There are actually two things to consider when determining if a device is capable of capturing a particular frequency.
1)Analog Bandwidth
2)Sampling Rate.
The front end of an analog sampling board such as a High Speed Digitizer can be modelled as a simple RC circuit which inherently acts as a lowpass filter. This will limit the bandwidth and ultimately the maximum frequency that can be measured. Frequencies which exceed this upper cutoff frequency will be significantly attenuated and cannot be measured accurately with that device. In addition, if the board is AC coupled then there will also be a highpass filter which limits the minimum frequency which can be measured. Frequencies below this cuttoff also cannot be measured accurately.
In the case of the NI PXIe-5622 it would not be an appropriate device to measure 150kHz as this is outside the bandwidth of the device.
The Nyquist theory that Travis quoted refers to the ability to measure a signal without aliasing. If you sample at least 2x the maximum frequency in your signal you will ensure that your signal is adequately sampled to prevent aliasing. However if you have a bandwidth-limited signal you can employ a technique of undersampling to accurately capture signals above the nyquist frequency. They will appear at the correct amplitude, however will appear to be a lower frequency than they actually are. If your signal is not bandwidth limited, it is possible for two frequencies to become aliased to the same frequency and it will be impossible to tell them apart. Refer to this tutorial for more information on the Nyquist theory, aliasing and undersampling.
PXIe with a dedicated x4 communication link will provide a theoretical data transfer bandwidth of 1GB/s. In practice data input can be streamed to disk at approximately 800MB/s. So for a device that has a maximum sample rate of 150MS/s, taking 2 Bytes per sample, data will need to be fetched at 300MB/s to support continuous streaming. This is within the streaming bandwidth of the PXIe technology. Therefore, yes you can measure continuously for hours at maximum sample rate. The limitation will generally be the ability to process or view the data in realtime. Generally this data is streamed to disk instead for later post-processing. You will need a large capacity storage method that is capable of keeping up with those rates such as the NI HDD- 8264 RAID Array. In this case you can stream the data until you run out of disk space.
Hope this helps.
Jennifer O.
Message Edited by Jennifer O on 01-02-2009 09:45 AM
