1.5 KHZ SSB OFFEST?

[Sonar]

Although i had this Madison retrofitted with a quasi vfo, and true frequency counter.about 4 years ago I've always been curious as to Mike's description about the 1.5 khz offset. What is a - 1.5 khz offset? My Madison's frequency counter shows a 1.5 khz offset to the + side on upper SSB, and -1.5 on lower SSB.
Mike seems impressed with the fact that the frequency counter shows this offset.
I've never seen another frequency counter show this offset. Can someone explain this 1.5 kilohertz offset to me?

 

[sp5it]

"Offset" is dependent to ssb filter used.
Mike

 

[Unit 75]

Hey, Sonar... I have 4 or 5 of those Madison's with the 858 chip that use Ralphs VFO/counter kit. The newer models have the encoder mounted in the display instead of being incorporated in that side mounted case on your radio.
sp5it is correct about the 7.8 MHz crystal filter in your unit.
Your radio has a carrier offset of +/- 1.5 KHz above and below your 7.8 MHz for USB and LSB. Ralphs VFO/counter is set up for that so it actually reads your SSB frequencies instead of just the AM frequency.

 

[Sonar]

Hey, Sonar... I have 4 or 5 of those Madison's with the 858 chip that use Ralphs VFO/counter kit. The newer models have the encoder mounted in the display instead of being incorporated in that side mounted case on your radio.
sp5it is correct about the 7.8 MHz crystal filter in your unit.
Your radio has a carrier offset of +/- 1.5 KHz above and below your 7.8 MHz for USB and LSB. Ralphs VFO/counter is set up for that so it actually reads your SSB frequencies instead of just the AM frequency.

Thanks for expanding of the first reply. Your detailed description has answered my question, and solved the mystery of the 1.5 offset my Madison's frequency counter shows. 73

 

[Unit 75]

No problem, Sonar. Here are a couple of pics.

 

[Mustang 131]

No problem, Sonar. Here are a couple of pics. View attachment 30647 View attachment 30648

Damm that looks good. Nice rig.

 

[Sonar]

No problem, Sonar. Here are a couple of pics. View attachment 30647 View attachment 30648

Two sweet looking rigs.

 

[Karmaduke]

Although i had this Madison retrofitted with a quasi vfo, and true frequency counter.about 4 years ago I've always been curious as to Mike's description about the 1.5 khz offset. What is a - 1.5 khz offset? My Madison's frequency counter shows a 1.5 khz offset to the + side on upper SSB, and -1.5 on lower SSB.
Mike seems impressed with the fact that the frequency counter shows this offset.
I've never seen another frequency counter show this offset. Can someone explain this 1.5 kilohertz offset to me?

Hi Sonar,

I know this is an old post, but I'm curious which VFO kit you had installed. Where did you get it?

 

[guitar_199]

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