[TenTec] Re: N4LQ and the 2.8kHz Pill
David E. Shelton
w4des@iolky.com
Sun, 3 Jan 1999 18:58:47 -0500
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Roy,
This is what Paul wrote concerning the Omni VI/VI+ problems read this and it
should explain rather well. David, W4DES
Paul, W9AC wrote previously:
Ten-Tec has two designs for the 9 MHz BFO Circuit Board: the older
generation uses the USB crystal for CW transmit, while the LSB crystal is
used for CW receive. The newer-generation board uses the LSB crystal for
both functions. Here's my theory for what it's worth:
I suspect the circuit change was made as a quiet attempt to improve the Omni
Six's CW note. Rather than switching between two crystals during the T/R
function, one crystal could be used, provided that logic changes were made
to components on the OMNI's Control Board. In Ten-Tec's attempt, the total
parts count, in fact, actually decreased. The new control board only works
with the new 9 MHz BFO board. Similarly, the old circuit boards must also
match together. Independently, I came to the exact same conclusion as
Ten-Tec and documented the changes. On paper, the new circuit appeared to
be a vast improvement. In reality, the new design is more prone to slight
chirp than the older design. Here's why: Ten-Tec did not change the value
of the LSB crystal. It remains 9,000.000 kHz. The Omni Six uses a fixed CW
transmit offset of 9,000.400 kHz. In CW transmit, the BFO crystal is
shifted *positively* to 9,000.400 kHz. These VXO circuits tend to produce
more audible chirp effects, albeit slight, when suddenly shifted positively
rather than negatively. When transmitting in CW, the poor LSB BFO crystal
is being rapidly frequency-shift-keyed and may or may not settle in time for
the QSK & T/R circuitry to quickly shift from receive to transmit.
To create a slight negative shift, I custom-ordered a 9,000.500 kHz crystal
and further changed the logic such that the crystal frequency is shifted *on
receive* rather than transmit. The initial "FSK" shock to the crystal is
only evident when switching from *transmit to receive.*
The older circuit board even with its inherent switching problems is still
superior in my opinion because although the USB crystal is shifted 2.7 kHz
on CW transmit, it represents a *decrease* in frequency as far as the VXO is
concerned.
That's the first problem. The second problem exacerbates the first. Here's
why:
The second problem is associated with the OMNI Six's 2.4 kHz @ 9 MHz filter.
As I've stated before and at the risk of sounding boring, the lower knee on
*some* Omni Six filters meets square-on with the Omni's CW fixed transmit
offset frequency of 9,000.400 kHz. A CW note that exhibits slight chirp
from the root-cause of problem #1 as stated above, becomes exacerbated from
the strangle-hold that the 2.4 kHz filter places on the BFO at 9,000.400
kHz. In theory, the 2.4 kHz filter is centered at 9,001.500 kHz. The
lower filter skirt, by design, cuts off at approximately 9,000.300 kHz. In
reality, the true lower filter skirt cutoff is not precise and its
variability can be attributed to imprecision in the filter's QC production
accuracy. The margin of filter error, even by design is only 100 Hz.
Imprecision during filter production can, and does, place the filter's knee
dangerously close to the CW transmit BFO frequency of 9,000.400 kHz.
THIS IS WHY SOME OMNI SIXES SOUND FINE ON CW WHILE OTHERS HAVE A SLIGHT
CHIRP AND DIRTY NOTE.
The Omni V does not manifest these problems even though the CW circuitry is
nearly 100% identical to that of the older-generation Omni Six. Here's why:
Although the Omni V and Six share the exact same 2.4kHz filter, the Omni V
uses a fixed CW BFO frequency of 9,000.600 Hz. This is 200 Hz higher than
the Omni Six's BFO frequency. When you're ever-so-close to that lower
filter cutoff frequency, 200 Hz makes all the difference in the world. I
confirmed this by slowly adjusting the BFO frequency while monitoring the CW
waveform on an Oscilloscope. On my Omni Six, a BFO frequency of less than
9,000.500 kHz resulted in a dirty CW note where the waveform begins to show
signs of visible distortion and a slight truncation of the waveform's
trailing edge. It's almost all in the filter accuracy, the remaining
difference rests with the BFO shift scheme. A BFO that chirps slightly,
will become much more intensified when it tries to pass through the
bottleneck of a sloppy filter skirt.
The absolute worst configuration is a newer Omni Six Plus, with an
inaccurate 2.4 kHz @ 9 MHz I.F. filter. (emphasis added to "inaccurate").
Listen to W1AW's 20-meter signal and you'll see and hear why. Perhaps we
can all take up a collection and present INRAD's 2.8 kHz filter to the W1AW
operating staff as a gift. I'll let Clark take up the cause from here!
-Paul, W9AC
-----Original Message-----
From: owner-tentec@contesting.com [mailto:owner-tentec@contesting.com]On
Behalf Of Roy Koeppe
Sent: Sunday, January 03, 1999 5:41 PM
To: tentec@contesting.com
Subject: [TenTec] Re: N4LQ and the 2.8kHz Pill
Tnx Steve, you wrote:
One who pays $3000 for a rig should be VERY PICKY. I would expect the BEST
sounding transmitter on the air, not the WORST. Even the 20 year old Triton
4 sounds better and for that matter, $99 TKIT sounds better than a $3000
Omni VI.
Interestingly, I noticed your keying had clicks on the lower frequency of
your carrier about 2khz away from center and was clean on the upper side.
This goes to further verify what Paul W9AC has been saying. He reports good
keying on his Omni VI after getting that new 2.8khz fitler from INRAD. You
may want to look in to that.
Anyway, it's too cold to be concerned about microchirps. I suppose even
colder there. Sould hit 9 degrees tonight and the snow is falling. 73
Yes,
I'd like an easy cure, and hope this is it. I have not heard
solid-sounding reports on here confirming it though?? Please advise.
But in my dim thinking, I don't understand the principle involved with the
2.8kHz filter. A transmitted carrier passing through a crystal filter skirt
is strictly a function of amplitude and not of frequency. So how can it
possibly be that it affects the chirp at all, since the very definition of
chirp is a frequency change?
I'm sure that my thinking is just too simplistic here, but it does trouble
me some. If any of you care to "halucinate" me about this please feel free
to insult me here!
Thanks again & 73 in 99,
Roy K6XK....CHIRP, CHIRP CHIRP! click, click, click!
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<DIV><SPAN class=3D670505523-03011999><FONT color=3D#0000ff face=3DArial =
size=3D2>Roy,=20
</FONT></SPAN></DIV>
<DIV><SPAN class=3D670505523-03011999><FONT color=3D#0000ff face=3DArial =
size=3D2>This=20
is what Paul wrote concerning the Omni VI/VI+ problems read this and it =
should=20
explain rather well. David, W4DES</FONT></SPAN></DIV>
<DIV> </DIV>
<DIV><SPAN class=3D670505523-03011999><FONT color=3D#0000ff face=3DArial =
size=3D2>Paul,=20
W9AC wrote previously:</FONT></SPAN></DIV>
<DIV><FONT size=3D2>
<P>Ten-Tec has two designs for the 9 MHz BFO Circuit Board: the =
older</P>
<P>generation uses the USB crystal for CW transmit, while the LSB =
crystal is</P>
<P>used for CW receive. The newer-generation board uses the LSB crystal =
for</P>
<P>both functions. Here's my theory for what it's worth:</P>
<P>I suspect the circuit change was made as a quiet attempt to improve =
the=20
Omni</P>
<P>Six's CW note. Rather than switching between two crystals during the =
T/R</P>
<P>function, one crystal could be used, provided that logic changes were =
made</P>
<P>to components on the OMNI's Control Board. In Ten-Tec's attempt, the=20
total</P>
<P>parts count, in fact, actually decreased. The new control board only=20
works</P>
<P>with the new 9 MHz BFO board. Similarly, the old circuit boards must =
also</P>
<P>match together. Independently, I came to the exact same conclusion =
as</P>
<P>Ten-Tec and documented the changes. On paper, the new circuit =
appeared to</P>
<P>be a vast improvement. In reality, the new design is more prone to =
slight</P>
<P>chirp than the older design. Here's why: Ten-Tec did not change the =
value</P>
<P>of the LSB crystal. It remains 9,000.000 kHz. The Omni Six uses a =
fixed=20
CW</P>
<P>transmit offset of 9,000.400 kHz. In CW transmit, the BFO crystal =
is</P>
<P>shifted *positively* to 9,000.400 kHz. These VXO circuits tend to =
produce</P>
<P>more audible chirp effects, albeit slight, when suddenly shifted=20
positively</P>
<P>rather than negatively. When transmitting in CW, the poor LSB BFO =
crystal</P>
<P>is being rapidly frequency-shift-keyed and may or may not settle in =
time=20
for</P>
<P>the QSK & T/R circuitry to quickly shift from receive to =
transmit.</P>
<P>To create a slight negative shift, I custom-ordered a 9,000.500 kHz=20
crystal</P>
<P>and further changed the logic such that the crystal frequency is =
shifted=20
*on</P>
<P>receive* rather than transmit. The initial "FSK" shock to =
the=20
crystal is</P>
<P>only evident when switching from *transmit to receive.*</P>
<P>The older circuit board even with its inherent switching problems is=20
still</P>
<P>superior in my opinion because although the USB crystal is shifted =
2.7=20
kHz</P>
<P>on CW transmit, it represents a *decrease* in frequency as far as the =
VXO=20
is</P>
<P>concerned.</P>
<P>That's the first problem. The second problem exacerbates the first.=20
Here's</P>
<P>why:</P>
<P>The second problem is associated with the OMNI Six's 2.4 kHz @ 9 MHz=20
filter.</P>
<P>As I've stated before and at the risk of sounding boring, the lower =
knee=20
on</P>
<P>*some* Omni Six filters meets square-on with the Omni's CW fixed =
transmit</P>
<P>offset frequency of 9,000.400 kHz. A CW note that exhibits slight =
chirp</P>
<P>from the root-cause of problem #1 as stated above, becomes =
exacerbated=20
from</P>
<P>the strangle-hold that the 2.4 kHz filter places on the BFO at =
9,000.400</P>
<P>kHz. In theory, the 2.4 kHz filter is centered at 9,001.500 kHz. =
The</P>
<P>lower filter skirt, by design, cuts off at approximately 9,000.300 =
kHz.=20
In</P>
<P>reality, the true lower filter skirt cutoff is not precise and =
its</P>
<P>variability can be attributed to imprecision in the filter's QC=20
production</P>
<P>accuracy. The margin of filter error, even by design is only 100 =
Hz.</P>
<P>Imprecision during filter production can, and does, place the =
filter's=20
knee</P>
<P>dangerously close to the CW transmit BFO frequency of 9,000.400 =
kHz.</P>
<P>THIS IS WHY SOME OMNI SIXES SOUND FINE ON CW WHILE OTHERS HAVE A =
SLIGHT</P>
<P>CHIRP AND DIRTY NOTE.</P>
<P>The Omni V does not manifest these problems even though the CW =
circuitry=20
is</P>
<P>nearly 100% identical to that of the older-generation Omni Six. =
Here's=20
why:</P>
<P>Although the Omni V and Six share the exact same 2.4kHz filter, the =
Omni=20
V</P>
<P>uses a fixed CW BFO frequency of 9,000.600 Hz. This is 200 Hz higher =
than</P>
<P>the Omni Six's BFO frequency. When you're ever-so-close to that =
lower</P>
<P>filter cutoff frequency, 200 Hz makes all the difference in the =
world. I</P>
<P>confirmed this by slowly adjusting the BFO frequency while monitoring =
the=20
CW</P>
<P>waveform on an Oscilloscope. On my Omni Six, a BFO frequency of less =
than</P>
<P>9,000.500 kHz resulted in a dirty CW note where the waveform begins =
to=20
show</P>
<P>signs of visible distortion and a slight truncation of the =
waveform's</P>
<P>trailing edge. It's almost all in the filter accuracy, the =
remaining</P>
<P>difference rests with the BFO shift scheme. A BFO that chirps =
slightly,</P>
<P>will become much more intensified when it tries to pass through =
the</P>
<P>bottleneck of a sloppy filter skirt.</P>
<P>The absolute worst configuration is a newer Omni Six Plus, with =
an</P>
<P>inaccurate 2.4 kHz @ 9 MHz I.F. filter. (emphasis added to=20
"inaccurate").</P>
<P>Listen to W1AW's 20-meter signal and you'll see and hear why. Perhaps =
we</P>
<P>can all take up a collection and present INRAD's 2.8 kHz filter to =
the=20
W1AW</P>
<P>operating staff as a gift. I'll let Clark take up the cause from =
here!</P>
<P>-Paul, W9AC</P></FONT></DIV>
<BLOCKQUOTE dir=3Dltr style=3D"MARGIN-RIGHT: 0px">
<DIV align=3Dleft class=3DOutlookMessageHeader dir=3Dltr><FONT =
face=3DTahoma=20
size=3D2>-----Original Message-----<BR><B>From:</B> <A=20
=
href=3D"mailto:owner-tentec@contesting.com">owner-tentec@contesting.com</=
A> [<A=20
=
href=3D"mailto:owner-tentec@contesting.com">mailto:owner-tentec@contestin=
g.com</A>]<B>On=20
Behalf Of </B>Roy Koeppe<BR><B>Sent:</B> Sunday, January 03, 1999 5:41 =
PM<BR><B>To:</B> <A=20
=
href=3D"mailto:tentec@contesting.com">tentec@contesting.com</A><BR><B>Sub=
ject:</B>=20
[TenTec] Re: N4LQ and the 2.8kHz Pill<BR><BR></FONT></DIV>
<DIV><FONT color=3D#000000 size=3D2>Tnx Steve, you wrote:=20
<DIV><FONT size=3D2>One who pays $3000 for a rig should be VERY PICKY. =
I would=20
expect the BEST sounding transmitter on the air, not the WORST. Even =
the 20=20
year old Triton 4 sounds better and for that matter, $99 TKIT sounds =
better=20
than a $3000 Omni VI. </FONT></DIV>
<DIV><FONT size=3D2></FONT> </DIV>
<DIV><FONT size=3D2>Interestingly, I noticed your keying had clicks on =
the lower=20
frequency of your carrier about 2khz away from center and was clean on =
the=20
upper side. This goes to further verify what Paul W9AC has been =
saying. He=20
reports good keying on his Omni VI after getting that new 2.8khz =
fitler from=20
INRAD. You may want to look in to that.</FONT></DIV>
<DIV><FONT size=3D2></FONT> </DIV>
<DIV><FONT size=3D2>Anyway, it's too cold to be concerned about =
microchirps. I=20
suppose even colder there. Sould hit 9 degrees tonight and the snow is =
falling. 73</FONT></DIV></FONT></DIV>
<DIV><FONT color=3D#000000 size=3D2></FONT> </DIV>
<DIV><FONT color=3D#000000 size=3D2>Yes,</FONT></DIV>
<DIV><FONT color=3D#000000 size=3D2></FONT> </DIV>
<DIV><FONT color=3D#000000 size=3D2>I'd like an easy cure, and hope =
this is it. I=20
have not heard solid-sounding reports on here confirming it though?? =
Please=20
advise.</FONT></DIV>
<DIV><FONT color=3D#000000 size=3D2></FONT> </DIV>
<DIV><FONT size=3D2>But in my dim thinking, I don't understand the =
principle=20
involved with the 2.8kHz filter. A transmitted carrier passing through =
a=20
crystal filter skirt is strictly a function of <U>amplitude </U>and =
not of=20
<U>frequency.</U> So how can it possibly be that it affects the chirp =
at all,=20
since the very definition of chirp is a frequency change?</FONT></DIV>
<DIV><FONT size=3D2></FONT> </DIV>
<DIV><FONT size=3D2>I'm sure that my thinking is just too simplistic =
here, but=20
it does trouble me some. If any of you care to "halucinate" =
me about=20
this please feel free to insult me here!</FONT></DIV>
<DIV><FONT size=3D2></FONT> </DIV>
<DIV><FONT size=3D2>Thanks again & 73 in 99,</FONT></DIV>
<DIV><FONT size=3D2></FONT> </DIV>
<DIV><FONT size=3D2>Roy K6XK....<EM><FONT=20
face=3D"Comic Sans MS" size=3D1>CHIRP, CHIRP =
CHIRP!</EM> click,=20
click, click!</FONT></FONT></DIV>
<DIV><FONT color=3D#000000 size=3D2></FONT> </DIV>
<DIV><FONT color=3D#000000 =
size=3D2></FONT> </DIV></BLOCKQUOTE></BODY></HTML>
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