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In a message dated 6/17/04 7:10:03 PM Eastern Daylight Time, [hidden email] writes:
I just picked up a copy of the June CQ in the airport bookshop and p.28
has a sidebar by K2MGA (CQ Publisher) on the history of SSB:
Regardless of how the SSB signal was
generated, the 455 kc USB signal was mixed up
to 9 Mc. Using a converted war-surplus
BC-458 transmitter...as a VFO, the
4.0 to 5.3 Mc output was either added
to or subtracted from the 9Mc SSB
signal. That produced a USB signal on
20 meters or an LSB signal on 75 meters.
(That's the origin of the world-wide
convention: LSB below 20 meters; USB on
20 meters and up. ..)
I'm sorry, but that's simply not what happens. K2MGA is mistaken.
Here's proof:
The tuning reverses. But the sideband does not! Here's why:
9 MHz filter and 5-5.5 MHz VFO
Start with a 9000 kHz USB signal (carrier at 9000, top end of audio at 9003)
Add a 5200 kHz VFO signal for 20 meters
(9000 + 5200 = 14200, 9003 + 5200 = 14203)
Result is carrier at 14200, top end at 14203 - still USB
Subtract a 5200 kHz VFO signal for 75 meters
(9000 - 5200 = 3800, 9003 - 3800 = 3803)
Result is carrier at 3800, top end at 3803 - still USB!
The numbers tell the story. In order to cause sideband inversion, the local oscillator must be above *both* the input and output frequencies of the mixer.
However:
5.2 MHz filter and 8.7 - 9.2 MHz VFO:
Start with a 5200 kHz USB signal (carrier at 5200, top end at 5203)
Add a 9000 kHz VFO signal for 20 meters
Result is carrier at 14200, top end at 14203 - still USB
Subtract the 5200 kHz USB signal from a 9000 kHz VFO signal for 75 meters
(9000 VFO - 5200 carrier = 3800, 9000 VFO - 5203 top end = 3797)
Result is carrier at 3800, top end at 3797 - now it's LSB!
On Thu, 17 Jun 2004 5:37pm, [hidden email] wrote:
> No, that's just not true. Urban legend.
> ...
> Now if you use a 5 MHz SSB generator and a 9 MHz VFO you *do* get
> sideband
> inversion.
Do the math and you'll see that the old urban legend is a myth. The standard of LSB/USB in ham radio was set at the very beginning - 1949 at the latest.
I wish it *were* true about the 5 MHz VFO/9 MHz IF sideband inversion - it would make some projects simpler! But the numbers don't lie.
Check out the heterodyne scheme of the K2 and you'll see the same principles in action. The only way you get sideband (not tuning direction!) inversion is if the LO is higher than both the input and output frequencies of a mixer.
73 de Jim, N2EY
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