[TenTec] Roofing Filters (LONG)

[Carl Moreschi]

I don't think that definition works for a single conversion receiver that
has just one filter in the IF.  This filter would certainly not be called a
roofing filter.  The purpose of the roofing filters in the Orion's are to
keep the DSP stage from overloading on signals that are not in the DSP
passband.  A roofing filter always has another stage of filtering in the AGC
loop following it.

Carl Moreschi N4PY
121 Little Bell Drive
Bell Mountain
Hays, NC 28635
----- Original Message ----- 
From: "Bill Tippett" <btippett at alum.mit.edu>
To: <tentec at contesting.com>
Sent: Monday, May 29, 2006 8:35 PM
Subject: [TenTec] Roofing Filters (LONG)


> Someone responded directly regarding the
> Omni's filtering scheme.  The optional Inrad roofing
> filter is inserted after the 1st mixer and BEFORE the
> original 15 kHz roofing filter.  The original filter
> is not switched out but it basically has few ill IMD
> effects because it is preceded by the optional filter
> (600 Hz or 2.4 kHz).  Here's a simple summary,
> where "etc" represents the following stages of the IF
> chain:
>
>
> 1.  Omni (original):
>
> 1st mixer > 15 kHz roofing > etc.
>
>
> 2.  Omni (Inrad roofing, either 600 Hz or 2.4 kHz):
>
> 1st mixer > Inrad roofing > 15 kHz filter > etc.
>
>
> 3.  Stock Orion (for 20k - 1k filters):
>
> 1st mixer > 20k - 1k selectable roofing > etc.
>
>
> 4.  Stock Orion (for 500-250 Hz filters):
>
> 1st mixer > 20k > 500 - 250 Hz selectable > etc.
> (this is how I understand it was configured)
>
>
> 5.  Orion (using Inrad 762):
>
> 1st mixer > 20k - 600 Hz selectable > etc.
>
>
> 6.  Orion II:
>
> 1st mixer > 20k - 300 Hz selectable > etc.
>
>
> A couple of issues come to mind:
>
> 1.  Orion / Inrad 762 and Orion II / 2001 600 Hz (#5 and
> #6 above) should have identical IMD performance using
> filters from 600 Hz and up since their block diagram is
> identical.  Orion II may have a small advantage using
> the 300 Hz at EXTREMELY close spacings (<300 Hz),
> but perhaps not realizable due to phase noise (Orion's
> and/or the offending station), clicks, etc.
>
> 2.  In the #4 case of Orion with the 500 & 250 Hz
> filters, it now seems fairly clear to me that the
> IMD problems were introduced because the 20k
> filter was left inline before the narrow filter stage.
> However no one has ever actually determined the
> source of the IMD problems so I cannot be sure.
>
> In summary I think the definition of a
> roofing filter being the first filter stage following
> the 1st IF holds true.
>
> 73,  Bill  W4ZV
>
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