Stuart, this noise discussion takes me back to the days of my National
HRO-5TA1. Single conversion and no roofing filters. Wonderful old radio -
for its time. It was 1947 if I remember when I got it, but the design dates
back to 1937 or 38 when it was first introduced.
Its forte' was extreme sensitivity coupled with a high-performance crystal
filter for single-signal selectivity. Mine even has a "noise limiter" on
it!
But it had very low dynamic range compared to today's receivers with their
quad-conversion architecture. I can remember how very noisy that radio was
- even with the antenna disconnected - and how frustrating it was trying to
work a weak CW signal down in that noise. I used to run the crystal filter
- a *real* crystal filter, not a bandpass filter implemented with crystals!
- cut way down in bandwidth just to reduce that noise somewhat. Sort of
like a modern notch filter set to peak instead of notch.
But, it was one of the reigning monarchs on the bands until the Collins and
Drake products came along.
I later on learned that much of the problem was (a) the use of TWO
high-gain r-f amplifier stages ahead of *the* very noisy mixer to build up
the sensitivity; (b) excessive IF gain which made the front-end device (r-f
amps and mixer) noise the predominant component; and (c) a relatively wide
open IF passband until the very last stage which implemented the crystal
filter driving the "detector" where the BFO got together with the IF signal
to produce the audio. Unfortunately, the filter was placed where all the
internal noise had already been maximized by the combined gains of the r-f
and i-f stages.
I know that the HRO design was far from the direct conversion and
single-conversion receivers that most of the guys are talking about, but I
just wanted to remark that the number of conversion stages is only part of
the story. Multiple conversions gain certain performance factors and cost
others.
The K2 is living testimony to how "good" a relatively simple
single-conversion receiver can be. But clearly there are market forces at
play here that do not reward "quiet" receivers or *all* receivers would be
of the K2 architecture if that were the magic design. Nothing succeeds like
success, they say, but I doubt very much that the new Orion bears much in
common with the K2 design. A much closer comparison will be between the K2
and the 516 QRP rig if it every assembles itself from all the vapor.
Good to see so many guys pitching into an interesting technical discussion
instead of griping about colors and knobs! <:}
73/72/oo, George W5YR - the Yellow Rose of Texas
Fairview, TX 30 mi NE of Dallas in Collin county EM13qe
Amateur Radio W5YR, in the 56th year and it just keeps getting better!
QRP-L 1373 NETXQRP 6 SOC 262 COG 8 FPQRP 404 TEN-X 11771 I-LINK 11735
Icom IC-756PRO #02121 Kachina 505 DSP #91900556 Icom IC-765 #02437
All outgoing email virus-checked by Norton Anti-Virus 2002
Stuart Rohre wrote:
>
> Noise in multiconversion rigs---(160M and 80M)
> It is the width of roofing filters and the overload properties of active
> mixers in the modern rigs. Plus, probably more strong signals today.
|