Re: LSB on 75 Meters

Posted by KU4AF on Nov 22, 2011; 11:22pm
URL: http://elecraft.85.s1.nabble.com/US-60-Meter-Band-Changes-Approved-by-FCC-CW-Issues-tp7012018p7022509.html

First, let's dispense with the red herring of backward tuning. The VFO does tune backward on 75 but it has nothing to do with sideband inversion. The USB/LSB business works the same way with a fixed LO as with a VFO, and it works as I described.

When you mix a 5 MHz signal with a 9 MHz signal, there's no need to stand on your head and pretend that you're subtracting 9 from 5 to get minus 4. Instead, you go back to second grade and subtract 5 from 9 and get 4.

Working the sideband part into it with a 9 MHz USB signal and a 5 MHz LO -
Suppressed carrier freq: 9.000 minus 5.000 = 4.000
3 kHz modulation tone: 9.003 minus 5.000 = 4.003

Just as a sanity check, take another look at your own negative number example, Joe, and 'splain to me how 5.000 minus 9.003 gives minus 3.987. OK, I'll grant you a typo and assume you meant minus 3.997, but you're still wrong. It's minus 4.003.

John, KU4AF
Pittsboro, NC

<quote author="Joe Subich, W4TV-4">
On 11/22/2011 4:04 PM, KU4AF wrote:
> Whatever the genesis of the LSB/USB convention on the ham bands,
> this conversion scheme wasn't it. Mixing a 9 MHz USB signal with a 5
> MHz VFO will produce a USB output on either 4 MHz or 14 Mhz. The
> sidebands only get inverted in a mixer when you subtract the SSB
> signal from a higher frequency

Try the math again ...

If you have a 5.0 - 5.5 MHz VFO and subtract a 9 MHz USB signal from
that VFO you end up with a LSB output that tunes from 4.0 - 3.5 MHz
in reverse.  Start at 5.0 and subtract the 9 MHz carrier frequency you
get *minus* 4.0 MHz (carrier frequency) when the highest modulating
frequency (3 KHz or 9.003 MHz) is used you end up with *minus* 3.987
MHz - *lower sideband*!  Do the same with the VFO at 5.5 MHz and you
will find LSB at *minus* 3.497 to 3.500 MHz.

A mixer can produce F2 +/- F1 just as easily as it produces outputs
at F1 +/- F2.  If you don't believe this ... spend some time reviewing
the early SSB transmitter and receiver articles in QST from the 1950s
or borrow a Drake 1A/2A/2B and learn why some bands "tune backward."

73,

    ... Joe, W4TV