[Discuss-gnuradio] Block & Specification Queries

[Michael Hill]

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?

https://www.ettus.com/product

 

 

**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

 

 

**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

https://www.ettus.com/product#tabs-3 

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 - rtprio 99  make a difference?