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.

Visible Light Communication Chronicles - Part IV - The Photodiode

In this post i will compare the new ambient light sensor that i got - the TEMD 6200FX01 by Vishay to the TEMT 6000 phototransistor [PT]. The TEMD6200FX01 is a PIN photodiode [PD]. More information can be found in its datasheet here. The circuit i hooked up with the PD is shown below.

The PD is reverse biased with a voltage of 3.3V and the output is taken across the 10K load resistor. Looking at the reverse voltage vs capacitance curve from the data sheet i approximated that the diode capacitance will be around  30 pF. The load resistor and the capacitance will determine the cut-off frequency and the time constant. Based on above values these turn out to be as shown below

Hence the cut-off frequency happens to be approximately around 530 KHz with this configuration of the PD.

The LED was modulated with different frequencies and the light was focused on both the PD and the PT. The below waveforms were observed on the scope. The orange trace is the PT output and the blue trace is the PD output. (In case of the PT the circuit is same as before and the output is obtained across a 10K resistor). It was pointless to go beyond 10KHz as the PT output faded away.

100 Hz square wave

1 KHz square wave

10 KHz square wave

The board i designed for mounting the TEMD 6200 0805 SMD so that i could breadboard the test circuit

In the next chronicle i will be hooking up the PD to a transimpedance amplifier...