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Just throwing this in.....

In a nominal "modulated AM signal" you will have the carrier at the center and will have both upper and lower "sidebands" usually filtered to about 3 khz wide. What I mean here is that the UPPER sideband starts at the carrier frequency and goes UP 3 khz and the LOWER sideband starts at the carrier frequency and goes DOWN 3 khz.   Again, in AM your radio sends out all of the above. (about 6 khz wide all together!)

In sideband you only want the sideband information from the "selected" side (upper OR lower).   The carrier and the opposite sideband need to be "shaved off" so that only the selected sideband goes out to the antenna and the world.

Since the audio sideband is about 3 khz wide a popluar means of doing the "shaving" is to use a filter with fairly steep skirts (edges) so that ONLY 3 khz worth of spectrum will fit through it. (think of it this way... if you have a 30" wide door... you can only fit 30" of something through it!).

If you don't apply ANY shifting at all..... what will be presented to the filter is... the carrier.. will be right smack in the middle... and the LOWER 1.5 Khz of EACH of the two sidebands.   That will be useless as you will get 1/2 of EACH sideband sent out with the carrier. This will leave 1.5 khz of each sideband OUTSIDE of the filter pass band.

So what is done is this:   If you happen to be wanting USB at the moment, the radio shifts the carrier frequency DOWN by 1.5 khz.   What this does is essentially a "shift left" that puts the carrier right at the LEFT EDGE of the filter into the skirts where IT and the lower sideband will be cut off.   What is NOW passing through the filter.... is 3 khz worth of upper sideband content.

When you want LSB... the carrier is shifted UP by 1.5 khz which NOW puts the carrier and upper sideband audio in or beyond the edge of the filter and THEY will be cut off.... leaving ONLY the 3 khz worth of lower sideband audio going through the filter.

So it is a "sliding window" effect... but you are not sliding the window (the filter!)... you are sliding the RF (the carrier and the sidebands) by half of the filter width to allow ONLY the sideband that you want to go "through the door".

I hope this makes sense.

(edited to add a "pencil whipped" illustration)

Bob

<blockquote data-quote="guitar_199" data-source="post: 846291" data-attributes="member: 18156">Just throwing this in.....In a nominal &quot;modulated AM signal&quot; you will have the carrier at the center and will have both upper and lower &quot;sidebands&quot; usually filtered to about 3 khz wide.&nbsp; What I mean here is that&nbsp; the UPPER sideband starts at the carrier frequency and goes UP 3 khz and the LOWER sideband starts at the carrier frequency and goes DOWN 3 khz.&nbsp;&nbsp; Again, in AM your radio sends out all of the above.&nbsp; (about 6 khz wide all together!)In sideband you only want the sideband information from the &quot;selected&quot; side (upper OR lower).&nbsp;&nbsp; The carrier and the opposite sideband need to be &quot;shaved off&quot; so that only the selected sideband goes out to the antenna and the world.Since the audio sideband is about 3 khz wide a popluar means of doing the &quot;shaving&quot; is to use a filter with fairly steep skirts (edges) so that ONLY 3 khz worth of spectrum will fit through it. (think of it this way... if you have a 30&quot; wide door... you can only fit 30&quot; of something through it!).If you don&#039;t apply ANY shifting at all..... what will be presented to the filter is... the carrier.. will be right smack in the middle... and the LOWER 1.5 Khz of EACH of the two sidebands.&nbsp;&nbsp; That will be useless as you will get 1/2 of EACH sideband sent out with the carrier.&nbsp; This will leave 1.5 khz of each sideband OUTSIDE of the filter pass band.So what is done is this:&nbsp;&nbsp; If you happen to be wanting USB at the moment, the radio shifts the carrier frequency DOWN by 1.5 khz.&nbsp;&nbsp; What this does is essentially a &quot;shift left&quot; that puts the carrier right at the LEFT EDGE of the filter into the skirts where IT and the lower sideband will be cut off.&nbsp;&nbsp; What is NOW passing through the filter....&nbsp; is 3 khz worth of upper sideband content.When you want LSB... the carrier is shifted UP by 1.5 khz which NOW puts the carrier and upper sideband audio in or beyond the edge of the filter and THEY will be cut off.... leaving ONLY the 3 khz&nbsp; worth of lower sideband audio going through the filter.So it is a &quot;sliding window&quot; effect... but you are not sliding the window (the filter!)... you are sliding the RF (the carrier and the sidebands) by half of the filter width to allow ONLY the sideband that you want to go &quot;through the door&quot;.I hope this makes sense.(edited to add a &quot;pencil whipped&quot; illustration)Bob</blockquote>

[QUOTE="guitar_199, post: 846291, member: 18156"] Just throwing this in..... In a nominal "modulated AM signal" you will have the carrier at the center and will have both upper and lower "sidebands" usually filtered to about 3 khz wide. What I mean here is that the UPPER sideband starts at the carrier frequency and goes UP 3 khz and the LOWER sideband starts at the carrier frequency and goes DOWN 3 khz. Again, in AM your radio sends out all of the above. (about 6 khz wide all together!) In sideband you only want the sideband information from the "selected" side (upper OR lower). The carrier and the opposite sideband need to be "shaved off" so that only the selected sideband goes out to the antenna and the world. Since the audio sideband is about 3 khz wide a popluar means of doing the "shaving" is to use a filter with fairly steep skirts (edges) so that ONLY 3 khz worth of spectrum will fit through it. (think of it this way... if you have a 30" wide door... you can only fit 30" of something through it!). If you don't apply ANY shifting at all..... what will be presented to the filter is... the carrier.. will be right smack in the middle... and the LOWER 1.5 Khz of EACH of the two sidebands. That will be useless as you will get 1/2 of EACH sideband sent out with the carrier. This will leave 1.5 khz of each sideband OUTSIDE of the filter pass band. So what is done is this: If you happen to be wanting USB at the moment, the radio shifts the carrier frequency DOWN by 1.5 khz. What this does is essentially a "shift left" that puts the carrier right at the LEFT EDGE of the filter into the skirts where IT and the lower sideband will be cut off. What is NOW passing through the filter.... is 3 khz worth of upper sideband content. When you want LSB... the carrier is shifted UP by 1.5 khz which NOW puts the carrier and upper sideband audio in or beyond the edge of the filter and THEY will be cut off.... leaving ONLY the 3 khz worth of lower sideband audio going through the filter. So it is a "sliding window" effect... but you are not sliding the window (the filter!)... you are sliding the RF (the carrier and the sidebands) by half of the filter width to allow ONLY the sideband that you want to go "through the door". I hope this makes sense. (edited to add a "pencil whipped" illustration) Bob [/QUOTE]

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