On 17/04/2023 23:34, Shenk, Trey E wrote:
Got it. Thanks very much, Marcus! I didn’t realize that the LO was a
square wave going in.
It's a fairly common practice these days. The VFO operates at a very
high frequency, and then a
eye-wateringly-fast divider chain produces the final frequency. VFOs
up at 12GHz in chips like
this aren't uncommon--so the PLL controls the VFO over a
relatively-small range, and the divider
does the rest.
But even when you're driving a mixer with a nice pure sine-wave, the
mixer tends to chop it, since mixers
are basically switching devices, so there'll always be some harmonic
content coming out of a mixer even
with sine-wave inputs. They'll just be at lower levels in my
understanding.
Thanks for the info and the tips about moving forward!
*From:* Marcus D. Leech <patchvonbr...@gmail.com>
*Sent:* Monday, April 17, 2023 8:02 PM
*To:* Shenk, Trey E <trey.sh...@pnnl.gov>
*Cc:* USRP-users@lists.ettus.com
*Subject:* Re: [USRP-users] Harmonic Distortion with B205mini
On 17/04/2023 22:49, Shenk, Trey E wrote:
Hi all,
Thanks for the input on this issue. I’m still trying to pin it
down. I’m using gnuradio to drive a B205mini. I’m seeing harmonics
of the carrier particularly at odd harmonics. The third harmonic,
3*carrier, is only 10dB down from the carrier itself, and I’m
seeing significant carrier leakage.
I’ve tried changing the sampling rate, as well as modulating a
QPSK signal (both direct conversion and using some off-tuning). I
still see spectral images at the odd harmonics.
Is this known behavior for the B205mini? I’m trying to decide if I
need to change how I’m driving it, use a RF filter at the output,
or switch to a different SDR.
Thanks,
Trey
Two things.
The LO leakage can be dealt with using an LO offset.
However, the AD9361 uses a square-wave LO into the mixer, which means
that odd harmonics will make it out of the device.
This has been discussed extensively in the ADI forums, and a couple
of folks on this list have discussed it as well with
spectrum analyzer screen dumps showing it. I confirmed it with my
own modest spectrum analyzer as well. Because it's
an odd-harmonic problem, any application operating above 2GHz will
naturally not have this problem, because the 3rd
harmonic (and all other odd harmonics) will fall outside the
operating range of the AD9361 chip.
The only solution is filtering.
Some USRPs have a built-in switchable filter bank that can help with
this in some cases, but it really depends on the
frequency scheme of your application. A discrete filter bank
cannot possibly deal with all applications.
I personally tend to think of SDRs as *components* in an overall
engineered RF *system*. It is certainly the case that
ready-for-type-acceptance radios, regardless of internal
architecture, will almost always have filters at the edges to
deal with architectural unpleasantness deeper in the radio
architecture. The B2xx series were designed to be
relatively inexpensive, and as such, they don't have a switchable
filter-bank on them--and as previously pointed out,
even THEN, that may not necessarily make the radio "application
ready" in terms of formal type-acceptance, etc.
*From:* Shenk, Trey E
*Sent:* Wednesday, April 12, 2023 4:21 PM
*To:* Marcus D. Leech <patchvonbr...@gmail.com>
<mailto:patchvonbr...@gmail.com>
*Cc:* USRP-users@lists.ettus.com
*Subject:* RE: [USRP-users] Harmonic Distortion with B205mini
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>
*Sent:* Monday, April 10, 2023 10:22 PM
*To:* Shenk, Trey E <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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