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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.=20
=20
Roy,
=20
I believe I can answer this for you and you're certainly not the =
first to ask the question. In a nutshell, more or less, Ten-Tec's stock =
2.4 kHz filter in the 9 MHz IF slot will excerbate a slight chirp =
eminating from the 9 MHz BFO circuit board. From my experiments earlier =
this summer, the degree of note degredation is a function of two =
factors: 1) Some Omni Sixes have a BFO which exhibits a slight degree of =
chirp, most don't. This can be confirmed by tuning an external receiver =
to 9 MHz and lightly coupling the BFO to the external receiver. 2) If =
the 2.4K filter has a sharp knee at the Omni Six's CW transmit BFO =
frequency of 9,000.400 kHz, the filter will not only magnify the effct =
of the chirp, but passing this frequency through the filter's sharp =
lower skirt can lead to a distorted CW waveform whose trailing edge =
becomes slightly truncated. I confirmed that the 2.4K filter was the =
sole contributor to this effct by augmenting the CW transmit BFO =
frequency slightly upward by 100 Hz, then downward by 100 Hz. On my =
particular Omni, a BFO frequency adjusted ever-so-slightly out of spec =
(or from drift) will cause noticable visual distortion of the waveform =
and audibly, this is when the note becomes dirty and soft. It bears =
pointing out that chirp is not the ONLY manifestation of poor 2.4K =
filter quality control. =20
=20
In an effort to resolve the problem, I took drastic measures: I =
purchased a used Omni V. I then compared every stage of circuitry and =
after several weeks of non-stop research, I came to the conclusion that =
although the Omni V and Omni VI's CW and critical transmit path =
circuitry is nearly identical, the only difference could be attributed =
to the fact that the Omni V uses a fixed CW transmited BFO offset that =
is designed exactly 200 Hz higher than the Omni VI. At first glance, =
200 hz seems insignificant. However, consider that the 2.4K filter with =
its 9,001.500 kHz center has a designed lower cutoff of 9,000.300 kHz. =
This leaves only 100 Hz of QC variability. This is why some Omni Sixes =
sound good on CW and some sound poor. The worst case, is like that of =
my own: my BFO circuit once chirped until I redesigned it with a =
switching scheme, and my stock 2.4 khz filter was very asymmetrical, =
placing the lower cutoff at 9,000.400 KHz. There's no magic here...no =
rocket science...just a rational reason, justified mathematically and =
confirmed with empirical testing.
=20
Anyone wishing to confirm and duplicate my tests can easily do so by =
adjusting the CW transmit BFO trimmer. By monitoring with a station =
monitor or oscilloscope and frequency counter, adjust the BFO trimmer to =
produce a value exactly at the designed frequency of 9,000.400 kHz. =
Next, slowly decrease the value to 9,000.300 kHz and observe what =
happens on the scope. Now listen to the note on an external receiver. =
Another useful test is to compare the CW note while alternating between =
the CW and FSK modes. In FSK, the CW circuitry is identical, only the =
sidetone is muted. You know you need a better filter if FSK produces a =
more pleasing CW note than in CW. Why? Because in FSK the transmitted =
BFO is shifted slightly UPWARD, well out of harms way of the lower =
filter skirt.
=20
The INRAD 2.8 kHz filter for the 9 MHz IF position was designed to =
provide for the same degree of passband freedom as that designed into =
the Omni V. The choice of 2.8 kHz was not arrived at recklessly. I =
wanted to duplicate the excellent CW characteristics of the Omni V. =
Becuase of the Omni Six's fixed CW transmit BFO of 9,000.400 kHz, =
nothing less than 2.8 kHz will do unless the filter is manufactured with =
exceptional QC standards. A side benefit of the INRAD filter is that =
the USB/LSB passband is much more predictable and symmetrical. There's =
an easy test to confirm filter asymmetry: if USB receive audio sounds =
slightly "thinner" than than that heard in LSB, you've got a defective =
2.4 kHz filter. =20
=20
I hope this answers your question.
=20
73,
=20
-Paul, W9AC
=20
=20
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!
=20
Thanks again & 73 in 99,
=20
Roy K6XK....CHIRP, CHIRP CHIRP! click, click, click!
=20
=20
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<BLOCKQUOTE=20
style=3D"BORDER-LEFT: #000000 solid 2px; MARGIN-LEFT: 5px; PADDING-LEFT: =
5px">
<DIV></DIV>
<DIV><FONT size=3D2>I don't understand the principle involved with =
the 2.8kHz=20
filter. A transmitted carrier passing through a crystal filter skirt =
is=20
strictly a function of <U>amplitude </U>and not of <U>frequency.</U> =
</FONT></DIV>
<DIV><FONT size=3D2></FONT> </DIV>
<DIV><FONT size=3D2>Roy,</FONT></DIV>
<DIV><FONT size=3D2></FONT> </DIV>
<DIV><FONT size=3D2>I believe I can answer this for you and you're =
certainly=20
not the first to ask the question. In a nutshell, more or =
less,=20
Ten-Tec's stock 2.4 kHz filter in the 9 MHz IF slot will excerbate a =
slight=20
chirp eminating from the 9 MHz BFO circuit board. From my =
experiments=20
earlier this summer, the degree of note degredation is a function of =
two=20
factors: 1) Some Omni Sixes have a BFO which exhibits a slight =
degree of=20
chirp, most don't. This can be confirmed by tuning an external =
receiver to 9 MHz and lightly coupling the BFO to the external=20
receiver. 2) If the 2.4K filter has a sharp knee at the Omni =
Six's CW=20
transmit BFO frequency of 9,000.400 kHz, the filter will not only =
magnify=20
the effct of the chirp, but passing this frequency through the =
filter's=20
sharp lower skirt can lead to a distorted CW waveform whose trailing =
edge=20
becomes slightly truncated. I confirmed that the 2.4K filter =
was the=20
sole contributor to this effct by augmenting the CW transmit BFO =
frequency=20
slightly upward by 100 Hz, then downward by 100 Hz. On my =
particular=20
Omni, a BFO frequency adjusted ever-so-slightly out of spec (or from =
drift)=20
will cause noticable visual distortion of the waveform and audibly, =
this is=20
when the note becomes dirty and soft. It bears pointing out =
that chirp=20
is not the ONLY manifestation of poor 2.4K filter quality =
control. =20
</FONT></DIV>
<DIV><FONT size=3D2></FONT> </DIV>
<DIV><FONT size=3D2>In an effort to resolve the problem, I took =
drastic=20
measures: I purchased a used Omni V. I then compared every =
stage of=20
circuitry and after several weeks of non-stop research, I came to =
the=20
conclusion that although the Omni V and Omni VI's CW and critical =
transmit=20
path circuitry is nearly identical, the only difference could be =
attributed=20
to the fact that the Omni V uses a fixed CW transmited BFO offset =
that is=20
designed exactly 200 Hz higher than the Omni VI. At first =
glance, 200=20
hz seems insignificant. However, consider that the 2.4K filter =
with=20
its 9,001.500 kHz center has a designed lower cutoff of 9,000.300 =
kHz. =20
This leaves only 100 Hz of QC variability. This is why some =
Omni Sixes=20
sound good on CW and some sound poor. The worst case, is like =
that of=20
my own: my BFO circuit once chirped until I redesigned it with a =
switching=20
scheme, and my stock 2.4 khz filter was very asymmetrical, placing =
the lower=20
cutoff at 9,000.400 KHz. There's no magic here...no rocket=20
science...just a rational reason, justified mathematically and =
confirmed=20
with empirical testing.</FONT></DIV>
<DIV><FONT size=3D2></FONT> </DIV>
<DIV><FONT size=3D2>Anyone wishing to confirm and duplicate my tests =
can=20
easily do so by adjusting the CW transmit BFO trimmer. By =
monitoring=20
with a station monitor or oscilloscope and frequency counter, adjust =
the BFO=20
trimmer to produce a value exactly at the designed frequency of =
9,000.400=20
kHz. Next, slowly decrease the value to 9,000.300 kHz and =
observe what=20
happens on the scope. Now listen to the note on an external=20
receiver. Another useful test is to compare the CW note while=20
alternating between the CW and FSK modes. In FSK, the CW =
circuitry is=20
identical, only the sidetone is muted. You know you need a =
better=20
filter if FSK produces a more pleasing CW note than in CW. =
Why? =20
Because in FSK the transmitted BFO is shifted slightly UPWARD, well =
out of=20
harms way of the lower filter skirt.</FONT></DIV>
<DIV><FONT size=3D2></FONT> </DIV>
<DIV><FONT size=3D2>The INRAD 2.8 kHz filter for the 9 MHz IF =
position was=20
designed to provide for the same degree of passband freedom as that =
designed=20
into the Omni V. The choice of 2.8 kHz was not arrived at=20
recklessly. I wanted to duplicate the excellent CW =
characteristics of=20
the Omni V. Becuase of the Omni Six's fixed CW transmit BFO of =
9,000.400 kHz, nothing less than 2.8 kHz will do unless the filter =
is=20
manufactured with exceptional QC standards. A side benefit of =
the=20
INRAD filter is that the USB/LSB passband is much more predictable =
and=20
symmetrical. There's an easy test to confirm filter asymmetry: =
if USB=20
receive audio sounds slightly "thinner" than than that =
heard in=20
LSB, you've got a defective 2.4 kHz filter. =
</FONT></DIV>
<DIV><FONT size=3D2></FONT> </DIV>
<DIV><FONT color=3D#000000 size=3D2>I hope this answers your=20
question.</FONT></DIV>
<DIV><FONT color=3D#000000 size=3D2></FONT> </DIV>
<DIV><FONT size=3D2>73,</FONT></DIV>
<DIV><FONT size=3D2></FONT> </DIV>
<DIV><FONT size=3D2>-Paul, W9AC</FONT></DIV>
<DIV> </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=20
about 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> =20
click, 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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