Seb Holzapfel

Seb Holzapfel

Hardware & Software Engineer

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A 2GHz, 2-port VNA adapter for SDRs


I thought it might prove an interesting challenge to attempt to use an off-the-shelf SDR (LimeSDR mini in my case), and build a 2Ghz-capable RF frontend for it to allow it to neatly do full 2-port characterizations of amplifiers and filters, like a true VNA, without having to wire up a bunch of external couplers.

What it looks like

The PCB I designed sits between the LimeSDR Mini and the device one wants to test:

Hardware overview

At a high level, the board has the following:

  • Directional couplers for return loss measurement on both ports.
  • A switch matrix to allow
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KiCAD Workshop (ELSOC '17)

(This workshop was given @ UNSW on 1 Sept. 2017. If you find any errors in this text, feel free to file an issue here )

Finished PCB

What You'll Need

  • A computer (Windows/Mac/Linux)
  • A 3-button mouse (Useful, but not necessary)
  • Motivation to build the best darn PCB in existance

Prelude - Installing KiCAD

Grab it here:

There are binaries available for all major operating systems.

Special note - Linux users

If you are using Linux, you may additionally want to install kicad-library & kicad-library-3d as these aren't included by default.

Creating a Project

  • Fire up kicad.
  • In
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Modular Synth VCO Design & Build

Needed an excuse to (finally!) learn KiCAD, so I decided to build a voltage controlled oscillator (sawtooth core). It's inspired by the early Elektor Formant synths, and designed to integrate with existing Eurorack synthesizer systems.

In the video I'll also talk about how I breadboarded up a MIDI to 1V/octave converter to go with it for keyboard/computer control.

See YouTube description for timecodes.

KiCAD design files:

Code I wrote for the PADI IoT stamp in the video for MIDI/CV conversion:

Hadn't originally planned on publishing

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LCD Upgrade for the HP 141T Spectrum Analyzer [Part 1]

In this video I demonstrate my progress so far on building an intelligent LCD display upgrade for the HP 141T.
I use an STM32F746G discovery board with a 4" touchscreen, and demonstrate a few neat features:

  • Real-time spectrum display with graticules etc, that works on the slowest up to the fastest timebase
  • A waterfall display mode
  • A normalization function that makes insertion loss measurements much easier.

I also take a brief look at the codebase (which is very hacky at the moment!)


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Rapid Prototyping RF Filters with Tape & QUCS

Was lucky enough to get this featured on Hackaday! See: Rapidly Prototyping RF Filters

A guide to simulating microstrip filters in QUCS and prototyping them with copper tape on blank FR4 sheets. These super-cheap materials are good up to a couple gigahertz, as you'll see in the video.

Following a quick real-time demonstration, I simulate and build 2 microstrip filters, a 1GHz stepped-impedance LPF, and a 10GHz end-coupled BPF.

(Checkout the youtube description section for active timecode links)

2:05 - 1/4 wavelength stub build & tests
3:46 - Radial stub build & tests
4:18 - Stepped

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14GHz Mixer Build & Experiments

Building a mixer based on the LTC5548 14GHz MMIC by LT, and a TCM1-83X+ by Mini-Circuits [plus bonus bodge wires ;)]
After measuring LO leakage, conversion loss and IIP3 at different frequencies, I build a poor-man's tracking generator with it.
I also talk (perhaps a bit too much) about HP's old signal identifier feature on my 8555A.

Thanks to LT and Mini-Circuits for providing samples of the components used in the video, I've definitely got bigger plans for these parts!

Design files are available at my other web page on this:

An article I did on the

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Vintage Crystal Oscillator Teardown & Experiments

Taking apart an old NDK 48MHz ovenized crystal oscillator from the 80s, making some measurements; and taking macro shots of the quartz.
I make some initial frequency measurements before taking it apart, and then after ripping out the crystal I run it through a tracking generator to look for resonance peaks.
Interestingly the crystal seems to operate at a slightly different frequency to what's written on the OCXO packaging, and the 'big' crystal package actually has a small one buried inside!

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