[Elecraft] I/Q

[Don Wilhelm]

Richard,

The I and Q signals contain baseband (audio frequency range) signals 
which vary in both frequency and amplitude.
The "magic" is that the I and Q channels are 90 degrees out of phase 
with each other.  Each channel is what could be referred to as a normal 
audio signal.  The 90 degree fixed relationship between the 2 signals no 
matter what the frequency or amplitude "does the deed" - each channel 
carries the same information, but the phase relationship 'does the 
magic'.  With the 90 degrees out of phase information one can create (or 
decode) any form of modulation wanted - I did that "way back when" in my 
EE classes using pure math - it is a bunch of sines and cosines with a 
lot of other things thrown it.  When you think of it that way, remember 
that the sine and cosine functions are 90 degrees apart.
My studies were back in the pure analog days and the math was "a bear", 
but today with ADCs available, those signals can be digitized and the 
math manipulated by a computer before sending the result to a digital to 
analog converter (DAC).
A full understanding may require in-depth study into modulation forms 
and techniques as well as skills in manipulating math equations 
involving sines and cosines.
De-modulation is similar, but the opposite of modulation.

Sometimes it is helpful to keep the pure math relationships in mind to 
fully understand what is going on.  For a simplified view, a spectum 
display can be created by using Fast Fourier Transforms (FFT) and also 
by other techniques which will transform from the time domain to the 
frequency domain.  That is what is being done to present the display on 
the PX3.  In the time domain, you see a signal like you would observe on 
an oscilloscope - amplitude vs. time.  In the frequency domain, you see 
the amplitude vs. frequency at any one instance of time.  The 90 degree 
out of phase signals are necessary to convert from the time domain to 
the frequency domain (or vice-versa).

If you want to dig into the math behind modulation techniques, then be 
my guest.  The nice thing about DSP is that the math can be done 
"perfectly" within the range of the resolution of the ADC and DAC 
devices used.  How good the DSP is depends on how faithfully it can 
represent the analog signal, and that depends on the number of bits of 
resolution that are needed for the task at hand.

Hopefully this information is helpful.  You are asking a complex 
question, and several others have given you guides to more detailed 
info.  You can either accept my simplified answer, or dig into it deeper 
and come out with the full details.

73,
Don W3FPR

On 3/3/2015 4:59 PM, Rstafford12 wrote:
> I am having trouble with I/Q modulation.
> 1) Do the I and Q components have a frequency, or are they just amplitudes. I would suspect the former.
> 2) How is the change in the phase difference for the I and Q symbols, compared to the reference frequency (carrier?)  determined? I believe I understand the math, but I don’t believe this is a Faraday statement that what you can see/sense can correspond to a physical model, but beyond that math has to do.
> Sorry if the is too basic, but I am really trying to understand my KX3 and PX3.
> Richard
>