Phase shift a channel and display

[Sharmi]

Hi, 

I am working on a project for which I need to display the I-Q constellation plot and normalize the data to fit it to a circle of unit radius. 

I am able to do this but the problem is sometimes the signal in one of the channels is very weak and so the plot moves close to the axes(please find attached the file, this is the best position, if one channel has a weak signal, the point moves to one of the axes). When this happens, I want to add a phase shift to the signal (45 degrees and multiples of it) to bring to back to the best position. I was trying to use the Extract Tone vi to get the phase , add 45 degrees but how do I reconstruct the signal wit the new phase?

Can anyone please help me?

Thanks so much,

Sharmi

[altenbach]

WHat is the data type? IQ data is most easily kept as a complex array. If it is not, combine the two channels into a single complex array, then multiply it with a scalar complex unit vector of the desired rotation angle. You can normalize by dividing with the average magnitude.

Can you attach your VI containing some typical data? (make current values default before saving)

[Sharmi]

The data is continuous complex data. Please find attached the vi.

I have already normalized the data by dividing with mean value.

This work is part of the transreceiver for which I am using PXIe-5663 and 5673 to transmit a single frequency RF carrier which is modulated by breathing/heartbeat and receive the signal. I want to add the phase shift before 'niRFSA Get IQ Components vi', i.e just after the complex data is acquired.

Please let me know if you need any other information.

Thanks so much,

Sharmi

[altenbach]

You did not attach the VI.

[Sharmi]

I am so sorry.

Please find attached

[altenbach]

What's inside the missing formula.vi?

[Sharmi]

The circle fit algorithm

[altenbach]

• To fit a circle, simply use "fitting on a sphere" with the Z values set to zero. No fancy math needed. 🙂
• To create a unit circle, use complex math, (see image). Much simpler!
• To rotate in the complex plane, multiply with a suiteable complex number. (See image). 
• XY graphs handle complex arrays directly, plotting IM vs RE. No need for clusters.

Attached is a quick draft. I only focused on the exact area of interest, leaving everything else as is. I did not check the rest of the code.

The control let's you shift the phase of the data from -pi to +pi.

[Sharmi]

Could you please attach the vi?

Thanks,

Sharmi

[altenbach]

Look again, It is already attached (LabVIEW 2011).