On 16/04/2023 21:13, David Bengtson wrote:
Yes, it's pretty typical that the output of an SDR needs additional
filtering and gain on the transmit side, and similarly filters and
gain on the receive side. I spent a substantial fraction of my time at
a previous job designing application specific front ends for Ettus SDR's.
Dave
In my other job, I do radio astronomy. "Filter early, filter often" is
what gets the job done in that domain. A many-octave
tunable filter with variable bandwidth would be a nice thing, except
for the ongoing miners strike at the
unobtanium mines...
On Wed, Apr 12, 2023 at 8:39 PM Marcus D. Leech
<patchvonbr...@gmail.com> wrote:
On 12/04/2023 20:28, Ron Economos wrote:
This is what I see with a B210 at 145 MHz fundamental. I had the
analyzer on a large RBW, so the even order harmonic are buried in
the analyzer noise.
I don't think there's any remedy except for external filters.
Ron
B210 harmonics
Thanks Ron. That looks like what my TinySA produced for 250Mhz
carrier. I need to learn more about it so that I can
record spectra with it. There's a Linux app for it that I need
to explore....
Anyway, the basic issue, having consulted some older Analog
Devices forum posts is that the LO produces a square-wave
signal, which means that all the odd-order harmonics will be
present in it, and naturally, mix with the baseband.
There's nothing that can be done other than, as you point out,
filter. As you move up in frequency, this becomes
very much easier of course, and at 2GHz, the 3rd harmonic is
outside the supported passband of the AD9361, and
you won't see it.
I didn't realize this about the AD9361 chip, and other Ettus
devices have automatically switchable filters that can
(often, not always) remediate this issue. The E3xx series, the
N3xx series, the TwinRX cards for the X310 and friends.
But I'll point out again that "built for purpose" radios
nearly-always have output (input) filtering to reduce or eliminate
unintended consequences of architecture choices deeper within
the radio. Since SDRs in general don't get to be
"built for purpose" either ever, or until some specific
application is using them as their "radio bits", it's hard to come up
with a universal RF filtering scheme that is suitable for all
applications.
I'd never noticed this issue because I don't really ever TX in my
"day job" use of these devices (radio astronomy), and on
the RX side, I always pre-filter anyway, usually rather
aggressively.
On 4/12/23 16:20, Shenk, Trey E via USRP-users wrote:
The original signal that I showed was an unmodulated carrier.
I tried a 10kHz complex exponential (plots included for several
harmonics). The carrier is clearly visible at all frequencies. I
can see copies of the 10kHz tone on odd multiples of the
carrier, but not on the even multiples.
*From:* Marcus D. Leech <patchvonbr...@gmail.com>
<mailto:patchvonbr...@gmail.com>
*Sent:* Monday, April 10, 2023 10:22 PM
*To:* Shenk, Trey E <trey.sh...@pnnl.gov>
<mailto:trey.sh...@pnnl.gov>
*Cc:* USRP-users@lists.ettus.com
*Subject:* Re: [USRP-users] Harmonic Distortion with B205mini
On 10/04/2023 14:28, Shenk, Trey E wrote:
I first tried turning the gain down. What I saw was that the
even harmonics (2*fundamental, …) did not change power, and
the odd harmonics decreased by the same amount as the
fundamental. This means that the dBc for the odd harmonics
stayed the same with decreasing gain.
Decreasing the baseband amplitude had the exact same effect.
Even harmonics stayed at the same power level, odd harmonics
decreased while maintaining dBc.
What is the nature of the modulating signal?
If you use an example app like "tx_waveforms" with, let's say,
10kHz SIN signal, what are the harmonic results?
*From:* Marcus D Leech <patchvonbr...@gmail.com>
<mailto:patchvonbr...@gmail.com>
*Sent:* Monday, April 10, 2023 8:39 AM
*To:* Shenk, Trey E <trey.sh...@pnnl.gov>
<mailto:trey.sh...@pnnl.gov>
*Cc:* USRP-users@lists.ettus.com
*Subject:* Re: [USRP-users] Harmonic Distortion with B205mini
Check twice before you click! This email originated from
outside PNNL.
Turn down the RF gain a bit as well as the baseband
amplitude. Does this make any difference?
Sent from my iPhone
On Apr 10, 2023, at 9:41 AM, Shenk, Trey E via
USRP-users <usrp-users@lists.ettus.com> 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
<carrier_freqsweep_fc0200M_gain55.png>
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