DC offset correction

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n_qrtu
Posts: 17
Joined: Fri Feb 12, 2016 3:59 am

Re: DC offset correction

Post by n_qrtu »

The errors values which I see are around 0.1 - 0.5, so cal dc rx seems to work fine for those VGA settings.

I will add an external LNA of 25dB soon, and like this I expect to see/decode satellites even when placing my antenna close to the window. (Right now I need to be on the roof or in an open space to see/decode at least 4 satellites and get my position).

Thank you again for your patience and help!

Nicolae
jynik
Posts: 455
Joined: Thu Jun 06, 2013 8:15 pm

Re: DC offset correction

Post by jynik »

Hi Nicolae,

That sounds good to me - we can only correct within whole-valued counts. There's not fine-grained controls anywhere in the current system to correct fractions of an LSB.

I'm happy to hear that things are progressing. Do you think you'll be able to share some more info about your project and progress once you get your antenna in? It'd be super exciting to see your methodology and some pictures of your setup!

Thanks for your patience as well -- I know I've been a little slow on forum responses as of late.

Cheers!
Jon
n_qrtu
Posts: 17
Joined: Fri Feb 12, 2016 3:59 am

Re: DC offset correction

Post by n_qrtu »

Hi Jon,

Sure, once I have everything in place I can share my setup / settings / pictures.

I have just ordered today the LNA, it should be shipped within the next few days.

We are using this for a hands-on Software Radio class, and the idea would be to let the students collect the samples and then process them in Matlab to decode the satellites data and get their position.

As a next step I would like to try to make everything from Matlab - setup / calibration / samples acquisition, so everything can be done nicely with scripts from Matlab.

Further on, we are thinking of getting another board and make a Tx / Rx system where we transmit / receive some QAM, etc. We can test this first within one board (internally and then connecting the Tx to Rx - with some appropriate attenuators of course :-) ). Once we get another board I think we should worry about clock synchronization, we have to implement that in Matlab.

Anyway, one thing at a time, for now I would like to have the GPS working robustly.

Cheers,
Nicolae
n_qrtu
Posts: 17
Joined: Fri Feb 12, 2016 3:59 am

Re: DC offset correction

Post by n_qrtu »

Dear Jon,

As promised (a long time ago :-) ), here is the setup for our GPS signal acquisition.
bladeRF_GPS_signal_acquisition_setup_small.JPG
We are using a BiasTee to provide current to the 2 LNAs (one external (cylindrical one) and the another one built within the antenna).
(bladeRF cannot provide enough current for them)

BiasTee:
DSA-546586.pdf
GPS antenna:
http://ch.farnell.com/fr-CH/tallysman-w ... dp/2056650

LNA:
http://ch.farnell.com/fr-CH/tallysman-w ... 7-00001003

Now, the bladeRF settings:

set frequency rx 1575420000
set samplerate rx 4092000
set bandwidth rx 1.5M
set lnagain 6
set rxvga1 28
set rxvga2 9

In order not to drop samples, we collect them in /dev/shm/
From there:
bladeRF-cli -i
bladeRF> cal lms
bladeRF> cal dc rx

One needs to collect at least 2000 bits (a bit more than 6 subframes = 1800 bits). The number of samples is computed as:
2000 bits x 20 CA/bit x 4092 samples/CA = 163680000.
bladeRF> rx config file=my_samples.sc16q11 format=bin n=163680000
Start the signal acquisition:
bladeRF> rx start

We then process the samples in Matlab. (convert them first to .mat format with https://github.com/Nuand/bladeRF/blob/m ... _sc16q11.m)
I cannot make the code public, since the students need to write part of it in the class :-)

In an open space, we can see up to 9-10 satellites. Just putting the antenna outside the window still gets 4-5 satellites.
If we place the antenna by the window but inside the room, we cannot really get/decode any satellites - it looks that the (double window) glass attenuation is too much...

Cheers,
Nicolae
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