I forwarded Art's comments to Rob Sherwood
of Sherwood Engineering. Rob has replied to
several points that were made. Curt K8IBQ
Durango, CO.
Subject: RE: Topband: Receiver recomm.
To: <topband@contesting.com>
Date: Wed, 26 Jan 2000 23:03:25 -0800
From: "Art Charette" <k6xt@arrl.net>
To: <Aidehua@aol.com>
CC: "Topband (E-mail)" <topband@contesting.com>
I think you'll be happier with the 775. The R4C
has relatively low dynamic range and TOI
compared to today's transceivers. Get
competitive with it (ie, contesting) and it'll
fold early. But, has really, really low
synthesizer noise:)
REPLY from Sherwood Engineering:
The DR of a Sherwood modified R-4C is 80 dB or
better at any spacing when using the CF-600/6 or
CF-2K/8 first IF filters to protect the 2nd mixer.
Typical measurements range from 80 to 85, a little
lower at 2 kHz than at 5 kHz. On the low bands,
80 and 160, the preselector makes the wider spaced
readings higher, at 20 or 50 kHz spacing due to the
front end selectivity. The real world experience of
many top banders is that a Sherwood modified C-Line
will perform as well as almost any other radio in a pile
up, often better. I have contested on 160 meters, and
found this to be true. (CQ WW) The IP3 of the C-Line
is only -10 dBm, but that holds up in close. Sure
modern radios boast +20 or greater IP3 numbers, but
in every case these numbers crater inside the first IF
filter "window". I have been able to operate on 160 CW
within 2 kHz of an S9 + 50 over signal, and not even
know he was there. Note: This was pre transmitted
phase noise problems of 20 years ago. Now transmitted
phase noise is a problem even if one had the "perfect"
theoretical radio.
As a point of interest, does anyone know of anyone
measuring receivers with close signal spacing, oh,
for example, INSIDE the first IF crystal filters?
Seems to me, first order, that the ARRL method of
using 20KHz or 50KHz spacing has little value on
160/80. Our interferers, that is each other, are
inside the roofing filters.
REPLY: While our chart is somewhat out of date, all
data on our web site lists both the close in spacing DR
and the 20 or wider DR. You are correct that the typical
DR at 20 kHz is of little value in a CW pile up, as most
up conversion radios have 30 dB LOWER DR within the
first IF filter than when tested at the wider optimistic
spacing. Sherwood and Passband to Worldband Radio (we do
the lab tests for them) is the only champion of the close
in method. We started publishing this back in 1977,
December issue of ham radio magazine. (Lead story, page 10)
QST never acknowledged the issue, as real world tests would
make the radios of their advertisers look bad. Read
between the lines !!!! Our web site is either of the
following two URLs: www.sherwood-engineering.com OR
www.sherweng.com
At the San Diego DX Club meeting tonight Icom
presented their new IC756Pro. It has 3 IF's each
with 15KHz xtal filters, leading to extensive DSP
(24 bit digitizer yielding DSP dynamic range of
144dB) at - if memory has lasted a few hours -
32KHz in the last IF. We'll measure one (inside the
xtal filters, you can be sure) as soon as we can
get hold of one.
REPLY from Sherwood Engineering:
I tested the JRC 545 DSP radio which has an 18
kHz filter at 455 kHz ahead of their DSP. The
spurious free DR INSIDE the protection filter is
45 dB !!!!!
The WJ HF-1000A or WJ-8711A is about 15 dB
better close in. The JRC DSP just goes berserk
inside the DSP when blasted with two strong
undesired signals outside the passband of the DSP
chosen bandwidth. The WJ, on the other hand,
only exhibited the expected 3rd and 5th order IM
products, not a spectrum full of spurious products.
It will be interesting to see how the Icom does.
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