Ron,
Your detailed explanation is one of the best that I have ever read.
Although I understand the reasoning for and the performance of preamps on
given HF bands, and the conditions of when to use and when to not use them,
your verbiage does extreme justice to ansering the question AND describing
in words most will understand the full sequence of events surrounding use or
non-use of preamps at HF.
Thanks for your time and effort to put forth this layman's explanation.
73, de Milt, N5IA
----- Original Message -----
From: "Ron D'Eau Claire" <r...@cobi.biz>
To: "'DOUGLAS ZWIEBEL'" <doug...@gmail.com>; "'Elecraft Reflector'"
<elecraft@mailman.qth.net>
Sent: Monday, December 15, 2008 11:14 AM
Subject: RE: [Elecraft] K3 sensitivity + preamp: looking for wisdom
Keith is quite right.
All receivers generate internal noise in their amplifiers. The goal is to
make sure this internal noise does not compete with the received signal so
the only thing that limits the ability to hear weak signals is the
unavoidable atmospheric or "antenna noise".
Of course, the amount of "antenna noise" you hear depends upon the filter
bandwidth used. Narrower bandwidths admit less noise. That's why narrow
band
modes (e.g. CW, PSK31, etc.) get through when wider bandwidth modes won't.
In order to keep very strong off-frequency signals from overloading the
amplifier stages in the receiver, the ideal receiver has as little gain as
possible before the filters that set the selectivity. But any noise
generated in the very first stages of a receiver (mixer, etc.) gets
amplified along with the signal and appears in the speaker or phones.
On lower frequencies (up to about 10 MHz) the "antenna noise" in the
quietest locations is still relatively high. That allows a receiver for
those frequencies to be designed for best dynamic range (minimum gain
ahead
of the filters) and still the antenna noise will dominate, even at the
narrowest filter setting. So the ability to hear weak signals it limited
solely by the "antenna noise" and not by the receiver's internal noise.
On the higher frequencies, the atmospheric (antenna) noise drops
dramatically. Depending upon the noise in one's location, somewhere
between
10 and 20 MHz the noise generated internally in most receivers designed
for
optimum dynamic range on the lower frequencies will start to compete with
the antenna noise. The K3 (and K2) are no exceptions.
The answer is to add more gain near the antenna using a low-noise
"preamplifier" that boosts the signal and antenna noise levels up above
the
receiver's internal noise. That's why the K3 (and K2) have built-in
"preamplifiers" that can be switched in on the higher frequencies. "Low
noise" refers to the fact that the preamplifier has low enough internal
noise that it won't compete with signals down at the level of the "antenna
noise". In general, the "quieter" (lower noise) the preamplifier is, the
more susceptible to overload it becomes.
And you don't want more gain than necessary ahead of the receiver's
filters.
Every dB of gain added reduces the dynamic range by bringing all signals
and
noise up nearer the clipping point of the subsequent amplifier stages in
the
receiver. That's why the preamplifiers are switchable. Smart ops turn them
off on the lower frequencies where they aren't needed. Indeed, that's why
there's an attenuator. Under extreme conditions, attenuating all the
signals
- wanted and unwanted alike - provides greater dynamic range and better
performance, as long as the antenna noise is still greater than that
generated internally in the receiver. That's a very common situation on
the
160, 80 and sometimes the 40 meter Ham bands for many Hams. It depends
upon
their antenna and the level of the "antenna noise".
The built in preamplifier in the K3 does a great job on frequencies up to
30
MHz or so but it's marginal for best weak-signal reception in many
locations
on 6 meters. That's why the PR6 was added. It's optimized for best 6 meter
weak-signal reception under most circumstances by providing enough gain to
bring the very low "antenna noise" and signals on 6 meters up above the
internal receiver noise.
Ron AC7AC
-----Original Message-----
Hi...
The following request for information is not specific to the K3 (or
even to Elecraft). Please craft your response to be generic.
Despite being an active ham/contester for over 40 continuous years
(but not being an ee), I just do not understand how a "preamp" works.
It is not a "smart" device, so how does it "know" to amplify the
signal and not the noise? I first used a preamp way back when
(1960's) when I was a teenager...it was an Ameco job. It was more
"fun" than useful, because I could give out "big" S meter reports.
Later, when listening in the noise became more important to my style
of operation, I never could notice a difference between putting the
preamp "on" and just turning up the volume (many rigs with built-in
preamps too). Sometimes, turning up the AF gain worked better.
So...HOW does a preamp help to "bring a signal out of the noise"
without the use of some fancy computer algorithms backed by lots of
computing power? Further, does a preamp really "improve" sensitivity?
I have yet to find a signal that I could not copy (or hear) with any
preamp "on" vs increasing the AF gain. PS...I am talking about HF. I
know that lots of guys use a preamp on 160m with their beverages, so
the effect must be real...right? HOW does a "dumb" device do it?
de Doug KR2Q
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