Passive Probe With Spectrum Analyzer

Is it possible to use a regular passive oscilloscope probe with a spectrum analyzer?

If not, what are possible choices of probes to use?

• Analyser regular inputs are nomally used with adapted 50 Ohms (or other) impedance line probe. But one can use, in some cases, also capacitive or inductive probe for measuring E or H for example (slotted line, antennas). Passive oscilloscope probe have a "large" capacity and ... resistor included ... there are not very usefull in such case.
– user288518
Jun 21, 2021 at 7:12

You can build a 30:1 voltage probe, using 1450 ohm and the 50 ohm of the SA. This is a standard voltage divider for which, if you want to go high frequency, you need some capacitive compensation for the unavoidable stray capacitance of the 1450 ohm arm: best off if you then build up the 1450 ohm as several series-connected smaller resistors and the overall stray capacitance will be reduced (besides handling higher voltages and higher power dissipation).

You get a good combination taking 2x 330 ohm and 2x 390 ohm. You may then fine trim to 1450 ohm.

One thing to add is a series capacitor to cut off mains voltage, so that this probe operates at about 10 kHz onward. 22nF at 5 kHz will make about 1500 ohm, so that at 10 kHz you will have half of it and the 1450 ohm resistive should prevail. At 50 Hz the reactance will be 150k isolating the mains quite effectively. A good capacitor with 1 kV rating should be used for safety (of your SA!).

simulate this circuit – Schematic created using CircuitLab

You can then house everything in a plastic pipe with a thin metallic foil as electrostatic shield, and then insulated again, e.g. by thermal shrinking tube.

It works and it's sold maybe as high as 1000 \$; honestly you can do a job as good for much less, and you learn something.

Be careful when calibrating with a tracking generator or VA because you have additional 50 ohm in series with the 1450 ohm.

Try and simulate it for very high frequency (e.g. 10 MHz and above) including parasitics, so to understand if you need compensation. Since the 50 ohm is inside the S.A., the you have a piece of cable between the 1450 ohm and the 50 ohm, that must be included (additional capacitance!)