Is there any limitation for PXI 5670 to generate QPSK signal?

[Ivan.Chen]

Hi,

I would like to select PXI 5670 to generate the QPSK signal in my system. How does is it make the QPSK modulation?As I understand, QPSK modulation would need to have two AWG to generate the I and Q signal. See below:

But PXI 5670 only integrated one AWG (PXI 5421). So how does it make the QPSK modulation? 

Do I need to download the user−specified baseband IQ waveform into the memory first,then playback it to generate the IQ signal and then upconvert it to the desired QPSK signal? if yes, the playback time will be limited by the onboard memory size. Even if I select the one of 256 MB of onboard memory , it can only 1.28 seconds of waveform data at the default sample clock rate of 100 MS/s.  Does it means the QPSK signal can only apear within 1.28 seconds? That's definitely not enough for me.

I am not sure if my understanding is correct or not. Any reply is appreciated.

BR,

Ivan Chen

[Ivan.Chen]

Anyone know about it? any advice is appreciated.

[lhaoxian]

Hello Ivan,

The NI 567x RFSG is typically used with the NI Modulation Toolkit for modulated signals applications. The IQ data is generated in software via the toolkit, and the complex data is written to the RFSG before being generated. An example is shown in the article here, under "Creating I and Q data".

For the NI 5670, it uses the NI 5421 AWG. Using a default IQ rate of 100MS/s, and assuming each waveform data is 16bits, and 256MB memory, we can store a waveform of maximum duration approximately 1.28s, as you have mentioned.

The NI 5671 uses the NI 5441 AWG, which has onboard OSP. One of the main advantages of the onboard OSP is that it can perform digital upconversion. We can specify a much lower IQ rate than 100MS/s (say, 1.25MS/s) for our waveform. The onboard OSP then digitally upconvert this baseband IQ waveform to IF. The result is that we can use a smaller amount of memory to represent the same waveform, and the onboard memory can store a longer waveform without repetition. Page 9 to 10 of the NI 5671 hardware manual explains this in detail, together with some comparison numeric figures.

Best regards,

Victor

NI ASEAN

[Ivan.Chen]

Hi Xian,

It's you again. Thanks for you reply.

Base on your information, it seems that the modulation toolkit create the IQ signal's waveform and than the AWG output the IQ mixed signal (contain the I and Q signals information ) in one  single channel,after that upconvert it to the RF IQ signal (which is we need) by the upconvertor. It sounds like the modulation is done by the software,let's said "sofware-modulation".How about the PXIe 5673? It should not be the same, right?As I see, I has two independent AWG output. So it should be no need to create the IQ signal wavefrom in the memory first, since it's capable of outputing the I and Q signal respectively,and then modulated by the upconvertor(let's said hardware-modulation).That means, for PXIe 5673, we should no need to worry about the play-back time which is limited by the memory size+IQ data rate.A simple diagram is as  below:

Is it correct? Please kindly correct me if I get any misunderstanding.

[Speedy_Badger]

Hello Ivan.Chen,

This is correct given the PXIe-5611 performs the complex modulation and direct upconversion in the physical analog domain. Typically, you will still be generating your waveform in software based upon I and Q however. The real I waveform will be generated on channel 0, and the real Q waveform will be generated on channel 1. The I and Q analog waveforms will then be modulated in a quadrature fashion and generated at the appropriate RF frequency.

The fundamental difference between the PXI-5670 and PXIe-5673 is that your the PXI-5670 eventually needs to received a waveform at full rate (100 MS/s) which has already been complex modulated while the PXIe-5673 only requires two real waveforms, I & Q, which can be at streamed to the device at a decimated rate since the onboard signal processing supports interpolation. This allows the PXIe-5673 to be an ideal streaming platform. It is also much faster in regards to tuning, and is capable of a 4x instantaneous bandwidth improvement - the PXI-5670 is 20 MHz, while the PXIe-5673 is > 100 MHz in bandwidth.

Check out the specifications for the PXIe-5673 for more detail on the overall device, as well as the PXIe-5450 specifications for more detail on the onboard signal processing capabilities.

[Ivan.Chen · Accepted solution]

Got it! I think I would use 5671 for my application since it's cheaper and equiped with OSP capability.

Thank you very much!