Electronic – Small ICs pitch and Oscilloscope probing

It would appear that LeCroy follow Agilent/Keysight in this respect (or, at least, Tektronix's presentation of what Agilent's method is). This can be seen from their probe manuals, for example for the ZS4000 active (single-ended) probe. They provide the probe impedance as a function of frequency and advocate that the user corrects for it when interpreting the measurement, using the formula:

$$V_\mathrm{out} = Z_\mathrm{probe}/(Z_\mathrm{probe}+Z_\mathrm{source}) \times V_\mathrm{in}$$

I avoid quoting further from their manual to avoid potential copyright issues (because it would require the whole section to be quoted to reproduce it properly here), but if you follow the link and read the manual, you will find that everything is quite clearly stated.

For the differential probes operating in the 10 GHz range (for example, the WaveLink D1030), their approach is slightly different to either of the ones presented in the Tektronix technical brief. The probes measure the loaded signal, as per Agilent, but they provide equalization software (Virtual Probe) to recover the unloaded signal. One models the circuit impedances and indicates the type and location of the probe, and the de-embedding is done accordingly. They summarize it as follows (quoting from the WaveLink probe manual):

Teledyne LeCroy probes are calibrated at the factory using a Vector Network Analyzer (VNA) to measure a system (probe plus test fixture) frequency response. The test fixture is de-embedded from the measurement using Teledyne LeCroy's Eye Doctor tools so the remaining frequency response is due to the combination of the test signal and the probe loading on the test circuit. The system frequency response is then calculated for these remaining circuit elements.

If you wish to de-embed the effect of probe loading on your circuit, you can use the appropriate equivalent circuit model ... and Teledyne LeCroy's Eye Doctor tools to accomplish this.

You can also use Teledyne LeCroy’s Virtual Probe option. This option allows you to select the probe tip from a list of supported tips. Your selection applies a corresponding s-parameter file that is derived from the equivalent circuit model of the tip.

However, I haven't actually used these probes, so I can't comment on how good the software is.

Most oscilloscopes have a fairly similar input impedance that fall within the adjustment range of 3rd party probes. Typically something like 1M\$\Omega\$ 20pF.

Here are the factors one supplier (TPI) mentions:

Several important factors must be taken into account when selecting the proper probe.

• The probe should have sufficient bandwidth and rise time for the test instrument and application. Choose a probe with at least an equal bandwidth as the scope it will be used with. For best performance a probe with twice the bandwidth as the scope should be selected.

• For oscilloscope probes, the input capacitance of your oscilloscope should be within the compensation range specification of the probe. In addition, if your oscilloscope has readout function, select a probe with this capability.

• For differential probes, make sure the maximum differential voltage is adequate for your application and the common mode rejection specification meets the requirements of the tests being performed. Refer to the oscilloscope and differential probe specification tables to select the correct probe for your application.

I would not bother with "twice the bandwidth" for an inexpensive used scope.

Typically you're going to want one that has 1:1 and 10:1 settings (ref setting is "nice to have"), an BNC to fit your oscilloscope and a tip that matches what you're planning on doing with it (usually a grabber that can be removed).

Usually the oscilloscope will have the input impedance marked right on the front near the BNC connector. You should ensure that the resistance is the same as the proposed probe is designed for and that the capacitance is within the compensation range of the proposed probe (eg. 10 ~ 35pF, which would include 20pF).

New 3rd party probes good for 100MHz will cost from $15 up, probably you can get good ones for $35-ish.