Poor Man’s 1-GHz Active Probe: DIY Saves a Pretty Penny （2）

Published:2011/8/3 3:30:00 Author:Amy From:SeekIC

By David Jewsbury

Construction

The PCB artwork is shown in Fig­ure 2. The board has been designed to allow fitting in a metal tube. All the components are surface mounted (SMD), but assembly by hand is rea­sonably easy with a fine tipped sol­dering iron and tweezers. The parts are assembled on one side of a dou­ble sided 1.6-mm thick PCB. Connec­tions are made between the ground plane on each side of the board with solid wire soldered on each side. Fly­ing leads take the power to the probe and a length of coax ending with a BNC plug take the output to the test instrument. Heat shrink coax is used to strain relieve the leads. The RF and Ground probe are made from steel pins filed to a point. Pins ’borrowed’ from the family’s clothes repair kit are excellent.



Testing and use of the probe

After connecting the probe leads to a suitable power supply, the probe should draw between 10 and 30 mA. If all is in order, connect the probe to a spectrum analyzer. Applying an RF signal to the probe should result in an output seen on the spectrum analyzer. To get accurate results it is important that the ground probe contacts an RF ground close to the probed point on the circuit. It is also important to hold the board by the edges to prevent stray effects from fingers on the circuit. If the imped­ance at the probed point is 50 £), then the peak on the spectrum analyzer should be about 20 dB less than the power at that point in the circuit. Commercial probes were notoriously sensitive to electrostatic discharge, but seem more robust these days. Although Tl has internal diodes to protect against ESD it is wise to take normal precautions against unwanted static, while using the probe, as for any sensitive electronics.

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