Oscilloscope probe

The basic tool for measuring time varying electrical signal is oscilloscope and probe.
A 10x oscilloscope probe is a passive pickup that couples a 10 x attenuated voltage signal to the oscilloscope 1M ohm input.

The simplified probe schematic in fig. 2 explain how the tap ratio happens. Effectively it is a voltage divider 1M / (1M + 9M) = 1/10.
Commercial probes like the one in fig.1 has additional impedance matching components both in the probe and a variable compensation in the connector.

It is a simple circuit, but it is not so easy to design it for high frequency. Here I share a design for a probe PCB that serves well at a hands-free solder in probe. It is practical to place a probe connected to the circuit under test and have the hands free. Oscilloscope probe kits come with a hook. One option is the solder a wire and connect the hook to the wire. Another solution is to use a probe holder. There are several styles offered by oscilloscope manufacturers. They are prices quite high. The business proposition is that you get an extra hand in the lab.
With 3D printing you can make your own probe holder. Fig. 3 is an example, the legs are M6 bolts. The weight of the bolts adds stability to the holder. One bolt is extended with a Nylon rod for fixing of the oscilloscope probe, using the thread of the bolt. The .stl and Freecad files can be downloaded below.

The third option is the make a small PCB with the probe circuit and use that at the end of a coaxial cable and solder signal plus GND wires.
Most oscilloscopes have 2 or 4 channels. The optimal is to combine the different probing options. With 4 channels two solder in probes, one probe in holder and one hand held probe is a good setup.
The following details the design for a solder in probe. The idea is a PCB with probe wires that are soldered to the test points on the board under test. The probe PCB is connected to the oscilloscope with a standard coaxial cable.

Fig. 4 is the schematic for the probe PCB. The input connector J1 is through hole pads for soldering wires. R1 and R2 adds to exact 9M ohm for precise 1 : 10 attenuation for DC. The variable capacitor C1 (5.5-30pF suggested population) allow tuning of the temporal response. R3 helps to suppress high frequency ringing. For further optimization R4 and C2 can be used to tune for particular coaxial cable and oscilloscope. The connector selected for the coaxial cable is an SMB edge mount connector. Example of coaxial cable assembly that will work well is BNC to SMB Amphenol RF 095-850-236M075.

A rendering of the populated PCB is found in fig. 6. The board has the dimension 34.6mm x 13.6mm. The probe PCB with coaxial cable and wires soldered to J1 should be calibrated like other probes using the oscilloscope calibration source.

Files for download