The most common filter that most used was the 217 500 hertz filter.
It's 500 hertz wide at 6 db points and 1000 hertz wide at 60 db points.
Just put that one in and don't worry about it.
Carl Moreschi N4PY
127 River Moss Way
Hertford, NC 27944
www.n4py.com
On 6/24/2020 9:29 AM, Petr Ourednik wrote:
Hi folks,
related to my preview post regarding the 1st IF 9MHz filters choosing (NAR
position) for Omni V (w/o reply) I tried to read as much as documents about it
but more study makes more confusion to me.
So I am trying to come over here and hope that someone more experienced will be
so kind to explaining it to me. Lets follows...
In Chapter 4, Operation hints (page 4-1) in par. 4-1.1 CW we can read "Actual RX frequency is
600Hz lower then displayed TX frequency...". Also mentioned there "this is due to BFO
shift to bring oscillator into filter passband". [1]
In Chapter 5, Operating Installation (page 5-1) in par. 5-1.3 Crystal filters we can read "A
fourth optional filters [Model 218 (1.8k), Model 217 (0.5k), Model 219 (0.25k) may be installed in
the 81410 9MHz IF board...". "This filter is selected by pressing the NAR key...".
[1]
Also in Chapter 6, Circuit descriptions (page 6-57) in par. 6-15 we can find "CW
transmit or TUNE, adjust C9 for 9.000600MHz". [1]
Finally in Chapter 3, Detailed Operating Instructions (page 3-8) in par. SPECIAL NOTES we
can read "The standard Model 217, 500Hz cw 9MHz filter, will not work on FSK because
it is optimized for 600Hz tones". [1]
Well from all of these facts in manual it seems that Omni V is using fixed BFO
offset 600Hz on CW which can not be changed and side tone should be set to
600Hz as same as the filters used in 1st IF must be centered at 9.000600MHz
right?
Well now confusion is going to start...
In the Ten-Tec Instruction sheet Pt. No. 74089 for Models 217, 218, 219 and 220
crystal filters we can see in Spec. table (page 1) for model 217:
Center frequency, MHz: 9.000750MHz (as same as Model 219 btw) [2] and these
filters are generally recommended by Ten-Tec for Omni radios, so I presume it
is also recommended for Omni V.
As I mentioned in preview part the Model 217 is recommended also directly in
Omni V manual In Chapter 5, Operating Installation (page 5-1) in par. 5-1.3
Crystal filters [1] even although these filters seems to be centered at
9.000750MHz so these are reliable for radios with 750Hz BFO offset right?
...and confusion is ongoing.
In my email from TT guru Tyler, N4TY years ago he is recommending the filter with
9.000600MHz center frequency. He says "So I would just plug the bare #753 Inrad
filter into NAR slot and operate with it for awhile". [3]
In harmony with this email communication it is navigating us to Inrad web sites
where we can read for filter #753:
400 Hz 9000.6 kHz CW 8-pole crystal filter.
This filter is a comparable replacement for the Ten-Tec 217, but for lower
pitch CW. [4, 5]
So even the Inrad talks about the "lower pitch" of their filter replacement for
Model 217 by Ten-Tec so it also presume that Model 217 from Ten-Tec is really centered at
9.000750Hz instead of 9.000600MHz.
Why Ten-Tec recommends this filter for Omni V, radio with 600Hz BFO offset?
At the end I found the eml threat on tentec@contesting.com list from 2003 named
"Confusion re Inrad filters" where we can read from NO7UP an article about similar
problem and we can read "In older rigs such as the Omni V, the offset is not adjustable -
it is fixed at 600 Hz. You would normally set your side tone to 600 Hz and match the tone of
the person you are working. Your signal would then be right on top of his signal. But when
using the #759 and #760 filters, which are fixed at 700 Hz, the station you are listening to
will be out of the passband. So most people set their side tone at 700 Hz so they will be in
the center of the 9MHz IF filter passband. But that creates another problem - since the radio
has a
fixed transmit offset of 600 Hz, you will NOT be right on top of the other
station - you will be 100Hz off. Not normally a big deal, but most people would
prefer to be right on top of the station they are working. That is why Inrad
came up with the new 9MHz filters that have the 600Hz offset - so on the
older rigs, everything lines up right." [6]
So it is confirming that Omni V has fixed BFO offset 600Hz and valid filter for
1st IF 9MHz stage is centered at 9.000600MHz isn't it? So again, why Ten-Tec is
recommending to install at that stage filters with 750Hz offset centered at
9.000750Hz?
This is end of this annoying post folks. Is there somebody who is willing and
kind to explaining it to me please? Am I doing some underlying or fundamental
mistake or is there something what I am missing please?
...oh well and back to the basic question from the beginning> which filter I
should purchase and install to the Omni V at the NAR position for highest
selectivity on CW to follow by my fully loaded 2nd IF stage?
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