules are configured for coupling in a series. 
     
     
         11 . A chassis for housing a plurality of electrical modules, the chassis comprising:
 a communication bus;   a plurality of slots configured to admit insertion of the electrical modules, respectively, wherein each of the slots is configured to couple a corresponding one of the electrical modules to the communication bus when that electrical module is inserted in the slot;   a calibration signal generator configured to generate a calibration signal having a stable frequency and amplitude, wherein at least one of the slots is configured to provide the calibration signal to a first of the electrical modules when that first electrical module is inserted into the slot;   memory that stores measured values associated with the calibration signal, wherein the measured values include a measured value of the frequency of the calibration signal and a measured value of the amplitude of the calibration signal;   control logic configured to access the memory and to provide the measured values to a computer in response to a request received from the computer.   
     
     
         12 . The chassis of  claim 11 , wherein the measured values have been determined by measuring the calibration signal at a facility that manufactures the chassis. 
     
     
         13 . The chassis of  claim 11  further comprising:
 an output port coupled to the calibration signal generator, wherein the output port has a connector that is configured to mate with an end connector of an external cable, wherein the output port is configured to output the calibration signal. 
 
     
     
         14 . The chassis of  claim 11  further comprising:
 a controller slot configured to admit insertion of the computer. 
 
     
     
         15 . An electronic module comprising:
 a calibration signal generator configured to generate a calibration signal so that a frequency and amplitude of the calibration signal are stable with respect to time and with respect to changes in temperature;   memory that stores measured values associated with the calibration signal, wherein the measured values have been determined by measuring the calibration signal at a facility that manufactures the electronic module, wherein the measured values include a measured value of the frequency of the calibration signal and a measured value of the amplitude of the calibration signal;   control logic configured to access the memory and to provide the measured values to a computer external to the electronic module in response to a request received from the computer;   a calibration output port coupled to the calibration signal generator, wherein the calibration output port is configured to output the calibration signal from the electronic module; wherein the module is constructed as a single package.   
     
     
         16 . The module of  claim 15 , wherein the module is configured for insertion in any of a plurality of slots of a chassis. 
     
     
         17 . The module of  claim 16 , wherein the calibration output port includes a connector that is configured to couple with a complementary connector that is part of a backplane of the chassis when the module is inserted in any one of the slots of the chassis. 
     
     
         18 . The module of  claim 16 , wherein the module is a PXI module or PXI Express module, wherein the chassis is a PXI or PXI Express chassis. 
     
     
         19 . The module of  claim 15 , wherein the module has a user access panel, wherein the calibration output port has a connector that is mounted on the user access panel, wherein the calibration output port is configured to make the calibration signal externally available via the connector. 
     
     
         20 . A method for operating a computer in order to calibrate a signal transformation path within an electronic module, the method comprising:
 (a) a computer sending a first instruction to the electronic module, wherein the instruction directs the electronic module to couple a calibration signal produced by a calibration signal generator to an input of the signal transformation path, wherein the calibration signal generator is located within the electronic module;   (b) the computer reading measurement values from a memory of the electronic module, wherein the measurement values include a measured value of the amplitude of the calibration signal and a measured value of the frequency of the calibration signal;   (c) the computer reading output power measurements from a power measuring device, wherein the output power measurements are measurements of power of an output signal produced at the output of the signal transformation path in response to said calibration signal being coupled to the input of the signal transformation path;   (d) the computer calculating a gain between the input and the output of the signal transformation path based on the measured value of the amplitude of the calibration signal and based on the output power measurements; and   (e) the computer storing the gain in a memory of the computer.   
     
     
         21 . The method of  claim 20 , wherein the measurement values have been determined by measurements of the calibration signal performed prior to said module being delivered to a user. 
     
     
         22 . The method of  claim 20 , wherein the computer performs (a), (b), (c), (d) and (e) in response to receiving a user command through a graphical user interface. 
     
     
         23 . The method of  claim 20  further comprising:
 the computer sending a second instruction to the electronic module, wherein the second instruction directs the electronic module to couple an external signal to the input of the signal transformation path, wherein the external signal is from a source external to the electronic module; 
 the computer reading additional output power measurements from the power measuring device, wherein the additional output power measurements are measurements of power of the output signal produced at the output of the signal transformation path in response to said coupling of the external signal to the input of the signal transformation path; and 
 the computer calculating a power of the external signal based on the gain stored in the memory of the computer and based on the additional output power measurements read from the power measuring device. 
 
     
     
         24 . The method of  claim 20 , wherein the memory of the electronic module also stores a set of factory-measured gain values corresponding to a set of frequencies that span a frequency range, wherein each of the factory-measured gain values is a gain of the signal transformation path at the corresponding frequency, wherein the frequency of the calibration signal is within the frequency range, wherein the method further comprises:
 the computer computing a gain value G* corresponding to the frequency of the calibration signal based on the factory-measured gain values;   the computer applying a translation to the set of factory-measured gain values based on a difference between the gain computed in (d) and the gain value G*;   the computer storing the translated set of gain values in the memory of the electronic module.   
     
     
         25 . The method of  claim 20 , wherein the calibration signal generator is configured so that a nominal frequency of the calibration signal is programmable, the method further comprising:
 the computer determining values G(f k ) of the gain corresponding respectively to distinct values f k  of the nominal frequency by repeatedly programming the nominal frequency of the calibration signal to each of the distinct values and performing (b) through (e) for each of the distinct values.   
     
     
         26 . The method of  claim 25 , further comprising:
 the computer interpolating a value of gain corresponding to a center frequency of a received signal based on the stored values of gain corresponding to two or more of the distinct values of nominal frequency;   the computer instructing the electronic module to decouple the calibration signal from the input of the signal transformation path and couple the received signal to the input of the signal transformation path;   the computer reading additional output power measurements from the power measuring device, wherein the additional output power measurements are measurements of power of the output signal produced at the output of the signal transformation path in response to the received signal being coupled to the input of the signal transformation path;   the computer adjusting the additional output power measurements using the interpolated gain value in order to obtain corrected power values for the received signal.   
     
     
         27 . The method of  claim 25 , wherein the distinct values of the nominal frequency cover a bandwidth of a received signal, the method further comprising:
 the computer computing an equalization filter based on the stored values of the gain corresponding to the distinct values of the nominal frequency, wherein the equalization filter is configured to compensate for the gain of the signal transformation path over the bandwidth of the received signal;   the computer instructing the electronic module to couple the received signal to the input of the signal transformation path and decouple the calibration signal from the input of the signal transformation path.   
     
     
         28 . The method of  claim 27 , further comprising:
 the computer sending the equalization filter to the electronic module, wherein the electronic module is configured to apply the equalization filter to the output signal produced at the output of the signal transformation path in response to said coupling the received signal to the input of the signal transformation path.   
     
     
         29 . The method of  claim 25 , wherein the memory of the electronic module also stores a set of factory-measured gain values corresponding to a set of frequencies that span a frequency range, wherein each of the factory-measured gain values is a gain of the signal transformation path at the corresponding frequency of said set, wherein the distinct values f k  of the nominal frequency occur within the frequency range, wherein the method further comprises:
 the computer deriving gain values g k  corresponding to the distinct values of the nominal frequency based on the factory-measured gain values;   the computer adjusting the set of factory-measured gain values based on differences between the gain values G(f k ) and the corresponding gain values g k ;   the computer stored the adjusted set of gain values in the memory of the electronic module.   
     
     
         30 . The method of  claim 20 , wherein the signal transformation path includes:
 a frequency down-conversion system configured to down-convert an input signal present at the input of the signal transformation path in order to obtain an intermediate frequency (IF) signal; and   a digitizer system configured to capture samples of the IF signal.