Generating Multiple Modulated Signals

[Troy]

I was a bit surprised I didn't see this on the forums, though I am new to the RF gear.  What I would like to do is generate two modulated signals, such as 802.15, on two different frequencies.  I am already able to create the signals individually using the modulation toolkit.  I am using a PXI-5671 RFSG.  I have found how to generate with interferance two CW tones in the examples, but do not see how to generate the two modulated tones.  I also need to be able to set one of the modulated signals to a lower power than then other.  I am trying to test adjacent channel interferance with modulated signals.  Any thoughts?

[Paul_C.]

Hi Troy,

In the example finder in LabVIEW, there are some great examples for multitone generation that will help you get started.  My recommendation would be to take a look at the example entitled "RFSG Multitone - Arbitrary Spacing.vi".  This example can be found by going to Help » Find Examples.  Once inside the example finder, go to Hardware Input and Output » Modular Instruments » NI-RFSG » Arbitrary Waveforms.  In the example, theres code for how they combine two signals into the complex multitone signal.  I believe this sounds like exactly what you're looking at doing.  Please let me know if I'm misunderstanding the application or you're unable to get it working.

Regards,

Paul C. 

[Yardov]

Hi Troy,

There are a few tricks you can play here but they are completely independent of hardware and more on signal processing. Basically you need to write both waveforms at the same time with different frequency offset.

1) Frequency Shift. Assume you have two completelly independent baseband signals s1[n] and s2[n]. Each one should be the same lenght (time duration).

One of them will be transmitted at Fc and the other one at Fc + df. s1[n] remains unchanged and s2[n] will be shifted by df using the following VI:

 

The trick here is to make sure that your sample rate is high enough to support the shifting, otherwise you will be aliasing. If you create your waveforms with Modulation Toolkit make sure they have enough samples per symbol.

Finally you need to sum all signals s[n] = s1[n] + s2[n].

2) Apply different Powers. Before you add the signals, you only need to scale the data correctly. To do this, I recommend to normalize both waveforms. This is s1[n]/max(sqrt(|s1[n]|). Thenscale the smaller power one. Finally, you will add them together.

Here is a spectrum of the composed signal:

 

I have attached an example to do this using 802.15 signals with a few modifications: I am not normalizing data but applying headroom to each signal and I hardcoded the number of samples per symbol so the maximum frequency offset is about 12 MHz. Should be helpful to get started.

Message Edited by Yardov on 10-17-2008 06:05 PM

[Troy]

Gerardo, thanks for the VIs, that is exactly what I needed.  I was able to easily incorporate them into development I have done to this point. 

Final question, is there any resources on NI's site covering the use of scripting with the RF gear?  I have used it to do things like we just did, but I understand from my local rep that it is a very powerful tool.  I would like to learn more about it.

[Yardov]

Hi Troy,

I am glad that the VI was helpful.

About scripting, I agree, this is a supper powerful tool that will make your generating life easier. There is not much to it in terms of API. To get started, you can go over the main definition of scripting base on this online document:  Advanced Waveform Sequencing and Triggering on Arbitrary Waveform Generator Even if is not NI-RFSG driver, it is exactly the same concept.

Once you have that concept clear, you can learn how to use it using the RFSG help file:

Figure 1: Sections that talk about scripting using NI-RFSG

Hope these documents help and let us know if you run into any issues.

Message Edited by Yardov on 10-22-2008 09:09 PM

[Siriuccs]

Hi Troy,

  I am student . I am doing senior project. I need labview driver for 8350 Sweep osccilator. I have seen in forum you are talking about 8350B . Couls you please provide me one if you have one.

Thanks,

Siri

[Troy]

Siri, can you repost under the Instrument Control part of the forums as that is where your request should go. 

That said, I am not sure about thread you refer to with 8350B.  The number sounds familiar but some further information would be helpful.  When you post this to the Instrument Control forum please post as much detail as you can about the gear such as full model number, and specifically what you need to do with it.  I/We will definately be able to help. 

[Siriuccs]

HI Troy,

Thanks for the reply. sure  i will post There also. I'm looking  a  Lab view driver for HP8350B Sweep oscillator.here is explanantion what i'm doing. i'm a student @ UCCS. We are doing Automation for all equipment in lab . this is one of the equipment i have to do.  As you know with out drivers which is difficult. Any Help would be appriciated.Thaks a lot for your help.

Thnaks  in advance,

siri