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DSB and SSB Radio

  1. Mar 20, 2016 #1
    Hey,
    I'm learning about DSB and SSB in my communications classes and I'm a little confused with the concept.

    So, We know that in a AM signal there is the Modulating signal (Information) and the Carrier signal (Transmission signal.) We also knows LSB and USB are created when the signals are combined

    in DSB signals it is said that the carrier is suppressed and only 1 side band remains (SSB) It also says that at the "min" peak in the cycles there is a phase shift.

    This confuses me a little bit because in my textbook they are only speaking of the transmission section of the am signals. Why would you want to eliminate the carrier if the signal hasn't been sent? It also says one of the main fall backs of SSB and DSB is that on the receiving end it is hard to remove the carrier signal. This all seems counter-intuitive of eachother....

    How I picture AM in my head is that the envelope of the carrier frequency is reshaped by the modulating signal. Due to my backround in audio I tend to think of the differences in sidebands, harmonics or anything of that nature in terms of the sound frequency spectrum and then just realize where it actually is on the spectrum. I understand that if the signal is single, you only need 1 side band which saves power but I don't understand what exactly is happening with the carrier.

    Typically in textbooks they always show things with sin waves, but if USB and LSB are different frequencies then how is the modulated signal shown with just a single frequency? (aside from showing the signals before they are joined)

    Hopefully these aren't all super silly,
    but I've been waiting for it to click together.
    Thanks in advance!
     
  2. jcsd
  3. Mar 20, 2016 #2

    davenn

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    no that is not quite correct

    just to clarify that for you. You would generally have DSB with suppressed carried
    (DSB with carrier is basically an AM signal)

    SSB as it suggests is just one of the sidebands upper or lower with a suppressed carrier

    the carrier isn't eliminated but rather suppressed it allows for a much narrower bandwidth of transmission, which is a much more efficient use of the available spectrum in a particular frequency band. It's also much more efficient use of the transmitter as it doesn't have to consume power producing a continuous carrier as AM does.
    EDIT: Ohhh and I forgot to mention, SSB offers a much better SNR ( Signal to Noise Ratio) than AM

    have a read of this ....
    https://en.wikipedia.org/wiki/Single-sideband_modulation

    and this
    https://en.wikipedia.org/wiki/Double-sideband_suppressed-carrier_transmission

    and this
    http://www.radio-electronics.com/in.../single-sideband-suppressed-carrier-ssbsc.php

    and finally this
    http://electronics.stackexchange.com/questions/79939/am-and-suppressed-carrier-ssb


    there's a lot of serious reading for you :smile:


    Dave
     
    Last edited: Mar 20, 2016
  4. Mar 20, 2016 #3
    What is not correct about it?
     
  5. Mar 20, 2016 #4

    Averagesupernova

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    The process of amplitude modulation is more than a combination of signals. The carrier is multiplied with the modulating signal. This gives us 2 new frequencies plus the originals. The 2 new signals are the upper and lower sidebands.
     
  6. Mar 20, 2016 #5
    Yes, I agree. But how does it make the op's statement wrong? He didn't use the word multiply? Or, maybe he used the word combined instead of multiply? Personally, I probably would have used the word mixed. But anyway, I think the op probably knows what's happening in this particular instance. Just a poor choice of words maybe.
     
  7. Mar 20, 2016 #6
    Well, that's the way it would appear on an oscilloscope. Try viewing it with a spectrum analyzer. You can probably find some examples with Google image search. Try "amplitude modulation spectrum".
     
  8. Mar 20, 2016 #7

    Averagesupernova

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    This is a case where there is a lot of confusion due to audio mixing which is summing. Frequency mixing is a completely different animal which of course you know. I just choose to avoid the confusion.
     
  9. Mar 20, 2016 #8
    Thanks both of you, I probably would have assumed it was similar to audio mixing. I'm going to read those articles, if I'm still foggy on stuff Ill report back.

    Since each sideband is a different signal does that mean in Audio transmission (say radio) which is a stereo signal L and R would each be in a sideband?
     
  10. Mar 20, 2016 #9

    davenn

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    No, each sideband has the same signal info within it

    and this is the exact reason you can get rid of the carrier and one sideband ....
    because you still have all the info ( voice signal) in the one remaining sideband
     
  11. Mar 20, 2016 #10
  12. Mar 20, 2016 #11
    Yes, That makes sense.

    But it also makes sense that it could be done.
    I imagine the modulating circuit is combined with the carrier separately L and then R and then later combined. There would still be harmonics I imagine. Probably not the best thing in the world.
     
  13. Mar 20, 2016 #12

    davenn

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  14. Mar 20, 2016 #13

    jim hardy

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    Hmmm


    mistaken post , hereby erased...

    as i said, corrections welcome =

    thanks Alex !

    old jim
     
    Last edited: Mar 20, 2016
  15. Mar 20, 2016 #14

    jim hardy

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    Okay, thanks Alex ! post deleted ...
     
  16. Mar 20, 2016 #15
    Thanks alot Jim!

    So when you have a modulation signal and you are designing the circuitry for modulation and demodulation is there principles/rules in which you choose a proper carrier frequency in relation to the modulation frequency (in many cases a modulation frequency range?)

    Thanks for the all the replies. This is helping a great amount.
     
  17. Mar 20, 2016 #16

    davenn

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    The two are not related ... eg the modulation freq is commonly the voice audio range, 300Hz to ~ 3 kHz
    The carrier freq can be kHz to 100's of GHz and anything in between

    one of my amateur radio transceivers has TX capabilities on a bunch of bands between 1.8 and 30 MHz, then 50 - 54 MHz, then 144 - 149MHz, 430 - 440MHz
    and finally 1250 - 1300MHz all in one box. It can do AM, FM, SSB and digital modes across all those bands.


    Dave
     
  18. Mar 20, 2016 #17

    jim hardy

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    What Dave said.....
     
  19. Mar 20, 2016 #18

    Averagesupernova

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    Not me. I have old(er) school stuff.
     
  20. Mar 20, 2016 #19

    davenn

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    The Kenwood TX2000X is a really nice bit of kit
    It replaced 4 separate radios in the shack
     
  21. Mar 20, 2016 #20

    Merlin3189

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    My maths is probably worse than Jim's, but: it starts ok, but if you do go further with the "mishmash" it just reverts to sin(ωc)sin(ωa) plain and simple.

    I agree that in practice you may get all sorts of other bits and pieces, IMO because the multipliers are not perfect and the original carrier may not be pure. So you do filtering as you say. But the maths says you could get a pair of perfect sidebands.

    I agree totally with the voila, 2 sidebands. But this is DSB. Notice, there is no carrier present, just the two sidebands. No need to suppress nor eliminate a carrier. It simply is not there, IF you can accurately multiply the carrier by the modulation.
    For SSB there is also a similar expression which generates a single sideband, without carrier nor other sideband. It too relies on accurate multiplication, but also on being able to generate quadrature signals for both carrier and modulation, which was very difficult before DSP.
    (In case anyone is interested, LSB = cos(ωc)cos(ωa) + sin(ωc)sin(ωa)
    and USB = cos(ωc)cos(ωa) - sin(ωc)sin(ωa) )

    The expression for AM is (carrier) x (1 + modulation) or sin(ωc )(1+sin(ωa))
    This is easy for old guys to remember, if you think of the old amplitude modulation class A circuit. It was done by modulating the HT line to the PA. Since this can't go negative, you add the modulation to the standing HT, so that it swings (up to) from near zero to near double.
    With no modulation, you get full carrier;
    With modulation amplitude equal to the HT, you get 100% modulation with full carrier and two sidebands, carrying between them power equal to that of the carrier;
    With greater modulation the valve cuts off during modulation peaks and you get splatter (mishmash?) and become very unpopular;
    With modulation amplitude less than the HT, you get undermodulation, with full carrier and two sidebands carrying less power than the carrier. This is what is happening most of the time, because only peaks can be allowed to reach 100% modulation

    A bit long winded, but OP seems to know about this signal. What you notice here is that in every case you are putting at least half your power into the carrier and this carrier is exactly the same whatever the modulation, if any. So it tells the receiver nothing by itself. The information about the modulation is all in the sidebands. But, as OP says, you can easily see the modulation in the envelope of the combined signal. And it is ridiculously easy to get the modulation back, just using a diode (or crystal and cats whisker.)

    So one motivation for DSB is to not waste power on transmitting carrier. If you can transmit 100W, instead of sending 50W of carrier and 50W of sidebands, send just the sidebands at the full 100W, then let the receiver generate a carrier signal and add it to the sidebands it receives. That will regenerate that simple AM signal in the receiver, but at twice the power.
    Doing this (in the receiver) was not trivial, which is one reason why DSB was not that common. You can work out what the carrier frequency is, because it is midway between the sidebands. And you can manually adjust your local oscillator to sit at the right frequency. But it is much more critical than tuning a simple AM signal.
    Since the two sidebands are like mirror images of each other, which both contain exactly the same information just arranged in a different way (if the whiz kids will allow that fuzzy desciption), you only need one of them, hence Single Side Band.
    One is sin(ωcm) the other sin(ωcm)
    Again they don't look nor sound like the original modulation, but if you reinject the carrier sinewave at the receiver, then you get the modulation back.
    Eg. Take the USB sin(ωcm) and multiply by sin(ωc)
    you get 1/2[cos(ωm) - cos(2ωc + ωm) ] which looks messy, until you realise the left cos is the modulation and the right cos is RF at nearly twice the carrier frequency, so can be filtered out to leave the modulation.
    Again you need to know what the carrier frequency is and it is a bit more difficult than with DSB. If the receiver knows what frequency is used at the transmitter it has to set it's oscillator to that frequency (and keep it there, used to be the difficult part!) One (common) technique was simply to have a matching crystal in both transmitter and receiver. Even without knowing the exact frequency, as with DSB, a skilled listener can tune the oscillator until the signal becomes intelligible when you get close enough to the original carrier.
    I think you mean "drawbacks" and "on the receiving end it is hard to reinstate the carrier".
    Yes. As said above, you need the carrier to make sense of an SSB or DSB signal. Recreating that was the problem. Crystal controlled oscillators was one solution. Phase locked loops was another. Reduced carrier, rather than suppressed carrier was a halfway house.
    Removing the carrier at the transmitter was also a problem, mainly met by crystal filters and balanced mixers until DSP came along.

    The modulated signal is generally not a single frequency (could be for SSB), and I don't think anyone would show a modulated signal as a simple sinewave.

    First using sinewaves in books, makes the maths easy to follow. They can write the formulae for any modulating signal - it just gets messier.
    Secondly, about 200yrs ago Fourier showed that a more complex signal is mathematically identical to a collection of sinewaves. You can work out what happens to any signal waveform, by working out what happens to each of those sinewaves and adding the results together.

    USB & LSB, are bands of frequencies.
    A single sinewave modulation produces an upper side frequency and a lower side frequency. Eg. 300Hz audio modulated onto a 1MHz carrier, gives 1.0003 MHz and 0.9997 MHz, spot frequencies. If you know that the carrier was 1MHz and you get either of these frequencies, you can work out what the audio frequency was. Try this: if you receive 1.0005 MHz, what was the audio frequency?
    When a radio station broadcasts a pop record, they want to send a complicated waveform which is a mixture of frequencies (at varying levels). So for each frequency you get an upper and lower side frequency. Together these make the two bands of frequencies, called sidebands. Converting these side bands of frequencies back to their original sinewaves and adding them together gives us the original complicated waveform.
    In the eg. above, if you had a sound containing a 300Hz tone and a 500Hz tone of twice the amplitude, it wouldn't look like a simple sinewave, but you'd get 1.0003MHz with one amplitude and 1.0005MHz with twice the amplitude. (and for DSB or AM, 0.9997 and 0.9995MHz) This modulated signal would not look like (nor be) a simple sinewave. When you added back the carrier frequency and recovered the 300Hz and 500Hz , you would get twice as much 500Hz as 300. When you added them together, they would again make the same waveform as the original mixture.
     
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