Seb Holzapfel

Seb Holzapfel

Hobbyist | Student | Engineer

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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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JDS6600 Function Generator Review & Teardown

I take a detailed look at a cheap $100 function generator that came out recently, from JunCe Instruments.
Measuring amplitudes, checking harmonic distortion, making some funky waveforms, and seeing what's inside!
Looking at the 25MHz model in this review.
There are a few things I'd still like to test (PC connectivity and measurement capability for example)

I have posted some high-resolution images of the teardown over at the EEVBLOG forum here:
(As well as some additional tests)

Repeated here for convenience:

Top of the main board

Top of the screen board:

Bottom of

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14GHz RF Synthesizer PCB + experiments

About a month ago I was frothing over a datasheet for a (pretty expensive) PLL synthesizer by Analog Devices, the ADF5355. It's capable of spitting out signals from 50MHz to 13.6GHz - widest I've seen!. It's got a VCO parked between 3.4 and 6.8GHz and internal dividers & doublers. Naturally, I requested some free samples of the $100 AUD (!!) part, to evaluate it - might be useful in a future project.

I then designed a board to house it - for a few reasons. Firstly, great practice for RF routing. Also, a good opportunity to create part

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