# Block & Specification Queries

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?

**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.

**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

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
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 - rtprio 99 make a difference?

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:

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.

**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

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

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

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