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How to Build a 1 GHz Scope Probe for Next to Nothing

Sparty's Favorite Recipes 015 Kiss Your ASIC Good-bye!
       

Instructions

Oscilloscope bandwidth is expensive, don't reduce it further with a slow probe. Expensive fast scope probes have high input impedance, which we do not need when looking at digital IC outputs. Oscilloscopes have more gain than we need for digital signals. So, let's sacrifice input impedance and voltage sensitivity, and save money.

Take a 3-ft piece of RG174 50-Ohm coax cable with a 50-ohm terminating BNC connector on one end, attach a 950 ohm (or 1 kilohm) small resistor to the other end and use a very short ground connection. Keep the ground-loop shorter than 0.5 inch.

Rise Time Budget  
Ground loop (half-inch diameter) 0.17 ns
3-ft coax 0.05 Ns
BNC connector with 50 Ohm termination 0.01 Ns

Total rise time of 0.2 Ns means a bandwidth of 2 GHz. Input impedance 1 kilohm, input capacitance < 2 pF, attenuation 20:1.

 

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