Generating Multiple Modulated Signals

Question

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?
Troy

Answer 1

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
Gerardo O.
RF Systems Engineering
National Instruments
Attachments:
frequency shift.jpg ‏9 KB
Generation Multiple Channels.zip ‏32 KB
spectrum.jpg ‏79 KB