Produce modulated signals is one the purposes of experiments with DSP included on OMAP-L137. The CODEC included in OMAP-L137 can support sampling (input and output) up to 96 KSPS. This allows for manipulation of signals up to 45 KHz (a little bit more).
So on this experiment I decided to experiment with digital modulations. For this purpose, two signals are synthesised (with samples), both exact divisors of main frequency to have a small number of samples. One of them, of higher frequency, will be the carrier. The other of a lower frequency will be the modulating signal.
This experiment explores techniques used in modulation of AM, DSB, USB/LSB signals. For AM and DSB, simple mathematics are used. For all of them an I/Q (with signlas in quadrature) modulator has been designed.
I/Q is a "complex" matter. Somtimes has a meaning sometimes other. So I plan in the near future add some explanation to the mathematics behind this matter.
¿How to test these signals?. Just connect the "Line-output" of the kit to the "Line-input" of your computer souncard, and use one of the many signal analyzer with spectrogram programs. I've used Spectravue, but others as PowerSdr can be used too.
You must be able to see lines of the spectrum corresponding to each modulation, considering that your soundcard rate and wideband is enough for all the presented signals.
If you have a receiver able to reach the low carrier frequency, inject the signal on receiver (Two twisted wires coupling -antenna receiver, and line out- are enough for receiving de signal). Avoid direct connection unless you can guarantee your receiver can withstand with near 1Vp-p without damage.
Probably you noted no FM modulation has been included, and expect audio modulation. For this we need more complicated processes. In the next entries we will introduce oscillators (for generation of any frequency non related to sampling) and filtering (useful for SSB generation from a real signal).
But now you can experiment with modulations downloading porject for CCSv4 and OMAPL137 KIT from here: http://sites.google.com/site/dspradiol137/downloads/100106a.zip
martes, 12 de enero de 2010
jueves, 7 de enero de 2010
Do you have a real feel of dB?
When working with digital signal processing, sooner or later you must deal with filters. Design a digital filter is an easy matter. Just install a filter designer, entry your design parameters and... voilà! you have your filter designed... or not.
If your specifications are kind, probably you can obtain the filter, but if your specifications are tight (or even crazy) probably is almost impossible or at least very difficult to implement.
Well... now you are facing a typical engineering problem: A compromise. A example: Designing a simple FIR digital filter, we can choose between a sharp roll-off on transition band or a strong attenuation on the stop-band for the same pass-band characteristics and number of taps. Are enough 35dB of attenuation? Should I sacrifice roll-off to get 10 extra dB?
We don't have the answer for this questionbut this experiment can help. This experiment compares one signal with an attenuated version of itself. Two type of signals can be compared: An internally generated tone and an external signal introduced through LINE-IN.
The result can be listened from LINE-OUT or HP-OUT on the same kit. Experiment shows attenuations of 3, 6, 10, 20, 30, 40, 50, 60 and 70 dB.
Runs on EVKIT for OMAPL137 under CCSv4 and the whole code with extra explanations can be obtained from: http://sites.google.com/site/dspradiol137/downloads/100101a.zip
If your specifications are kind, probably you can obtain the filter, but if your specifications are tight (or even crazy) probably is almost impossible or at least very difficult to implement.
Well... now you are facing a typical engineering problem: A compromise. A example: Designing a simple FIR digital filter, we can choose between a sharp roll-off on transition band or a strong attenuation on the stop-band for the same pass-band characteristics and number of taps. Are enough 35dB of attenuation? Should I sacrifice roll-off to get 10 extra dB?
We don't have the answer for this questionbut this experiment can help. This experiment compares one signal with an attenuated version of itself. Two type of signals can be compared: An internally generated tone and an external signal introduced through LINE-IN.
The result can be listened from LINE-OUT or HP-OUT on the same kit. Experiment shows attenuations of 3, 6, 10, 20, 30, 40, 50, 60 and 70 dB.
Runs on EVKIT for OMAPL137 under CCSv4 and the whole code with extra explanations can be obtained from: http://sites.google.com/site/dspradiol137/downloads/100101a.zip
sábado, 2 de enero de 2010
2010: A year for SDR
Hi everyone!
Almost one year ago I began to write a blog about SDR. At that time the purpose was to put in order mi own ideas and knowledge about SDR.
At the same time a work with a development kit of processor OMAP-L135 (A device form Texas Instrumens) began too. During past year I acquired some theoretical background and a little practice with SDR and DSP. Now, the moment to dig deep into practice has arrived.
Main purpose of this blog is to share and comment all of the experiments and practices I made with the OMAP kit and other componentes about SDR. Another purpose is to improve my english. I apologize for any "spanglish" and promise to become better on my english skills.
As I did one year ago, the blog must serve to organize my ideas an practice before I can finally design my own and complete SDR. Do I need another year for this? I'm allways in a hurry but time is limited. Really I don't consider when or when to arrive. The important thing is to enjoy the ride.
I give you a warm welcome. Consider you invited to share this travel!
EA2DFH, Javier Romera
Almost one year ago I began to write a blog about SDR. At that time the purpose was to put in order mi own ideas and knowledge about SDR.
At the same time a work with a development kit of processor OMAP-L135 (A device form Texas Instrumens) began too. During past year I acquired some theoretical background and a little practice with SDR and DSP. Now, the moment to dig deep into practice has arrived.
Main purpose of this blog is to share and comment all of the experiments and practices I made with the OMAP kit and other componentes about SDR. Another purpose is to improve my english. I apologize for any "spanglish" and promise to become better on my english skills.
As I did one year ago, the blog must serve to organize my ideas an practice before I can finally design my own and complete SDR. Do I need another year for this? I'm allways in a hurry but time is limited. Really I don't consider when or when to arrive. The important thing is to enjoy the ride.
I give you a warm welcome. Consider you invited to share this travel!
EA2DFH, Javier Romera
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