We’ve done quite a bit with Google Sheets and signal processing: we’ve generated signals, created filters, and computed quadrature signals. We can pull all that together into an educational model for two SDRs talking to each other, but it’s going to require two parts: modulation and demodulation. Guess what? We can do that with a spreadsheet.
The first step is to generate a reference clock for the carrier. You’ll need a cosine wave (I) and sine wave (Q). Of course, you also need the time base. That’s columns A-C in the spreadsheet and works like other signal generation we’ve seen.
The Plot Thickens
Think about the IQ plot of what we have so far. Plotting IQ means putting I on the X axis of a graph and Q on the Y axis. When I=1, Q=0. That’s a spot right on the X axis at X=1. The same thing happens, but flipped around when I=-1. It also happens when I=0 and Q=1, but rotated 90 degrees. The key is that, in each case, the length of the vector is 1. Every spot, in fact, will have a length of 1 because sin2(x)+cos2(x)=1. If you look at the first 12 rows of the spreadsheet with default values, you’ll see that X and the Y move between 1 and -1 in such a way that the sum of their squares is always 1.
What does that mean? It means the IQ reference signal will draw a circle of radius 1 on the IQ plot. If we multiply both by the same amount, the radius will change, but you’ll still get a circle ..
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