Just a few clarifications for the never ending probe topic and especially my tests for the SDS1000X-E.
1. Source impedance
Of course the tests have been performed with 25 ohms source impedance, as explained in reply #59, where I quoted from the original thread where the idea for this test was born (replies #520, 521):
https://www.eevblog.com/forum/testgear/siglent-sds1104x-e-in-depth-review/msg1434847/#msg1434847As everyone can see, reply #521 even contains circuit diagrams representing the test setup.
One could also expect that using the wrong source impedance would usually mean just 50 ohms – and using a source impedance higher than 25 ohms would actually lead to worse performance figures.
2. Differences in frequency response between PP510 and PP215 below 100 MHz
This doesn’t exceed 0.66 dB (at 40 MHz) - not very significant for a cheap oscilloscope probe indeed. It might be because of slight differences in the LF-compensation (because of the fiddly trimmer capacitors).
3. Test with a 200 MHz DSO
Of course. The test was about appropriate probes for the SDS1200X-E.
I’ve also tried to explain that a probe has to be matched with the actual input impedance of the DSO in question, which in turn is a lot more complex than just 1 megohm in parallel with a few picofarads. My tests showed that the 300 MHz rated probes for the e.g. previous SDS2000X do not perform well on the SDS1000X-E machines.
For the ones who still like such demonstrations, I’ve posted the frequency response of the PP510 on a 1 GHz SDS5104X, see reply #191 in this thread:
https://www.eevblog.com/forum/testgear/siglent-sds1104x-e-in-depth-review/msg3290546/#msg32905464. No significant difference between 100 and 200 MHz rated probes?
I’ve tried to explain it already in the above posting:
… if you build a passive probe properly, with the correct HF compensation for the scope input it's intended to be used with (or adjustable HF compensation like e.g. the TesTec TT-MF312), then you can expect a probe bandwidth of several hundred MHz and you would have to deliberately add internal filtering to limit the bandwidth to e.g. 100 MHz.
In other words: it would take either the intention of a deliberate bandwidth limit or a serious amount of incompetence to design a probe that does not perform up to at least 300 MHz. I also talked about former tests, where a humble 100 MHz Tektronix probe outperformed the 500 MHz Keysight probe on a SDS2304X.
People should understand that probe specifications are only valid in conjunction with the scope they are intended to be used with.
Furthermore, people should understand that the probe together with the scope input is a complex and severely mismatched system from the outset and the frequency response can be anything (and certainly needs not resemble a lowpass behavior).
Finally, it should be clear that specifications just tell you what the manufacturer is willing to guarantee; there is absolutely no implied information what the performance outside the specifications will be.
5. What does all that mean for the SDS2000X plus?
I once measured the frequency response of my old PP215 probes on the SDS2354X plus (with the 500 MHz option). The -3 dB system bandwidth was about 420 MHz. From past experiences, I expect the PP510 to give similar results.
Probing techniques are important and it is indeed rather doubtful that passive “high Z” probes will cut it at high frequencies – at least in cases where the source impedance is considerably higher than 25 ohms. From this point of view it’s hard to understand where all the worries about the probes come from.