G4ILO wrote:
GM3SEK wrote:
What amazes me is the persistence of the belief that it's all because a
9MHz SSB generator and a 5MHz VFO produce opposite sidebands on 14MHz
and 3.5MHz. Anyone who can rub two numbers together can see that isn't
true... but that makes no difference at all.
9+5.0=14.0. 9+5.5=14.5.
9-5.0=4.0. 9-5.5=3.5.
Sorry, I don't see how that isn't true. I'm sure I can also recall some
published transceiver designs that took advantage of this fact to provide
two band coverage using a single VFO and 9MHz IF.
This particular mixing process gives 3.5MHz and 14MHz very conveniently
- but it gives the SAME sideband on both bands.
Imagine a phasing-type SSB generator with a 9.000MHz suppressed carrier
frequency. When configured for USB, an audio tone at 1kHz gives output
in the upper sideband, at 9.001MHz. Then:
9.001 + 5.300 = 14.301 - that's 14.300MHz USB
9.001 - 5.300 = 3.701 - that's 3.700MHz USB
We have the SAME sideband on both 20m and 80m!
To swap sidebands in this example requires the SSB generator to be
switched to LSB. With a 1kHz tone, the output switches to 8.999MHz. Then
the same calculations give:
8.999 + 5.300 = 14.299 - that's 14.300MHz LSB
8.999 - 5.300 = 3.699 - that's 3.700MHz LSB
Once again, we get the SAME sideband on both 20m and 80m.
A 9MHz filter exciter is slightly different, because it's the filter
passband that is centred on 9.000MHz. Such exciters swap sidebands by
switching between carrier oscillators. For USB the CO is below the
filter passband at 8.9985MHz, or for LSB the CO is above the filter at
9.0015MHz. The arithmetic is a bit more messy, but the result is exactly
the same - generating SSB at 9MHz does NOT automatically swap sidebands
between 80m and 20m.
That is clear mathematical proof that the 9MHz SSB exciters were NOT
responsible for the ham USB/LSB convention. On the contrary, when
changing between 20m and 80m they were forced to switch sidebands at
9MHz in order to *follow* that convention! There is also plenty of
historical proof that the USB/LSB convention had been in existence for
several years before the first published 9MHz design came along in 1956.
--
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek
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