Tweet
Ground Balancing
The block diagram above shows how I am going to ground balance the detector.
The transmit signal is an AC signal that can be described as having an amplitude T and varying with time as sin(t). The receive signal, S sin(t+a), has amplitude S and is phase shifted by an angle a.
The demodulator samples the receive signal at times p and p+90°. It removes t from the signal and gives DC outputs S sin(p-a) and S sin(p+90°-a). But sin(p+90°) is also cos(p), so we will call the second output S cos(p-a).
Then we rotate this output by the angle -p giving S sin(-a) and S cos(-a) which we call x and y. We can easily calculate the phase angle a from these values. X/y is sin(-a)/cos(-a) which is tan(-a). So we can find a by taking arctan(x/y).
Then we rotate x,y by the angle g, giving the outputs G = S sin(g-a) and GQ = S cos(g-a). If g is the phase angle of the ground signal then the ground signal will produce G = S sin(0) which is 0, and GQ = S cos(0) which is S. So all of the ground signal appears in GQ and none of it appears in G. G is known as the ground balanced signal.
Non discriminating or "All Metal" detectors only need the signal G, so they usually only have one demodulator and they phase shift the demodulator clock to get G directly out of the demodulator.
Discriminating detectors need more information so they can calculate the phase shift a. They may have 3 demodulators with clocks shifted to give x, y, G directly from the demodulators. Or they may have two demodulators giving x,y and rotate those to get G. Or they may have two demodulators that give G and GQ directly. This last option is commonly used in detectors with a factory set ground balance.
I originally planed to use the x,y,G signals for discrimination. That requires motion filters for 3 channels. Now I am leaning towards using the G,GQ signals which would only require filtering two channels. In that case I would not need the x,y signals and could rotate the raw data just once by the angle g-p to get G,GQ. But during development I would prefer to have x,y available because they are familiar and I can get a good feel for how the system is working by looking at those values.
Robert Hoolko