Hi Steve,
Let's address this one issue at a time and I believe it will make sense. As
I've been advocating, the 2.8 kHz filter may only be a partial solution,
depending on just how badly the Omni chirps. Newer Omni Six's have a
greater propensity to chirp for reasons I've explained before and will
repeat again below. The 2.4 kHz bandbass is the root-cause of the dirty
sounding portion of the CW note, notwithstanding any chirp. The CW BFO
frequency of 9,000.400 kHz passed through he filter's edge only exacerbates
what you and I have been saying all along: that the primary cause of chirp
is rooted in the 9 MHz BFO Circuit Board. On to the Q&A...
>The chirp seems to originate in the BFO of the tx-audio board. I spent
>nearly 3 months verifying this both in the shack and on the air. You can
>verify this easily by simply listening to your 9mhz signal on another
>receiver as you key the Omni VI. Use a hank of wire for the receiver's
>antenna and just jam it inside the Omni's cabinet through the vent holes
>on the left side of the rig. You will hear the actual signal from the BFO
>before filtering. If you doubt this, simply remove the filter then listen
>to it.
Absolutely. This is the cause of the majority of the chirp. There is yet a
second source of chirp that occurs because of inadaquate PLL synthesizer
settling time WHEN LARGE OFFSETS ARE USED (greater than 10 kHz). The Omni V
is immune from this because the synthesizer is allowed a longer settling
time in the T/R transision. On large CW offsets, this becomes noticable on
the Omni V. It throws my timing off, but even with large offsets, the Omni
V stays chirp-free. The Omni Six is faster in this regard, only the
tradeoff is that its synthesizer is not given adaqaute settling time,
particularly when the synthesizer must shift between large offsets.
>I've done this test with 2 Omni VI's here in the shack. Now switch to the
>FSK mode, retune the reciever and listen to the difference. This is how it
>should sound.
This is what made me a believer and became the impetus for finding the root
cause of the dirty CW note and the chirp. The CW note in FSK is shifted
just as it is in CW, only the offset frequency in the FSK mode is well above
that of CW. I found I could make CW in the CW mode sound every bit as good
as CW in the FSK mode, simply by adjusting the CW BFO trimmer to match the
FSK offset.
>Correct me if I"m wrong here but I beleive in the Omni VI, when you key
>the rig, you are not only starting the LSB oscillator from dead zero but
>also switching a capacitor across it's crystal to shift it's frequency.
>This is really asking a lot of an oscillator! I believe the old Corsairs,
>Tritons etc just switched in the capacitor across the already running
>oscillator resulting in a fairly clean signal. In the Omni VI, I believe
>this is what happens in FSK mode, just a capacitor switched across an
>xtal.
Right again Steve, only with older Omni Six's the USB crystal is shifted and
used on CW transmit. CW receive uses the LSB crystal. This becomes part of
the sequencing dilemma that can cause chirp. Now look at the Omni V
schematic. The two 9 MHz BFO schematics are nearly 100% identical. You
will see some slight circuit differences where the Omni Six uses a 4.7K
resistor and the Omni V uses a small molded inductor for switching
isolation. These components are part of the CW shift switch. At first, I
made the two circuit boards 100% identical, thinking that this was the cause
of the chirp and that it was somehow related to crystal loading of the
resistor versus the inductor. No true. Even when I purchased a used Omni V
and removed its entire 9 MHz BFO Circuit Board and placed it into the Omni
Six, the chirp still followed. I firmly believe that an ever-so-slight
amount of chirp exists on many Omni V rigs, only we don't hear it becuase of
the Omni V's use of the 600 Hz offset, thereby placing the CW transmit BFO
well out of harms way of the 2.4 kHz filter edge. Again, the filter
magnifies the effect of chirp.
When the older version of the 9 MHz BFO Circuit Board is modified or if you
purchase a new one which uses the LSB crystal for both transmit and receive
functions, it's still necessary to change the LSB crystal to allow for a
negative BFO shift. I had several 9,000.500 kHz crystals made by JAN
Crystals in several different loading capacities, and picked the one with
the best loading characteristics. Now, my BFO shifts on RECEIVE, not
transmit, which minimizes the crystal's "shock" effect as the crystal
rapidly frequency shifts to the rythym of your fist. Further, I now only
have a 100 Hz shift from transmit to receive. My 9 MHz BFO board is very
happy and is chirpless.
Is the above mod all that necessary? I firmly believe that on older BFO
boards, the answer is no. The Omni V functions just fine with the USB
crystal shifting negatively more than 2 kHz. This point bears repeating:
The Omni V's 600 kHz fixed offset saves this transceiver from audible chirp.
>For you who want to hear what you sound like without that 2.4khz filter,
>just yank it out and stick a 100pf capacitor between the input and output
>holes. If it sounds the same then don't expect the 2.8khz xtal to help.
And when you and I had a QSO after all my mods, this is exactly the
configuration you heard and remained that way until a few days ago when I
added the INRAD filter. Actually, I didn't even bother with a coupling cap.
A two-inch buss wire is all I used in the 2.4 kHz filter's place.
>Next question.
>
>If the 2.4khz xtal is really the cause of all this, why can't the cw ops
>just yank it out, bypass it and leave it that way? SSB ops who do
>occasional cw would have more to contend with of course. Perhaps a diode
>switching system could bypass the xtal during ssb operation.
Right again Steve. The ONLY reason I asked George to customize the new
filter was for SSB transmit, where the filter is needed to limit occupied
spectrum. I would have lived with the jumper in the filter position if I
didn't occasionally use SSB. In fact, I then placed my 2.4 kHz filter in
the N-1 position and my 500 Hz filter in N-2. It worked great that way on
receive!
>My guess is: It will still chirp.
Probably, but again, it's much more imperceptable, just like the Omni V.
Remember that the two BFO designs are virtually identical. Even so, I ruled
this out as a possibility by inserting an Omni V BFO board into the Omni
Six. Nothing changed until I bypassed the 2.4 kHz filter. Here's another
important point that had to be ruled out: chirp caused by the synthesizer.
All my testing was conducted by keying on the TX IN RCA connector. Recall
that Ten-Tec uses this jumper on the back of many of their rigs to
facilitate QSK timing with their own amplifiers. Chirp caused by any
synthesizer offsets occurs PRIOR to this jumper. This way, even if I
neglected to turn off a large frequency split or offset, I knew it was not a
variable in the testing.
Let me know if I need to clarify anything here Steve. Thanks for all your
input and suggestions!
-Paul, W9AC
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