Abstract:
A frequency response measurement circuit includes a generation circuit operative to provide an input signal having a voltage and programmable frequency characteristics in response to a frequency control signal. A decision circuit is coupled to the generation circuit and is operative to sample the input signal at predetermined intervals in response to a sampling clock signal and determine the amplitude characteristics of the input signal relative to a variable threshold signal. A control circuit is coupled to the decision circuit, and is operative to determine the frequency response characteristics of the input signal at varying frequencies and threshold voltages in response to the frequency control signal.

Description:
[0001]     The present application claims the benefit of U.S. Provisional application Ser. No. 61/614,024, filed Sep. 27, 2004. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention generally relates to diagnostic and test equipment for analyzing high-speed data bit streams and, more particularly, to devices and methods capable of measuring, among other things, the analog input performance of a high-speed data communications receiver.  
         [0004]     2. Description of the Related Art  
         [0005]     Frequency response is a common measure for diagnosing and analyzing a communication channel. Among other things, the frequency response of a communication channel defines the analog bandwidth that limits the channel capacity. Understanding the frequency response is a key element to diagnosing problems with individual devices and entire systems.  
         [0006]     There exists presently commercial instruments known as network analyzers or vector network analyzers. These analyzers measure frequency response directly from the channel under test. These devices typically output sine waves at various frequencies and measure the sine wave power output from a device under test (DUT). The analyzer then is able to display the ratio of input power to output power versus frequency for the DUT.  
         [0007]     It is known to build similar functionality from separate devices or instruments such as a synthesizer (sine wave generator) and an RF power meter for a more manual test. Power meters can be replaced with spectrum analyzers in this application, as well. The power meter then is capable of measuring the output power of the device. In all cases, the test system generates a sine wave of known amplitude and presents this to the DUT. Then the DUT outputs a sine wave back to the test system and the test system calculates and presents the ratio of output power to input power.  
         [0008]     However, none of the aforementioned systems or analyzers truly tests the input frequency response of a decision circuit. For example, the goal of a decision circuit is to digitally sample the input signal and output a logical one or zero as a result of the decision. This is compatible with measuring the output of a sine wave for a power measurement to be measured by a power meter. Also decision circuit outputs are often highly integrated with other functions including serial to parallel shift registers that aggregate the binary decisions into multi-bit parallel words that, again, cannot be presented to a power meter for a power measurement.  
         [0009]     There presently exists an urgent need to be able to measure the analog frequency response of the DUT, so that a proper measurement of that point of a digital receiver can be made. Thus, there still exists the need to be able to measure such digital receivers in transceiver devices and receiver-only devices.  
       SUMMARY OF THE INVENTION  
       [0010]     A frequency response measurement circuit includes a generation circuit operative to provide an input signal having a voltage and programmable frequency characteristics in response to a frequency control signal. A decision circuit is coupled to the generation circuit and is operative to sample the input signal at predetermined intervals in response to a sampling clock signal and determine the amplitude characteristics of the input signal relative to a variable threshold signal. A control circuit is coupled to the decision circuit, and is operative to determine the frequency response characteristics of the input signal at varying frequencies and threshold voltages in response to the frequency control signal.  
         [0011]     A frequency response measurement method includes receiving an input signal to sample. Next, sample the voltage characteristics of the input signal relative to a threshold voltage value at a corresponding frequency. This may be accomplished, for example, by comparing the input signal voltage to a threshold voltage value. Next, adjust the threshold voltage value at the corresponding frequency. Then sample the voltage characteristics of the input signal relative to the adjusted threshold voltage value at the corresponding frequency. This may be accomplished; for example, by comparing the input signal voltage to the adjusted threshold voltage value. After performing the adjusted voltage comparison, repeat the previous sampling and adjusting operations until the input voltage and threshold voltages intersect. Then, plot the intercept values relative to the corresponding frequency.  
         [0012]     It is a general object of the present invention to provide a method and apparatus for measuring an analog input frequency response at the decision point of a digital receiver.  
         [0013]     It is an additional object of the present invention to provide a method and apparatus for measuring an analog input frequency response at the decision point of a digital receiver which is capable of being integrated with present digital receivers.  
         [0014]     It is an additional object of the present invention to provide a method and apparatus for measuring the an analog input frequency response at the decision point of a digital receiver wherein such apparatus and the method of using the same is a stand alone test device requiring little if any modifications of presently existing digital receivers.  
         [0015]     An advantage provided by the present invention is the ability to measure an analog input frequency response at the decision point of a digital receiver.  
         [0016]     Another advantage provided by the present invention is to provide a method and apparatus for measuring the analog input frequency response at the decision point of a digital receiver using a separate measuring instrument that requires only minor modifications of the digital receiver.  
         [0017]     In an exemplary embodiment, there are various mechanisms for measuring the maximum and minimum amplitude of an input sine wave. In the exemplary embodiment, the maximum and minimum amplitudes are determined by moving the decision threshold voltage of the decision circuit and then measuring the change in the resulting decisions. It will be appreciated, that as a result of performing the method of the present invention that the measurement may be done synchronously or asynchronously to the sampling rate of the decision circuit.  
         [0018]     The asynchronous mode of the exemplary embodiment employs sampling of the applied known-amplitude sine wave. In this case, when the decision point is “inside” the sine wave&#39;s amplitude, the output decisions are nominally 50% true and 50% false. However, as the voltage threshold is moved above the applied sine wave input, all applied signal voltages are below the threshold so all logic falses are output. Similarly, when the voltage threshold is moved below the applied sine wave input, all applied signal voltages are above the threshold so all logic trues are output. By moving the threshold voltage to a very high voltage and then stepping it down to the point where all logic falses are no longer received, the maximum amplitude can be measured. Similarly, by moving the threshold voltage to a very low voltage and then stepping it up to the point where all logic trues are no longer received, the minimum applied sine wave amplitude can be measured.  
         [0019]     Embodiments that use asynchronous sampling can easily measure both low and high frequency responses outside the range of the frequencies of decision making supported by the DUT. Embodiments that use synchronous sampling (such that the applied sine wave frequency is synchronous—in some octave—to the sampling rate of the decision circuit) restricts the flexibility of the applied sine wave stimulus and complicates the setup. Furthermore, synchronous setups must also step the sampling time throughout the applied sine wave input frequency to further find where the maximums and minimums of the input sine wave occur. In cases where the decision circuit only operates on a very narrow-band input frequency (i.e. at one frequency), an asynchronous sampling style would generate the most wideband result. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]     For a further understanding of the objects and advantages of the present invention, reference should be had to the following detailed description of the invention, taken in conjunction with the accompanying drawing, in which like parts are given like reference numerals and wherein:  
         [0021]      FIG. 1  is a schematic block diagram of an exemplary frequency response circuit configured to operate in an asynchronous mode;  
         [0022]      FIG. 2  is a schematic block diagram of an exemplary frequency response circuit configured to operate in a synchronous mode;  
         [0023]      FIG. 3  is a timing diagram illustrating the output of the circuit of the present invention operating in asynchronous mode; and  
         [0024]      FIG. 4  is a timing diagram illustrating the output of the circuit of the present invention operating in synchronous mode. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0025]     An exemplary embodiment of the present invention will now be described in greater detail with reference to  FIGS. 1-4 . The present invention may, however, be embodied in many different forms and should not be construed as limited to the exemplary embodiments set forth herein; rather these embodiments are provided so that this disclosure will be thorough and complete and will fully convey the invention to those skilled in the art.  
         [0026]     It will be understood by those of ordinary skill in the art that the frequency response of a system is a graph of the ratio of the output of the system compared to the input of the system versus frequency of input. Such a graph cannot be created using conventional systems because the output of a decision point of a digital receiver is not accessible as an analog output ready to make a power measurement on. Rather, the decision circuit itself must be used to make the measurement right at the decision point. This is done by stimulating the input of a decision circuit with a sine wave of a particular frequency and then controlling the decision threshold voltage up and down to find what threshold voltages corresponds to the maximum and minimum of the input stimulus. This result represents the amplitude of the received signal at the input to the decision circuit and can then be plotted against the known applied sine wave amplitude (which can be known because it is calibrated) for all frequencies of interest.  
         [0027]     There are several mechanisms that allow a decision circuit to measure the maximum and minimum of the input sine wave. These include moving the decision threshold voltage of the decision circuit and measuring the change in resulting decisions. This can be done synchronously or asynchronously to the sampling rate of the decision circuit.  
         [0028]      FIG. 1  is a schematic block diagram of an exemplary frequency response circuit  20  configured to operate in an asynchronous mode. The frequency response measurement circuit  20  includes a programmable frequency sine wave generator  22  that provides an input signal  23 , for example, a sine wave, to a first input of a decision circuit  25 , also referred to as a device under test (DUT)  25 . The input signal  23  has an amplitude V in . In order to perform the calculations in accordance the invention V in  must be known value. V in  can be pre-measured during a calibration or initialization phase of the frequency response measurement circuit  20 , or a larger circuit or system to which the frequency measurement circuit  20  forms a part.  
         [0029]     The decision circuit  25  includes a comparator  24 , having a first input and a second input, and a latch or memory component  125 , for example, a D-type flip flop. The input signal  23  is coupled to the first (e.g. positive) input of the comparator  24 . the second (e.g. negative) input of the comparator  24  is coupled to a threshold control signal  29 , provided by a threshold control circuit  28 .  
         [0030]     A sampling clock generator  26 , for example, for example, a commercially available synthesizer or any other suitable frequency oscillator, provides sampling clock signal  27 , which is coupled to the trigger or clock (CLK) input of the memory component  125  and counter  34 . The sampling clock signal  27  triggers the memory component  125  of the decision circuit  25 , thereby, causing the memory component  125  to sample the input signal  23  and transmit the logic result (e.g. logic one or logic zero) to the first input of logic (e.g. exclusive-OR) gate  32 . the logic result is the output provided by the comparator  24 , which is the value of the comparison of the input signal amplitude value, V in , against the applied threshold voltage signal  29  value. In an exemplary embodiment, if the input signal  23  amplitude is greater than the threshold signal  29  value, a logic one is provided on the output of the decision circuit  126 ; otherwise, a logic zero is provided on the output of the decision circuit  126 .  
         [0031]     The sampling clock signal  27  frequency can be set to any frequency sufficiently different from the input signal  23 , a sine wave or any octave of that sine wave frequency (input sine wave  23 ). In an exemplary embodiment, the sampling clock signal  27  frequency is set to 9/100 of the data frequency. This restriction assures that there will be an even distribution of samples at various phases of the input sine wave  23  which meets the requirement that the clocking be asynchronous If the sampling clock signals are too close to each other, the sampling may dwell at only one phase of the input signal  23  and generate an erroneous result. If frequency response measurements are desired near the frequency being applied to the input sampling clock, then the frequency of the sampling clock signal  27  can be moved to assure sufficient asynchronous behavior. The amount of frequency deviation between the input sine wave  23  and the sampling clock (or any octave) signal  27  depends on how long each measurement is taken. For very short measurements, a farther more un-related frequency is needed. For longer measurements, clock frequencies that are closer can be used.  
         [0032]     A threshold control circuit  28 , for example, a digital to analog converter and operational amplifier/buffer used to convert digital values to desired analog threshold levels provides a threshold control signal  29  to a second input of comparator  24 . The magnitude (e.g. voltage) of the signal provided by the threshold control circuit  28  is controlled by a set threshold signal  38 . The threshold control voltage signal  29  can be varied depending upon the measurement that is sought.  
         [0033]     In application, the threshold voltage  29  applied to the decision circuit  25  is set to a level greater than any part of the applied voltage level of the input signal  23 . At this point, the output of the decision circuit  126  should be all zeros indicating that at all sample times defined by the sampling clock signal  27 , of the input sine wave  23  is below the applied threshold  29 . Next, the threshold signal voltage  29  is decreased in small increments until the output of the decision circuit  126  is no longer all zeros. This voltage is defines V hi .  
         [0034]     A processor or control circuit  30 , for example, a microprocessor, mirco-controller, dedicated hardware (e.g. ASIC) or software code executed on one or more processors, sets the voltage level of the threshold control signal  29  by applying a set threshold signal  38  to the threshold control circuit  38 . In order to define V low , the control circuit  30  sets the set threshold signal  38  to a low value, for example, two times lower than the expected V low , of the signal. This value is so low that the output of the decision circuit  126  is always a logic one.  
         [0035]     Once this step is accomplished, the control circuit  30  incrementally increases the voltage applied by the threshold control circuit  28  until the output of the decision circuit  126  is no longer all ones. This threshold voltage is deemed the V low . The amplitude of the applied voltage before the decision circuit  25 , V out , is then calculated as V hi  minus V low  and can be plotted by the control circuit  30  at the frequency of the input signal  23  as a ratio of the V out /V in . This entire process is then repeated for all sine wave frequencies of interest. For example, for a 12.5 Gbit/sec input for a high-performance bit error rate tester decision circuit, the frequencies of interest might range from 1 MHz to 26 GHz.  
         [0036]     In order to determine when the decision circuit output  126  no longer generates all ones or all zeros, a one-bit comparator (in the form of an exclusive-OR gate)  32  is used. The first input of the exclusive-OR gate  32  is coupled to the output  126  of the decision circuit  25 . The second input of the exclusive-OR gate  32  is coupled to the output  31  of a compare level control circuit  30 . The output of the exclusive-OR gate  33  will be a logic  1  when the two input signals disagree. This means that a compare level control circuit  30  can set which logic level would produce an enable signal  33  to the awaiting counter  34  downstream. When the output of the compare level control circuit  31  is a one, then any logic zero output from the decision circuit  25  would be counted. Alternatively, when the output of the compare level control circuit  31  is a zero, any logic one output of the decision circuit  25  would be counted. In this way, the control circuit  30  can monitor the count value in the counter  35  to know when the appropriate threshold value is reached.  
         [0037]     In an alternate embodiment, the control circuit  30  may directly control the sample clock frequency. After stepping through all the sine wave frequencies of interest and measuring the V lo  and V hi  signals, the control circuit  30  can display a graph (illustrated below control circuit  30 ) or present in tabular format the frequency response of the input to the decision circuit.  
         [0038]      FIG. 2  is a schematic block diagram of an alternate embodiment of an exemplary frequency response measurement circuit  40 , configured to operate in a synchronous mode. In the synchronous mode, the input signal  123  frequency and the sampling clock signal  127  frequency are the same.  
         [0039]     As mentioned above, it is somewhat more involved to perform a synchronous response measurement and it is somewhat more limited. However, the synchronous embodiment is important and is required in certain circumstances. For example, when the sampling clock signal  127  and input data signal  123  frequency are required to be related as in the case of a built-in clock and data recovery unit.  
         [0040]     In the synchronous embodiment  40 , a sine wave and clock generator circuit  122  is coupled as a first input (to comparator  24 ) to decision circuit or DUT  25 . The synchronous embodiment  40  also includes a threshold control circuit  28 , a control circuit  30  and a counter  34 , which all perform analogous functions to the earlier described asynchronous embodiment  20  and are connected in a manner consistent with the earlier described asynchronous embodiment  20  except noting the differences defined by synchronous rather than asynchronous clocking.  
         [0041]     In the synchronous embodiment  40 , the input sine wave signal  123  of a known incident amplitude is generated along with a corresponding synchronous sampling clock signal  127  by the sine wave and clock generator  122  in response to the set frequency control signal  37 . The relationship between the sampling clock signal  127  frequency and the input (e.g. sine wave) signal  123  frequency may be an octave (higher) or harmonic (lower) integer value. For example, the input signal  123  may be split with a power splitter and then a divide-by-N circuit used to create a lower-frequency sampling clock signal  127  that would be applied to the clock (CLK) or trigger input of the memory component  125  of the decision circuit  25  that is responsible for sampling the output signal of the comparator  24 , which corresponds to the result of the comparison of the input signal  123  and the threshold voltage value  29 . In an alternative embodiment, not shown, two clock generators may provided which support a common locking reference frequency, for example, 10 Mhz.  
         [0042]     Measuring the V lo  and V hi  in the synchronous measurement embodiment  40  also requires that the phase of the sampling clock signal  127  as compared to the input signal  123  be adjusted. This is because all samples of the decision circuit  25  as compared to the threshold control signal  29  will be at one repeating phase of the input signal  123  and one must sweep through all possible phases of the input signal  123  in order to find the highest and lowest points where the output of the decision circuit  126  changes from all ones to all zeros, in order to determine the amplitude of the input signal  123  to the decision circuit  25 . This extra sweeping process, not required in asynchronous measurements is due to the fact that the asynchronous measurements will naturally make samples at all phases of the input signal  123 , is an extra complication for the synchronous application. This means that the control circuit  30  must sweep the threshold control voltage  29  from a high level where the decision circuit output  126  is all zeros to incrementally lower values until the decision circuit output  126  is no longer all zeros. Then, the control circuit  30  must remember these thresholds for all increments of phase of the input sine wave signal  123  and, when measurements are complete for all phases of the input sine wave signal, search the memory to find the highest and lowest thresholds. These two thresholds are subtracted to find the V out  in the synchronous case.  
         [0043]     After performing the aforementioned operations, the control circuit  30  can then present in either a graphical or tabular presentation the ratio of V out /V in  versus all the input signal  123  frequencies to show the frequency response of the input to the decision circuit  25 .  
         [0044]      FIG. 3  is a timing diagram  50  illustrating the output of the decision circuit of the present invention operating in asynchronous mode. In this timing diagram  50 , the Q-outputs are represented from the decision circuit for various hi, low and middle voltage thresholds. As shown, when the applied threshold voltage is set to V min , the Q-output of the decision circuit is all ones. Thus all parts of the input signal are above the threshold. When the applied threshold voltage is set to V max , the output of the decision circuit is all zeroes.  FIG. 3  also shows that all samples of the input signal are below the threshold control voltage at V max .  
         [0045]     When the threshold control voltage is set to V mid , an alternating sequence of ones and zeros is provided by the decision circuit. As shown in the side graph which corresponds to the average duty cycle measurement versus the threshold control voltage, depending on exactly where the V mid  threshold is compared to the edges of the input sine wave, the average duty cycle of the decision circuit output ranges from 0% to 100%. For example, when in the middle of a sine wave, the average duty cycle would be nominally 50%.  
         [0046]      FIG. 4  is a timing diagram  60  illustrating the output of the decision circuit of the present invention when operating in a synchronous mode. As shown, the synchronous embodiment of the present invention defines and identifies the V lo  and V hi  of the input signal. As noted above, many measurements must be done at each time offset, T i , to find the highest and lowest voltage present in the input signal. At each time offset within the cycle of the input signal, the threshold control voltage signal is swept from V max , where the decision circuit will output all zeros, incrementally lower until the decision circuit output is no longer all zeros. The control circuit then recalls this voltage threshold level. Once measurements have been made for all time-offset increments that define one cycle of the applied input signal, the control circuit can search the measured results to find the lowest and highest measured values, V lo  and V hi .  
         [0047]     At any given time increment, and depending on where the threshold control voltage is set with respect to the input signal, the duty cycle of the decision circuit output will vary from 0% to 100%. In this case, which is different from the asynchronous case, the duty cycle variation will go completely from 0% to 100% in the short amount of voltage thresholds that represent the noise on the single trace of the sine wave input as illustrated using the side graph on  FIG. 4 . A noisy input signal will have a gentler slope going from 0% to 100% duty cycle where as a clean (or non-noisy) input signal will present a very steep slope going from 0% to 100% duty cycle.  
         [0048]     The control circuit  30  is required to control, measure, compute and display the results of the frequency response measurement of the present invention. Exemplary psuedo-code for an algorithm that can be used to implement the necessary asynchronous measurement functions is provided below:  
                                                                                                     Array: FreqResponse[]           Vin = 1Vpp           For(freq= Fmin; freq&lt;Fmax; freq += Fstep)                {Set frequency of Sine Generator to freq           Set amplitude of Sine Generator to Vin           RESET count           Set Compare Level = 1           For (thresh = Vmax; count == 0; thresh −= Vstep)                {}                Vhi = thresh           thresh = Vmin           RESET count           Set Compare Level = 0           For (thresh = Vmin; count == 0; thesh += Vstep)                {}                Vlo = thresh           FreqResponse[l] = (Vhi − Vlo) / 1Vin           }                      
 
         [0049]     Once this algorithm is completed, the array “FreqResponse” will hold all the frequency responses that correspond to the frequencies of interest between Fmin and Fmax at a resolution defined by Fstep.  
         [0050]     A similar algorithm exists for the synchronous embodiment  40  of the present invention. This alternative approach must include a further step whereby it searches through all phase offsets of the input sine wave signal to find the highest and lowest signals. For example, an algorithm which implements the synchronous frequency response measurement functionality of the present invention is provided below:  
                                                                                                     Array: FreqResponse[]           Vin = 1Vpp           Vhi = Vmin           Vlo = Vmax           For(freq= Fmin; freq&lt;Fmax; freq += Fstep)                {           Set frequency of Sine Generator to freq           Set amplitude of Sine Generator to Vin           For(phase=0;phase&lt;2*pi; phase+= PhaseStep)                {           RESET count           Set Compare Level = 1           For (thresh = Vmax; count == 0; thresh −= Vstep)                {}                if(thresh &gt; Vhi) Vhi = thresh ;           if(thresh &lt; Vlo) Vlo = thresh ;           }                FreqResponse[l] = (Vhi − Vlo) / 1Vin           }                      
 
         [0051]     Once this algorithm is completed, the array “FreqResponse” will again hold all the frequency responses that correspond to the frequencies of interest between Fmin and Fmax at a resolution defined by Fstep.  
         [0052]     In summary, the present invention discloses exemplary methods and apparatus for measuring the analog input frequency response of a decision circuit by moving the digital decision point while measuring differences in the resulting decisions to measure the amplitude of an applied input signal. Moving the decision point can involve just moving it in voltage (up and down) if asynchronous sampling is used or may involve moving it in voltage and in time if synchronous sampling is used. Synchronous or asynchronous measuring allows for sweeping the input sine wave stimulus to cover all frequencies of interest while plotting the measured amplitude as compared to the applied amplitude to result in the frequency response.  
         [0053]     While the foregoing detailed description has described several embodiments of the method and apparatus in accordance with this invention, it is to be understood that the above description is illustrative only and not limiting of the disclosed invention. It will be appreciated that the embodiments discussed above and the virtually infinite embodiments that are not mentioned could easily be within the scope and spirit of this invention. Thus, the invention is to be limited only by the claims as set forth below.