Hi Sverre,
I and Q are Real and Imaginary parts of your complex baseband signal – the digital signal you send to the USRP sink! z is just the formula symbol often used for complex numbers. As this is a bit of an important concept in SDR, I'd recommend reading up on complex signals. There's a lot of good textbooks on signals and systems theory, but maybe Michael Ossmann's online tutorials might be a quick start: http://greatscottgadgets.com/sdr/ is a good thing. It builds up gradually, so I recommend starting from the 1. "Welcome" chapter. Your signal's amplitude values are simply to large. You need to reduce them, otherwise the USRP sink doesn't know what to do. Best regards, Marcus On 03/17/2017 01:26 PM, Sverre Turter Sandvold wrote: > > Hi, > > > I'm using a USRP N200 with a CBX daughterboard. > > > I'm new to USRPs and GNUradio, but what do you mean with z, I and Q? > Where do I change them? > > > Sverre > > > > ------------------------------------------------------------------------ > *Fra:* USRP-users <usrp-users-boun...@lists.ettus.com> på vegne av > Marcus Müller via USRP-users <usrp-us...@lists.ettus.com> > *Sendt:* 16. mars 2017 19:01 > *Til:* usrp-us...@lists.ettus.com > *Emne:* Re: [USRP-users] Problems with USRP Sink/Source > > > Hi Sverry, > > > there's a couple of things wrong here: > > > First of all, no, it can never be exactly the same! > > There's random phase offset between RX and TX, unless you use a USRP > and a daughterboard that have deterministic phase after tuning, and > use timed commands for tuning (which you don't). Also, the amplitude > is not going to be the same. Also, some noise will happen. That's > basically why we can't have infinitely fast internet everywhere – your > channel is never 100% known, so you can never fully compensate it. > Digital Comms :) > > > Then, you're using different RX and TX sample rates, so the RX signal > would at the very least have to look differently "stretched" over the > same number of samples. > > > Then: The signal going to the USRP must have |z| ⩽ 1. Your signal has > I and Q magnitude 25, and that makes for a |z| = sqrt( I² + Q²) = > sqrt(2)·25 > 1, so your signal gets wrapped already at the TX. > Multiply it by a factor of 1/(sqrt(2)·25), or better, much more. > > > Also, make absolutely, 100%, sure that you're not killing your RX with > your TX power. I don't know which USRP you're using (you forgot to > tell us, please try to put as much relevant info into your problem > descriptions as possible) , but USRP frontends' RX LNSa are typically > sensitive and get damaged (with a few exceptions) at powers above > -15dBm. Most our TX frontends can put out something around +20 dBm. > That's more than a thousand times as much as permissible for RX! So > always, always use attenuators on direct loop back. > > > Then, while this might work on an B2xx and on an X3xx, not all devices > support both the 5 and 8 MHz sampling rate – a sampling rate MUST be > an integer factor of the device's master clock rate, and with 5 and 8, > that means the MCR must be a multiple (including 1) of 40 MHz. That's > the case for B2xx, because it has adjustable MCR and can use 40 MHz > directly, and for X3xx, because its MCR is fixed at 5·40 MHz, but for > the other devices, it won't work (but you'd be notified in the shell > window). > > > Best regards, > > Marcus > > On 03/16/2017 10:29 AM, Sverre Turter Sandvold via USRP-users wrote: >> >> Hi! >> >> >> Having some problems in GNU Radio with USRP Sink and USRP Source. I'm >> sending a vector to a USRP Sink on TX/RX and trying to receive the >> same signal on RX2. Why is it a big difference between the >> transmitted and the received signal? Shouldn't it be the exact same? >> >> >> See screenshots: >> <https://www.dropbox.com/s/vtspcmoszmrt06t/Screenshot%20from%202017-03-16%2010-01-00.png?dl=0> >> >> https://www.dropbox.com/s/vtspcmoszmrt06t/Screenshot%20from%202017-03-16%2010-01-00.png?dl=0 >> >> https://www.dropbox.com/s/dvmlx8u3gb7qi65/Screenshot%20from%202017-03-16%2010-01-26.png?dl=0 >> >> >> SverreTS >> >> >> >> _______________________________________________ >> USRP-users mailing list >> usrp-us...@lists.ettus.com >> http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com >
_______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
