Thursday, December 11, 2014

Visible Light Communication Chronicles Part VI

I have finally finished a working prototype of my VLC system and it works like a charm ;) This prototype has a maximum bandwidth of 1 Mbps and works to a distance of roughly over 2 meters using a collimated light source. After several design iterations i have used newer components like the S1223 30MHz photodetector from Hamamatsu and operational amplifiers from Analog Devices. Analog Devices has some great tools and application notes that simplify designing. The picture below shows the finished prototype of the VLC receiver. The micro-controller interface uses a Cortex-M4 micro-controller from Freescale - The Teensy 3.1 development board from PJRC. The circuit is basically a photodiode transimpedance amplifier using the AD8651 followed by a AD8561 high speed comparator for wave shaping.

Going forward with this project i shall be increasing the bandwidth and experimenting with smaller form factors, controllers, PDs, to facilitate audio, video and data streaming applications...



The final specifications had the feedback resistor with a value of 68 K, feedback capacitor of 2.4 pF for an output voltage swing of 3.3V. The PD was reverse biased by 5V resulting in a capacitance of 20pF. The AD8651 has a Gain Bandwidth Product of 50 MHz and powered by a 3.3V single supply. For the select Rf the 45-degree phase margin bandwidth turned out to be 1.82 MHz and the Cf with a value of 1.2 pF. Since the desired value of Cf was greater than the value of Cf that produces 45 degree PM the circuit was deemed stable. The dynamic range turned out to be 91 dB with total RTO noise of 96 uVolt rms. Since the slew rate of the AD8651 is 41 V/us, the output voltage max swing was chosen as 3.3V which when plugged into 2πfv for f = 1MHz gives approx. 20 V/us, which is less than the max specification


Output signal from the receiver. The transmitter is a single white LED driven by a 1 MHz clock signal.

Below are some graphs i plotted in Matlab to visualize component selection based on the CN0272 circuit note by Analog Devices




Feedback resistor value effect on bandwidth.



18 comments:

Ahemed said...

Dear sir i have recently found your blog and then i sent FB request to you.... I have taken VLC as a interested area and i took a project batch for work on that...I want to start from the first step so i recently ordered the components which you used in your circuit...waiting to get the delivery. And also waiting for your response please kindly acknowledge me. And give a part for sharing your knowledge in this area. Thankyou.
ahmedali@sasi.ac.in

Rishi said...

Sure...what would you like to know ??

Ahemed said...

Thank you sir for the reply...I am planning to implement the circuit which is shown in the "Visible Light Communication Chronicles Part II" page without an arduino kit...Just directly connecting the audio signal to the MOSFET.

Rishi said...

what mosfet are you using and how are you giving it audio input ????

Ahemed said...

PHOTODIODE,BP W34
MOSFET,2N7000TA
Cree,LED,C513AMSS-CV0Y0132

Ahemed said...

The transmitter figure shown in "Visible Light Communication Chronicles Part II" ill use audio input. Directly from 3.5mm jack connected to mobile/laptop at one end other end is sleeve less one wire is at input terminal and other wire at ground sir.

Ahemed said...

If it is not possible please let me know thank you...

Rishi said...

I see that the gate threshold voltage for your fet is 2.2V (max). If you are driving the gate directly with the audio signal your signal will clip below 2.2V assuming you have offset it appropriately. Besides why use a switching device for this ? Please check this link - http://electronics.stackexchange.com/questions/66332/mosfets-or-bjts-for-using-as-switch-for-audio-signals

Ahemed said...
This comment has been removed by a blog administrator.
Anonymous said...

please do not mention mail-ids on posts

Ahemed Ali said...

i did one circuit sir im sending to you sir

Ahemed said...

Hi sir i was waiting for your response ...thankyou

Dovlet Durbayev said...

I am also doing this project as final project... can u plzz mail me the details or send me circuit diagram plzzz durbayev73@gmail.com Thank you very much..

Naira said...

I am interested in doing this project as a final project, please mail me the details of the circuit diagram at: always.naira@gmail.com .. thank you so much

Mahmut said...

I am interresting this project. we can try different thing. Can u please mail me the details or send me circuit diagram plzzz mahmutd@yandex.com

Thank you very much

Niraj Khadka said...

Dear Sir, i have my final year project choosen as VLC. I have done the transmitter oart using max3967a SFP led driver IC, which is used to drive the LED. But i am working on with the recieving part. I have already got the ad8561 and ad8651 chips on hand and i am thinking on finding a detail on how you implemented the project according with the receiveing program.
My main concern here ia the impedance and the maximum operating frequency of the reciever ckt.
The effect of photodiode capcitance on overall performance. Please guide me how you tackled on these things. The other concern is how are you achiving the data transfer from the PC to the teensy for more than 1Mbps. Please guide me through. you can reply me on at console1913@gmail.com
Thanking you.

Niraj Khadka said...

and btw, i have not got the hamamatsu diode but a od-1wb model. It has a 2pf capcaitance at 10v reverse bias mode and worst rise time of 15ns so basically if we take equal rise and fall time the maximum possible operating frequency is 1/(15+th+15).
where th is the holding time for the level. we can put th=0 for maximum frequency..
So basically the frequnecy we can operate the photodiode is 33.3333 MHz.
Well that means considerin g the 50 mhz bandwidth of the opamp and the 7ns comparision time of the comparator i can get good results.. Now what i need to know is how to lay the ckt so that i have a controlled impedance line for the photodiode and lowest possible inductacne and capacitance of photodiode to the opamp.. Please guide me through.
Thanking you once again.

Abderrahim Jebali said...

hi sir i'm interested by this project. i'm student at the university of New Brunswick , Canada.
If you can provide me please with the final circuit for comparison purpose. that's will be appreciated.

Best Regards