On 09/10/2018 10:48 PM, Ron Economos via USRP-users wrote:
It seems like it was correct before and wrong now. It's always been
said that baseband levels must be restricted to -1.0 to 1.0. Now it's
something different?
I consider this issue to be the same as a patch to the Linux kernel
that breaks user space. And you know how Linus feels about that.
Ron
Yes, I can see where you're coming from here, and I'll leave it to
Michael West to comment further on the "fix".
But there has *NEVER* been a guarantee that in any randomly-chosen
situation a baseband level near MAX can always be guaranteed not
to drive analog components into non-linear operating states. It's
just a convenient scaling ceiling. Now, I'll allow as how moving from
"close to 1.0" to 0.67 as the highest achievable baseband level is a
significant jump, and perhaps more attention needs to be paid
to whatever scaling fix was implemented.
Consider for example that mixers and RF amps are being asked to operate
over a couple of decades of bandwidth. So a mixer baseband level
that is "nicely maxed without unpleasant artifacts" at one frequency
might be "ouch" at another. And these edge conditions are notoriously
manufacturing-batch sensitive, and to a certain extent
usage-sensitive as well. Theoretically, one could iterate over the
entire parameter space
in an exhaustive calibration exercise, and have calibration tables
that "hide" all of that. But it would be an expensive exercise, I'm
willing to bet.
On 09/10/2018 07:26 PM, Marcus D. Leech via USRP-users wrote:
On 09/10/2018 10:21 PM, Ron Economos via USRP-users wrote:
I have to reduce the baseband amplitude from 0.96 to 0.67 to achieve
the same IMD.
This is a very annoying issue for me, especially since I've set
levels in the Gnu Radio gr-dtv example flow graphs based on previous
behavior.
Ron
I think it was Michael West who worked on the scaling issues in
B2xx. The internal implementation had been wrong for a long time,
from what I understood. This has the unfortunate side-effect that
previous carefully-set baseband levels may no longer work properly.
Unfortunately, there's no "set the baseband magnitude and RF gain as
loud as you can without producing excessive IMD" operator in UHD.
On 09/10/2018 05:37 PM, Marcus D. Leech via USRP-users wrote:
On 09/10/2018 08:34 PM, Ron Economos via USRP-users wrote:
I'll try that. It's going to take a while though since I have to
rebuild.
Ron
Understood. Sorry for the PITA....
On 09/10/2018 05:30 PM, Marcus D. Leech via USRP-users wrote:
On 09/10/2018 08:27 PM, Ron Economos via USRP-users wrote:
No change with 3.13.0. Haven't tried 3.14, since it's not
released yet.
Ron
Is this effect independent of baseband magnitude?
On 09/10/2018 05:21 PM, Marcus D. Leech via USRP-users wrote:
On 09/10/2018 08:17 PM, Ron Economos via USRP-users wrote:
I had previously noted that the B210 TX power had increased by
about 5 dB going from UHD 3.11.1.0 to UHD 3.12.0. Today, I was
doing some two tone IMD testing and noticed that the IMD
performance has degraded *severely* from UHD 3.11.1.0 to UHD
3.12.0.
Here are the spectrum analyzer plots at roughly the same power
level. The 3rd order two tone IMD level changes from almost
-50 dB in 3.11.0.1 to a hideous -20 dB with 3.12.0. Something
is terribly wrong.
I know that there had been some "futzing" with scaling in the
B2xx at around that time. What about 3.13 or 3.14?
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