I've been reading this thread with interest. There's a couple of things
that I'd like to point out.
First, there is clearly the need to run to audio gain at a level for
comfortable listening. However, running the gain "wide open" or at a very
high setting does leave one exposed to likely ear damage, headphone damage
or speaker damage in the event of some accidental feedback condition that
might develop. Be careful, headphones are about the most dangerous device
one can place over or in our ears.
As to RF gain, keep in mind that the A-D and D-A processing clearly has a
limit as to the dynamic range it can accurately handle. Exceed this range
and the system will produce all sorts of rude sounds and results. Now to
keep the A-D and D-A systems within their correct signal handling range
there is an internal AGC function that is not user accessible. At the same
time in order to maintain the highest S/N ratio for reception, some type of
gain control system is required, thus enter the user AGC system. That
system too has its limits of input dynamic range vs. output dynamic range.
Now to operate within that range comes the "human interface" thus the
dreaded manual RF Gain control.
The noise floor of these radios is extremely low. Clearly many dB below the
atmospheric noise and antenna thermal noise. This is necessary to "hear the
weak ones". At the same time the gain of the radio, input to output is
quite high. Again needed to hear the weak ones. Typically I find that
after setting the AF gain at a modest level, it is then quite easy to find a
RF gain setting for the band conditions and signals present. I really find
that I don't have to "ride the RF Gain" when tuning the bands. The internal
AGC systems handle the signals quite well. Changing bands or antennas
systems or even sometimes beam heading changes the noise floor and thus some
minor RF gain adjustment is needed.
Perhaps we are not being clear in out terminology between AVC and AGC. In
case you thought so, they aren't the same. Today's digital radios don't
have AVC systems so don't expect the AGC system to do what a AVC system
does.
I'm still not convinced that the PRE AMP on condition is really needed for
most HF operation as it provides some 6 to 24 dB of gain. In most cases on
HF, again atmospheric noise will be the weak signal limiting factor. There
are certain exceptions that apply. However, adding gain via the PRE AMP
will raise the signal and will also raise the noise thus reducing the
dynamic range that can be handled by the receiver AGC systems. In my case
the use of a PRE AMP is one that is mounted at the antenna and its purpose
it to make up for the signal loss in the feedline between the antenna and
the receiver thus maintaining or improving the NF of the receiver. I
believe we must get away from the thinking that more gain will allow us to
hear weaker signals. Actually it is more gain and less noise that
accomplishes this task. Thus, reduction in RF Gain is the objective.
73
Bob, K4TAX
----- Original Message -----
From: "Doug" <douglas.shock@gmail.com>
To: "Discussion of Ten-Tec Equipment" <tentec@contesting.com>
Sent: Friday, February 01, 2008 9:27 PM
Subject: Re: [TenTec] ORION II CW pile up readability
I have been playing with the max af, ride the rf, pre-map on idea all
evening. My "ears" tell me that there is an improvement. The NR actually
works better than it ever has.
The problem I have with this is that I am not sure the ergonomics's of
the radio were designed with this setup in mind. So perhaps it is just a
work around until TenTec blesses us with another release of SW.
Merle Bone wrote:
Jerry Volpe (KG6TT) said:
---------------------------------------------------------------------------------------------------------------------------------
"We discussed the A-D converters in the Orion and that it was their
practical
sampling limit.... or max range of ability.... that made it important to
keep
the Preamp IN. The object there is to make sure that there is sufficient
signal
available to the A-D converters so that the conversion produced a better
sampling overall (think of early audio CDs). If the RF energy applied to
the
A-D converters is too weak then the conversion process generates too few
samplings... resulting in continual sampling errors which many of us
experience
as 'distorted' weak signal audio (I certainly had)."
------------------------------------------------------------------------------------------------------------
I continue to return to the V1 firmware for my Orion for a number of
causes - always
hoping that a new release of V2 will be as good as the V1 firmware. And,
I certainly have no
reason to doubt what Jerry heard from one of the Ten-Tec engineers about
the
A/D converter issue with the V2 code for the Orion and the Orion II. To
look for that issue
in the V1 code, I ran some basic "minimum discernable signal" tests
tonight - similar
to those run by the ARRL for their test reports. I started with a
calibrated signal generator,
on 20 meters, and use a calibrated attenuator pad to further reduce the
signal. I used
Spectrum Lab software, and most importantly my ear, to find the "minimum
discernable
signal" and listened carefully to thee beat note from the signal
generator through the Orion.
With the preamp off I got to a -121dbm signal that I judged to be the
"minimum discernable signal"
from the ARRL procedures - that is about .224 microvolts for those that
think better in those
terms. The RF gain of the Orion was set to 86. The note from the
generator was
clear and undistorted.
I then turned on the Orion preamp and inserted additional attenuation
looking for the
minimum discernable signal. With the preamp on I got to a signal
of -133dbm - or
about .047 microvolts. The Orion RF gain was set to 80. The note from the
generator
was clear and undistorted.
So I think it is safe to say that the V1 code doesn't appear to have any
issues with A/D
errors at the level of signals that we use in communications through the
Orion. Of course
the noise level on all but the highest HF bands is much above the levels
of these - and the
ARRL's - tests. So, the signal levels passing through the Orion are
significantly above the
test levels.
-----I stopped writing for a couple of minutes to work EL2DX on 160M - My
QTH, the radio
black hole of urban St. Louis Missouri ----- :-)
As I just worked the EL2 on 160M, my preamp was off, my RF gain was on
78, my AGC
threshold was on .37 microvolts, AGC Decay was at 45dB/S and my hang was
set to
.04. You could not ask for a cleaner signal to work - all be it close to
the noise here in
Missouri!
Maybe we will hear more from Ten-Tec about this A/D converter error with
the V2
code and the "AGC overshoot." And maybe even get a new V2 code release
for the
Orion and Orion II with some more of the issues resolved!
73, Merle - W0EWM
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