Abstract:
A mixed signal integrated circuit includes a signal source to inject a test signal into the signal path of the mixed signal integrated circuit, a feedback loop and a signal comparator for determining characteristics of a resulting signal. Conveniently, the test signal may be a digital signal injected upstream of a digital to analog converter (DAC). By connecting the output to the input, the entirety of the signal path and the majority of the integrated circuit may be tested. The signal may be condition or manipulated in the feedback loop. By incorporating test signal generation and measurement into the mixed signal integrated circuit, the cost of test equipment and the test duration for each device under test may be reduced.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority under 35 U.S.C. §119(e) from the provisional U.S. patent application Ser. No. 61/298,295 filed on Jan. 26, 2010, entitled “MIXED SIGNAL INTEGRATED CIRCUIT, WITH BUILT IN SELF TEST AND METHOD” the contents of which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to integrated circuits, and more particularly to integrated circuits that digitally process analog signals (referred to as mixed signal circuits) and self-tests therefor. 
     BACKGROUND OF THE INVENTION 
     Modern electronic circuit design relies heavily on the use of digital components. Digital circuits can typically be designed, integrated and manufactured with greater predictability and accuracy than analog circuits. Moreover, modern fabrication techniques make the production of digital circuits straightforward and relatively cost-effective. 
     Nevertheless, certain electronic applications still require the processing of analog signals. For example, audio and video processing often relies on the processing of a received analog signal and the provision of an analog output signal. 
     To take advantage of digital design and fabrication techniques, such circuits may be implemented using a mixed digital/analog design. Typically, in such mixed circuits, received analog signals are converted to digital format. Signal processing is performed in the digital domain and the resulting processed digital signal is converted again to an analog output. In this way, the predictability and fabrication ease of digital signal processing cores may be combined with conventional digital to analog and analog to digital converters in order to perform sophisticated signal processing on analog signals. 
     Integrated mixed signal circuits are, for example, commonly used in television and radio receivers, audio devices, cellular (in particular 3G) telephone, and in power over Ethernet applications. A particular example of a mixed signal circuit is contained in U.S. Provisional Patent Application No. 61/294,092 that discloses a television tuner, for use in a television, personal video recorder, set-top box or the like. 
     Such mixed signal designs must often be tested to ensure design and/or production quality. To this end, such circuits may be superficially tested, by applying power and measuring a response to an input signal. More sophisticated tests may be performed by applying a known analog signal to the circuit input and measuring the quality and characteristics of the analog output. 
     Naturally, superficial testing may not diagnose imperfections. The more sophisticated tests require relatively complex analog signal detection equipment and an analog source. This makes integration of testing circuitry as part of the mixed signal integrated circuit cumbersome. 
     Accordingly, there is a need for an improved method of testing mixed signal circuits and, more particularly, integrating components allowing on-chip testing and self-testing of such circuits. 
     SUMMARY OF THE INVENTION 
     Exemplary of an embodiment of the present invention, a mixed signal integrated circuit includes a signal source to inject a test signal into the signal path of the mixed signal integrated circuit, a feedback loop and a signal comparator for determining characteristics of a resulting signal. Conveniently, the test signal may be a digital signal injected upstream of a digital to analog converter (DAC). By connecting the output to the input, the entirety of the signal path and the majority of the integrated circuit may be tested. The signal may be conditioned or manipulated in the feedback loop. 
     In an embodiment, the integrated circuit may be a component of a television tuner. The injected signal may be a two-tone test signal that may be mixed with a carrier. Characteristics of the mixed signal may be assessed by a signal comparator. 
     In an embodiment, a mixed signal integrated circuit includes an analog to digital converter (ADC); a digital signal processing (DSP) core; and a DAC. The integrated circuit further includes a built-in test circuit that interconnects the output of the integrated circuit to its input, in feedback, and injects a digital test signal downstream of the DSP core. A resulting digital signal generated as a result of the injected digital test signal is compared downstream of the ADC. 
     Signal to noise (SNR), distortion, power, and other metrics of the resulting digital signal may be measured. The signal may be injected for a test interval to test the integrated circuit. Test signal(s) may be generated from stored representations of sine-waves; expected signals and expected values may also be stored and/or loaded onto the integrated circuit. 
     Conveniently, by incorporating test signal generation and measurement into the mixed signal integrated circuit, the cost of test equipment and the test duration for each device under test [DUT] may be reduced. 
     In accordance with an aspect of the present invention, there is provided a mixed signal integrated circuit comprising: an input; an output; an analog to digital converter (ADC) to convert an analog signal derived from a signal at the input to a digital signal; a digital signal processing (DSP) core for receiving the digital signal and to provide a digitally processed digital signal; a digital to analog converter (DAC) to convert the digitally processed digital signal to an analog signal for provision to the output; a self test circuit comprising: a digital signal generator for generating a digital test signal to be provided to the DAC; a feedback loop to selectively connect the output to the input, in feedback; and a digital signal comparator to compare a digital signal downstream of the ADC to an expected digital signal, resulting from the digital test signal. 
     In accordance with another aspect of the present invention, there is provided a method of performing a test of a mixed signal integrated circuit. The mixed signal integrated circuit comprises: an input; an output; an analog to digital converter (ADC) to convert an analog signal derived from a signal at the input to a digital signal; a digital signal processing (DSP) core for receiving the digital signal to provide a digitally processed signal; a digital to analog converter (DAC) to convert the digitally processed signal to an analog signal for provision to the output; the method comprising: connecting the output in feedback with the input; injecting a digital test signal downstream of said DSP core; comparing a resulting digital signal generated as a result of the digital test signal, downstream of the ADC to an expected digital signal. 
     Other aspects and features of the present invention will become apparent to those of ordinary skill in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the figures which illustrate by way of example only, embodiments of the present invention, 
         FIG. 1  is a simplified schematic diagram of a conventional mixed (i.e. analog and digital) signal integrated circuit; 
         FIG. 2  is a simplified schematic diagram of a mixed signal integrated circuit, exemplary of an embodiment of the present invention; and 
         FIG. 3  is a simplified schematic diagram of a specific mixed signal integrated circuit, exemplary of an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates a conventional mixed signal integrated circuit  10 . As illustrated, integrated circuit  10  includes an analog processing block  12 , receiving an analog input signal at input  11 , and providing a processed analog output signal to an analog to digital converter (ADC)  14 . The output of ADC  14  feeds a digital signal processing (DSP) core  16 , the output of DSP core  16  is provided to a digital to analog converter (DAC)  18  that provides analog output to a final analog processing block  20 . 
     In operation, an analog input signal is provided to analog processing block  12 . Analog processing block  12  may pre-process the analog signal by, for example, filtering, amplifying, or otherwise manipulating the analog signal in the analog domain. The output of analog processing block  12  is provided to ADC  14  that provides a digital equivalent of the processed analog signal. The digital equivalent is provided to digital signal processing (DSP) core  16 . 
     DSP core  16  may include any variety of conventional digital signal processing blocks. DSP core  16  may, for example, include finite impulse response filters (FIRs), infinite impulse response filters (IIRs), interpolators and the like, in order to perform desired digital signal processing on a received signal. The output of DSP core  16  is provided to DAC  18 , which is typically complementary to ADC  14 . DAC  18  reconstructs an analog signal from the now processed digital signal. The output of DAC  18  is provided to a final analog processing block  20  that may post process the digitally processed signal in the analog domain, and provide an analog output signal at output  21 . 
     As will be appreciated, the testing of the design and manufacture of integrated circuit  10  typically involves the application of an analog input signal to input  11  of analog processing block  12  and a comparison of the produced analog output signal provided by analog processing block  20  at output  21  to a desired or expected output signal. 
       FIG. 2  accordingly illustrates a mixed signal analog/digital circuit  100 , exemplary of an embodiment of the present invention. As illustrated in  FIG. 2 , circuit  100 , like conventional circuit  10  ( FIG. 1 ) includes an analog processing block  102  that receives an input analog signal at input  101 . 
     Analog processing block  102  may be formed of a variety of active and passive analog components, and may for example, act as one or more of an analog mixer, filter, demultiplexer, amplifier, automatic gain control circuit, and the like. The analog output of analog processing block  102  is provided to an ADC  104  that feeds a DSP core  106 . The output of DSP core  106  feeds a DAC  108  that in turn, feeds a further analog processing block  110 , that post-processes the signal output by DSP core  106  in the analog domain, to provide the desired analog output signal at output  111 . 
     ADC  104  may be any suitable A/D converter, chosen with suitable sampling rates, and bit depth to provide one more digital signal streams to DSP core  106 . ADC  104  may for example be a linear or non-linear ADC, with any desirable resolution, and sampling rate, and may be chosen to avoid spectral aliasing, and provide dither if desirable. ADC  104  may over or under-sample the analog signal. In the event that analog signal has been demultiplexed into several analog signals, ADC  104  may be formed of multiple A/D converters. The digital output of ADC  104  may have any desirable bit-depth. For example, ADC  104  may output a single digital bit stream, or multiple n-bit streams. ADC  104  may be clocked by an external clock (not shown) that may be the same as, or phase locked to other clocks used to clock circuit  100 . 
     DSP core  106  receives the one or more digital streams provided by ADC  104 . DSP core  106  like DSP core  16  may include any variety of conventional digital signal processing blocks, such as for example, FIR filters, IIR filters, interpolators and the like, in order to perform desired digital signal processing on a received signal. DSP core  106  may further include multiplexers, mixers, and other functional blocks operating in the digital domain. DSP core  106  may also be clocked by the clock clocking ADC  104 , or a clock derived (e.g. phase locked, and a frequency multiple thereof) from that clock. DSP  106 , in turn outputs a digitally processed signal, at a rate determined by this clock. 
     DAC  108  is complementary to ADC  104  and DSP core  106 , and receives one or more digital streams provided by DSP core  106  at its clock rate. DAC  108  performs the opposite operation of ADC  104 . As such, DAC  108  may have any desirable resolution, sampling frequency, and may be formed in any manner understood by those of ordinary skill. For example, DAC  108  may use pulse width modulation, oversampling, or interpolation, or a resistive ladder to form an analog output signal, proportional to a digital input value. Again, DAC  108  is suitably clocked. 
     Analog processing block  110  post processes the analog signal produced from DSP  106  and DAC  108 . Analog processing block  110  may again be formed of a variety of active and passive analog components, and may for example, act as one or more of an analog mixer, filter, demultiplexer/multiplexer, amplifier, clipper, automatic gain control circuit, or the like. The output of analog processing block  110  provides an analog output at output  111  of circuit  100 . 
     In order to allow testing, circuit  100  further includes built-in test circuit including a digital test signal generator  124  in communication with the signal path downstream of DSP core  106 , and a digital test signal comparator  126 , located upstream of digital signal generator  124 , and downstream of ADC  104 . Further, output  111  is interconnected to analog input  101  in feedback, by way of switch  120  and a further analog processing block  122 . Components  120 ,  122 , and  124  may be formed wholly or partially on (or off) the integrated circuit embodying the remainder of circuit  100 . 
     Analog test signals, which may for example be sine waves, may be digitally generated by circuit  100  to be processed and sent back into the part under test where the results will be measured using the DSP core  106  to compare to expected values such as those for power levels, distortion, or gain. An output signal is generated at test output  130  to indicate a pass or failure based on values loaded into memory. Output may be generated to gather statistical data. 
     In alternate embodiments, characteristics of the received signal may be computed by signal comparator  126 . For example, signal comparator  126  may determine the presence of a signal or of a certain characteristic of the received signal. For example, signal comparator  126  may determine whether or not a signal is present at certain frequencies; the SNR of the received signal; and/or a signal distortion ratio (SDR) by measuring signal levels at specific frequencies; or the power of the received signal. 
     In normal operation, circuit  100  functions in much the same manner as circuit  10 . Analog signal received at line  101  is processed in analog domain by an analog processing block  102 . The analog signal provided by block  102  is provided to ADC  104  to produce a digital representation of the output of analog processing block  102 . This digital representation is fed to digital signal processing core  106  that manipulates the digital signal in the digital domain. The processed digital signal is then provided to DAC  108 . The analog output of DAC  108  is provided to an analog processing block  110 . Analog processing block  110  provides the desired signal output at output line  111 . In this mode of operation, switch  120  is maintained in its open position. Analog processing block  102 , ADC  104 , DSP core  106  and DAC  108  may perform the functions of circuit  100  in normal operation. For example, circuit  100  may be a component of a television tuner or demodulator, as for example detailed in U.S. Provisional Patent application No. 61/294,092. 
     In a second—test—mode of operation, switch  120  is closed and a digital signal generator  124  provides (or injects) a digital test signal to digital to analog block  108 . The second—test mode—may be initiated by applying an external test enable signal. Test signal generator  124  may be clocked to provide a digital test signal at the rate expected by DAC  108 . At the same time, no external input signal is provided to input  101 . The injected digital signal generated by test signal generator  124  is then converted to analog form by DAC  108 . Again, analog processing block  110  further processes the analog equivalent of the digital signal in the analog domain. 
     Now, in this test mode, analog processing block  122  is selectively connected in feedback between output  111  and input  101  by switch  120 , and provides a further processed version of the analog signal to input  101  of circuit  100 . This analog signal is further processed by analog processing block  102 . The output of analog processing block  102  is fed to ADC  104  that in turn, feeds test signal comparator  126 . 
     In the depicted embodiment, signal comparator  126  is located upstream of DSP core  106 . However, a person of ordinary skill will readily appreciate that test signal comparator could be located downstream of DSP core  106 . If located upstream of DSP core  106 , as illustrated in  FIG. 2 , comparator  126  receives or intercepts the output of analog to digital converter  104  in the second mode of operation. Signal comparator  126  compares the digital signal provided by analog to digital converter to an expected signal reflective of the signal provided by signal source  124 , as processed by analog signal processing blocks  102 ,  110 , and  122 . In this way, signal comparator  126  can ensure that DAC  108 , analog processing block  110 , analog processing block  122 , analog processing block  102 , and ADC  104  process a known signal provided by signal generator  124  in an expected manner. In this embodiment, signal comparator  126  does account for (and therefore test) signal processing by DSP core  106 . If located downstream, signal comparator  126  may also test the signal processing by DSP core  106 . Of course downstream and upstream are logical concepts, and signal comparator  126  may formed as part of DSP core  106 . 
     In an alternative embodiment, test signal comparator  126  may be interconnected with an output of DSP core  106 , downstream of DSP core  106 . In this embodiment, test signal comparator  126  tests the operation of DSP core  106 . 
     Signal comparator  126  may be formed in many ways, and may form a comparison metric between a signal actually received at comparator  126 , and a signal expected at comparator  126 . As noted, comparator  126  may calculate any one of a number of metrics to identify the quality of the resulting digital signal. 
     Signal comparator  126  may, for example, calculate one or more of third order inter-modulation products (IM3); signal to distortion ratios (SDR); second order distortion products; and/or signal to noise ratios (SNR) of the received signal. 
     As illustrated, test signal generator  124  and test signal comparator  126  are illustrated as functional blocks, external to DSP core  106 . As will be appreciated, these could easily be integrated into DSP core  106 . Of course many other ways of forming test signal generator/comparator  124 / 126  will be known to those of ordinary skill in the art. 
     Optionally, integrated circuit  100  could further include functional blocks to represent a test sequence of symbols in memory  140  over multiple test intervals to further ensure proper function of integrated circuit  100 . 
     As will be appreciated, integrated circuit  100  may be formed and manufactured in conventional ways. For example, a specific integrated circuit  100  may be formed as an application specific integrated circuit (ASIC) using suitable electronic system-level (ESL); RTL design and physical design tools. Integrated circuit  100  could further be integrated with other digital, analog or mixed signal circuits. 
     In a specific example embodiment depicted in  FIG. 3 , circuit  100  may take the form of television tuner component, as for example disclosed in U.S. Provisional Patent Application No. 61/294,092, the contents of which are hereby incorporated by reference. In this embodiment, circuit  100  is designed to receive and process a television channel tuned to an intermediate frequency. The intermediate frequency may for example be at 36 MHz. The television channel is typically band-limited to 8 MHz. The television channel may be an NTSC, PAL, SECAM, DV-B or similar television channel. 
     In this embodiment analog processing block  102  may include a band-pass filter  154  and a fixed gain amplifier  156 , as illustrated. 
     Exemplary of an embodiment of the present invention, test signal generator  124  is formed to produce a two tone signal—to be mixed in the analog domain, by a mixer  150  formed as part of analog processing block  122 . Analog processing block  150  further includes a local oscillator  152  to provide a mixing frequency. The resulting mixed signal resembles a channel tuned to an intermediate frequency. In this embodiment, test signal comparator  126  further includes an SNR meter that may be formed as part of DSP core  106 . 
     In the depicted embodiment, test signal generator  124  may form a ⅚ MHz two tone test signal, and local oscillator  152  may provide a 28 MHz mixing signal. As noted, test signal generator  124  may be formed wholly or partially on (or off) the integrated circuit embodying the remainder of circuit  100 . 
     Now in test mode, switch  120  is closed, and test signal generator  124  generates the two tone signal. Local oscillator  152  generates a 28 MHz signal, and provides it to mixer  150 . Mixer  150  further receives an analog signal corresponding to the test signal (as formed by DAC  108 ) produced by test signal generator  132 . 
     The mixed signal, will include components at 28±5 MHz (33 MHz); 28±6 MHz (34 MHz). The mixed signal may also include components of harmonics and beats of the test signals and their harmonics. For example, the mixed signal may include components at 28±(5+6) MHz; 28±(2*5−6); and 28±(2*6−5). 
     Image signals at 28−5 MHz; 28−6 MHz; 28−(5+6) MHz; 28+(2*5+6) MHz; 28+(2*6+5) MHz; and 28−(2*5−6) may be filtered by band pass filter  154 . 
     As such, ADC  104  may receive signals at 28+5 MHz=33 MHz; 28+6 MHz=34 MHz 28+(5+6) MHz=39 MHz; 28+(2*5−6)=32 MHz; and 28+(2*6−5)=35 MHz. 
     As may now be appreciated, the resulting mixed signal resembles, in some ways, a television carrier signals tuned to an intermediate frequency. Conveniently, using a 28 MHz mixing frequency with signals at 5 and 6 MHz yields output frequencies around 36 MHz as input signals to the demodulator which, in turn, generate distortion products inside the bandwidth of interest so that they can be easily measured by the DSP  106 . 
     Now, test signal comparator  126  may measure power at 32/33/34/35 and 39 MHz and calculate third order intermodulation products (IM3), and a signal to distortion ratio (SDR). Also, test signal comparator  126  may determine the signal to noise ratio (SNR) across an entire band of interest—for example between 32 and 40 MHz in 1 MHz intervals. If either the IM3 or SDR is too high, or the SNR across the band is too high, an error condition may be signalled, and circuit  100  may be deemed defective and discarded. 
     Conveniently, the cost of automated testing of circuit  100  is reduced, as test duration and the complexity of automated test equipment may be reduced. Simple test signals (like the two tone signal) generated by test signal generator  124  and complementary tests ease measurement and evaluation. Similarly, the cost of test equipment is reduced as test success and failure may be signalled by test signal comparator  126 . 
     Conveniently, a failed test may signal a failing component. Performance levels need not be measured. This further relaxes the precision requirement in testing to one which is sufficient to determine when operation is outside the expected design bounds indicating that a manufacturing failure has occurred. 
     Of course, the above described embodiments are intended to be illustrative only and in no way limiting. The described embodiments of carrying out the invention are susceptible to many modifications of form, arrangement of parts, details and order of operation. The invention, rather, is intended to encompass all such modification within its scope, as defined by the claims.