The 17 meter problem in the Omni C was described here yesterday as being
second harmonic of the IF. That's not quite true. Its actually second
harmonic of the PTO injection.
I a transmitter with 9 MHz IF, the use of a 5 MHz VFO is handy. If the
IF is creating USB, then 9 + 5 makes 14 and USB while 9 - 5 makes 4 MHz
and LSB. And back in the day 2 of SSB, many rigs used the phasing method
for generating that 9 MHz SSB signal. In theory one merely inverts the
audio to one of the two mixers to swap the sidebands. In nearly
universal practice, the audio phase shift networks weren't quite 90
degrees, some were as much as 10 degrees off. Even with that error, one
could adjust the RF phase shift and achieve maybe 40 dB of sideband
suppression, but when you hit the invert switch, the suppression the
other way was barely detectable. Often as poor as 10 dB. So as the
result once the exciter (homebrew or commercial) was balanced for good
suppression on one sideband at the 9MHz IF it wasn't switched. So USB on
20 and LSB on 75 became the standard.
In the Central Electronics rigs, harmonics of the 5 MHz VFO were used
for bands other than 75 and 20.
In the vintage Tentecs the 5 MHz VFO is mixed with crystals for other
bands. And the PTO is mixed to 9 MHz for 18 MHz. Now the PTO signal to
the transmit mixer is probably ten times the signal from the 9 MHz IF
and is there all the time so that is what makes the second harmonic
spur. More recent Tentecs mix the PTO to 27 MHz to take the difference
for 18 MHz which puts the normal sideband on lower which doesn't match
the rest of thee world, but it removes nearly all of the transmitter
spur.
--
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer
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