Hi Jerry,
.....that is, before the Japanese manufacturers started using DDS.
As far as I am aware, Icom pioneered DDS in amateur transceiver design. One
of the first high-end Japanese HF transceivers to employ a DDS 1st L.O. was
the IC-781. It achieved some of the best reciprocal noise-mixing "numbers"
in the industry in its day; on 14100 kHz, an undesired signal with 10 kHz
offset raised the idle-channel noise floor by 3 dB for an input level of 11
mV (RF preamp off) and 10 mV (RF preamp on).
Current-generation Icom HF radios such as the IC-756Pro and Pro II employ a
DDS chipset which is several generations newer (and thus appreciably
quieter) than that used in the IC-781. The Pro II 1st L.O. phase noise
is -125 dBc at 2 kHz offset (compared to -105 dBc for the IC-781).
The Paragon, being general-coverage, used a "classical" PLL synthesiser
design.
Incidentally, it is noteworthy that the hambands-only Icom IC-740 used a
frequency-generation scheme similar to that of the Ten-Tec Omni V/VI - a
divided-down VHF PLL synthesiser for the 2nd L.O. Instead of a crystal bank,
though, the 1st L.O. was a single-loop PLL with a step size of 500 kHz (as I
recall). This large step size ensured that the phase-noise contribution of
the 1st L.O. would be negligible. The ARRL product review of the IC-740
(QST, September 1983, p. 39) indicated that the IC-740 frequency generator
was much quieter than any of the conventional PLL implementations previously
tested, and that the measured dynamic range even exceeded that of some
analogue VFO transceivers. Of course, the compromise which had to be struck
here was hambands-only coverage - a fact which locked the IC-740 design out
of non-amateur markets.
The logical "next step" in a down-converting general-coverage architecture
using high-side 1st L.O. injection was to replace the relatively noisy
"classical" PLL with a DDS approach. This was, and is still, the road to the
best of all worlds - a low 1st L.O. noise pedestal (which gets lower as DDS
hardware improves) together with the continuous frequency coverage required
for access to commercial, governmental and similar markets. (In fact, a
defence contract funded the original IC-781 development. Then well-heeled
hams discovered the radio, and bought every 781 offered in the retail
market.)
In closing, could I request that we avoid the use of pejorative terms such
as "ricebox"? They do not enhance a debate.
Best 73,
Adam, VA7OJ/AB4OJ
North Vancouver, BC, Canada
http://www.qsl.net/ab4oj/
Note new e-mail address:
mailto:farson@shaw.ca
-----Original Message-----
From: tentec-admin@contesting.com [mailto:tentec-admin@contesting.com]On
Behalf Of Dr. Gerald N. Johnson, electrical engineer
Sent: August 26, 2002 20:16
To: Steve Ellington
Cc: KE4TEG; tentec@contesting.com
Subject: Re: [TenTec] Re: synthesizers was Omni C
Tentec designed the Omni V, and VI (and I don't know about the Paragon)
with the synthesizer at VHF and divided it down to reduce the phase
noise. Its not perfect but its better than most rice boxes that don't
use that phase improving technique.
73, Jerry, K0CQ
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