I/Q

[Rstafford12]

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
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[Bill Frantz]

Kevin Reid, AG6YO has a description of I & Q as part of his
visual introduction to SDR and DSP at
<http://visual-dsp.switchb.org/>. He will be presenting the same
material at the March 11 meeting of the West Valley Amateur
Radio Association (www.wvara.org) in San Jose, California.
Anyone interested is welcome to attend. (See the web site for details.)

73 Bill AE6JV (WVARA president)

On 3/3/15 at 1:59 PM, [hidden email] (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.
-----------------------------------------------------------------------
Bill Frantz        |Security, like correctness, is| Periwinkle
[phone]      |not an add-on feature. - Attr-| 16345
Englewood Ave
www.pwpconsult.com |ibuted to Andrew Tanenbaum    | Los Gatos,
CA 95032

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[tomb18]

Hi
Here is a good starting point.
http://www.dspguru.com/dsp/tutorials/quadrature-signals
73's Tom

-----Original Message-----
From: Rstafford12
Sent: Tuesday, March 03, 2015 4:59 PM
To: [hidden email]
Subject: [Elecraft] I/Q

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
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[Don Wilhelm-4]

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
>

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[alorona]

Hi, Richard,
 
These are very good questions. You have already been given links to tutorials on the subject, but here are some short answers to your questions to get you thinking.
 
1) The I and Q signals do have a "frequency", but it's called 'sample rate'. I and Q are constantly changing, but they are being sampled, or measured, at a regular rate which is the sample rate. By the time I and Q appear at the input of your soundcard they are considered 'baseband' or 'audio' and no longer have a carrier frequency associated with them because they've been demodulated. Think about this: does CW coming out of your speaker have a frequency? Well, not a carrier frequency, because it's been removed in the detector or demodulator, and besides we can't hear at the carrier freq, but the CW definitely has a 'words per minute' rate which your ear locks on to when it copies the CW. This is kinda like sample rate. You could call it 'data rate'.
 
2) I and Q are always 90 degrees out of phase. But their absolute phase is unknown. So once you see I and Q, if they aren't squared up you can rotate them artificially so that they line up on the X and Y axes that you see in all the math books. This is easily done with a phase shifter, which is just adding a delay to I and Q. If you take a picture of a football field, and your camera wasn't perfectly parallel to the chalk lines, your mind automatically 'adds phase' so that the lines are nice and square in your mind. That's kinda how it works in a demodulator. I hope I answered the question you had.
 
Al  W6LX
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[Edward R Cole]

not going to repeat what everyone has already posted:

One begins with an RF signal which has modulation (easiest to
consider the modulation SSB and even easier if you take the example
of a two-tone test signal for SSB).

The first step in producing IQ output is to spit the RF into two
parallel mixers that use the same frequency LO, but one is shifted
90-degrees in phase.  The LO is the same frequency as the RF so one
gets audio as an output (same thing one does in an SSB
demodulator).  The two resulting audio outputs are 90-degree out of
phase with each other.  In the typical SDR the next step it convert
to digital using a analog to digital converter (ADC)(hams are using
their soundcard as a ADC).  The resulting two digital signals are out
of phase with each other and can be processed by a computer (called
an DSP = digital signal processor).

Due to the unique mathematical properties of the two digital signals
one can recover CW/DSB/SSB/AM/FM/PSK modulation types (and I may have
left out others).

This is a very rudimentary (non-mathematical) description.  The true
beauty of using IQ is its versatility.  No longer does the receiver
have to have a product detector, discriminator, envelope detector,
etc.  The DSP does it all in digital form and re-converts the
demodulated signal back to audio (or displays the signal on a screen).
http://www.kl7uw.com/MAP65.htm

the diagram may help understanding.

73, Ed - KL7UW
http://www.kl7uw.com
     "Kits made by KL7UW"
Dubus Mag business:
     [hidden email]

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[Mike Markowski-2]

Me neither, but here's a page with both nice illustrations and description:

   http://whiteboard.ping.se/SDR/IQ

73,
Mike ab3ap

On 03/03/2015 08:49 PM, Edward R Cole wrote:
> not going to repeat what everyone has already posted:
> [...]
>
> 73, Ed - KL7UW
> http://www.kl7uw.com
>      "Kits made by KL7UW"
> Dubus Mag business:
>      [hidden email]
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[Bob W7AVK-2]

An interesting discussion of I and Q signals 90 degree phase shift from
each other and what can be done with them check any ARRL Handbook in the
1950 and look for "Phasing SSB".

73  Bob  W7AVK

On 3/3/2015 5:56 PM, Mike Markowski wrote:

> Me neither, but here's a page with both nice illustrations and
> description:
>
>   http://whiteboard.ping.se/SDR/IQ
>
> 73,
> Mike ab3ap
>
> On 03/03/2015 08:49 PM, Edward R Cole wrote:
>> not going to repeat what everyone has already posted:
>> [...]
>>
>> 73, Ed - KL7UW
>> http://www.kl7uw.com
>>      "Kits made by KL7UW"
>> Dubus Mag business:
>>      [hidden email]
> _

.
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