On 10/04/2023 09:39, Shenk, Trey E via USRP-users wrote:
I'm using a B205mini to transmit signals. When transmitting, I can see
copies of the SOI at harmonics of the center frequency. I ran some
measurements of total harmonic distortion, and found it to range from
39% with a 100MHz to 23% with a 1GHz carrier. The second harmonic is
<-50dBc, but the third harmonic is usually around -10dBc.
My main concern is for the lower frequency carriers, like 100MHz,
because multiple harmonics will show up on a spectrum analyzer set to
a wideband. I've looked at putting an RF filter at the output, but I
need the system to be able to switch transmit center frequencies in a
range from 100MHz to 5GHz.
Is it possible to reduce the harmonics by some hardware setting
(driving with gnruadio)?
Thanks,
Trey
I just did some tests myself, using a couple of tones with modest magnitude.
Indeed, the odd harmonics of the carrier are not particularly well
suppressed, which surprises me somewhat--the mixer in
the AD9361 is fairly good.
But what is true is that only SOME of the USRP line have a switchable
filter bank that the UHD software automatically switches
into place, which would (in many cases) handily suppress harmonic
content.
I tend to think of SDRs as *components* in an overall, engineered, RF
systems design. In radios-for-a-particular-purpose, there
are almost ALWAYS application-specific filters to eliminate spurs and
harmonic mixer output content from reaching the antenna
at levels that are significant. An SDR, being very general-purpose,
has no opportunity to do that (although, as I've said,
SOME USRP models include a handful of different filters in the TX and
RX chains).
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