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From: ewong9 on 24 Mar 2010 08:49
I would appreciate if anyone can answer my question.
Let's say that I have two arrays of 200 samples each.
One array is I data and the other is Q data. The sampling
rate for these samples is 1 Mhz. I know that I = A*cos(theta)
and Q = A*sin(theta). Also that tan(theta) = sin(theta)/cos(theta)=
Q/I. But how do I know which array is I data and which array is Q
data from analysing both arrays? Thanks in advance.



EEE


From: Tim Wescott on 24 Mar 2010 09:54
ewong9 wrote:
> I would appreciate if anyone can answer my question.
> Let's say that I have two arrays of 200 samples each.
> One array is I data and the other is Q data. The sampling
> rate for these samples is 1 Mhz. I know that I = A*cos(theta)
> and Q = A*sin(theta). Also that tan(theta) = sin(theta)/cos(theta)=
> Q/I. But how do I know which array is I data and which array is Q
> data from analysing both arrays? Thanks in advance.

In general you don't, so it's better to keep track of which is which.
Assuming you don't start out with a phase reference, the only real way
to tell is that if you have them swapped your spectrum will be reversed
-- but that's the same symptom as if you have them right way 'round and
one of them has been multiplied by -1 somewhere along the line.

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com
From: Jerry Avins on 24 Mar 2010 09:55
ewong9 wrote:
> I would appreciate if anyone can answer my question.
> Let's say that I have two arrays of 200 samples each.
> One array is I data and the other is Q data. The sampling
> rate for these samples is 1 Mhz. I know that I = A*cos(theta)
> and Q = A*sin(theta). Also that tan(theta) = sin(theta)/cos(theta)=
> Q/I. But how do I know which array is I data and which array is Q
> data from analysing both arrays? Thanks in advance.

Suppose there is a complex number z = x + i*y. You are given two
numbers, a and b, and told that one is the value of x and the other is
the value of y, but not which is which. You can't determine which is
which without knowing something distinctive about the nature of z.

Jerry
--
Discovery consists of seeing what everybody has seen, and thinking what
nobody has thought. .. Albert Szent-Gyorgi
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From: glen herrmannsfeldt on 24 Mar 2010 17:41
ewong9 <ewong1(a)n_o_s_p_a_m.cfl.rr.com> wrote:
> I would appreciate if anyone can answer my question.
> Let's say that I have two arrays of 200 samples each.
> One array is I data and the other is Q data. The sampling
> rate for these samples is 1 Mhz. I know that I = A*cos(theta)
> and Q = A*sin(theta). Also that tan(theta) = sin(theta)/cos(theta)=
> Q/I. But how do I know which array is I data and which array is Q
> data from analysing both arrays? Thanks in advance.

If you get it wrong, unless you have an absolute phase reference
(in which case you wouldn't be asking) the result is the same
as time going backwards. If you can't tell the sign of time,
then you can't tell I from Q. (or Q from -I).

-- glen
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