DSB-SC demodulator

Discussion in 'Wireless & RF Design' started by Kerim, Mar 3, 2011.

  1. Kerim

    Thread Starter Member

    Mar 3, 2011
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    Hi all,

    Would you please let me know if there exists an IC that can be used to demodulate a double-sideband suppressed carrier signal while the carrier is frequency modulated at the source?

    The given data are:

    The average frequency of the suppressed carrier is 455 KHz.
    The AM modulating signal has an audio spectrum from 100 to 7000 Hz.
    The suppressed carrier changes its frequency at a rate of 6Hz (sinusoidal) between 405 to 505 KHz.

    The formulas are:
    V_if = Vm * cos(Wc*t) * f(t)
    where:
    Wc = 2 * pi * 455000 * [ 1 + 50000*cos(2*pi*6*t) ]
    and
    f(t) is an audio signal with zero average voltage from 100 to 7000 Hz.
    For simplicity, f(t) could be expressed as a single 1 KHz tone:
    f(t) = cos(2*pi*1000*t)

    Thank you in advance for any information you can provide me.

    Kerim
     
  2. t06afre

    AAC Fanatic!

    May 11, 2009
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    This one perhaps http://www.datasheetcatalog.org/datasheet/motorola/MC1496D.pdf
     
  3. Kerim

    Thread Starter Member

    Mar 3, 2011
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    Thank you for your reply, t06afre.

    MC1496 is indeed a very good IC (though very old too).
    But I am afraid this IC is rather a modulator.

    Of course, MC1496 can be used as a synchronous demodulator as well but the received signal of interest has no carrier (hence the expression 'suppressed carrier").

    So the IC I am looking for, if it exists, should have the ability to recover this suppressed carrier first; frequency and phase. Only then the demodulation (synchronous here) would be possible which, in turn, recovers the audio signal.

    Kerim
     
  4. Ron H

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  5. Kerim

    Thread Starter Member

    Mar 3, 2011
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    Thank you Ron,
    I will study the datasheet of FN3652 to see if it works fine in case the carrier frequency is not constant and changes in a relatively wide range.

    Truth be said, once I built this demodulator using some old ICs as CD4046 and LM339. It was able to restore the missing carrier from the two side bands only and be locked to it while it varies from say 405 KHz to 505 KHz with a rate of 6 Hz (even 10 Hz).
    Lately I had a talk about this (by emails) with a few professionals in telecommunications who claimed there are many new low cost ICs that can do the same job and mentioned as you did the Costas Loop.

    I think the IC you have suggested is typical in this field.
    I will see if it can do (or not) the same job of my DSB-SC demodulator and let you know. On the other hand, it is clear that such a complex IC should cost much more than the conventional PLL as CD4046 but this is not important for the time being.

    Thank you again for your contribution.

    Kerim
     
  6. Kerim

    Thread Starter Member

    Mar 3, 2011
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    Hi Ron,

    After many hours of searching and reading, I got this conclusion.

    Costas Loop wasn't a practical solution when the signals to deal with had to be analogue.
    Then with the introduction of digital filters and the digital VCOs providing I & Q outputs (from ROM tables), it became possible to implement/integrate Costas Loop in a well balanced configuration and even add many other control functions in the same IC.

    But on the other hand, I couldn't find a practical circuit to build and be based on these new digital ICs. Obviously there are some evaluation kits for them but I don't think they are for radio amateurs or even students.

    So it seems the very simple analogue circuit I built as a real DSB-SC demodulator is not known yet officially!

    Is it possible I am the only one in the world to know how a conventional PLL (VCO, phase comparator and low pass filter) can be made to lock to the missing carrier, just by a simple modification?!

    I am afraid it may be the case because I couldn't find any reference talking about it even as an idea.

    Finally, it is sad that in all universities around the world, teachers keep telling their students:
    "A SSC-SC signal is much easier to demodulate than DSB-SC one".
    But the common sense says recovering the baseband from two side bands SHOULD BE much easier than from just one side! In fact to prove this truth, I designed my simple DSB-SC demodulator which can be built by any amateur/student at a cost of about $1... Chinese prices :)

    Kerim
     
  7. bertus

    Administrator

    Apr 5, 2008
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  8. Kerim

    Thread Starter Member

    Mar 3, 2011
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    Thank you Bertus, it is indeed a good reference for various interesting methods for both RF modulation and demodulation.

    But... :) Do you personally think that it could be possible, based on the actual known methods and components, to say that a DSB-SC signal is easier to demodulate than SSB-SC one?

    As we know, SSB-SC may carry a hi-fi music signal for example which can be recovered by inserting at the receiver side a local carrier at the same frequency of the missing one. What makes this method easy to implement is that the local oscillator doesn't have to be in phase continuously with the suppressed carrier. But on the other hand, the receiver should know in advance the exact frequency of the suppressed carrier to minimize the possible frequency shift of the recovered baseband.

    Let us suppose the same hi-fi music signal is transmitted as DSB-SC instead hence taking twice the bandwidth of the previous SSB-SC. Isn't supposed that who is able to build the previous SSB-SC detector can also build, with the same ease, an analogue DSB-SC demodulator as well?

    Unfortunately this is not the case yet and I am real surprised that what I did (explained in a previous post) as an MS thesis 30 years ago couldn't be found in any reference till our days. I have always believed that whatever idea might come to my mind will likely come to many other's as well around the world. So I keep searching... [repeat: searching for an analogue DSB-SC demodulator using a conventional PLL, not as Costas Loop with I & Q, that locks to the missing carrier by a simple modification].

    Kerim
     
  9. mapinfold

    New Member

    Jul 31, 2011
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  10. Kerim

    Thread Starter Member

    Mar 3, 2011
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    Hi Mike,

    Thank you for your kind reply.
    As you know, it is accepted in literature that:

    “Demodulating a SSB-SC signal is much easier to achieve than in case its two sidebands are present (that is DSB-SC).”

    When I have heard this while I was a graduate student, it sounded to me like a dilemma. It is weird that getting more info that takes more bandwidth lets the signal detection be harder!!! I was sure that there is something missing to the scientific world.

    So in 1979, I had the idea to find a very simple way to demodulate a DSB-SC signal as an MS thesis project. After 5 months of failures, I ended up recovering the missing carrier from the two sidebands by using a simple conventional PLL (phase detector, LPF and VCO with a minor modification, the trick). This means there is no need for any LC circuit as in the squaring technique, and no need for quadrature signals as in Costas Loop. Therefore the novel demodulator could be integrated as an analogue IC (and when its simple concept will be known by the world, it can be converted to a digital IC as well). And my thesis project has proved, once for all, that the above statement (SSB-SC versus DSB-SC) is no more valid, at least to me ;) Obviously, these days after I had the chance to download and install a simulator (LTspice), sometimes I try analyzing it more in depth and even updating some parts of it to get better settling time and optimum lock range... etc (though this depends on the application).

    Since a few months ago, I had the idea to contact some professionals in electronics and communications. Those who worked for companies have no time updating their scientific knowledge if not allowed on their schedule. And those who were graduate students have preferred not to upset their professors by working on a totally novel project, while it is assumed that such a simple DSB-SC demodulator cannot exist in the first place.
    For instance, this DSB-SC demodulator works obviously for any AM index as well; from zero to infinity (the case of suppressed carrier).

    Yes, you are right. It is unusual (if not impossible) that a private engineer who was born then lived/worked in Syria, knows a scientific technique not known yet by any other engineers in the world, mainly in USA :)

    Cheers,

    Kerim
     
  11. mapinfold

    New Member

    Jul 31, 2011
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    Kerim do you have a circuit for this demodulator we can see ?? , if you are using just a pll and ancillary circuit , sounds like it may be just operating in squaring loop mode with the phase detector providing the square law mixing function ? and the rest of the loop locking onto the stable upper and lower sidband mixing product , one of which will be halfway between the side bands ?? cheers Mike ZL1BTB
     
  12. Kerim

    Thread Starter Member

    Mar 3, 2011
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    Mike,

    Of course there is a circuit.
    Long ago (in the 80's) when I needed a private link (between home and work, when I was rather poor), I used this concept as a sort of scrambling. That is, on the AM band, I let the carrier vary at a low rate (about 6Hz) in a 50 KHz bandwidth. On the FM band, I modulated the voice signal on 32 KHz subcarrier as DSB-SC and without adding a pilot while keeping the audio baseband spectrum blank (silent).

    You are right, the trick is a simple auxiliary circuit which I found by chance, like it happened to Archimedes :). So, by just adding this simple circuit (that has no LC or filter) the PLL was able to lock easily with the suppressed carrier as if it is present!

    Cheers,

    Kerim

    Added:
    Now you have a clue on how to do it ;)
     
    Last edited: Aug 1, 2011
  13. mapinfold

    New Member

    Jul 31, 2011
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    Kerim , how about publishing a circuit so we can build it and see how it actually performs ?? cheers mike
     
  14. Kerim

    Thread Starter Member

    Mar 3, 2011
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    Hi Mike

    Your proposal is surely a good one for someone living where science is supported. In my case, the time I will publish it, I will be out of the game once for all and in all respects.

    The irony is, I already did it in the early 80's, and Wireless World published it and I got for it a check of 40 Sterling Pounds (as it was the case for every circuit I sent and be published during this period of time). But it happened that no one took it seriously at that time, thinking it was just another circuit idea (from Syria) of a known concept.

    So since I will have no role after this simple solution will be introduced to the world, I prefer to reveal it to just one person who can take advantage of it much better than I. For example, I thought that it could be a good gift to a graduate student in communications. And his project concept will be added to the literature at last.

    For instance, do you know a close person to you studying communications? :)

    Cheers,

    Kerim
     
  15. mapinfold

    New Member

    Jul 31, 2011
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    God day kerim yes it would be good to see it and you would have your name attached to it by virtue of the the public awareness of the internet probably almost as good as a patent which as you know anyone can modify it , change and claim to be theirs ha ha but once lots and lots of people see your circuit and it with your name no one can lay claim to it originality .. its has been seen to be yours by so many . Sorry I do not know of any communications students no universities of that calibre anywhere near here ... for me it is just a hobby as a sideline from amateur radio , I am much happier to make and study circuits and how they work than talk all around the world , dsb is interesting as it is easy to produce as has a signal advantage over ssb but alas there is not much about on reception to take advantage of the extra infornmation and signal redundancy of the technique , still a lot of qrp Ham radio build little qrp transmitters and it would be nice to have a companion true dsb receiver to compliment and take advantage , however i am trying to make the circuitry as simple and functional as possible thats the challange cheers Mike ZL1BTB !
     
  16. Kerim

    Thread Starter Member

    Mar 3, 2011
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    Hi again Mike,

    Oh... I didn't know that such a demodulator could have a practical use even in these days.

    An editor for an actual magazine of electronic designs wrote me back that even FCC has decided recently to ban new systems using DSB-SC. Naturally, the FCC group had a good reason to do it because, based on the present knowledge, its reception is either not reliable or relatively costly (using complex digital ICs) hence it is useless practically.

    What I like most in DSB-SC is its power efficiency; no input signal gives zero output power as well! But if its detection has to be rather complex, this benefit won't be a real advantage even its generation is very simple (relative to other modulations).

    In my opinion, DSB-SC fits best in the aviation RF communications (but not before FCC hears about the new simple concept for its detection ;) ). The interference would be much lower than of the conventional DSB AM system which is still in use (for voice). It is likely that more than one plane (near and far) turn on their transmitters simultaneously and on the same frequency. But by using DSB-SC, a captain would be able to control his transmitter output power by his own voice! And since all captains won’t keep talking all the time, hearing from all planes, mainly the weak signals from the relatively far ones, would become possible even without using directive antennas. Of course, the same can be achieved by using SSB-SC which has another advantage of taking half the bandwidth but its system will have to be rather complex (at both sides; TX and RX) to get the same accuracy in the carrier regeneration (so usually a pilot is added).

    In any case, FCC makes the rules not I.
    Alas, its group still needs time to be real good in communications. :D

    Cheers,

    Kerim

    Note:
    Do you know a group who could be interested specifically in DSB-SC system?
     
    Last edited: Aug 2, 2011
  17. mapinfold

    New Member

    Jul 31, 2011
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    hi Kerim, you have the whole aviation industry is all vhf AM and hf SSB , there is no way any of the whole systems going to change , for any other performance reasons as it would be prohibitively expensive , and in case you didnt know just because it is used in aviation, its price is 3 times more than the land based version ( I have a pilots licence) its is ridiculous but thats how it goes , I am afraid my friend DSS-Sc is merely a historical curiosity now for only technical and technical understanding reasons , it had its day so many years ago , SSB is now king but its overall complexity is offset by the volume of inexpensive components to achieve the performance and you have to admitt SSB does work well nowadays .inexpensive frequency stability is leaps ahead of what it was 20 yrs ago , and now we have all digital systems that enable any mode including dsb to be enabled at the press of a button with impressive perormance to boot .DSB is a dinosaur .. Visitors who hear the DSB demodulator system are perplexed when i screw the tuning of the receiver plus or minus a few hundred hertz and they are baffled as to why the pitch of the received voice doesnt change , they havent quite grasped how the demodulator works ha ha orhow the carrier osc is generated from the sidebands , you say you had the curcuit pblished in WWorld in the 80's I must have a look to see if i can find the article Id be interested to read it and study the circuit . I m still interested in old analogue rf techniques , I also have a 1973 Norton motorcyle so i must be clinging to the past ha ha cheers Mike
     
  18. Kerim

    Thread Starter Member

    Mar 3, 2011
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    Hi Mike,

    I agree with you on all what you have said. Indeed, you have well described the present situation concerning the fate of DSB-SC systems.


    On my side, I found another fact which is there is NO institution in the world that still cares about pure science. I even contacted so-called professors in communications who teach in great universities and their reply was just: “Sorry, we have no time to change what we teach, based on what we already have in literature!”.

    In fact and based on my rather long experience, I noticed that a ‘formal Science’ is much like a ‘formal Religion’ (formal -> addressed to the masses). In both, people have to follow the teachings (said scientific or religious) by faith, and in the way the persons in charge have decided to present them! And no matter these teachings could be true or not, the majority in the world will keep accept them as they are, till the Elite group (scientific or religious) decides on something else.

    So to me in the least, only the ‘individual Science’ and the ‘individual Religion’ could be based on 'pure logic'. Obviously, one of the basic teachings in both formal Science and Religion is that a person should not rely on his own logic. But if other’s reasoning has to be more reliable than mine, I wonder why I got a brain in the first place! Is it given to me just to discover other’s Elite Brains?!!!

    Back to the DSB-SC, I think you get now my point that the best way to introduce its novel concept is by an enthusiastic graduate student in communications with the hope his thesis supervisor will be an open minded professor who enjoys both teaching and learning... as I do :)

    Cheers,

    Kerim
     
  19. RRITESH KAKKAR

    Senior Member

    Jun 29, 2010
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    @ I was born and I had to live on a planet called earth

    same with other, most of them not all......
     
  20. Kerim

    Thread Starter Member

    Mar 3, 2011
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    You too?! what a nice coincidence :cool:
     
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