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
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