Monday, December 27, 2010

FPGA development station in the works

The Spartan 6 development board powered up OK.
A fresh install of Windows on the development machine has been completed.
The development environment (ISE Webpack 12.3) has been installed.

Next up: update the ISE to 12.4 and figure out how to share the station. 
 -Michelle W5NYV

Wednesday, December 22, 2010

Spartan-6 hardware purchased, ISE 12.4 released from Xilinx, and filter solution provided by Bob

I bought us a development kit in order to provide some hardware to work on (you all might recall that the loaner DSP-centric development kit didn't work out for us). 

The goal is to put it on the net so that any of us can access it and do development. The way to accomplish this is probably through a VPN and/or screen sharing setup. I'll have a computer dedicated to the station that will (hopefully) stay up and be accessible all the time!

Here's a link to the kit:

And, 12.4 ISE (we use the free webpack version) has just been released. This is the software that allows you to develop on Xilinx FPGAs. Here's the download link:

Please update your ISE and stay tuned for more FPGA updates   :+)

Also, Bob McGwier has provided a filter solution (which I'm sorry to say I haven't had a chance yet to do much with quite yet). Thank you Bob!

-Michelle W5NYV

Wednesday, December 15, 2010

Greetings everyone!

I have a question. I'm confused on page 176 in the Six-Port Technique book. I'm working on an Octave model of six-port modulation and demodulation. It takes four reflection states, modulates, then demodulates. I am expecting to be able to show how a six-port device works with this model.

Determining the demodulation coefficients is an important step. There's three matrices. A, B, and X. A is composed of power out for all ports and all reflection states. B is the constellation points defined. X is the demodulation coefficients. 

The book says to do a least squares solution for X that minimizes the norm of B=A*X. 

The problem I'm having is understanding what the right sizes for the matrices are. 

For QPSK, it would seem to me that B and X are both 8x1. However, the size for A isn't 8x8, which is (I think) what it would have to be in order to multiply A times X. 

In my case, the number of constellation points M (for QPSK) is 4. This means that X has to have 8 values. There are four sets of two values that determine the constellation points.

I'm implementing a six-port model, so the number of ports N is 4. The notation seems to indicate that the matrix A is 4x8, but multiplying a 4x8 times a 8x1 results in a 4x1 matrix, and not a 8x1 matrix.

Either I have the size of B wrong, or I have the size of A wrong. I'm pretty sure I have the size of X right.

I think the size of B for QPSK should be 8x1 because of how it's used on page 166. Each of the Powers (from 4 ports) is multiplied by a particular demodulation coefficient. If I have 4 "I" and 4 "Q", then that should be eight demodulation coefficients in all, for an 8x1 matrix.

So, given all that, I'm thinking that I'm not understanding the notation for the matrix A, or else not understanding what the vertical and horizontal bars mean within A, B, and X.

Please let me know where I'm botching this - I'm getting a constellation out at the end using an abridged X of four elements (in order to match the size and sort of limp in with a fake BPSK), but it's severely distorted, and I think getting the correct A would make the ideal-case model to behave correctly and result in a milestone being met.

I attached the model as a text file.  I know it's difficult to interpret other people's code, but here is the current version for reference.

I also attached the most recent demodulation result so you can see that the output isn't quite there yet - but it's close. I'm not sure why the title keeps getting cut off - there is possibly something I need to do to make gnuplot give me more margin so this doesn't happen, but I've been concentrating on getting the model corrected and have put off polishing the graphics for later.

Working on this is very exciting, and we've been able to make progress. I'm looking forward to making some hardware, and have found some practical references for combiners and couplers. I am still working on finding use of HFSS, but did track down someone with an older copy of the program who is supposed to bring it to me at one of the next microwave meetings, and I think Roger AD5T had a lead on a copy too. 

Using a local company's version didn't work out due to the license agreement that the company had with the vendor of HFSS.

More soon,
-Michelle W5NYV