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 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 > [image: 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> > <patchvonbr...@gmail.com> > *Sent:* Monday, April 10, 2023 10:22 PM > *To:* Shenk, Trey E <trey.sh...@pnnl.gov> <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> <patchvonbr...@gmail.com> > *Sent:* Monday, April 10, 2023 8:39 AM > *To:* Shenk, Trey E <trey.sh...@pnnl.gov> <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> > > _______________________________________________ > USRP-users mailing list -- usrp-users@lists.ettus.com > To unsubscribe send an email to usrp-users-le...@lists.ettus.com > > > > _______________________________________________ > USRP-users mailing list -- usrp-users@lists.ettus.com > To unsubscribe send an email to usrp-users-le...@lists.ettus.com > > > _______________________________________________ > USRP-users mailing list -- usrp-users@lists.ettus.com > To unsubscribe send an email to usrp-users-le...@lists.ettus.com > > > _______________________________________________ > USRP-users mailing list -- usrp-users@lists.ettus.com > To unsubscribe send an email to usrp-users-le...@lists.ettus.com >
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