Wes,
"A "Roofing filter" is simply a filter in the radio's first IF through which all signals must pass before they
will be "seen" by later receiver stages. The narrower this filter is, the less exposure later stages will have. Thus a
"narrow" roofing filter is desirable -- but "narrow" is relative, as I'll explain."
What Elecraft said (above) is exactly what I said. Moreover, Elecraft's
explanation is required because the term roofing filter is now applied to
up-conversion in multiple conversion radios (with relatively wide first I-F
filters compared to what is achievable at a low first I-F) which is what the
term initially sought to rebuff in the first place, also my point.
73, Will, wj9b
CWops #1085
CWA Advisor levels II and III
http://cwops.org/
--------------------------------------------
On Thu, 6/14/18, Wes Stewart <wes_n...@triconet.org> wrote:
Subject: Re: [Elecraft] Field Day rig experience
To: elecraft@mailman.qth.net
Date: Thursday, June 14, 2018, 4:47 PM
Will,
First of all I have said before and will repeat
it, I detest the term "roofing
filter." That said, by the generally
accepted definition, you are wrong. See
Elecraft's take on this:
http://www.elecraft.com/K3/Roofing_Filters.htm
If you will think in
Wayne's terms, the post-mixer filter is a
"protective"
filter, not a
mode-specific filter. So the question becomes, how much
protection is necessary? In
Elecraft's case, quite a lot, IMHO. With its QRP
DNA, Elecraft uses post crystal filter
circuitry that minimizes current
consumption. The trade off for this is the
need for a bank of pricey crystal
filters
to limit the frequencies that the circuitry is exposed
to.
Now what if the
subsequent circuitry doesn't require this much
protection
because it is more robust? We
now have direct-sampling radios that can digitize
a whole ham band with good performance. If the
BW was limited to 10-15 kHz in an
up
conversion configuration they should be even better. The
limitation now
becomes LO phase noise, but
newer synthesizer designs overcome that obstacle.
Another thing to note is that IMD in crystal
filters is reported to be inversely
proportional to BW. So a wider filter might
actually be better from that
perspective.
Some Elecraft filters exhibit passive IMD BTW.
Wes N7WS
On 6/14/2018 8:01 AM, WILLIE BABER wrote:
> Hello Wes,
>
> I took a look. Both designs are using
the idea of "roofing filter" to refer to
up-conversion radios similar to the use of up-conversion
3khz filters as roofing filters in Icom radios.
>
> "Roofing
filter" (a mode specific filter after the first mixer
including narrow cw filters) only makes sense in the
context of the history of superhet design and in
particular the use of one broad 15 khz first I-F (so that
all modes may pass through it) typical of all Japanese
radios until recently. Calling a 45 mhz filter at the
first I-F a "roofing filter" as noted in the info
you sent entirely misses the point of what roofing filter
means. Or, to put it another way, all Ten-Tec radios had
roofing filters in them (and were ssb and cw only) well
before the term roofing filter was coined! Which is why an
Omni C will out perform any wide (15 khz) first I-F Japanese
radio, even those built well after the 1980 vintage Omni
C.
>
> Unless mode
specific up-conversion crystal filters can be made and as
narrow as 200 hz (this is possible with down-conversion)
then "roofing filter" and up conversion
doesn't make sense historically or in reality.
>
> Actually, Icom says
that did it with 1.2khz filter at 64 mhz in the Icom 7851,
though I'm not convinced the filter is that narrow, and
1.2khz is far from the 200hz filter that my K3 has in it
(however, the placement of this filter is why the 7851 is
among the best radios in Sherwood's chart, on cw).
>
> It is possible to
make very narrow and precise crystal filters as narrow as
the 200 hz inexpensively, and this is the point of having
multiple roofing filters at the first I-F. So, this is the
origin of the term roofing filter---in comparison to the
barn-door up conversion first I-F.
>
> 73, Will, wj9b
>
> CWops #1085
> CWA
Advisor levels II and III
> http://cwops.org/
>
>
--------------------------------------------
> On Wed, 6/13/18, Wes Stewart<wes_n...@triconet.org>
wrote:
>
> Subject:
Re: [Elecraft] Field Day rig experience
> To:elecraft@mailman.qth.net
> Date: Wednesday, June 13, 2018, 3:08
PM
>
>
Certainly not to disparage the
> K3(S)
architecture (I have two of them) there is
> nothing inherently wrong with an
up-conversion
> receiver, if modern
hardware is used.
>
> See:https://martein.home.xs4all.nl/pa3ake/hmode/g3sbi_intro.html
>
> and my friend
Cornell's,
> Star-10
transceiver.
>
https://pdfs.semanticscholar.org/eb33/5c12858779a653d9b9b93ca20120aebb7616.pdf
>
> Wes N7WS
>
>
> On 6/13/2018 11:38 AM, WILLIE
BABER
> wrote:
>
> Robert is talking about the
>
crystal filters, also known as roofing filters now-days,
> that are typically placed after the
first mixer (I
> mistakenly typed
"ahead" but I meant
>
"after" as Robert notes), though there is a
post
> amp and NB before these filters
in K2 and K3.
> >
> > The idea is that a
> crystal filter right after the first
mixer gives high
> dynamic range
because high selectivity comes before the
> receiver has developed stages of gain
that otherwise could
> cause blocking
or IMD, especially when selectivity is
> postponed to the second mixer while
ignoring gain
> distribution in prior
stages of the receiver. This basic
>
idea was popularized in Solid State Design for the Radio
> Amateur, and it was applied to Ten-Tec
radios for decades
> (at a 9 mhz
I-F).
> >
>
>
> Roofing filter gets defined in
relationship to Japanese
> radios that
had up conversion 15 khz filters at the first
> I-F, and generally lower dynamic range
as a result, (but you
> got all modes,
general coverage, and optional crystal
> filters at the second I-F).
> >
> > Good
for everyone radios.... but with
>
lower dynamic range and phase noise from the early
> synthesizers. This is why Ten-Tec
radios were so popular
> among
contesters, especially Omni V and VI (modified with a
> narrow cw filter at the first I-F).
> >
> > 73,
Will, wj9b
> >
>
>
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