ni5660 streaming data examples

[Om]

hi,

I searched through your forum and discovered that for ni5660, I cannot do streaming a 60sec data for a BW > 1.25MHz.  See the link below:

http://forums.ni.com/ni/board/message?board.id=290&message.id=345&jump=true

Its is said that streaming "may" be possible for BW < 1.25MHz.  So is it really possible.  If possible do u have any examples???

thanks.

[David S.]

Hi Om -

The "may" part refers to all the system dependencies and limitations that accompany streaming in a computer.  When you fetch data from the 5620 digitizer, that data is DMA'd across the PCI bus to RAM.  This process is limited to the PCI bus's 133 MB/s bandwidth, which is closer to 100 MB/s in reality.  Once this data gets there, the CPU has to scale it and form it into the datatype you want out.  This takes time and a few memory transfers, as well.

If you're streaming to disk, you then have to write the data from RAM to your hard drive.  Hard drives are incredibly slow compared to the other parts of a PC, and you may only be able to sustain 20-25 MB/s.  This can be increased by using new technologies like SATA2, a RAID array, or drives with fast spindle speeds (like 10k rev/min).

And all of that assumes you won't need to process the data as it's fetched, which further reduces your speed.

Attached are two simple benchmark programs that you can run to find your system's streaming rate.  One of them streams to RAM, and the other to disk.  The disk example returns unscaled raw data, so it should represent the fastest possible streaming rate to disk.  The RAM example scales the data first.  You will probably not be able to sustain a 1.25 MHz bandwidth when streaming to disk, though a newer controller is likely to be okay if streaming to RAM.

[Om]

thanks for the examples.  I ran the "PXI-5620 Stream to RAM" example.  Actually I want to capture a FM modulated signal with center frequency 433.92MHz.  in the example the IQ data is given by "MT Convert IF to IQ" VI.  but when I demodulate this IQ data i do not get back my unmodulated signal.  Plz help!!! 

I have attached the modified VI.

[David S.]

Hi Om -

The PXI-5660 is made up of two modules - the 5600 analog downconverter and the 5620 digitizer.  The benchmark VIs I posted are minimal in that they don't use the downconverter at all; they only use the digitizer to sample data.  Your signal is centered around 433 MHz, but the digitizer's sampling rate is only 64 MS/s.  So it's impossible to sample your modulated signal directly without the downconverter.  This is why the data coming out of your VI is meaningless.

The benchmark that my VIs provide is to show how quickly data can be retrieved from the digitizer and stored either in RAM or on disk.  They comprise only a piece of a full application, and they're intended to benchmark solutions that will use offline processing to demodulate or analyze acquired data.  Demodulating in the acquisition loop slows the loop severely, and these VIs are meant to show the best-case performance.

I recommend that you use the shipping example ni5660 Demodulate FM instead, which shows how to configure the downconverter with NI-Tuner in addition to using the digitizer.  The combination of these devices will let you downconvert the 433 MHz data to an IF frequency with the 5600, then digitally downconvert that IF data to the baseband, where it can be properly sampled by the 5620.

Message Edited by David S. on 04-13-2007 10:08 AM