Steve,
Not having the schematic of the Flex isn't as important as knowing
they are using a 16 bit A/D. In a perfect world, the rule of thumb is
6db/bit, 96 db may be accounted for. However, an A/D that can cover all
the way to 2 meters has to be very fast > 296 msp, if you believe
Nyquist. If you apply hardware AGC in any form, it is reflected across
the entire spectrum. And to add further insult, high speed A/Ds have
noise figures great enough to mask ambient noise at mid and above HF.
That means you need to add a low noise amplifier ahead of the A/D with
enough gain to overcome system noise, and that will start to bite into
those 16 bits by a function of the gain of the amplifier. You can do
some creative things like use tapered gain amplifiers that have a
reverse taper so that max gain is at the high end of the spectrum;
Typically signals and noise are greater at the lower end of HF even if
SNR appear to be greater at the high end.
The use of a 24 bit A/D allows for using a simpler approach and yet
maintain a high dynamic range. It also disallows using A/Ds that can
cover the entire spectrum as they don't yet exist.
Unlike math and physics, EEs are more artists than scientists. I
suspect the discussions at Elecraft over the architectures would have
been very interesting. There are advantages and disadvantages to direct
sampling and to the hetrodyne SDR's. Which is better? It's a function of
how you want to handle the requirement set which may impact price. Why
do I call engineers artists? The sciences are absolute and engineers
have to craft a solution that accounts for the compromises, and few
engineers always agree on the very same solution. One may like red and
another may like blue so the creation comes out as a work of art. The
bottom line is that the chosen implementations of Elecraft radios are
Gee Whiz and exceed what the casual user needs and appears to be the
best for contesting. That last bit is my opinion and I'm sticking with
it.
Just for the record, I use a KX3. It is lower on Sherwood's
rankings, but I think it is the best overall radio out there. It
satisfies a greater number of my requirements. And, that is how I
colored my solution. ;-)
73,
Barry
K3NDM
------ Original Message ------
From: "Steve Ellington" <steven...@gmail.com>
To: k6...@foothill.net
Cc: "Elecraft Reflector" <elecraft@mailman.qth.net>
Sent: 11/10/2015 8:51:16 PM
Subject: Re: [Elecraft] Sherwood Receiver Test Data - K3S Added Today
Part of the problem here is that Flex won't release the schematic to
the
6000 series so who knows?
I suspect that the AGC-T (Agc threshold) control actually adjust the
input
level to the ADC....somehow.
This control is manual and must be fiddled with per-band as conditions
change. Strong signals will sound distorted and you must manually
compensate often.
N4LQ
On Tue, Nov 10, 2015 at 8:42 PM, Fred Jensen <k6...@foothill.net>
wrote:
OK. Remembering that I'm the one who disappointed Mom and Dad when I
got
A's in Math and B's and C's in EE, and finally transferred to the
Math Dept
...
I didn't want to imply it was a problem, it was just a question.
First
off, I believe a direct-sampling SDR is one that essentially does
nothing
to the entire RF envelope being received except maybe bandpass limit
it to
the ham band of interest. No guarantee that's correct, and if it
isn't,
you might as well delete this now.
But if it is what direct-sample means, and since the RF envelope is
both +
and -, in an 8-bit ADC, 127 would be zero, 255 [all 1's] would be the
maximum along with all 0's for the negative parts. If the RF
envelope is
allowed to go above the value that digitizes to all 1's, it will
still
digitize to all 1's and it's excursions above all 1's are lost [i.e.
clipped]. In my experience as a ham, this is rarely if ever a good
thing.
OTOH, you want as much dynamic range as possible, so you want the
strongest input to the ADC be at the all-1's level so the parts of
the
envelope below that level will digitize to something other than zero
[127
in my 8-bit example]. Any signals below that level will digitize to
127
and you'll never hear them.
If the gain of the RF stage(s), and I'm assuming there is at least
one RF
stage, is such that the maximum of the RF envelope is below the
clipping
point, then it seems to me that signals you might have heard won't be
heard
because they never got digitized.
My question was [and is], do direct sampling receivers employ some
sort of
AGC to keep the max RF envelope at the clipping point? If the answer
is
"yes", I have a second question in the wings waiting to be asked.
Mom and Dad never recovered from their mathematician's defection from
EE,
despite having been a wireless addict since age 12.
73,
Fred K6DGW
- Northern California Contest Club
- CU in the Cal QSO Party 1-2 Oct 2016
- www.cqp.org
On 11/10/2015 5:06 PM, Barry LaZar wrote:
Fred,
When you have a 24 bit A/D and not looking at the entire
spectrum,
this should not be as big a problem as you imply, if gain
distribution
is correct. 24 bits should yield a great enough dynamic range to
cover
greater than ~ 95% of time; that last ~5% covers your friend next
door
with a KW or that thunderstorm over head. 24 bits also allows for
some
amplification to overcome the down stream noise figure that may be
greater than ambient.
On the other hand, the Flex uses a really high speed A/D, but
it is
only 16 bits, if memory serves. That architecture is far more
sensitive
to gain distribution. Those who have suggested that the Flex may
have a
problem with all the bits going to 1 in a stress environment may be
very
correct. IMHO: With the state of the A/D art as it is, I believe
Elecraft has the better practical architecture. As soon as low
noise,
high speed A/Ds become available with greater than 16 bits, my
opinion
may change.
73,
Barry
K3NDM
------ Original Message ------
From: "Fred Jensen" <k6...@foothill.net>
To: elecraft@mailman.qth.net
Sent: 11/10/2015 7:32:56 PM
Subject: Re: [Elecraft] Sherwood Receiver Test Data - K3S Added
Today
Question from a mathematics major [whose parents wanted him to be a
EE]:
In a direct-sampling receiver, how do you control the input level
to
the ADC to achieve maximum available dynamic range without clipping
at
the ADC?
73,
Fred K6DGW
- Northern California Contest Club
- CU in the Cal QSO Party 1-2 Oct 2016
- www.cqp.org
On 11/10/2015 3:58 PM, Steve Ellington wrote:
Looks like the K3S should be above the Flex because the Flex
achieved
2DB
higher due to:
Footnote Y --- "This is a testing anomaly of a direct-sampling
receiver."
Congratulations K3S
Steve N4LQ
______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:Elecraft@mailman.qth.net
This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html
Message delivered to k3...@comcast.net
-----
No virus found in this message.
Checked by AVG - www.avg.com
Version: 2016.0.7227 / Virus Database: 4460/10976 - Release Date:
11/10/15
______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:Elecraft@mailman.qth.net
This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html
Message delivered to steven...@gmail.com
______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:Elecraft@mailman.qth.net
This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html
Message delivered to k3...@comcast.net
______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:Elecraft@mailman.qth.net
This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html
Message delivered to arch...@mail-archive.com
