Photodiode Amplifier Design

I recently designed an infrared sensor board (dubbed “IRis”) for my friend’s Defcon talk. This video walks through the circuit design of the photodiode amplifier, and discusses some of the pitfalls associated with photodiode amplifier design.

Schematics, BOM, and KiCAD design files for the described IRis board are available on github.

References and additional reading:

Description Reference
The bible of photodiode amplifier design Photodiode Amplifiers: Op Amp Solutions, Jerald Graeme
Good overview of photodiode design concerns Common Photodiode Op-Amp Circuit Problems and Solutions, Digi-Key
Excellent photodiode amplifier reference design Photodiode Amplifier Reference Design, Texas Instruments, John Caldwell
Bob Pease’s musings on transimpedance amplifiers What’s All This Transimpedance Amplifier Stuff Anyhow?, Bob Pease
JFETs as ultra-low leakage diodes Current Sources and Voltage References, Linden T. Harrison, Chapter 6.6

RC Filter Design Basics

An introduction to passive and active low pass filters, filter requirements and design concerns, and the evolution from a simple single-pole RC filter to a multi-stage Sallen-Key topology.

Description Reference
Single and dual pole passive RC filters Low Pass Filters, ElectronicsTutorials
Qualitative and quantitative Sallen-Key analysis Analysis of the Sallen-Key Architecture, Texas Instruments
Sallen-Key filter design and operation Sallen-Key 101 Whiteboard Series, Matt Duff, Analog Devices

Quadrature Mixers

I’s and Q’s and negative frequencies, oh my! Today we discuss mixers and frequency conversion, in particular, quadrature mixers and Tayloe detectors: what they are, how they work, why you might want one, and what do we use all this I and Q stuff for anyhow?

Additional references:

Description Reference
Introduction to RF mixers Basics of RF Mixers in Radio Receivers, Alan Wolke (W2AEW)
Mixer theory and switching mixer operation Downconversion Mixers, Gino Giusi, Ph.D
Introduction to IQ signals and phasor diagrams Basics of IQ Signals and IQ modulation & demodulation – a tutorial, Alan Wolke (W2AEW)
IQ signal processing fundamentals, Euler’s identities A Quadrature Signals Tutorial: Complex, But Not Complicated, Richard Lyons
IQ phase relationships at positive and negative frequencies IQ Modulation, Keysight Technologies
The Tayloe quadrature detector Ultra Low Noise, High Performance, Zero IF Quadrature Product Detector and Preamplifier, Dan Tayloe

Amplitude Demodulating BPSK31

BPSK31 has become a very popular digital communication mode among amateur radio operators due in no small part to its efficiency and narrow bandwidth. You can easily demodulate BPSK31 signals with readily available DSP software, or even using an Arduino.

Never the less, I thought it would be fun to build an analog BPSK31 demodulator that would output digital bits via simple two wire bus, and hit upon the idea of doing this relatively easily using amplitude demodulation.

Amplitude demodulating phase modulated data??

Amplitude demodulating phase modulated data??

I know, I know, a proper PSK demodulator would implement a costas loop or similar phase detection circuit. This approach is a hack for sure, and won’t work as well as a proper phase demodulator, but has the advantage of being easily constructed with a hand full of “jellybean” components that are probably already sitting in your parts bin.

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