The swapped side bands on 17m is a product of the 9 MHz IF and 5 MHz VFO
scheme used in Tentecs for nearly all recorded history. That comes from
some of the earliest home brew rigs for SSB. And is why we use mostly
LSB on 80 and 40 and USB on higher bands.
In the days of the phasing rigs, changing sidebands was theoretically
simple, just invert one audio line, but in practice that never worked
because the 90 degree phase shift networks weren't exact and when tuned
for maximum suppression one sideband it wasn't adequate on the other
sideband. It was closer to not detectable unwanted sideband suppression,
so with the 9MHz IF and 5 MHz VFO it was tuned for USB on 20m using the
sum of the two signals and got LSB on 75 using the difference. This was
expanded to other bands by mixing the VFO with suitable crystals. Low
side injection to the signal mixer gets USB, high side gets LSB.
Works fine for 160 through 10 except for 17m. There with the 9 MHz IF
the mixed VFO is also 9 MHz for low side injection. Unfortunately the
mixer is imperfect and the strongest spur is the second harmonic of the
local oscillator coming out at 18 to 19 MHz while the dial tunes from 5
to 5.5 MHz. Likely only 10 or 15 dB down from the desired SSB signal. No
amount of filtering can keep that from transmitting a carrier, in or out
of the band. That's solved by using high side injection at 27 MHz
instead of 9 MHz at the cost of inverted side bands.
Some earlier Tentec built when 17 was a few years in coming did plan on
using the 9 MHz mixer injection and those rigs don't meet spurious
requirements on 17 meters and so should not be used to transmit on 17.
The Corsair takes care of that spur.
73, Jerry, K0CQ
On 4/16/2011 8:04 AM, tubsc66@aol.com wrote:
Hola everyone, Ive had my Corsair a few weeks now and have finally had some
time to get it completely
checked out. All is good for the most part, but I do have an issue on 17 meter
operation. Th sidebands
are reversed, in other words, I have to have the mode switch in "SB-R" to copy
signals. Is this a common
issue? Hopefully its an easy fix, I just received a 500hz filter for it (thanks
Al!) so I'll have the cover off soon
to install that. Any advice would be appreciated, thanks! 73's
Chris KC5IIE
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