That's why I love the Shewood system...I can flip a switch to have
front end roofing filter(s) or go back to the original 8KC....then the
NB works OK.
I choose day to day depending on what I am doing.
By the way...I have a R-4B here that I'm working on for a friend...and
I noticed the filters are very soft...meaning I can copy SSb on the .4
setting...seems a little too wide of skirt? But the audio on the B is
sweet!
IMHO the ideal setup is to have the Sherwood C line for serious
contest/DX...then have a B line for the casual oprating and , sweet
audio.
Would be nice to actually have a nice way to switch the T-4XC to either
the R-4C or R-4B?? Then 1 TX would be all I need.
73,
Lee
-----Original Message-----
From: Garey Barrell <[email protected]>
To: drakelist <[email protected]>
Sent: Tue, Feb 1, 2011 10:10 pm
Subject: Re: [Drakelist] Wanted 4NB Noise Blanker
What I SAID was that 'the CW roofing filter is particularly bad' _WITH
RESPECT TO GROUP DELAY AND PULSE STRETCHING_ which makes the NB
useless. I'm not disparaging your expensive filter, group delay and
pulse stretching are a fact, the more poles, (sections,) the worse the
problem. As I also said, _changing ONE parameter of a system often
has
unintended consequences for the REST of the system.
If I'm concerned about dynamic range, i.e., low band contesting for
example, then yes, dynamic range is more important than a functioning
NB. On the other hand if I do most of my operating on today's ham
bands _outside_ of a contest weekend and have a neighbor with an
electric fence, then the reverse is true. IT'S a SYSTEM, and there are
trade-offs.
By the way, the 'standard' filter is 8 kHz wide.
73, Garey - K4OAH
Glen Allen, VA
Drake 2-B, 2-C/2-NT, 4-A, 4-B, C-Line
and TR-4/C Service Supplement CDs
<www.k4oah.com>
Don Jones wrote:
KO7I RESPONSE - "The CW roofing filter is particularly bad... but the
4NB is
good..."
SAY WHAT? I could not disagree with you more.
A unmodified R-4C has a Narrow Spaced Dynamic Range of 58dB while the
Sherwood modified R-4C has a dynamic range of 84dB. So in affect what
you
have just said is that having a noise blanker that works is more
important
than having your receiver blown away by adjacent QRM. I'll take the
roofing
filter thank you. :-)
FWIW, the mod's to my "Sherwood R-4C" do allow you to use the
standard 6kHz
front end filter. The 4NB noise blanker has only been marginally
successful,
at best, even when I revert to the standard filters.
Don Jones KO7i
Message: 1
Date: Tue, 01 Feb 2011 16:26:07 -0500
From: Garey Barrell<[email protected]>
To: [email protected]
Subject: Re: [Drakelist] Wanted 4NB Noise Blanker
Message-ID:<[email protected]>
Content-Type: text/plain; charset=windows-1252; format=flowed
Don -
You answered your own question/statement. I believe the words 'heavily
modified' were tossed around! :-)
The Sherwood mods essentially render the 4NB useless, as you have
discovered. The CW roofing filter is particularly bad, as it narrows
the
noise bandwidth seen by the NB, and delays pulses through it so they
no
longer match up with the blanking pulses.
The NB is very effective as some have stated, when used with the
original
receiver topology. Like so much in the world of RF and electronics in
general, a receiver is a 'system'. The original design was optimized
(within
cost constraints) as a whole. Anything that alters that design takes
the
risk that it 'may' alter that basic, optimized design, 'possibly'
improving
one characteristic. Unfortunately the characteristic it improves
almost
always causes another characteristic to decline, perhaps obviously,
otherwise not so much.
The basic Drake NB is the same design from the R-4A through the R-7,
and
most find them VERY effective on high level, short duty cycle impulse
noise.
As soon as the noise pulses are 'stretched', either by source
characteristics or group delay through filters, the effectiveness
drops off
quickly.
73, Garey - K4OAH
Glen Allen, VA
Drake 2-B, 2-C/2-NT, 4-A, 4-B, C-Line
and TR-4/C Service Supplement CDs
<www.k4oah.com>
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