# Block & Specification Queries **URL:** \ **Category:** GNU Radio\ **Created:** [June 25, 2012, 2:00pm UTC](https://www.ruby-forum.com/t/block-specification-queries/220921 "2012-06-25T14:00:20Z")\ **Posts on this page:** 3\ **Page:** 1
**Author:** ![aris](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/258eb7/32.png) [@aris](https://www.ruby-forum.com/u/aris)\ **Post date:** [June 25, 2012, 2:00pm UTC](https://www.ruby-forum.com/t/block-specification-queries/220921/1 "2012-06-25T14:00:20Z")
I have a 5 questions that are probably relatively straightforward so I thought it’d be easier to lump them all in together. Some of these have been touched on in some other disussion threads I found but I wanted to clarify the answers… To put things in context, I am doing radar research and am looking at replicating simple signals such as CW, Doppler Pulses, FMCW Chirps etc. Thanks in advance, I’m very new(ish) to alot of signal processing and even linux so its been a steep learning curve for me! \*\*Amplitude Parameter The Amplitude parameter affects the Amplitude dB output, but from what i’ve determined / read, it is only linear up to a certain level. E.g. I thought I read somewhere that 1 should be considered the maximum digital amplitude but (for example) that for the RFX1200 the maximum should be considered to be 0.2 before the dB amplitude increase as a factor of the amplitude parameter starts to be come non-linear. Is this due to the fact that the maximum power output is stated as being 200mW? E.g. Can I therefore correctly assume that for the XCVR2450 that 0.1 should be the maximum value for the linear region? Also is the amplitude (db) fft plot the idealised / theoretical power I assume? > **[USRP Software Defined Radio (SDR) online catalog - Ettus Research](https://www.ettus.com/products/)** > > Ettus Research specializes in software defined radio (SDR) systems. The USRP platform addresses a wide range of RF applications from DC to 6 GHz. \*\*Freq Modulation Block. This block has a sensitivity parameter, but is this effectively just a tuneable parameter, e.g. if I put in a 4 how does that differ from a 2 or a 1? E.g. Does that mean the frequency will be modulated to a frequency 1/4 of the original, as opposed to 1/2 or 1/1? Over what time frame will this change take place? I’m aiming to create chrips using existing blocks. I’ve read that feeding a saw tooth into a Freq Mod block is the best option… but i’d like to be able to control the time and compression factor… i guess the time is affected by the period time for my ramp, and amplitude is my sensitivity but again… what does this equate to? 😕 I’m just not sure. [http://gnuradio.org/doc/sphinx/gr/modulation\_blk.html](http://gnuradio.org/doc/sphinx/gr/modulation_blk.html) \*\*Bandwidth The XCVR2450 has a bandwidth of 33MHz according to the first link below, but according to the second… it says it can cover Tx: 24, 36, 48 MHz; Rx 15, 19, 28, 36 MHz. I’m a bit confused by this > **[USRP Software Defined Radio (SDR) online catalog - Ettus Research](https://www.ettus.com/products/)** > > Ettus Research specializes in software defined radio (SDR) systems. The USRP platform addresses a wide range of RF applications from DC to 6 GHz. [http://www.ettus.com/content/files/kb/Selecting\_an\_RF\_Daughterboard.pdf](http://www.ettus.com/content/files/kb/Selecting_an_RF_Daughterboard.pdf) \*\*Sampling Rates When running some grc files via gnuradio companion with my USRP N210 (with a Basic Tx Daughterboard), I often find that I get an output saying something to the effect of Target Sampling Rate = 22Msps Actual Sampling Rate = 20Msps. In fact when testing through a bunch of sampling rates I find it almost selecting sampling to rates such as 20Msps, to 25Msps or sometimes 33.333Msps. What impacts the selected sampling rate? E.g. is this a factor of the clocking frequency of my computer, the USRP, both or something else? \*\*Real time Scheduling I still can’t seem to get Real Time Scheduling going despite having added the @usrp - rtprio 50 to /etc/security/limits.conf (and also setting Real Time Scheduling to “on” in my top block) My computer is a 3GHz Quad Core, so i’m surprised I can’t this isn’t working Would chaning it to @usrp [t@usrp](mailto:t@usrp) - rtprio 99 make a difference? ---
**Author:** ![teddio](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/b3f665/32.png) [@teddio](https://www.ruby-forum.com/u/teddio)\ **Post date:** [June 26, 2012, 11:01am UTC](https://www.ruby-forum.com/t/block-specification-queries/220921/2 "2012-06-26T11:01:01Z")
On Mon, Jun 25, 2012 at 09:29:01PM +0930, Michael H. wrote: > correctly assume that for the XCVR2450 that 0.1 should be the maximum value for > the linear region? No, you can’t. All of this is rule-of-thumb, trial-and-error and experience. Some call it black magic. In theory, amplitude==1 would saturate you ADC (much earlier than the d’board), and such a signal would radiate (using the d’board) at approx. 200mW. > Also is the amplitude (db) fft plot the idealised / theoretical power I assume? It is not theoretical, but it is not calibrated to physical values. See the FAQ: [http://gnuradio.org/redmine/projects/gnuradio/wiki/FAQ#How-do-I-know-the-exact-voltagepower-of-my-received-input-signal](http://gnuradio.org/redmine/projects/gnuradio/wiki/FAQ#How-do-I-know-the-exact-voltagepower-of-my-received-input-signal) > Over what time frame will this change take place? I’m aiming to create chrips > using existing blocks. I’ve read that feeding a saw tooth into a Freq Mod block > is the best option.. but i’d like to be able to control the time > and compression factor.. i guess the time is affected by the period time for my > ramp, and amplitude is my sensitivity but again.. what does this equate to? 😕 > I’m just not sure. Hm, are you sure about this? Wouldn’t an NCO do what you want? > \*\*Bandwidth > > The XCVR2450 has a bandwidth of 33MHz according to the first link below, but > according to the second.. it says it can cover > > Tx: 24, 36, 48 MHz; Rx 15, 19, 28, 36 MHz. I’m a bit confused by this You should read the datasheets. The IC powering the XCVR2450 has switchable filters, and you can set them to any of the bandwidths mentioned above. If using GRC, see the ‘bandwidth’ field in the UHD sink. > In fact when testing through a bunch of sampling rates I find it almost > selecting sampling to rates such as 20Msps, to 25Msps or sometimes 33.333Msps. > > What impacts the selected sampling rate? E.g. is this a factor of the clocking > frequency of my computer, the USRP, both or something else? USRPs don’t run at any sampling rate; they have to be integer fractions of their clock rate. So if you need 22 Msps, you have to resample on the host (say, to 25 Msps) and then use that on the USRP. Make sure to correct for resampling filter delay in your radar measurements. > Would chaning it to @usrp - rtprio 99 make a difference? Do you actually get an error message stating that RT doesn’t work? On a side note, I’ve done OFDM radar measurements using the N210/XCVR2450 combo and it worked fine. I’d be interested to hear about your results. MB – Karlsruhe Institute of Technology (KIT) Communications Engineering Lab (CEL) Dipl.-Ing. Martin B. Research Associate Kaiserstraße 12 Building 05.01 76131 Karlsruhe Phone: +49 721 608-43790 Fax: +49 721 608-46071 www.cel.kit.edu KIT – University of the State of Baden-Württemberg and National Laboratory of the Helmholtz Association ---
**Author:** ![teddio](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/b3f665/32.png) [@teddio](https://www.ruby-forum.com/u/teddio)\ **Post date:** [June 26, 2012, 6:36pm UTC](https://www.ruby-forum.com/t/block-specification-queries/220921/3 "2012-06-26T18:36:59Z")
Thanks very much Martin! \*\*Amplitude Parameter Ah. ok! I guess i’ll have to look into the variation, I assume it would be the same for multiple copies of the same USRP model. \*\*Freq Mod I’m using GNU Radio 3.6.0, I dont think there is a NCO block… has this been surpassed by the functionality of another block? \*\*Real Time Scheduling I’ll check to see what my error was tomorrow… It might be a while before I get some results but i’ll be sure to post what I get! -Michael