Maximum Frequency problem using PXI-4461 for Swept Frequency Application

[Steve SO]

Hi All,

Using the sound and vibration 3v0 toolkit for swept sine application and the maximum sampling frequency of PXI-4461 is 204.8 KHz.  When i want to sweep freq from 100Hz to 100KHz, i found that the "Stop Frequency" is limited to 79999 Hz and error is occured. To debug it, i found that the sampling freq will be divided by 2.5 comparing with maximum modulating frequency (i.e 100 KHz in my case.) in "SVT Start Swept Sine(DAQmx).vi".  I guess that this is for anti-aliasing purpose according to Shannon sampling theorem. (i.e. Sampling freq is twice or above of modulating frequency. ). If i change the divider from 2.5 to 2, have any problem to be concerned that i have ignored?

Thanks&Regards,

Steve 

[Ali M]

Hello Steve,

At 2.5, the Alias-free bandwidth (passband) is DC to 0.4fs while having the the Low-Frequency Alias Rejection Enabled (Default) or DC to 0.4535fs (if Disabled). Since the frequency response of the NI PXI 4461 (or NI 446x) digital filters have a cutoff of 0.484fs (with the Low-Frequency Alias Rejection Enabled)/0.491fs (if Disabled); changing the divider to 2 (0.5fs) will attenuate your signal i.e. it will not fall within the specs recommended above.

Additionally, please refer to the following resources:

1. NI 446x User Specifications (pg 3): http://www.ni.com/pdf/manuals/373770j.pdf

2. NI Dynamic Signal Acquisition User Manual (pg 63/64): http://digital.ni.com/manuals.nsf/websearch/0A30E9AB873ED45586257508004D645F

Regards,

Ali M

Applications Engineer

National Instruments 

Message Edited by Ali M on 02-11-2010 04:38 PM

[Steve SO]

Hi Ali M,

Thanks for your information. BTW, i hope that you can explain a little bit for following questions. thx~

1. What is the different of Low-Frequency Alias Rejection when enabled and disabled? From the datasheet, it seems that it has a better tolerance if Low-Frequency Alias Rejection is disabled.

2. What is the aliasing rejection minimum? [i.e. 104 dBc (enabled) , 120dBc (disabled)]. I believe that it is the attenuation.

Thanks for your help~~

Thanks&Regards,

Steve

[Ali M]

Low Frequency Alias rejection is recommended when the sampling rate is less than 25.6KS/s. Due to noise at certain multiples of the sampling rate that may pass through the digital filter, an analog filter with a fixed cutoff frequency above the maximum sampling rate is placed before the digital filter. We can enable this feature, which forces the actual sampling rate to never fall below 25.6kS/s. Additionally, the feature Enabled/Disabled graphs of the frequency response with Low-Frequency Alias rejection (pg 63/64 of the DSA User Manual); display the minimum Aliasing Rejection (attenuation, as you correctly stated) occuring at values greater than 0.5fs.

Please refer to example calculations and details of how the driver would automatically set the sampling rate to an integer multiple of the desired sampling rate:

1. http://digital.ni.com/public.nsf/allkb/8BD1190B07BDC43286256FD9006DC52F?OpenDocument

Regards,

Ali M

Applications Engineer

National Instruments

Message Edited by Ali M on 02-12-2010 01:12 PM