how to fetch 60s data in RFSA with bandwidth 2MHz?

[yakgg]

hi

I have a problem in using the 5620 digitizer to fetch real time data

We have a project to demodulate GPS signal  wich center frequency at 1.57542GHz and bandwidth 2MHz.
In first step, I used " ni5660 fetch IQ data.vi" by modify the bandwidth to be 2MHz and fetched about 0.2 second data. I succesfully demodulated the GPS signal. 

But I found with this vi, I can not get longer time, for example, 60seconds data.  So I used the "nituner getting started.vi" to mannually downconverte the signal to IF and "use niScope EX Stream to Disk.vi" to fetch the data to disk.  Now the niTunner: frequency shift 1.561GHz.
So, the IF frequency is 1.57542GHz-1.561GHz=14.42MHz. Then I set the digitizer frequency to 6.4M/s which means that the the signal  is aliased to 14.42MHz-6.4MHz*2=1.62MHz.  With the signal bandwidth 2MHz. Now the aliased signal is from 1.62MHz-1MHz to 1.62MHz+1MHz.  But now, I can not demodulate this signal !!!

Could anybody help me about this?

Thanks

[Tica]

Hello Yakgg,

You should be able to use the "ni5660 fetch IQ data.vi" to acquire for 60 seconds as well since this is doing a continuous acquisition of the data. All you need to do is change the "time duration" input to match the 60 seconds you want to acquire for.

If there is another reason why you are not using this example please let me know and we'll look at other ways for acquiring the data.

Regards,

Tica T

Applications Engineer

[yakgg]

no, I can not fetch 60s data.

Since the GPS signal has bandwidth 2MHz. I have to change the bandwidth in the fetch iq data.vi. After doing that, I can not get more than 10s data.

Guang CHEN

[Tica]

Hello Guang Chen,

I'm sorry for the misunderstanding but after looking at the details of the PXI-5660 I realized that this is not going to be possible. For a bandwidth of 2 MHz, the Digital Downconverter (DDC) on the 5660 will not be enabled. This means that the digitizer will be sampling at a rate of 64 MS/zec, and since it is a 14 bit digitizer, the data rate is two time the same rate -128 MB/sec. This transfer rate is too hight to sustain continuous streaming across the PCI bus.

The PXI-5660 comes with two memory options - 32 MB and 64 MB but since there are two bytes for every sample, the memory options can be looked at as 16 MS or 32 MS. If you are sampling at 64 MS/sec, then you can ither capture a data record that is 250 ms or 500 ms long depending on your memory option; so capturing 60 s of data into onboard memory is clearly not an option.

Another thing is that the niScope Sream to Disk example is optimized for streaming while the ni5660 example is not. The niScope example fetches binary samples while the other fetches scaled voltage values.

Finally, capturing 60 s of continuous, gap-free data is not possible. If you can use a bandwidth of <= 1.25 MHz, then this may become possible since this is when the DDC turns on and effectively reduces the sample rate of the acquisition. If this is an option and you need further assistance please let me know.

Regards,

Tica T

Applications Engineer