Why?

[guitar_199]

Now, my screen graphic abilities are the absolute WORST and I had to "pencil whip" this one and do a little editing. Hopefully it will get show my point adequately.

The first image is kind of what the "frequency spectrum" of an AM waveform looks like. This is ( a LOUSY representation of ) what a spectrum analyzer might show.

Things to notice: The big peak in the middle is the carrier for whatever channel you are on. The squiggles to the right are the "voice content" in the upper sideband. The squiggles to the left are the exact same "voice content" in the lower sideband. The thing to catch here.... the audio frequency that modulates this waveform represents the same frequency on EITHER SIDE of the carrier.

So if I am whistling at 1 Khz.... there will be content at Carrier + 1 Khz AND Carrier - 1 KHz. If I am whistling at 2 Khz ... there will be content at Carrier + 2 Khz AND Carrier - 1 Khz. The trick is.... on BOTH SIDES... the LOWER the audio frequency IS.....the closer it is to the carrier. And THAT means.... the LSB side is mirror image/backwards from the USB side. As the audio frequency goes up....the USB signal moves to the right BUT THE LSB signal moves to the left.

Now the NEXT image..... I just grabbed the USB squiggle for this one.

In THIS image it is important to remember TWO things.....

THiS LITTLE WAVEFORM (modulated on RF)... is ALL that comes out of your radio. The other sideband AND the carrier were chopped off!!!! So just THIS LITTLE package of RF is flittering about in the sky. So all we have is a modulated squiggle....flying around... finding resonant antennas... and crawling down into the attached radio.

When it gets into the radio, the reciever can bring in the squiggle, it can amplify it, but it has no idea what the modulating signal intelligence is. The radio has to be set to a "mode". Lets say we have the radio switched to USB. As the signal comes in, the receiver's demodulator is trying to match this signal up with an inserted carrier. Since we told the receiver we are wanting upper sideband.... it is going to try to match up ON THE LEFT SIDE of the squiggle. meaning....as we adjust the clarifier up or down it is "in a way" attaching the squiggle to the RIGHT SIDE of the inserted carrier. Since this IS a USB squiggle.... the lower frequency is to the left...and the higher frequency is to the right. AND THAT IS GREAT.....because.....when we adjust the clarifier and get that inserted carrier just in the right spot..... the frequency content in the squiggle are on the correct side of the carrier....so we get good audio.

Same squiggle (USB) but NOW let's say the radio is in LSB. The radios demodulator is NOW trying to match the squiggle up on the LEFT SIDE of the inserted carrier. Eventually, by careful dialing of the clarifier you WILL get the inserted carrier matched to the RIGHT side of the squiggle....the problem is.... the RIGHT side of the squiggle is HIGH frequency content and the LEFT side of the squiggle is the LOW frequency content.... which is completely backwards from a NOMINAL LSB squiggle. Consequently....the frequencies carried in the squiggle are DEMODULATED BACKWARDS. The message content is NOT backwards....but the frequencies in the complex waveform ARE backwards.

Hence....... "duck sound city"! The radio might be set for LSB and STILL FIND a USB squiggle.....but the encoding is backwards. So you will HEAR something.....but it will not be intelligible.

Those with more technical savvy might certainly see that this presentation may not be TOTALLY correct from a technical point of view.... but I am hoping that the concepts are close enough that it might help some understand what is going on in the world of SSB demodulation.

Of course...... I might get thrown OUT of here too!!!!!!

just see what you think.....

 

[Handy Andy]

Here's another fun part...

The Superheterodyne...

The WIKI of this article can be found here...
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The mixing of two signals - that are close in frequency, you get a third - a DERIVATIVE.

You can then extract the Audio signal from the two frequencies "interfering" with each other.

It's how the IF-Based BFO - your Clarifier or Voice Lock - works -

So by using the wrong mode, as @guitar_199 -describes, you hear it sounds like a "Duck, or backwards" is correct- because of the MODE your in.

The Radios IF mixes the signal in a way that if you are in the wrong mode, won't work - it's inverted, reversed or just plain unintelligible. To hear the mode - it's in how you apply and use a Frequency to obtain or extract audio information from the received signal.

So again, back to the "Why AM operators - I can hear them in USB nd LSB - becasue of the MODE - but if you try to use the wrong mode and say, use LSB to decode the USB - the frequency you may be able to get it there, but the method to detect it, is opposite to what you need to use.

It is why IF has "offsets" in your SSB transceiver.- to use the correct "image" to the correct BFO frequency, to hear the signal.