[TenTec] Re: N4LQ and the 2.8kHz Pill

Paul Christensen paulc@mediaone.net
Sun, 3 Jan 1999 18:47:47 -0500


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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>&nbsp;</DIV>
    <DIV><FONT size=3D2>Roy,</FONT></DIV>
    <DIV><FONT size=3D2></FONT>&nbsp;</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.&nbsp; 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.&nbsp; 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.&nbsp; This can be confirmed by tuning an external =

    receiver to 9 MHz and lightly coupling the BFO to the external=20
    receiver.&nbsp; 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.&nbsp; 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.&nbsp; 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.&nbsp; It bears pointing out =
that chirp=20
    is not the ONLY manifestation of poor 2.4K filter quality =
control.&nbsp;=20
    </FONT></DIV>
    <DIV><FONT size=3D2></FONT>&nbsp;</DIV>
    <DIV><FONT size=3D2>In an effort to resolve the problem, I took =
drastic=20
    measures: I purchased a used Omni V.&nbsp; 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.&nbsp; At first =
glance, 200=20
    hz seems insignificant.&nbsp; 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.&nbsp;=20
    This leaves only 100 Hz of QC variability.&nbsp; This is why some =
Omni Sixes=20
    sound good on CW and some sound poor.&nbsp; 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.&nbsp; 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>&nbsp;</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.&nbsp; 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.&nbsp; Next, slowly decrease the value to 9,000.300 kHz and =
observe what=20
    happens on the scope.&nbsp; Now listen to the note on an external=20
    receiver.&nbsp; Another useful test is to compare the CW note while=20
    alternating between the CW and FSK modes.&nbsp; In FSK, the CW =
circuitry is=20
    identical, only the sidetone is muted.&nbsp; You know you need a =
better=20
    filter if FSK produces a more pleasing CW note than in CW.&nbsp; =
Why?&nbsp;=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>&nbsp;</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.&nbsp; The choice of 2.8 kHz was not arrived at=20
    recklessly.&nbsp; I wanted to duplicate the excellent CW =
characteristics of=20
    the Omni V.&nbsp; 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.&nbsp; A side benefit of =
the=20
    INRAD filter is that the USB/LSB passband is much more predictable =
and=20
    symmetrical.&nbsp; There's an easy test to confirm filter asymmetry: =
if USB=20
    receive audio sounds slightly &quot;thinner&quot; than than that =
heard in=20
    LSB, you've got a defective 2.4 kHz filter.&nbsp;&nbsp; =
</FONT></DIV>
    <DIV><FONT size=3D2></FONT>&nbsp;</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>&nbsp;</DIV>
    <DIV><FONT size=3D2>73,</FONT></DIV>
    <DIV><FONT size=3D2></FONT>&nbsp;</DIV>
    <DIV><FONT size=3D2>-Paul, W9AC</FONT></DIV>
    <DIV>&nbsp;</DIV>
    <DIV><FONT size=3D2></FONT>&nbsp;</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 =
&quot;halucinate&quot; me=20
    about this please feel free to insult me here!</FONT></DIV>
    <DIV><FONT size=3D2></FONT>&nbsp;</DIV>
    <DIV><FONT size=3D2>Thanks again &amp; 73 in 99,</FONT></DIV>
    <DIV><FONT size=3D2></FONT>&nbsp;</DIV>
    <DIV><FONT size=3D2>Roy&nbsp;&nbsp;&nbsp;&nbsp; K6XK....<EM><FONT=20
    face=3D"Comic Sans MS" size=3D1>CHIRP, CHIRP =
CHIRP!</EM>&nbsp;&nbsp;&nbsp;=20
    click, click, click!</FONT></FONT></DIV>
    <DIV><FONT color=3D#000000 size=3D2></FONT>&nbsp;</DIV>
    <DIV><FONT color=3D#000000 =
size=3D2></FONT>&nbsp;</DIV></BLOCKQUOTE></BODY></HTML>

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