Quadrature

[Anonymous]

Hi all
I have been trying to understand a number of key concepts that seem to underpin the technology, and while trying to describe them I have realised that I do not <EM>really</EM> know what they mean. One of these concepts is that of quadrature. OK a sinusoidal signal presented to the receiver has a real and imaginary part, the in-phase real part is straightforward enough, but what about this quadrature? I have scoured books, websites and forums to find hands on info and as yet have found nothing useful. How about starting from a pure sinusoidal and introducing a target, then a description of the departure from pure wave to distorted wave. Lastly, how does the theoretical description and derived values relate to the real world?
Thanks a bunch guys, what we all do without you! Play golf probably... <img src="/metal/html/lol.gif" border=0 width=15 height=15 alt=":lol">

 

[Anonymous]

Hi Roy,
"In Phase" and "Quadrature" are terms used in continuous wave induction balance detectors. They are not terms used in PI. There is a page on the Geotech forum which describes them.
Eric.

 

[Anonymous]

Hi Roy,
Eric is right, the term "quadrature" is generally used when referencing the sampling done in a VLF.
On a PI, sampling is done in terms of time and generally is referenced to the end of the pulse on time.
On a continuous wave type detector such as the VLF, there is no beginning or end so any sampling has to be referenced to some point or angle of the frequency.
The Wikipedia dictionary on the net mentions this about quadrature;
In telecommunications, the term quadrature has the following meanings:
1. The state of being separated in phase by 90

 

[Anonymous]

around my way they are the latest buzzwords in PI! Its all the rage, haven't you heard? <img src="/metal/html/wink.gif" border=0 width=15 height=15 alt="">

 

[Anonymous]

The others are right, quadrature is used in continuous-wave detectors (VLF), not PI. Although some of the multi-frequency designs sorta merge the two.
In a nutshell: A sinusoid has amplitude and frequency. It can also be said to have a "phase", but this can have two meanings. One is an instantaneous vector phase angle, and the other is an overall relative phase shift. In both cases, quadrature demodulation can be applied. The first case is common in digital communications, such as a cell phone or a modem. The second case is used in metal detectors.
Every sinusoid A*sin(wt + phi) can be split into pure sine and cosine components a1*sin(wt)+a2*cos(wt). The first term is the I, the second is the Q. This is just plain ol' trig. In circuits, we can take a phase-shifted sinusoid and compare it to a reference sinusoid, and use this math to determine the phase angle. This can be done with an analog mixer, as it was in the mid-70's, or by using sampling as it is done now. Two samplers are used, one (the I) clocked by the reference signal, the other (the Q) clocked by a 90-degree shifted reference signal. The (theoretical) result is 2 DC voltages that represent sine and the cosine of the phase angle.
This really needs some diagrams for clarity.
- Carl

 

[Anonymous]

I and Q are the most common example of "quadrature". If you take any vector, and then draw another vector at right angles to it, those two vectors are "in quadrature".
One of the things about the beeper industry that takes a bit of getting used to, is that there is no central clearinghouse for standardizing nomenclature and descriptions, or for that matter a standard way of visualizing and explaining how the darn things work. So each engineer comes up with his own way of doing it.
It can get a bit frustrating when you find yourself in a situation where you're having to work with another engineer, and you have difficulty communicating some of the simplest stuff-- for instance what does "zero degrees" represent? The resistive component (zero electrical loss angle) or the reactive component (zero magnetic loss angle)?
A little over a year ago we had a lengthly debate on this forum about what "flyback" is, and what it had to do with the ability to detect targets in a PI machine. It took us awhile to figure out that we were actually pretty much in agreement, because our ways of thinking about the process and describing it were so different, at first they sounded entirely contradictory.
The first time many years ago I saw George Payne's "motion discrimination patent", I was thrown by the fact that he described it as though it were a direct conversion single-sideband receiver. At first I thought he did it to make the patent as confusing as possible (not an uncommon tactic), but after studying it awhile, I realized that his way of looking at it would make perfectly good sense to someone whose background was in radio engineering, whereas my background was in industrial process control where we're used to looking at meters and charts that provide information in the time domain.
--Dave J.