On 10/03/2018 05:22 PM, Lundberg, Daniel via USRP-users wrote:
A bit of background:
I have been using a B205i mini to transmit a variety of pre-computed
waveforms. These have ~5 MHz of BW, and I’ve been streamin binary
files sampled at 6.25 MS/s. I use a modified version of the
tx_samples_from_file example, using a host laptop via a USB 3.0 to
Ethernet adaptor. The manner in which it is operated allows the
process to be slow, i.e., it’s OK for me to manually launch a new
script that selects a different waveform binary file on the host
laptop each time I want to start transmitting something else. This
all works fine.
However, the quality of the waveform is important, and the B205 has
some noticeable limitations with respect to IQ imbalance resulting in
unwanted amplitude modulation, phase noise, a 12 bit DAC, etc… This
is OK, because it is a <$1000 device, but now it’s time to do better.
I have an N310 to try, and I am looking for feedback on the best path
forward. I understand that the clock rate selections are somewhat
limited on the N310, but that I should be able to run it in a
comparable way to the B205 if I allow the N310 to perform
interpolation, i.e., 125 MS/s with an interpolation factor of 20 for a
6.25 MS/s input. This is an OK first step.
So now to my question:
I would like to investigate the transmit quality without the N310’s
interpolation, because I have no idea exactly how it does this signal
processing. So I think the normal path forward there is to set up an
interface on a host that can stream at 125 MS/s, which is not trivial
(thunderbolt adaptors, a computer with the right cards, etc…). I am
curious if it is possible for me to run the N310 in embedded mode,
have the waveform files loaded locally on the N310, and issue commands
to transmit in a loop. These files are currently ~80MB each, because
they are ~1.5 seconds sampled at 6.25 MH/s. They would scale up to
~1.6 GB if sampled at 125 MS/s with no interpolation. Is this
something the N310 hardware could handle in embedded mode, i.e.,
streaming from its own hardware resources, rather than through the
SFP? I am finding documentation on the embedded mode of the N310 hard
to find, but I may just be looking in the wrong places.
Thank you,
DL
There's no way that the embedded CPU on the N310 will be able to stream
at 125Msps, however the platform supports RFNoC, which means you can
go from stream to radio without an intervening DUC.
Something to be very much aware of is that the RFIC on the N310, the
AD9371 is the same overall "family" as the AD9361, but with a couple more
bits in the DAC.
Have you tried things like offset tuning on the B2xx to reduce the
effects of the DC-offset removal algorithms, etc? Are you sure that
you're not
saturating things, and causing non-linearity on the RF output? This
may not be an issue of "it's only a $1K device", but rather, simple
inadequate
"tuning" of your application.
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