Understanding IQ modulation as used in software defined radio

Discussion in 'Wireless & RF Design' started by RICHARD A PHILLIPS, Mar 1, 2015.


    Thread Starter New Member

    Jan 22, 2015
    Does anyone have any suggestions on articles or books that explain quadrature modulation and or IQ modulation in a more intuitive fashion. I would also love to find some way of simulating some of the circuits and ideas using Multisim or Lt-spice.
    Background: precalc and trig, but not calculus. I am reviewing precalculus and trig. at this time; some understanding of complex numbers. But a little trouble with complex numbers expressed as e^(2 pi f t); I have a partial understanding of IQ modulation from reading some articles, but I don't understand about negative frequency or how to represent rf signals using vectors- especially why some vectors move clockwise and others move counter-clockwise. (Basically a piece meal understanding of the subject).
    Thank you, Richard Phillips
  2. bertus


    Apr 5, 2008
    RICHARD A PHILLIPS likes this.

    Thread Starter New Member

    Jan 22, 2015
    Thanks. I'll check it out. I just started to look at it. I'll probably be spending some time there.

    Thread Starter New Member

    Jan 22, 2015
    Thank you
  5. BR-549

    Well-Known Member

    Sep 22, 2013
    Richard Phillips,
    I understand and agree with your frustration.

    I believe the reason is because instructors and textbooks still teach that an electrical signal is sinusoidal. A sine wave.

    This causes mass confusion. The cosine and the sine of the signal is sinusoidal. But the signal is helical.

    If you try to study I Q signals with vectors on paper....remember that when you rotate....the vector is moving in or out of the paper.

    Think of a signal as the outer edge of threads of a machine screw/bolt.

    The thread of a machine bolt forms a helix.

    The diameter of the thread is the amplitude. Actually it's half that....the radius. If we vary the radius....we are amplitude modulating the helix.

    The pitch(number of threads per inch) of the thread is the frequency of the helix. If we vary the pitch...we are frequency modulating the helix.

    And like the threads on a bolt....the helix rotates. A bolt and the helix can rotate(screw) in and they can rotate(screw) out. And if we modulate that in and out........we get phase modulation.

    If you study this you will find that by controlling the AMPLITUDES of the cosine(I) and sine(Q) of the helix, you can modulate any way you want. OR any combination you want.

    You start with an oscillator, that gives you a helical signal. You feed the helical signal into a phase splitter. The splitter breaks the helical into the cosine and sine components. Now you can adjust the amplitudes of the cosine(I) and sine(Q) independently.

    When completed....we re-combine the cosine and sine....back into the complete helix.....and presto...you have any kind of modulation that you want.

    Modulation is easy. The real value is in the demodulation.

    I suggest the TAYLOE detector.
    nsaspook likes this.

    Thread Starter New Member

    Jan 22, 2015
    BR-549, Thanks for the reply. Wow, there is a lot to think about. I was studying the website suggested by bertus: http://www.fourier-series.com/IQMod/index.html . It helped me visualize frequency modulation and amplitude modulation by showing the rotating vector. But, as always, there is some things that I don't quite understand. The bolt analogy gives a more complete picture, but I have to think about it for a while. I am trying to associate a lot of things that I have studied in the past about this and put it all together. Maybe I am making this too complicated. In some explanations they describe a sine wave as y= sin (2 pi f x) and the cosine wave as cos (2 pi f x). I guess that the cosine wave could be expressed as y= sin (2 pi f x + (pi/2)). This would show show the output of the oscillator with the 90 degree or pi/2 phase shift. The website went along to show how the sine and cosine wave ( the I and the Q signals) varied as the vector rotated. They put the I signal on the X axis and the Q signal on the Y axis. This kind of confused me because I thought that the X axis represented the cosine value and the Y axis represented the sine value. But forgetting that, you could see how both the I signal and the Q signal changed as the vector rotated. Now, how to apply these ideas to a three dimensional picture. I can visualize a bolt. Your explanation described it well, but I am having a little trouble visualizing this. I will check out the TAYLOE detector. I have not heard of that. I was also wondering about demodulation.

    Again, thank you for your help,
    Richard Phillips
  7. BR-549

    Well-Known Member

    Sep 22, 2013
    Take a dozen or so turns of a slinky and spread them apart horizontally up in front of you. Or imagine so.

    If you snapshot it, it appears to be a sine function. (I)

    This is because you are looking at a helix or spring from the side of it.

    Now imagine a center line going thru the center of the slinky.

    Now rotate the slinky one quarter turn around the center line.

    If you take another snapshot, you will get another sine function. (Q)

    When you look at the end of the slinky, you see a circle.

    Look at pi at the end and look at pi from the side.

    Do the two pi look the same?

    This is because you are trying to plot pi in 2-D.

    An electric pi is a 3-D entity.

    The origin moves as the arc of pi is struck.

    This means pi has a curved wing or curved blade area.

    This REAL pi area is tilted thru your graph paper.

    Again, using a 2-D plot to describe a 3-D event is always incomplete and distorted.

    I re-read your original post and I apologize for recommending the Tayloe. I see you only want to simulate and not build.

    Remember....simulators use 2-D math.

    Using I/Q demodulation, one can detect any type of signal.

    We take a snapshot of the unknown, this will give us I component.

    Wait 90 degrees and take another snapshot...that gives us Q.

    You can analyze any unknown signal.

    And you can tell a lot about the source of the signal.

    Radar and sonar.

    To draw a circle or cycle on paper, you only need one radius.

    An electric circle or cycle needs two radii. One for the electric and one for the magnetic. These vectors are orthogonal.

    I can only give you an intuitive view. I do not believe the current equations show the proper relationship between the electric and the magnetic and angular momentum.

    I will leave the rest to you and others. You have enough to vary the vectors and see the result.

    BY the WAY.......if anyone built these Tayloe detectors (Mitch Electronics perhaps)......they would sell like hotcakes.

    There are lots of people on this site that use simulators and will gladly go thru the current math with you.

    Good luck with your studies, Richard.
    RICHARD A PHILLIPS likes this.

    Thread Starter New Member

    Jan 22, 2015
    Can you tell me more about pi. I can visualize everything that you said, but I'm not sure about pi. I am not that great at math, I got C's in both pre-calc. and trig., and I felt lucky to get them. But by going over the math involved with complex numbers, I am getting a better understanding of what we studied in both math courses. I have a hard time learning this stuff as fast as the class go. But, by working with it, I am learning better.

    I am so glad that you mentioned about the Tayloe detector. Boy, do I want to build the circuits. There is nothing like having the circuit live in front of you. It would be nice to have a working encoder and decoder circuit (transmitter and receiver). If I get enough of an understanding, I would like to demonstrate this for our Amateur radio club. We have had other talks about software defined radio and IQ modulation. The part about IQ becomes very vague. I don't need an engineering level understanding on how it works, but I would love to know how it works in a basic way and play with it. It has been quite an adventure and that is what I love about electronic communication.

    I briefly looked at some of the Tayloe websites. I have not had time to really investigate them. But, they are sure intriguing.

    Thanks again.
  9. BR-549

    Well-Known Member

    Sep 22, 2013
    This detector might be a little heavy for a newcomer. Surface mount components. Plus you need a oscillator at 4 times the operating frequency...which will be surface mount..i.e. SI570.

    For a demo to a ham club, I would use this.


    I bought one off of eBay for ~$12 a couple of years ago.

    This site will tell you all about the different chips and all of the free software available.


    These chips are different from the Tayloe, but give an output in I and Q.

    This output is fed into your audio card....via usb.

    There are also modifications to these dongles for different frequency ranges.

    My dongle does quite well with a short indoor antenna.

    The airport, fire and police, WX and many ham repeaters are 5X5.

    Have fun.
  10. KL7AJ

    Senior Member

    Nov 4, 2008
    Try out Digital Signal Processing Technology by Doug Smith, available from the ARRL bookstore.
  11. alfacliff

    Well-Known Member

    Dec 13, 2013
    this is a software version of the phase shift modulation scheme for generating and detecting ssb signals. used to be used in hardware, phase shifting the audio and rf to produce usb, lsb, am and other modes.
  12. w2aew


    Jan 3, 2012
    Maybe my video on the basics of IQ signals and modulation will help:
    macgregor_sailor likes this.
  13. alfacliff

    Well-Known Member

    Dec 13, 2013
    the phasing type of ssb generation can be used also for generating fm and other modes, by changing the phase of the I and Q signals. the modern methosd is to use the dounb board in a pc and software to do the detection of the phase of the I and Q signals using the digital signal signal processisor on the sound board., there are quite a few programs on line for doing this.
  14. nsaspook

    AAC Fanatic!

    Aug 27, 2009
    I just had a discussion (that someone learning radio now does need to understand I/Q signals) about this on another site and this was used to demonstrate the helical cosine(I) and sine(Q) signal.

    BR-549 likes this.
  15. recklessrog


    May 23, 2013

    Excellent links!!!