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
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
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