... I did find this video useful for bandwidth info.. =)
Yet this bandwidth-demonstration is rather flawed in all possible ways. The good news is that the actual probe bandwidth will most likely be even higher than the claimed 215 MHz.
1. The signal source should be a levelled signal generator with a amplitude flatness of ideally better than +/-0.5 dB. The tracking generator of a cheap Tiny SA (where the frequency response hardly matters, as it will be normalized before each SNA measurement anyway) hardly qualifies as a trustworthy signal source for bandwidth measurements.
2. The signal source has to be terminated into 50 ohms, i.e. there has to be a 50 ohms inline terminator between generator and probe, making the total source impedance 25 ohms. From the video, I cannot clearly see if a terminator is present, but if not, then this means a -3 dB point of the signal source at about 270 MHz already. With the industry standard 25 ohms source impedance, it is twice as high.
3. What has been shown in the video, is the system bandwidth of a SDS2505X Plus together with the probe – not the probe bandwidth. Even at only 200 MHz, there will be small deviations in the frequency response of a “600 MHz” (actually 570 MHz) oscilloscope.
4. Actually, the probe builds a unit with the input buffer of the DSO-frontend and we always need to look at the combination of the two. Since the frontend of an SDS2000X Plus is clearly different to the SDS800X HD series, there’s o guarantee that the performance of the probe will be the same in both cases.
5. The shown way with pocket calculator and level measurements at just a few frequencies is a rather clumsy and inaccurate one – especially when taking the low-quality signal source into account. Why not take advantage that these scopes (other than some competition) do have a usable FFT with Max Hold and Average modes and correct window functions, to present the measurement in a precise and comprehensive way?
If one doesn’t have the equipment to meet the requirements listed above (usually the biggest challenge will be a proper high quality signal generator), the best approach to get meaningful results is comparing a direct coax connection with the probe. This way, the frequency flatness of the signal source becomes less significant.
We would start with a direct connection (using a high quality low-loss coaxial cable as short as possible) between signal generator and 50 ohms oscilloscope input. There should be an at least 10 dB inline attenuator between generator output and cable, in order to improve the VSWR, hence the ripple in the response curve, which is usually rather bad for most (even expensive) signal generators.
Here the first problem arises with an instrument that doesn’t provide 50 ohms input impedance. The next best thing is to place an inline terminator at the scope input, but then we have 50 ohms in parallel to about 22 pF instead of pure 50 ohms, which will naturally have a negative impact on the frequency response.
We plot the frequency response using FFT in Max Hold mode and store the result as a reference waveform. Then we perform the probe test as described earlier in this article, i.e. the inline terminator is moved from the scope input to the generator output. The attenuator is now removed (if we had used a 20 dB attenuator for the direct coax measurement, we would now get the exact same levels without attenuator using a 10x probe). Then we can compare the two curves and determine how well the probe performs over the entire frequency range.
All this has been explained numerous times, also in this very thread, if one can be bothered to use the forum search and look it up:
https://www.eevblog.com/forum/testgear/siglent-sds800x-hd-12-bit-dsos-coming/msg5403110/#msg5403110In fact, the following link contains all that’s needed to know:
https://www.eevblog.com/forum/testgear/sds800x-hd-review-demonstration-thread/msg5351024/#msg5351024The measurements for the SDS824X HD have been carried out with an old 100 MHz PP510 and a 500 MHz PP5050A probe, using a proper signal source with better than 0.3 dB flatness. We can see that the frequency response actually begins to drop at about 120 MHz, but only ~1.5 dB, and then stays relatively flat up to some 270 MHz.
“270 MHz isn’t much more than the 215 MHz from the video, and this is a 100 MHz probe instead of just 70, some might think. The main fact is that this bandwidth has been reached with the SDS824X HD, not an SDS2504X Plus. The SDS824X HD has about 250 MHz bandwidth when using a direct coax connection, so the 100 MHz PP510 actually extends the system bandwidth. Compare this with the claims from all those textbook-afficionados, who will insist that the combination of a 100 MHz probe together with a 200 MHz scope will give you a system bandwidth of just 90 MHz…
I firmly believe that the 70 MHz probe will behave nearly identical (the PP215 certainly does). In general, a probe with higher bandwidth rating will not give much of an advantage, especially when considering the limited usability of high-Z passive 10x probes at frequencies above some 100 MHz (due to high capacitive load). If you have such a low impedance circuit node, it is still better to make this a 50 ohm test port and use a direct coax connection once again. If the node is high impedance, then you’re out of luck and might need an active probe solution. Best way is to design proper 50 ohms test points into your circuit right from the start.
The 500 MHz PP5050A performs barely better than the cheap PP510. The 1.5 dB drop at 120 MHz is there as well (so this might be a property of the frontend) and the -3 dB bandwidth is only insignificantly higher at ~290 MHz.
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