PCBite 200Mhz Probes and Rigol DS1054z

[kelemvor]

Greetings,

I saw a recent video on the eevblog where someone sent Dave the PCBite 200Mhz oscilloscope probes and thought they were awesome!   I've got a riglol ds1054z scope.   I ordered the probes and they arrived yesterday.  Opened the box and found the full color advertisement even showed them being used with my very scope!  Great!

When I got to the specs, I noticed they are rated for 10MΩ and 15-45pF.  The scope is clearly labeled 1MΩ 13pf on the inputs.

The probes seem to work okay in my initial testing.  Should I be concerned?

[pigrew]

This specification describes the range of adjustment of probe compensation guaranteed by the manufacturer. Since your Oscilloscope is outside the range of the probe, you might not be able to properly compensate it to achieve flat frequency response. Because of manufacturing tolerances, the probe should be compatible with a slightly wider range of capacitances then advertised

Connect your probe to the square wave terminal of your oscilloscope, and check that the square waves look square. If you are able to adjust the probe's trimmer capacitor to make the square waves look square, then the probe is perfectly fine to use.

[Hydron]

I had this same concern - my RTB2004 is even lower with a 9 or 10pF specification (LCR meter says more like 10.5). Another forum member kindly checked the adjustment range and it seemed like it went beyond the spec so I bought them and sure enough, works very well with my scope inputs.

A bit annoying though that the spec doesn't match with reality and causes unnecessary concern (also, who has 45pF input C on their scope?).

[kelemvor]

Yeah I thought that's maybe something high end Techtrox or Rohde & Schwarz might have. I wouldn't know lol.

Thanks for the help, guys. The PCBite probes are working great on my lowly Rigol.

[pigrew]

Yeah I thought that's maybe something high end Techtrox or Rohde & Schwarz might have. I wouldn't know lol.

It's quite the opposite: High-end scopes will tend to have lower capacitances.

My older scope (Tek 2335) has 20 pF inputs, and if you browse TekWiki, you'll find scopes from the 50s and 60s which have inputs with ~40 pF capacitance.

Most modern scopes I use have inputs between ~10 pF and ~14 pF, but switchable to 50 ohm (with negligible capacitance). The really high-end scopes have only have 50-ohm inputs (as 1 Mohm inputs are bad for high frequencies).

My speculation is that old oscilloscopes used larger transistors (or valves) which have large input capacitances. Also, these mostly were used with lower frequencies (<20 MHz BW) so there was little reason to design lower input-capacitance inputs.

More speculation: Because the input capacitance of transistors varies with temperature and part-to-part, a discrete capacitor (often a trimmer) is added at the input so that the input capacitance will be more similar between ports and across temperature.

[kelemvor]

Kind of makes me wish Dave would make a video on the subject.  I'd like to understand it better.