Extremely low frequency filters












2












$begingroup$


I have a data acquisition board(A/D+Digital Signal Processor) and I want to check if a digital high-pass filter(implemented in DSP) at an extremely low cut-off frequency(0.05Hz) is actually working.



If this was a frequency I could generate with a signal generator it'd be easy to check, but 0.05Hz is too low and I can't generate it. How do engineers check this kind of filters?






digital-filter







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$endgroup$

















    2












    $begingroup$


    I have a data acquisition board(A/D+Digital Signal Processor) and I want to check if a digital high-pass filter(implemented in DSP) at an extremely low cut-off frequency(0.05Hz) is actually working.



    If this was a frequency I could generate with a signal generator it'd be easy to check, but 0.05Hz is too low and I can't generate it. How do engineers check this kind of filters?






    digital-filter







    share|improve this question









    $endgroup$















      2












      2








      2





      $begingroup$


      I have a data acquisition board(A/D+Digital Signal Processor) and I want to check if a digital high-pass filter(implemented in DSP) at an extremely low cut-off frequency(0.05Hz) is actually working.



      If this was a frequency I could generate with a signal generator it'd be easy to check, but 0.05Hz is too low and I can't generate it. How do engineers check this kind of filters?






      digital-filter







      share|improve this question









      $endgroup$




      I have a data acquisition board(A/D+Digital Signal Processor) and I want to check if a digital high-pass filter(implemented in DSP) at an extremely low cut-off frequency(0.05Hz) is actually working.



      If this was a frequency I could generate with a signal generator it'd be easy to check, but 0.05Hz is too low and I can't generate it. How do engineers check this kind of filters?






      digital-filter




      digital-filter






      share|improve this question













      share|improve this question











      share|improve this question




      share|improve this question










      asked 2 hours ago









      MediocreMediocre

      296414




      296414






















          2 Answers
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          active

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          $begingroup$

          I guess it depends on several factors, among others the order of the filter, but you have a few possibilities:




          1. Find a signal generator that gets there. These are rather inexpensive nowadays.

          2. Trust the math. This is a digital filter and as such it scales with sampling rate. If you can increase the sampling rate by two orders of magnitude you would have a filter with a 5Hz cutoff, much easier to measure. Likewise, if the limiting factor becomes the ADC you can isolate it from the filter and feed in some artificial digital data.

          3. Use a step response (many wideband signals would do). Calculate the step response of your desired filter and compare with the result. Or, alternatively, compute the frequency response by means of the FFT of the step response.


          We use a variation of alternative 3 in some of our test setups, not because we cannot generate the slow waveforms required, but because the <0.01Hz cutoff of our analog filters would take way too long to characterize if we tried even a rough frequency sweep. This reduced the testing time from more than an hour to mere minutes.






          share|improve this answer









          $endgroup$





















            5












            $begingroup$

            I would use my Agilent function generator, which goes down to 1$mu$Hz, a fairly unremarkable (and obsolete) Model 33522A. My Rigol DG4102, I think, similarly has 1$mu$Hz resolution and cost less.



            Unfortunately, you can't get that low with cheap DDS (eg. AD9850) modules because the tuning word is only 32 bits and the clock is typically 125MHz, so that's 0.03Hz resolution. I suppose it would give you a few data points (0.0291/0.0582/0.0873 Hz)



            You also could feed it a step and look at the time domain response.






            share|improve this answer











            $endgroup$









            • 1




              $begingroup$
              If your DDS module can take an external clock signal, simply underclock it! A DDS is really a very fancy divide-by-N at heart....
              $endgroup$
              – ThreePhaseEel
              1 hour ago










            • $begingroup$
              @ThreePhaseEel You can do that but it won't be a very nice output in general unless you redesign the output low pass filter, which is typically something like a 5-pole LC elliptical filter. In the case of OP, since the maximum frequency is so low, he or she could probably just add a 1Hz low pass RC filter to the output and get a nice signal.
              $endgroup$
              – Spehro Pefhany
              1 hour ago










            • $begingroup$
              Agreed that you'd need a postfilter -- AD9850s have a minimum clock of 1MHz btw, which is a bit limiting, but still plenty enough to get the results the OP wants
              $endgroup$
              – ThreePhaseEel
              1 hour ago










            • $begingroup$
              @ThreePhaseEel Sure, even 8 or 10 MHz would yield resolution of 0.002 or 0.0024Hz.
              $endgroup$
              – Spehro Pefhany
              47 mins ago











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            $begingroup$

            I guess it depends on several factors, among others the order of the filter, but you have a few possibilities:




            1. Find a signal generator that gets there. These are rather inexpensive nowadays.

            2. Trust the math. This is a digital filter and as such it scales with sampling rate. If you can increase the sampling rate by two orders of magnitude you would have a filter with a 5Hz cutoff, much easier to measure. Likewise, if the limiting factor becomes the ADC you can isolate it from the filter and feed in some artificial digital data.

            3. Use a step response (many wideband signals would do). Calculate the step response of your desired filter and compare with the result. Or, alternatively, compute the frequency response by means of the FFT of the step response.


            We use a variation of alternative 3 in some of our test setups, not because we cannot generate the slow waveforms required, but because the <0.01Hz cutoff of our analog filters would take way too long to characterize if we tried even a rough frequency sweep. This reduced the testing time from more than an hour to mere minutes.






            share|improve this answer









            $endgroup$


















              2












              $begingroup$

              I guess it depends on several factors, among others the order of the filter, but you have a few possibilities:




              1. Find a signal generator that gets there. These are rather inexpensive nowadays.

              2. Trust the math. This is a digital filter and as such it scales with sampling rate. If you can increase the sampling rate by two orders of magnitude you would have a filter with a 5Hz cutoff, much easier to measure. Likewise, if the limiting factor becomes the ADC you can isolate it from the filter and feed in some artificial digital data.

              3. Use a step response (many wideband signals would do). Calculate the step response of your desired filter and compare with the result. Or, alternatively, compute the frequency response by means of the FFT of the step response.


              We use a variation of alternative 3 in some of our test setups, not because we cannot generate the slow waveforms required, but because the <0.01Hz cutoff of our analog filters would take way too long to characterize if we tried even a rough frequency sweep. This reduced the testing time from more than an hour to mere minutes.






              share|improve this answer









              $endgroup$
















                2












                2








                2





                $begingroup$

                I guess it depends on several factors, among others the order of the filter, but you have a few possibilities:




                1. Find a signal generator that gets there. These are rather inexpensive nowadays.

                2. Trust the math. This is a digital filter and as such it scales with sampling rate. If you can increase the sampling rate by two orders of magnitude you would have a filter with a 5Hz cutoff, much easier to measure. Likewise, if the limiting factor becomes the ADC you can isolate it from the filter and feed in some artificial digital data.

                3. Use a step response (many wideband signals would do). Calculate the step response of your desired filter and compare with the result. Or, alternatively, compute the frequency response by means of the FFT of the step response.


                We use a variation of alternative 3 in some of our test setups, not because we cannot generate the slow waveforms required, but because the <0.01Hz cutoff of our analog filters would take way too long to characterize if we tried even a rough frequency sweep. This reduced the testing time from more than an hour to mere minutes.






                share|improve this answer









                $endgroup$



                I guess it depends on several factors, among others the order of the filter, but you have a few possibilities:




                1. Find a signal generator that gets there. These are rather inexpensive nowadays.

                2. Trust the math. This is a digital filter and as such it scales with sampling rate. If you can increase the sampling rate by two orders of magnitude you would have a filter with a 5Hz cutoff, much easier to measure. Likewise, if the limiting factor becomes the ADC you can isolate it from the filter and feed in some artificial digital data.

                3. Use a step response (many wideband signals would do). Calculate the step response of your desired filter and compare with the result. Or, alternatively, compute the frequency response by means of the FFT of the step response.


                We use a variation of alternative 3 in some of our test setups, not because we cannot generate the slow waveforms required, but because the <0.01Hz cutoff of our analog filters would take way too long to characterize if we tried even a rough frequency sweep. This reduced the testing time from more than an hour to mere minutes.







                share|improve this answer












                share|improve this answer



                share|improve this answer










                answered 2 hours ago









                Edgar BrownEdgar Brown

                3,9851626




                3,9851626

























                    5












                    $begingroup$

                    I would use my Agilent function generator, which goes down to 1$mu$Hz, a fairly unremarkable (and obsolete) Model 33522A. My Rigol DG4102, I think, similarly has 1$mu$Hz resolution and cost less.



                    Unfortunately, you can't get that low with cheap DDS (eg. AD9850) modules because the tuning word is only 32 bits and the clock is typically 125MHz, so that's 0.03Hz resolution. I suppose it would give you a few data points (0.0291/0.0582/0.0873 Hz)



                    You also could feed it a step and look at the time domain response.






                    share|improve this answer











                    $endgroup$









                    • 1




                      $begingroup$
                      If your DDS module can take an external clock signal, simply underclock it! A DDS is really a very fancy divide-by-N at heart....
                      $endgroup$
                      – ThreePhaseEel
                      1 hour ago










                    • $begingroup$
                      @ThreePhaseEel You can do that but it won't be a very nice output in general unless you redesign the output low pass filter, which is typically something like a 5-pole LC elliptical filter. In the case of OP, since the maximum frequency is so low, he or she could probably just add a 1Hz low pass RC filter to the output and get a nice signal.
                      $endgroup$
                      – Spehro Pefhany
                      1 hour ago










                    • $begingroup$
                      Agreed that you'd need a postfilter -- AD9850s have a minimum clock of 1MHz btw, which is a bit limiting, but still plenty enough to get the results the OP wants
                      $endgroup$
                      – ThreePhaseEel
                      1 hour ago










                    • $begingroup$
                      @ThreePhaseEel Sure, even 8 or 10 MHz would yield resolution of 0.002 or 0.0024Hz.
                      $endgroup$
                      – Spehro Pefhany
                      47 mins ago
















                    5












                    $begingroup$

                    I would use my Agilent function generator, which goes down to 1$mu$Hz, a fairly unremarkable (and obsolete) Model 33522A. My Rigol DG4102, I think, similarly has 1$mu$Hz resolution and cost less.



                    Unfortunately, you can't get that low with cheap DDS (eg. AD9850) modules because the tuning word is only 32 bits and the clock is typically 125MHz, so that's 0.03Hz resolution. I suppose it would give you a few data points (0.0291/0.0582/0.0873 Hz)



                    You also could feed it a step and look at the time domain response.






                    share|improve this answer











                    $endgroup$









                    • 1




                      $begingroup$
                      If your DDS module can take an external clock signal, simply underclock it! A DDS is really a very fancy divide-by-N at heart....
                      $endgroup$
                      – ThreePhaseEel
                      1 hour ago










                    • $begingroup$
                      @ThreePhaseEel You can do that but it won't be a very nice output in general unless you redesign the output low pass filter, which is typically something like a 5-pole LC elliptical filter. In the case of OP, since the maximum frequency is so low, he or she could probably just add a 1Hz low pass RC filter to the output and get a nice signal.
                      $endgroup$
                      – Spehro Pefhany
                      1 hour ago










                    • $begingroup$
                      Agreed that you'd need a postfilter -- AD9850s have a minimum clock of 1MHz btw, which is a bit limiting, but still plenty enough to get the results the OP wants
                      $endgroup$
                      – ThreePhaseEel
                      1 hour ago










                    • $begingroup$
                      @ThreePhaseEel Sure, even 8 or 10 MHz would yield resolution of 0.002 or 0.0024Hz.
                      $endgroup$
                      – Spehro Pefhany
                      47 mins ago














                    5












                    5








                    5





                    $begingroup$

                    I would use my Agilent function generator, which goes down to 1$mu$Hz, a fairly unremarkable (and obsolete) Model 33522A. My Rigol DG4102, I think, similarly has 1$mu$Hz resolution and cost less.



                    Unfortunately, you can't get that low with cheap DDS (eg. AD9850) modules because the tuning word is only 32 bits and the clock is typically 125MHz, so that's 0.03Hz resolution. I suppose it would give you a few data points (0.0291/0.0582/0.0873 Hz)



                    You also could feed it a step and look at the time domain response.






                    share|improve this answer











                    $endgroup$



                    I would use my Agilent function generator, which goes down to 1$mu$Hz, a fairly unremarkable (and obsolete) Model 33522A. My Rigol DG4102, I think, similarly has 1$mu$Hz resolution and cost less.



                    Unfortunately, you can't get that low with cheap DDS (eg. AD9850) modules because the tuning word is only 32 bits and the clock is typically 125MHz, so that's 0.03Hz resolution. I suppose it would give you a few data points (0.0291/0.0582/0.0873 Hz)



                    You also could feed it a step and look at the time domain response.







                    share|improve this answer














                    share|improve this answer



                    share|improve this answer








                    edited 2 hours ago

























                    answered 2 hours ago









                    Spehro PefhanySpehro Pefhany

                    206k5156412




                    206k5156412








                    • 1




                      $begingroup$
                      If your DDS module can take an external clock signal, simply underclock it! A DDS is really a very fancy divide-by-N at heart....
                      $endgroup$
                      – ThreePhaseEel
                      1 hour ago










                    • $begingroup$
                      @ThreePhaseEel You can do that but it won't be a very nice output in general unless you redesign the output low pass filter, which is typically something like a 5-pole LC elliptical filter. In the case of OP, since the maximum frequency is so low, he or she could probably just add a 1Hz low pass RC filter to the output and get a nice signal.
                      $endgroup$
                      – Spehro Pefhany
                      1 hour ago










                    • $begingroup$
                      Agreed that you'd need a postfilter -- AD9850s have a minimum clock of 1MHz btw, which is a bit limiting, but still plenty enough to get the results the OP wants
                      $endgroup$
                      – ThreePhaseEel
                      1 hour ago










                    • $begingroup$
                      @ThreePhaseEel Sure, even 8 or 10 MHz would yield resolution of 0.002 or 0.0024Hz.
                      $endgroup$
                      – Spehro Pefhany
                      47 mins ago














                    • 1




                      $begingroup$
                      If your DDS module can take an external clock signal, simply underclock it! A DDS is really a very fancy divide-by-N at heart....
                      $endgroup$
                      – ThreePhaseEel
                      1 hour ago










                    • $begingroup$
                      @ThreePhaseEel You can do that but it won't be a very nice output in general unless you redesign the output low pass filter, which is typically something like a 5-pole LC elliptical filter. In the case of OP, since the maximum frequency is so low, he or she could probably just add a 1Hz low pass RC filter to the output and get a nice signal.
                      $endgroup$
                      – Spehro Pefhany
                      1 hour ago










                    • $begingroup$
                      Agreed that you'd need a postfilter -- AD9850s have a minimum clock of 1MHz btw, which is a bit limiting, but still plenty enough to get the results the OP wants
                      $endgroup$
                      – ThreePhaseEel
                      1 hour ago










                    • $begingroup$
                      @ThreePhaseEel Sure, even 8 or 10 MHz would yield resolution of 0.002 or 0.0024Hz.
                      $endgroup$
                      – Spehro Pefhany
                      47 mins ago








                    1




                    1




                    $begingroup$
                    If your DDS module can take an external clock signal, simply underclock it! A DDS is really a very fancy divide-by-N at heart....
                    $endgroup$
                    – ThreePhaseEel
                    1 hour ago




                    $begingroup$
                    If your DDS module can take an external clock signal, simply underclock it! A DDS is really a very fancy divide-by-N at heart....
                    $endgroup$
                    – ThreePhaseEel
                    1 hour ago












                    $begingroup$
                    @ThreePhaseEel You can do that but it won't be a very nice output in general unless you redesign the output low pass filter, which is typically something like a 5-pole LC elliptical filter. In the case of OP, since the maximum frequency is so low, he or she could probably just add a 1Hz low pass RC filter to the output and get a nice signal.
                    $endgroup$
                    – Spehro Pefhany
                    1 hour ago




                    $begingroup$
                    @ThreePhaseEel You can do that but it won't be a very nice output in general unless you redesign the output low pass filter, which is typically something like a 5-pole LC elliptical filter. In the case of OP, since the maximum frequency is so low, he or she could probably just add a 1Hz low pass RC filter to the output and get a nice signal.
                    $endgroup$
                    – Spehro Pefhany
                    1 hour ago












                    $begingroup$
                    Agreed that you'd need a postfilter -- AD9850s have a minimum clock of 1MHz btw, which is a bit limiting, but still plenty enough to get the results the OP wants
                    $endgroup$
                    – ThreePhaseEel
                    1 hour ago




                    $begingroup$
                    Agreed that you'd need a postfilter -- AD9850s have a minimum clock of 1MHz btw, which is a bit limiting, but still plenty enough to get the results the OP wants
                    $endgroup$
                    – ThreePhaseEel
                    1 hour ago












                    $begingroup$
                    @ThreePhaseEel Sure, even 8 or 10 MHz would yield resolution of 0.002 or 0.0024Hz.
                    $endgroup$
                    – Spehro Pefhany
                    47 mins ago




                    $begingroup$
                    @ThreePhaseEel Sure, even 8 or 10 MHz would yield resolution of 0.002 or 0.0024Hz.
                    $endgroup$
                    – Spehro Pefhany
                    47 mins ago


















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