Saturday, 18 February 2017


I have been using the constant current version of my LDBiasDriver and I felt that it would be handy to be able to control the current, monitor it, etc. via a PC. To this end, I designed and have so far had the PCB manufactured for my "LDBiasController".

This new board provides a variable resistance to two LDBiasDrivers as well as monitors the current using an Arduino Nano. I plan to create a serial protocol to do the control and monitoring but I will create a MATLAB or C# based GUI.

In terms of specifications, initially I was going to use a digital potentiometer but the cost of versions with more than 256 steps was prohibitive. I considered (and even designed) a version with two pots to create about 16 bits of resolution but it would have been non-linear and thus hard to control accurately. In the end, I went with the MCP4726 Digital to Analog Converter (DAC) which provides 12 bits of resolution from 0 to 2.5V.

I also wanted an external input capability which can be switched in (I used a 4066 IC for this) to modulate the laser drivers at up to 1 kHz (10 kHz is possible with higher noise version of the MLD203 drivers from ThorLabs). There is no way to guarantee or even sometimes generate a 0-2.5V external signal, so I opted to put a digital pot to attenuate this signal if need be. I am able to switch the output of the DACs through the digital pots too which further enhances the resolution to about 20 bits (I hope)!

Here's the GitHub link to the design files:

As a side note, I had these manufactured at Elecrow. I'm super impressed with them as they added slots to the DC power jack on the board on their own accord saving me a huge potential headache in future!

Thursday, 9 February 2017

Simulink 2x1 MIMO Channel Estimation Test

I have been working on a MATLAB Simulink based Alamouti testbed for USRP software defined radio. I am using the Ettus B210.

I have implemented a very simplistic channel estimation scheme whereby I transmit each of the four QPSK constellation points on each antenna consecutively. I then receive using a single antenna, and after all of the frequency and phase synchronisation I divide what was received by the ideal constellation leading to a simple H-matrix.

Check out this video where you can see the pilot constellation change as I move the antenna! Awesome!

Home Assistant Hue Emulation

My Home Assistant setup uses nginx to proxy the web interface to HTTPS. I bought a Google Assistant for black friday and I want it to be abl...