DIY 1GHz Active Probe For Under 20$
So, you’re an electronics hobbyist and want a 1GHz* active probe, or a professional and tired of blowing your 2000$+ active probe in sketchy circuits when all you wanted to know are the whereabouts of some RF-signal?
Then stay with me!!
This Instructable will show you how to build a 1GHz* Fet-based Active Probe, the Fetprobe, for about 10$*, provided you have access to an electronics lab. It is based on an Elektor-magazine article (see the pdf’s addendum, section literature in my thesis) beside some other designs. However, as topic of my bachelor-thesis I wanted to find out how good these designs really are and how far one can push them.
If you go down this road, you need access to a lab that is equipped for some RF-fun, a cheap way for ordering RF-components and some rapid prototyping capabilities. Options some DIY-electronic-aficionados may not have. Although, my Bachelor-thesis comes with a lot of measured performance data, you will get pretty similar results if you stick to this tutorial.
In case you want to learn some more and don’t mind a more scientific approach than have a look at my thesis included as PDF-file in last step.
Equipment needed to build this probe:
- PCBs (512µm Rogers RO4003 w. 17µm copper dual-sided or similar) based on my gerber files
- SMD soldering gear and tweezers!!
- oscilloscope for debugging and testing
- DC power supply
- access to RF rated SMD component-kits (caps, resistors, inductors)
Components you need to get:
- bf998 dual-gate Mosfet(s) SC-61B 4pin
- RF-Rated 0603 caps around 1pF
- 10M 0603 resistor (bias)
- spring-loaded tips often called pogo pins (RF_in and GND)
- voltage regulator, i.e. LM317LCDR SOIC 8pin
- SMA or BNC connector (RF_out)
add. Equipment needed to design & develop your own probe:
- PCB mill, capable of SMD boards 125µm track-gap or similar etching process
- VNA (Vector Network Analyzer)
- swiss-army-knife RF software like AWR Microwave Office from National Instruments (layouting, spice-sim, RF-sim)
*: handles 1MHz-500MHz really well, usable up to 1GHz though (if you can accept a bit of dispersion and amplitude error, as shown in the S21-graphs in the last step
**: means you may need a membership in a laboratory, or acquire some people’s affection who do (i.e. via cold beverages). Also, the 20$ price tag suggests you only need to buy the special components, not the standard things which lie around in an RF-lab anyway.
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