I dunno about the VI+, but the TX Audio board in the VI can be improved by
replacing the factory crystal with more stable crystals. After I modified
my VI/Opt1 with new BFO xtals and a TXCO upgrade I am able to stay within 10
Hz during long transmits in full duty modes like PSK31 and my rig stays
within 10 Hz of WWV once warmed up.
See my earlier posts in the archives about the changes I made. I would be
willing to bet that newer versions of these boards would benefit from better
crystals, and don't understand why Ten-Tec didn't spec better crystals
(actually I do... the price difference would add about $50 to the cost of
the rig, which means at least $100 to the retail price).
Ten-Tec will tell you that this rig was meant to provide the lowest noise
floor, not have the best frequency stability, because they considered
staying within 50 Hz to be acceptable. The factory-supplied crystals will
let the rig stay well within this range while using CW or SSB, but not when
using full duty cycle modes like RTTY or PSK31. Note that RTTY via FSK
using the Mark/Space inputs may be (should be) more frequency-stable since
they each have their own BFO xtal that isn't pulled very much, unlike the
USB xtal. So, the real 'problem' here is that this rig's design pre-dates
the rise of soundcard software and AFSK popularity, so a very prominent mode
of operation today was very rarely used in the early '90s. One can't fault
Ten-Tec for ignoring this aspect of design since it certainly wasn't
applicable at the time.
Of course, to me the idea solution would have been using the 10 Mhz
frequency standard as a reference thru a feedback loop as was done on the
PLL board. That way, the entire rig would be as stable as that 10 Mhz
oscillator... and as some hams have modified their VIs to use a GPS-sync'ed
clock-referenced signal, then you could get the VI to be stable to under 1
Hz.
I don't have the engineering skills... but the BFO board is fairly simple in
design and it shouldn't be hard for someone to come up with a replacement
that WOULD use the 10 Mhz signal as a reference. I know I'd spend $100 for
a kit (PC board, components, I build it)... how many others out there would
do this for their VI?
- jgc
John Clifford KD7KGX
Heathkit HW-9 WARC/HFT-9/HM-9
Elecraft K2 #1678 /KSB2/KIO2/KBT2/KAT2/KNB2/KAF2/KPA100
Ten-Tec Omni VI/Opt1, Centaur, 238B
Alinco DR-605TQ, DJ-V5
Icom T90A
IRLP #3978
email: kd7kgx@arrl.net
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