Lyle Johnson wrote:
Anybody been around long enough to explain the theory behind the use of LSB
on the lower bands vs. USB higher up? What is the advantage to doing so?
This is a classic detective story, with more than the usual share of red
herrings!
Early filter rigs used 9 MHz crystal filters. With a 5.0 - 5.5 MHz VFO
(often from a surplus AN/ARC-5 or SCR-274N "Command Set"), you got 80 m
and 20 m. Only needed one BFO crystal that way.
The 9MHz filter rigs were relative latecomers; the ham USB/LSB standard
was at least 10-20 years earlier. Far from helping to create the USB/LSB
standard, the 9MHz filter rigs involve extra complications to meet that
standard.
A 9MHz xtal filter with a single (low side) carrier xtal and 5.0-5.5MHz
VFO will give USB on *both* 80m and 20m. To generate LSB on 80m, these
rigs needed a second (high side) carrier xtal, which meant another
switch to throw when changing bands.
A much earlier influence on the ham standard was the 80/20m phasing
exciter by Norgaard, W2KUJ, which generated SSB on 5MHz and mixed with a
9MHz VFO. This frequency plan does invert the sideband, and decisions
between early experimenters using this rig seem to have been the true
origin of the ham standard.
When SSB experimenters in the USA and Europe began to work each other,
the world-wide ham standards grew out of those very first QSOs -
suddenly, everyone around the world had to agree which sideband to use
on which band. (British experimenters had actually been using USB on
80m, and on an agreed date they all changed to the new international
standard of LSB.)
Meanwhile, at around the same time in the late 1940s, the big telephone
and telegraph companies were starting to use ISB (independent sideband)
for their international radio traffic. ISB carries two completely
separate channels on opposite sidebands, so the transmitters generated
separate USB and LSB signals sharing the same suppressed carrier
frequency, about 2MHz. The two SSB signals on opposite sidebands were
simply combined to create the ISB signal. This ISB signal was then
upconverted to a range of crystal-controlled working frequencies which
could be anywhere up to 30MHz. To keep the two ISB signals consistently
'the same way up' and avoid inadvertently swapping channels between the
two ends of the link, the upconversion oscillator would have to be
consistently on the high side of the IF... but at the upper end of the
HF band, high-side injection would involve difficult and expensive
overtone oscillators (this was half a century ago, remember).
To help get around this problem, the companies involved agreed to use
high-side oscillators for working frequencies up to 10MHz, and low-side
oscillators for higher frequencies. Therefore all international ISB
links inverted (swapped) their sidebands when the working frequency
changed above or below 10MHz, and later this became adopted by CCIR as a
formal world standard. As more ham bands became available, IARU needed
to decide what the formal turnover frequency between USB and LSB should
be... somewhere between 7MHz and 14MHz obviously; but where exactly? We
discovered the existing CCIR standard for ISB, and IARU decided to adopt
the same turnover frequency of 10MHz.
So there we are. The ham standards for USB/LSB seem to originate from a
mixture of technical considerations and what happened way back in the
very first experimental QSOs... unless someone else has even better
information!
Commenting on Ron's posting, with today's DSP-based rigs there is now
*less* reason than at any time in history for hams to change to the
marine standard of using USB on all frequencies. All it needs to swap
sidebands is a few changes from "+" to "-" in the DSP code, and to link
that to the band selector. It's all done for us in the K3... but are you
*sure* you got those signs right, Lyle? :-)
--
73 from Ian GM3SEK
http://www.ifwtech.co.uk/g3sek
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