I'm a student of Electronics and Telecomunications and I would like, if possible, to have my questions and doubts answered.
I'm using C6713 to create a laser comunication system and i'm also using Labview DSP Module 2.0 and LabView 8.2.1.
The system consists of emitter and receiver, and the objective is to transmit voice over laser.
For the emitter I have a microphone, a c6713, a signal conditioning circuit connected to the output of the DSP (opamp which add a DC offset and amplifies and a bc548 to drive the laser) and a common red laser <5mW.
I'm modulating voice with PWM and the frequency is 12kHz.
For the receiver, I have a phototransistor with integrated lens that captures the modulated laser, a c6713 and a pair of speakers.
My questions are:
1 - My square wave frequency is about 12kHz but at this value the result is more like a sum of sine waves. I'm aware that the square wave is formed by infinite sum of sine waves, but what is the maximum frequency value for a square wave, I can create using c6713 and Labview DSP module?
2 - It's possible to output a signal on the c6713 with offset?
3 - At this point I have the emitter setup almost ready, and I'm about to pass to the receiver but how can I connect two DSP kit's in the same PC? I've tryed to program my emitter vi to the flash memory of the c6713 and I could do it sucessfully, but the VI doesn't run when I power-up or reset the DSP. I can only put the VI to work when I have LabView opened and run the VI on my PC. What am I doing wrong?
4 - When I run the VI through the PC to the c6713, apparently the resulting output signal in the osciloscope doesn't seem to be as expected. This is a very strange effect and I don't know it's origin. The expected is a square wave at 12kHz with 50% duty-cycle assuming no voice and noise at the microphone, but in the initial seconds after I have started the VI the output is some kind of random signal. If I disconnect the front panel indicators the random output signal simply disapears and I can't have anymore that 0V. I also noticed that after some time (can be seconds or minutes) the random signal is replaced by the correct waveform and if I disconnect the front panel indicators the resulting waveform becomes even better as expected. This sounds a little silly :s I remembered that this effects can be caused by some initialization of the c6713 hardware. My VI only have a infinite while loop which should start after power-up. Doen anyone know anything related to this problem?
5 - Finally, as I begun to think in the VI for the receiver I was forced to stop after I realised that I would need to measure the duty-cycle of the received signal. The Timing Transition and Measurements Express block would resolve this problem but in the c6713 block set there aren't any blocks that I know, to deal with time and transitions measurement.
Can anyone sugest a simple way of doing the duty cycle measurement?
In the image:
yellow: output of DSP
blue: after opamp (DC offset added and amplification)
green: laser signal (collector BC548)
pink: signal captured by phototransistor
Thank you very much and sorry for the long text
Did you solve your problems?
Could you phone to the technical support for your different issues? I think it would be better.
National Instruments France
Finally an answer! Yes I partially solved my problems.
First let me say that in my searchings through the web and through this forum, I couldn't find any topics that could answer my questions. Anyway, after spending some time thinking about the earlier questions I decided to return to the Texas Instruments CCS instead of continuing my project in LAbView and DSP Module. Maybe I'm not experienced enough to use eficiently the DSP Module or simply the LabView DSP Module hides too many details. I'm not saying LabView is a bad tool, far from it. It's a great software that I used in other projects related to robotics (I had the opportunity to use NIDAQ USB 6008, DAQCArd 6062E and use the Lego NXT Mindstorms Module).
1 - I changed the approach to the initial problem. Instead of trying to use PWM modulation, I changed to FM. I made this choice after understanding that C6713 couldn't create a decent square wave with a high frequency. If you open the image in the first post, in yellow you can see the output of the DSP. This wave is clearly a bad square wave with aproximately three harmonics. This happened because the AIC23 input and output are limited in frequency by an anti-aliasing and anti-image (I'm not sure if this is the correct name in english) filters. The cutoff frequency of this two filters is half the sampling frequency, which in my case was 96kHz. So the filters would cut all the frequency's above 48kHz. Considering that I was creating my square wave at 12kHz the next harmonic was at 36KHz, the next at 60KHz, 84KHz, etc. My square wave was very far away from being an infinite sum of odd multiple harmonics, resulting the signal in the refered image. The new approach I've made, uses sine waves, because this type of signal is composed by it's fundamental harmonic only. So idealy the C6713 can generate a sine wave at 48KHz. In my project the carrier is a sine wave with 28KHz, because I'm limited by the photodiode (I've changed from a phototransistor BPX43, 3.5€ to a photodiode BPW34, 1.2€) speed (if anyone knows a better model, with a low price let me know); Because I'm still using a laser, after adding offset and amplifying the signal is putted through a comparator which creates a 28kHz square wave that drives the laser. For this I'm using a JFET (ZVN3310).
2 - Because the AIC23 Codec input and output are decoupled, the offset is added and the signal amplified (LF351);
3 - At this point I haven't payed much attention to this minor problem. It would be very usefull to have my program in the DSP flash memory and run it on power on, but I couldn't put this to work neither in LabView or CCS. (Only the post example is running on power on).
4 - A complete mystery. I assumed that in LAbView DSP Module the C6713 needed some kind of hardware initialization procedure. The the signal remained with the strange effect until I changed to CCS.
5 - At this stage I'm starting the receiver software and for the FM demodulator I'm thinking using a PPL. The signal received by the photodiode e filtered by a modified Deliyannis filter. With this setup, I'm able to recover the 28kHz sine wave and input it to the DSP without the danger of having this signal altered by the anti aliasing filter.
Finnaly I would like to leave a sugestion: It would be very helpfull if a sub board related only to the LAbView DSP Module and the supported harware existed.
In the image:
yellow - output of emitter DSP;
green - laser signal (drain of ZVN)
blue - photodiode signal
pink - input of receiver DSP
Nuno Baltazar Santos