Re: [Discuss-gnuradio] Block & Specification Queries

[Martin Braun (CEL)]

On Mon, Jun 25, 2012 at 09:29:01PM +0930, Michael Hill wrote:
> **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?

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

> 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.

> **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?

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.

> **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 address@hidden - 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 Braun
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

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