The trick would be to oversample. I'm not sure how you would do that
with just UHD, but with GNU Radio, you would just use a rational
resampler block. That is, setting the sample rate to 20 Msps and using a
2X resample ratio.
Ron
On 7/17/24 08:38, Brais Ares Fernández wrote:
I've just tested exactly the same setup but using a *B210*, and it
does *not *happen.
So it definitely seems to be what Ron pointed out.
@Marcus: yes, the signal used is sampled at 10e6 and I can see its
spectrum in Matlab and it's a clean OFDM. Actually we tried one OFDM
signal modulated by us, and other completely different one by 5GMAG
<https://github.com/5G-MAG/rt-mbms-modem> and that effect always appear.
Then... Using N210, is there no way that I can filter out that
aliasing stuff?
------------------------------------------------------------------------
*De:* Ron Economos <w...@comcast.net>
*Enviado:* miércoles, 17 de julio de 2024 16:05
*Para:* Brais Ares Fernández <ba...@gradiant.org>;
usrp-users@lists.ettus.com <usrp-users@lists.ettus.com>
*Asunto:* Re: [USRP-users] Re: Replicas out of band in OFDM signal
(N210+CBX)
Yeah, I think UHD sets it to some calculated value that can't be
overwritten.
BTW, here's what the spectrum looks like with a very wide analog
filter. So the default bandwidth setting on the N210/CBX is just a
little too wide to fully suppress that first alias.
Ron
On 7/17/24 06:39, Brais Ares Fernández wrote:
Hello, Ron. Thank you for the answer.
I've already tried but it seems to do nothing:
$ sudo /lib/uhd/examples/tx_samples_from_file --freq 1500e6 --rate
10e6 --gain 20 *--bw 10e6* --type float --file ~/OFDMsignal.raw
--args="name=sdr1"
I even tried weird values like 100e3 or 100e6, but no apparent effect
over the spectrum at all.
------------------------------------------------------------------------
*De:* Ron Economos via USRP-users <usrp-users@lists.ettus.com>
<mailto:usrp-users@lists.ettus.com>
*Enviado:* miércoles, 17 de julio de 2024 15:00
*Para:* usrp-users@lists.ettus.com
<mailto:usrp-users@lists.ettus.com> <usrp-users@lists.ettus.com>
<mailto:usrp-users@lists.ettus.com>
*Asunto:* [USRP-users] Re: Replicas out of band in OFDM signal
(N210+CBX)
Those are aliases. Try setting the analog bandwidth with the --bw
parameter.
Ron
On 7/17/24 03:58, Brais Ares Fernández wrote:
Hello,
This might be a common effect but I don't comprehend why it happens
and if it is avoidable.
I'm using a N210+CBX to transmit a 10 MSPS *OFDM signal*, with Fc =
1.5 GHz. When transmitting, I can see some kind of "replicas" at
+-7.5 MHz from Fc, always with a 40 dBc difference.
$ sudo /lib/uhd/examples/tx_samples_from_file --freq 1500e6 --rate
10e6 --gain 20 --type float --file ~/OFDMsignal.raw --args="name=sdr1"
I've already tried changing lo_offset and executing the calibration
utilities for IQ imbalance, but none of that seems related to the
problem.
Could anyone provide any insight?
Regards.
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