PXI-5660

[T Sreedhar]

I am having the following doubts regarding PXI-5660.
a.Timing Constraint: If the signal is acquired at a high speed of 2GHz and since the sampling rate is also very high, the hard disc of 40GB will be full with in a matter of a few of seconds. So, up to what maximum time can the systems log the data? What is the solution available for this if the data is logged for a span of one hour continiously?
b.Waveform Analysis Functionality: There is no ADC converter available, which can cater for such a high sampling rate. Therefore downconversion takes place at the ADC level, so at the time of downconversion, will there be any loss of data. Can the system retrieve the waveform exactly?
c. What is the meaning of 20MHz real time bandwidth?

[Andy_Hinde]

a.Timing Constraint:
The PXI-5660 is composed of an analog RF downconverter which places the downconverted RF spectrum at an IF frequency span between 5 and 25 MHz. The spectrum from 5-25 MHz is what the second module, the PXI-5620 64 MS/sec digitizer sees. The digitizer has 14 bits of resolution, so its data acquisition rate is 128 MB/sec (64 MS/sec * 2 bytes/sample). This is almost at the theoretical limit of the PCI bus, and transfer rates this high cannot be sustained. However, a solution exists if signals <= 1.25 MHz in bandwidth are being analyzed. In this situation a Digital Downconverter (DDC) chip ppresent on the PXI-5620 turns on and downconverts the IF input of the 5620 to 0 Hz and outputs baseband IQ data. This process includes digital downconversion, filtering, and finally decimation, with the decimation being key to sustained continuous acquisition. The sampling rate drops from 64 MS/sec to 2 MS/sec for spans <= 1.25 MHz and goes below 2 MS/sec depending on how narrow your configured bandwidth is. The main point though is that when the DDC is on, continuous data transfer is possible and the space required is easily calculated based on sampling rate and hard disk size.

b.Waveform Analysis Functionality:
I am not sure I understand this question. There is a 64 MS/sec 14 bit ADC present on the PXI-5620 which is more than ample to sample the 5-25 MHz IF output of the PXI-5600 RF Downconverter. Analog downconversion takes place in the PXI-5600, and digital downconversion takes place when the DDC on the PXI-5620 is enabled (configured span <= 1.25 MHz). There is no loss of data and the exact IF waveform is acquired when the DDC is off, and the exact IQ waveform is acquired when the DDC is on.

c. What is the meaning of 20MHz real time bandwidth?"
That is simply the bandwidth of the PXI-5600 RF downconverter. A passband of 20 MHz in the frequency range of 9 kHz to 2.7 GHz can be downconverted to 5-25 MHz IF output for the PXI-5620. Modulated signals can be completely captured by the wideband, vector architecture, as opposed to swept-tuned spectrum analyzers which have a narrow RBW filter. If a spectrum of span > 20 MHz is desired, the PXI-5660 simply acquires 20 MHz 'chunks' of spectrum and concatenates them appropriatly, creating a great performance advantage over narrowband, swept-tuned architectures.

[Development]

Hi Andy,

Please find the two replies posted by u which I felt as quite contradictory. So Still I have one doubt. Please let me know clearly whether is it possible to acquire 22 MHZ signal using PXI 5660.

(If a spectrum of span > 20 MHz is desired, the PXI-5660 simply acquires 20 MHz 'chunks' of spectrum and concatenates them appropriatly).

There is no way to acquire 22 MHz of real-time data with the 5660.


What is the meaning of 20MHz real time bandwidth?"
That is simply the bandwidth of the PXI-5600 RF downconverter. A passband of 20 MHz in the frequency range of 9 kHz to 2.7 GHz can be downconverted to 5-25 MHz IF output for the PXI-5620. Modulated signals can be completely captured by the wideband, vector architecture, as opposed to swept-tuned spectrum analyzers which have a narrow RBW filter. If a spectrum of span > 20 MHz is desired, the PXI-5660 simply acquires 20 MHz 'chunks' of spectrum and concatenates them appropriatly, creating a great performance advantage over narrowband, swept-tuned architectures.


The 5600 RF downconverter has a passband bandwidth of 20 MHz. It does not matter what digitizer is acquiring the data - the data coming out of the 5600 has a real-time bandwidth of 20 MHz. There is no way to acquire 22 MHz of real-time data with the 5660. The problems with acquiring a 22 MHz wide signal with a 20 MHz wide input are that you will attenuate the 1 MHz of data on either end of othe BW.

Thanks in advance

[Andy_Hinde]

There is a difference between the real time bandwidth of the PXI-5660 and the bandwidth of a FFT/power spectrum that can be acquired with the PXI-5660. If you are acquiring a spectrum, this spectrum can be > 20 MHz even though the RT BW of the 5660 is 20 MHz because we will just grab segments of the requested bandwidth in 20 MHz segments.

For example, if you want a spectrum with a center frequency of 100 MHz and a 100 MHz span, the 5660 SW will acquire 5 spectrums with center frequencies of 60, 80, 100, 120, and 140 MHz and concatenate them together. The SW will control the tuning of the 5600 downconverter module and the acquisition performed by the 5620, as well as the proper concatenation such that te spectral lines are correctly aligned. Please note that the 5 sub-spectrums are acquired serially and therefore do not represent one instant of time, which is the same principle behind a spectrum analyzer sweeping a RBW filter across a span.

This is entirely different from modulation or communication data. In these situations you cannot concatenate data from one 5600 center frequency position to the other becuase the nature of these applications cannot tolerate gaps in time or phase in the data.

So to answer your question, you can acquire a spectrum of your 22 MHz signal as long as it is continuous in nature. If you are trying to acquire your 22 MHz signal for demodulation (i.e. IQ data) then you cannot do this.