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How does an amplifier inadvertently demodulate a radio signal?

  1. May 2, 2016 #1
    I know sometimes speakers/amps will pick up a radio signal. I'm confused as to how the signal is demodulated accidentally. Are the signals picked up exclusively AM, or do FM signals get picked up as well?
  2. jcsd
  3. May 2, 2016 #2


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    A common way is via rectification. If you rectify an RF signal, you will generally get the envelope information, which can be audio. Certainly this can happen for AM signals, but it can happen for FM signals as well, if there is some filtering going on in the rectification path.
  4. May 2, 2016 #3

    I'm quite new to the subject. So still trying to gain an intuitive understanding...
    Last edited: May 2, 2016
  5. May 2, 2016 #4


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    and just to expand on Berkeman's rectification comments

    Two common ways this happens
    1) is by the amplifier interconnect cables picking up the RF signal and the diode action of the transistors in the preamp stages
    rectify and demodulate the signal which is then passed along the amplifier stages along with the wanted signal
    2) is by similar process as above but when the RF is picked up by the long speaker cables the process is confined to the rectification /demodulation, of the RF signal within the output transistor stages of the amplifier

    so in that last scenario, the RF is making its way into the amplifier via the speaker cables and into the
    output transistors, getting rectified and amplified and sent back out to the speakers

  6. May 3, 2016 #5

    jim hardy

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    We have no idea of your background.
    Have you ever studied radio or electronics at all? Ever build a crystal set as a kid ?
    Do you know what the guys meant by "rectification" ?

    A picture might be worth a thousand words.... and some vocabulary in order.
    from http://www.radio-electronics.com/in...plitude-modulation-detection-demodulation.php

    Left waveform is an AM radio signal. It is a high frequency AC wave, usually sinusoidal , and of the Radio station's Frequency . It's called RF for short.. actually "RF carrier" because it "carries" the lower frequency information that is to be conveyed.

    It is "amplitude modulated" meaning its amplitude shrinks and swells as shown, at the much lower rate of the information it's carrying maybe voice, music or data.. That one looks rather like it's modulated with a sine wave, maybe it's a pure note from a violin or Madonna's pretty voice singing something.
    The tips of the RF wave form the "Envelope" so called because when you look at it with an oscilloscope the individual RF waves are packed so close together they just look like a solid mass , ie they are 'enveloped'(surrounded) by the modulating signal.

    " Demodulation" at its simplest is a two step process, Rectify and filter.
    "Rectify" means chop off and discard half of it , in this case we've removed everything below zero volts, the negative half, leaving just the positive half of the RF wave. A simple diode will rectify the RF .
    That leaves you with the middle signal. Usually the rectification process is called "Detection" and the rectifier is called a "Detector" because there are other methods of demodulation. This one is the simplest and has been around for a hundred years.

    Filling in the gaps between the rectified RF peaks completes the demodulation. A simple capacitor that's not too big will accomplish this. It is sized large enough to hold charge between the RF peaks but not large enough to hold charge between the slower peaks of the envelope.
    That capacitor connects the RF peaks just like connecting dots, leaving you with just the envelope frequency.
    So the RF has been "filtered out" , completing the demodulation.
    You're left with the third waveform.

    Now - does demodulation have to be perfect ? of course not. You don't need an ideal diode,, any nonlinearity will distort the RF and partially demodulate it.

    So when a wire from a microphone or speaker carries a radio signal into an amplifier,
    the first transistor junction it encounters is a nonlinear device and it will partially rectify , hence imperfectly demodulate, the RF signal.
    That's why the sound you hear from such accidental demodulation is not very clear. The ear and brain do a very good job of recognizing even severely distorted signals though so it's quite distracting.

    A radio receiver can be this simple, note detector diode and capacitor driving headphones just as described above
    read about them here http://www.hws.org.au/Crystalset 2000/mystery.html

    hope this helps

    if you're more advanced than i thought - no offense meant.

    old jim
    Last edited: May 3, 2016
  7. May 3, 2016 #6
    I know what a rectifier does in terms of converting to one polarity, but I don't yet understand it's relevance to this application.

    I'm fairly knowledgeable in terms of wave physics, overtones, modulation, etc (just from working with audio), but know very very little about electronics and circuits. So an explanation based in wave physics would be very helpful.

    Edit: Thank you for your explanation. I'm starting to get it, but I don't yet understand some things. My understanding (possibly incorrect) is that the audio signal is just summed onto the carrier frequency (or is that not the case?) If so, why does the RF frequency need to be filtered, wouldn't it be inaudible anyway? And why is the rectification necessary (why do we have to remove everything below 0 volts)?

    Thank you!
    Last edited: May 3, 2016
  8. May 3, 2016 #7

    jim hardy

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    Sorry, i'm not well versed in high math or wave functions, i barely passed vector calculus 50 years ago and know better than to tangle with it now.

    Any help ?

    old jim
  9. May 3, 2016 #8
    Thanks, the multiplication bit is helpful in trying to get an intuitive understanding.

    Based on my username, you can probably parse that I'm coming at this from a musical background. So maybe using this scenario would be helpful (for me)?

    So if, for the sake of my understanding... If I had a carrier frequency of 10 khz, and was trying to, using AM, transmit a sine wave of 4khz, how would I do that (would I even be able to?). My understanding is that just summing the two waves would cause the amplitude of the 10khz carrier to be modulated up and down at a rate of 4khz, but is that not what happens?

    EDIT: Or I guess the modulation is more like having a tremolo effect at the modulating frequency, which would obviously produce different results...?
    Last edited: May 3, 2016
  10. May 3, 2016 #9


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    Rectification, and low pass filtering, the filtering provided by the bandwidth response of the amplifier.
  11. May 3, 2016 #10

    jim hardy

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    Glad you mentioned tremolo. I was about to flub up a mathematical attempt.
    Nowadays there's a field of EE called "Digital Signal Processing" , DSP.
    Have you studied Fourier Analysis?
    DSP uses it intensely. I don't go there much.

    Addition and multiplication of sinewaves don't give the same result.
    Addition just gives sin(a) + sin(b), the original frequencies.
    multiplication gives

    and observe taking the product gives you two new frequencies, sum and difference of the originals.
    Those are called the "sidebands" because they are centered on the mean and differ by twice the modulating frequency.
    That mean is the carrier, sum and difference are the sidebands. They sum to that enveloped wave in post #5.

    To help visualize the difference in sum and product
    i figure this picture saves a thousand words
    it's a picture of sinewave addition
    from http://clas.mq.edu.au/speech/acoustics/waveforms/adding_waveforms.html
    that i annotated


    multiplication of any two numbers has to give a zero result whenever either one is zero, doesn't it ? as in leftmost wave of post 5 ? Here you can SEE that isn't the case for addition.
    So addition of two isn't amplitude modulation. It's addition.

    When you want to Amplitude Modulate a sinewave, you multiply it by another one.
    That's not too difficult with tubes or transistors
    but nowadays everybody uses specialized integrated circuits
    (well radio stations probably still use giant tubes, maybe our Ham radio friends on PF will elucidate)
    from http://www.analog.com/media/en/technical-documentation/data-sheets/AD633.pdf
    Note the multiplier gives just the sidebands, in agreement with the trig identity up above for products, and they add the carrier back in at pin 6.

    That's why i like electonics , it is so close to math.
    But unlike normal people i have to use electronics to understand math not vice versa.

    Last time i tried to derive ths from trig identities i had to be rescued by a kindly PF member whose math is better than mine.

    I hope this helps you. If you need to see the the DSP math, i'd suggest post a question in the math forum....
    sinewaves are fascinating. Euler was a genius.

    old jim
    Last edited: May 3, 2016
  12. May 3, 2016 #11

    jim hardy

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    How a real analog multiplier works

    It is easy to make a controlled current source using transistors
    and it is easy to make another transistor behave like an adjustable resistor
    so place them in series and by ohm's law and you have multiplied .

    that's called a "transconductance " multiplier.

    There are other types.
  13. May 3, 2016 #12


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    Why don't you just do something up in a spreadsheet and graph it or write a quick program? You can get a good feel for what is happening by doing that. AM is most certainly multiplication of the modulating signal and the carrier.
  14. May 3, 2016 #13
    Thank you for clarifying the multiplication bit.

    I guess I was trying to understand it in a less mathematical way. For example my understanding of FM is that you basically convert the signal to a "vibrato" which then modulates the carrier frequency. The detector then outputs 0v when the carrier frequency is at exact pitch and the voltage goes up and down depending on the degree of pitch change, so the signal gets recreated. I probably wrote that poorly, but it seems like kind of an intuitive way of understanding how FM works, and I was hoping to get a similar level of understanding for AM.

    But in this case, maybe just thinking mathematically is more straightforward.
    Last edited: May 3, 2016
  15. May 4, 2016 #14

    jim hardy

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    i don't know much about music, looked up vibrato and it seems the right concept

    Your understanding of FM is exactly right.
    FM broadcast in US(your 88 to 108 mhz radio stations) shifts the carrier frequency ±150khz for full modulation.
    That'd be like modulating the pitch of a sound.
    FM was used also to record analog data in scientific investigations before the computer based dataloggers came into existence. An audio tone is frequency modulated per the measurement and recorded on an old fashioned tape recorder.
    I had an Ampex 32 channel FM tape recorder in the power plant, allegedly portable it weighed a hundred pounds but was a monument to precision.

    AM of course adjusts the amplitude instead of the frequency
    that same site i linked earlier has more on modulation


    Suoernova had a good idea above-
    Are you good with spreadsheets ?
    Try setting a cell for sin(t) and another for sin(t/50)
    t in degrees not radians
    and a third cell for their product cell1 X cell2

    run t from 0 to 36000 and plot third cell
    it should plot 100 cycles of amplitude modulated carrier showing 2 cycles of envelope, resembling the picture above.

    then run it again showing sum instead of product

    i always did mine in Qbasic... xcel only taunts me .

    old jim
    Last edited: May 4, 2016
  16. May 4, 2016 #15


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    To confuse you even farther, the carrier frequency itself does not change in amplitude with AM. It appears so but it is not the case. The composite signal of the carrier and the upper and lower sidebands make it appear like the carrier itself is changing but it is really not.
  17. May 4, 2016 #16
    So is tremolo not a good analogy in that case, if the carrier frequency isn't changing in amplitude?
  18. May 4, 2016 #17

    jim hardy

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    The Fourier transfer term for carrier doesn't change amplitude

    but as you can see in the pictures all RF cycles do not remain the same height.
    Algebraically the carrier and modulation are multiplied.
    But in the Fourier series of the result , sideband terms are added.

    Fourier and Laplace transforms are tools that i learned to use for cracking textbook problems, without understanding their underlying math.
    Just as a crow learns to drop a clam from altitude to crack it open, without understanding the math of momentum and impulse.

    I looked up Tremolo in Webster and found it refers to either pitch or loudness.
    So i left that one lone.

    old jim
  19. May 4, 2016 #18
    I'm familiar with fourier transforms in the sense that I know what they are and (in some cases) what they're used for. And just generally looking at spectral analyzers, looking at waveforms, playing with tools/toys like this http://www.falstad.com/fourier/

    So I'm quite familiar with fourier transforms, but in more of the practical/applied sense than in the math formula sense.

    Pitch is incorrect, tremolo and vibrato are often used interchangeably but they're not the same thing. Pitch(vibrato) would be frequency modulation and volume (tremolo) would be amplitude modulation. That's why I thought it would be applicable(?)
  20. May 4, 2016 #19
    When I have a bit of free time, I'm going to see if I can mess around with my synthesizer/recording software to try and see if I can encode/decode AM that way. Would help me understand it intuitively.
  21. May 4, 2016 #20


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    Matlab seems to be software of choice at the moment for communications work, at least in academia. One can generate a signal, perform any kind of modulation, characterize a channel, dump statistics, and plot resulrs all in a few dozen lines of code.
  22. May 4, 2016 #21

    jim hardy

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    in that case you have it right.
  23. May 4, 2016 #22

    jim hardy

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    is it hard to learn ? Excel was nothing but frustration and led to serial mouse-icide.
  24. May 4, 2016 #23


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    Easy to learn and powerful, that's not the drawback. Matlab is proprietary, costs few hundred $ for non-students, more for some of the plugin packages, which often causes problems I find as soon as one wants to ship code around to others outside your own shop. Also, its not quite a proper programming language, so as soon as a problem with some complexity and size comes along, requiring many pieces to cooperate, or requires high, C type compute performance, or something that requires longer term maintenance, Matlab becomes troublesome. For these reasons, the last serious team project I worked used Matlab to prototype all the technically challenging algorithms, and then converted the most successful Matlab algorithms to C++.

    Last edited: May 4, 2016
  25. May 4, 2016 #24
    Playing around with my synthesizer, I am able to see the sidebands develop as I increase the rate of modulation (up to a point)

    The only clumsy part is I'm having to do it with a triangle wave, which is the closest I can get to a Sin.

    But I can still see the sidebands develop around the fundamental of the triangle.
  26. May 4, 2016 #25

    jim hardy

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