Re: Signal reconstruction

GNU Radio
1

Hi,

Firstly my apologies for posting known quantities, I guessed that
members
would like to see in what way I tinkered with usrp_rx_cfile.py. point
taken
though.

Screen shots of the MATLAB plots as well as the usrp_oscop.py output can
be
found at:

https://rrsg.ee.uct.ac.za/members/aadil/

I’m pretty sure the MATLAB is correct.

I have upped the input freq to 250KHz, now attempting a square wave (in
desperation) and
set the options.freq to 0 in —>self.u.tune(0, self.subdev,
options.freq)

I’m Still not getting sensible plots.

Thanks again,

Aadil

2

Hi,

Use center frequency =0 in the usrp scope. If you use center frequency =
250K, you will shift the signal to 0 Hz.

What is your signal input level in dBm ?

Firas

Aadil V. [email protected] wrote: Hi,

Firstly my apologies for posting known quantities, I guessed that
members would like to see in what way I tinkered with usrp_rx_cfile.py.
point taken though.

Screen shots of the MATLAB plots as well as the usrp_oscop.py output can
be found at:

https://rrsg.ee.uct.ac.za/members/aadil/

I’m pretty sure the MATLAB is correct.

I have upped the input freq to 250KHz, now attempting a square wave (in
desperation) and
set the options.freq to 0 in —>self.u.tune(0, self.subdev,
options.freq)

I’m Still not getting sensible plots.

Thanks again,

Aadil

3

Hi,

I’ve updated the scope screenshot on the website. Two shots are
available,
notice there seems to be some kind of modulation??? I’ve tested the
input
signal with an external scope and the input signal is fine. I don’t know
why
the usrp scope is “misbehaving”

The input signal level is set at -16.02dBm

Aadil

4

Hi,

Input signal level is ok.The scope pictures was not generated by
usrp_oscope.py. What is the program used to generate it ? Test The
signal with usrp_oscope.py

Firas

Aadil V. [email protected] wrote: Hi,

I’ve updated the scope screenshot on the website. Two shots are
available, notice there seems to be some kind of modulation??? I’ve
tested the input signal with an external scope and the input signal is
fine. I don’t know why the usrp scope is “misbehaving”

The input signal level is set at -16.02dBm

Aadil

On 10/11/2007, Firas abbas <[email protected] > wrote:Hi,

Use center frequency =0 in the usrp scope. If you use center frequency =
250K, you will shift the signal to 0 Hz.

What is your signal input level in dBm ?

Firas

Aadil V. < [email protected]> wrote: Hi,

Firstly my apologies for posting known quantities, I guessed that
members would like to see in what way I tinkered with usrp_rx_cfile.py.
point taken though.

Screen shots of the MATLAB plots as well as the usrp_oscop.py output can
be found at:

https://rrsg.ee.uct.ac.za/members/aadil/

I’m pretty sure the MATLAB is correct.

I have upped the input freq to 250KHz, now attempting a square wave (in
desperation) and
set the options.freq to 0 in —> self.u.tune(0, self.subdev,
options.freq)

I’m Still not getting sensible plots.

Thanks again,

Aadil

5

The Scope was definitely usrp_oscope.py Used Directly from the from the
examples folder:

sar23@sar23:~/Desktop/GnuRadioResources/gnuradio-3.0.2/gnuradio-examples/python/usrp$
./usrp_oscope.py

Aadil

6

It seems I need to hit return after i’ve entered the values, my error.

the new plot is on the website:

https://rrsg.ee.uct.ac.za/members/aadil/

7

Hi,

Sorry about the scope file, I using a modified version and it seems that
I being confused with the standrad one. However, there is something
strange in the data shown in your picture. Since you are using
decimation =8, the data coming across the USB should be complex 8 Msps.
But your scope is showing (Fs@USB: 4M) which is wrong. another thing,
the DDC be should equal 0 , while in your picture it is being (DDC :
-8M) !!!.

Firas

Aadil V. [email protected] wrote: The Scope was definitely
usrp_oscope.py Used Directly from the from the examples folder:

sar23@sar23:~/Desktop/GnuRadioResources/gnuradio-3.0.2/gnuradio-examples/python/usrp$
./usrp_oscope.py

Aadil

On 10/11/2007, Firas abbas [email protected] wrote: Hi,

Input signal level is ok.The scope pictures was not generated by
usrp_oscope.py. What is the program used to generate it ? Test The
signal with usrp_oscope.py

Firas

Aadil V. [email protected] wrote: Hi,

I’ve updated the scope screenshot on the website. Two shots are
available, notice there seems to be some kind of modulation??? I’ve
tested the input signal with an external scope and the input signal is
fine. I don’t know why the usrp scope is “misbehaving”

The input signal level is set at -16.02dBm

Aadil

On 10/11/2007, Firas abbas < [email protected] > wrote:Hi,

Use center frequency =0 in the usrp scope. If you use center frequency =
250K, you will shift the signal to 0 Hz.

What is your signal input level in dBm ?

Firas

Aadil V. < [email protected]> wrote: Hi,

Firstly my apologies for posting known quantities, I guessed that
members would like to see in what way I tinkered with usrp_rx_cfile.py.
point taken though.

Screen shots of the MATLAB plots as well as the usrp_oscop.py output can
be found at:

https://rrsg.ee.uct.ac.za/members/aadil/

I’m pretty sure the MATLAB is correct.

I have upped the input freq to 250KHz, now attempting a square wave (in
desperation) and
set the options.freq to 0 in —> self.u.tune(0, self.subdev,
options.freq)

I’m Still not getting sensible plots.

Thanks again,

Aadil

8

It seems that the signal is ok. If you use a pure sinusoidal signal you
will see a much clearer picture.

Firas

Aadil V. [email protected] wrote: It seems I need to hit return
after i’ve entered the values, my error.

the new plot is on the website:

https://rrsg.ee.uct.ac.za/members/aadil/

9

Ok,

I still have the problem with the reconstruction though.

When I perform the FFT on the sample set, the plots are not as you would
expect for a pure 250KHz signal?

This is the major concern for me and the crux of my problem.

10

Thanks for responding and for your input, I answer your questions below,

You’re transmitting a 250kHz square source which is the summation of all
odd harmonics of >250kHz. I believe that is why you were told to switch
over to a sine wave which you would >probably just see a single tone.

I have switched over to a sine wave, but I still don’t get the single
peak
on the FFT as one would expect, that’s the source of this conversation.

I am sorry if I am unfamiliar, but what exactly are you trying to
accomplish, and how do you expect to do it with a square wave?

What I want to do (for now) is be able to correctly digitise a signal
and
write it to file (thereafter I will do various signal processing). My
short
term objective is to digitise “off air FM signals” and perform ambiguity
measurements.

I’m using the sine wave as input and plotting it’s FFT to see if I was
able
to correctly digitise it.
This should be a fairly trivial 5min process but I’ve been at it for a
week!!

Thus far non of the suggestions have helped cure my problem.

Images of my matlab plots are available here:
https://rrsg.ee.uct.ac.za/members/aadil/

I hope you can help, I feel hopeless right now.

11

On Sat, Nov 10, 2007 at 02:34:11PM +0200, Aadil V. wrote:

The Scope was definitely usrp_oscope.py Used Directly from the from the
examples folder:

sar23@sar23:~/Desktop/GnuRadioResources/gnuradio-3.0.2/gnuradio-examples/python/usrp$
./usrp_oscope.py

I note that you are using gnuradio-3.0.2. That’s almost a year old.

I suggest that you consider using the latest stable code, the 3.1.1
release, available at
ftp://ftp.gnu.org/gnu/gnuradio/gnuradio-3.1.1.tar.gz

Also, it’s not clear to me what you’re actually trying to do.
If you want to plot data in real time, please use either the
usrp_fft.py or usrp_oscope.py applications. You can capture data
to disk using the usrp_rx_cfile.py example.

I suggest you start with these, then do your own experiments.

Eric

12

On Nov 10, 2007 9:39 AM, Aadil V. [email protected] wrote:

Ok,

I still have the problem with the reconstruction though.

When I perform the FFT on the sample set, the plots are not as you would
expect for a pure 250KHz signal?

You’re transmitting a 250kHz square source which is the summation of
all odd harmonics of 250kHz. I believe that is why you were told to
switch over to a sine wave which you would probably just see a single
tone.

Due to bandwidth limitations, transmitting square waves over the air
doesn’t work quite as one may expect.

This is the major concern for me and the crux of my problem.

I am sorry if I am unfamiliar, but what exactly are you trying to
accomplish, and how do you expect to do it with a square wave?

Brian

13

Also, it’s not clear to me what you’re actually trying to do.

I’d like to capture FM signals and perform ambiguity measurements on
them.

at the moment, i am failing to capture data correctly.

If you want to plot data in real time, please use either the

usrp_fft.py or usrp_oscope.py applications. You can capture data
to disk using the usrp_rx_cfile.py example.

I suggest you start with these, then do your own experiments.

These are the files that I have been using.
usrp_rx_cfile.py to write to file and usrp_fft.py and usrp_oscope.py for
realtime examination of the data and Matlab for post processing, based
on
the utils in gnuradio-core.

Eric