I think I can answer this quite simply Steve. As much as the 9 MHz BFO
circuit should be optimized for use with the LSB crystal and a smaller shift
(The USB crystal as used on older Omni Sixes requires 2.4 kHz of shift
between RX and TX), the primary difference among all the Omni Sixes
is......with the passband tolerance of the 9 MHz @ 2.4 kHz filter. I have
no doubt that if we were to sample 50-100 of these filters, we would find
those that exacerbate the chirp and some that sound clean.
This point is crucial:
Since the Omni Six's BFO is fixed at 9,000.400 kHz, the CW TX BFO is placed
right on the ragged edge of a VERY steep filter skirt. Some filters through
sloppy QC, probably have a slightly better response that allows JUST enough
of the CW TX BFO to fall within a linear portion of the filter. The Omni V,
using the same 2.4 kHz filter has its BFO placed in area out of harms
way....exactly 200 Hz above the knee of the crystal filter. Remember, the 9
Mhz @ 2.4 kHz filter is the ONLY filter used on SSB, FSK or CW transmit. it
shares double-duty on receive.
Here's additional proof in what I am asserting: On those Omni Six's with
perceived chirp, switch to the FSK mode, where the BFO is placed within the
flat, linear range of the 2.4 kHz filter. When in the FSK mode, CW still
functions exactly the way it does in its normal mode, including the
integrator for waveshaping. The CW note will sound better on another
receiver when keying in FSK. The ONLY difference is that the sidetone is
lost, and the TX BFO frequency is different (higher). This is exactly what
led me to the path of discovery on the 9 MHz Filter Board. Further
refinement to the 9 MHz BFO circuit (i.e, using the LSB crystal and shifting
the BFO down, rather than up) cleans up and residual manifestations of
chirp.
Here's another test for the adventurous: Remove the 9 MHz @ 2.4 kHz crystal
and jumper it with a small piece of wire. Now, critically listen on another
receiver while in the CW mode. This is for CW testing only. Do not attempt
to transmit on SSB without this filter in place because of spectral
occupancy considerations.
-Paul, W9AC
-----Original Message-----
From: Steve Ellington <n4lq@iglou.com>
To: Paul Christensen <paulc@mediaone.net>
Cc: Michael Little <kb4kkl@worldnet.att.net>; Ten Tec
<tentec@contesting.com>
To: <tentec@contesting.com>
Date: Monday, September 21, 1998 4:04 PM
Subject: Re: [TenTec] Re: fix for Omni
>
>
>This is all good stuff but I have one burning question. How come all Omni
>6's dont sound bad? Or what little quirk makes such a big difference?
>Furthermore, why does it take such a drastic mod on some and not all Omni
>6's?
>
>If and it's a big IF....some OMni 6's sound so good....then HOW COME???
>
>Here's something I've observed which are hard to explain. The sound of an
>Omni 6's keying on the receiving end is affected by:
>
>1. Band conditions
>2. type of receiver
>3. cw speed
>
>When band conditons are poor the chirp is not noticible.
>A Ja receiver with ringy xtal filters tends to smooth out the bad keying.
>Listening to a 6 on my 5 almost always sounds bad.
>QRQ speeds of 40+ wpm sound better.
>
>Pauld , have you any ideas? Could this be something to do with the tx
>filter bandwidth you mentioned?
>
>Steve N4LQ
>
>
>
>
>> FAQ on WWW: http://www.contesting.com/tentecfaq.htm
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>>
>>
>
>Steve Ellington N4LQ@IGLOU.COM Louisville, Ky
>
>
>--
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>
>
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