My Understanding of SSB Demodulation

145

02:16:40

This works by multiplying (the XNOR gate works as an analog multiplier) the carrier (red) by the single sideband (blue) to get a mixed signal (orange), and then filtering out the high frequency component to produce an audio signal (green).

An online example of this that I made using JavaScript in conjunction with ProcessingJS, is on the website http://js.do/blog/processing/editor/ using this code:


background(0,0,0);

stroke(255,255,255)
line(0,15,100,15);
line(0,50,100,50);
line(0,85,100,85);

noStroke();

var x=0;
var y=0;
var z=0;
var a=0;

while(x<100){

     fill(255,0,0);
     y=sin(x/3)*10/-1+15;
     ellipse(x,y,1,1);

     fill(0,0,255);
     z=sin(x/4)*10/-1+50;
     ellipse(x,z,1,1);

     fill(0,255,0);
     a=((y-15)/10*((z-50)/10)-sin(x*7/12-89.5)/2)*10+85;
     ellipse(x,a,1,1);
     x+=0.1;
}


Note: if you know JavaScript, and you know how the math here works, you may be wondering why the value 89.5 is used instead of 90. The answer is that I found this value to work better out of trial and error, and that's probably because of inaccuracies in the sin() function.

Please tell me if there is anything that can be improved, and if this is indeed a theoretically possible method to demodulate an SSB signal.

published 8 years ago

hurz

8 years ago

Right, the Problem is here the mirror at 1250Hz get also demodulated. So the effort is the input filter which must be very sharp, or there is a technic which uses a second multiplier with 1kHz carrier but 90degree shifted in phase, this will if fine balanced multiplier are available suppress the second sideband or upper sideband.

jason9

8 years ago

Yes, but isn't the whole point of SSB that there is no second sideband for the demodulator to demodulate?

hurz

8 years ago

Yes, but maybe the next radiostation and noise is always present 😉

jason9

8 years ago

But this isn't about a receiver receiving only one SSB station, it's about demodulating the SSB signal the receiver has received, right?

hurz

8 years ago

Demodulating is receiving, dont know what you mean. Anyway, the principle is in both the same and called quadratur modulator. Multiplying an input signal on path with "sin" and a parallel path with "cos" and add both results together. This is the job of receiver and also from transmitter. But attention, this is only one out of tens to do SSB.

hurz

8 years ago

What you probably mean in your headline is "how to mirror audio spectrum"? But this is a subproblem from SSB and would even from be vailid in old scrambler circuits police used.

jason9

8 years ago

By receiving vs. demodulating I mean receiving the radio signal with an antenna vs. demodulating that radio signal into an audio signal. I fail to understand the similarity between SSB and QUAM. Also, I'm not sure what you mean by "how to mirror audio spectrum", and how this is a subproblem of some type of old scrambler.

hurz

8 years ago

You have any questions?

jason9

8 years ago

No.

sslz

2 months ago

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