On 2022-10-25 18:15, Brian Padalino wrote:
I have an application where I am using both channels of the TwinRX
without using LO sharing. I am using channel 0 as a single frequency
channel, and I am using channel 1 to tune to different frequencies.
I am noticing that some transients happen on channel 0 - the fixed
frequency channel - as I am changing channel 1. This happens with even
terminated inputs on both channels. I also notice that if I change
the rate at which I am changing channel 1, the spectrum, on average,
is much cleaner but the transients stay there from a "max hold"
perspective. So once the LO has settled, things don't seem to be as
noisy.
My setup does not install the LO sharing cables, so those MMCX
connectors are left floating/open.
My question is if Ettus has seen this or knows about this? As I stated
previously, I don't need the LO sharing feature of the TwinRX and I am
worried that constantly retuning the PLLs might be causing the noise
and distributing it to the fixed frequency channel? If that is the
case, are there some resistors or modifications I might be able to
make to the TwinRX that could remove this as a source of noise knowing
I never want to perform the LO sharing? If I didn't connect the MMCX
LO sharing ports, am I already removing this as a possible noise source?
Lastly, a thought is that the noise might be coming from digital
switching noise to reprogram the LOs. How feasible is this?
Thanks,
Brian
I wonder if this is a radiative coupling thing? Like the synthesizer
for the other channel briefly sweeps across the stable tuned
frequency on the other channel, and a small amount of energy is
bouncing around inside the box and couples to the input.
The TwinRx has an internal heatsink/shield, and I would expect very
few instances where that shield is "waveguiding".
It is absolutely the case that with modern electronics with lots of
high-speed digital logic, it's nearly impossible to make
all your spurs go away. This is compounded by the fact that
front-ends in receivers have become about 10dB more
sensitive in terms of average noise figure over the last decade or
two. So internal "spurs" that would previously never
be seen are now showing up well above the noise floor.
My own approach to internal spurs is to make sure the outside-world
signal is strong enough to overwhelm them. This
isn't always practical, of course.
I have an X310 I can do some experiments with also. But I won't be able
to get to it until end of the week or maybe even
next week.
_______________________________________________
USRP-users mailing list -- [email protected]
To unsubscribe send an email to [email protected]
