I decided to reply here to leave the fine demo thread alone.
Thank you very much for considering this, though I don’t see this as off-topic. Nowadays, where even entry level oscilloscopes offer bandwidths >200 MHz, the topic of probing / input matching is very important.
When I got my scope, I was mainly interested in HF frequencies and below. HF is 3 MHz - 30 MHz. So, 30 MHz and below. Will using the 1M inputs with the 10X probe or the BNC-T with 50-ohm termination suffice with my SDS814X HD?
My test shows that the BNC-T with end terminator reaches a VSWR of 1.5:1 at ~78 MHz. Fortunately, a VSWR of 1.5:1 only means 2% amplitude error (-0.17 dB).
At 30 MHz, the error should be just 0.32 % (-0.028 dB).
The 10X probe on a 50 Ω dummy load would be an alternative, and the capacitive loading could be slightly less (which reduces the error even further), yet I do not recommend this. Passive High-Z probes introduce additional errors because it proves difficult to connect them to a bigger test object (dummy load with some power handling capability) without additional parasitic elements, mainly inductances – especially the ground connection. And then there is the divider ratio, which might already be off by 2%.
Maybe a year or more I could consider a better scope that has 50-ohm inputs, but I was fortunate to get the Siglent SDS814X HD when I did. Does a SDS1000X HD have 50-ohm inputs?
Yes, the SDS1000X HD have 50 Ω inputs.
For a 50-ohm input, what would the maximum power-level you would be willing (not what the specification says) to measure? I ask because you never want to get near the maximum power handling capability of a device - you always want to leave room.
A sensible limit would be 2.5 Vrms (125 mW). The lowest sensitivity on a DSO with 50 Ω input is 1 V/div, any higher V/div values are not available. Consequently 7.07 Vpp is a sensible maximum for the input signal amplitude.
Specification permits up to 5 Vrms (500 mW) and I think the internal termination is actually rated higher than this, but the signal would exceed the screen height, hence wouldn’t be measurable anymore and excessive heating would degrade accuracy.
Or what about the normal 1M input using a standard 10X probe? Not what the spec says, but the max you would be willing to measure? Or the max using a BNC-T with 50-ohm termination?
Since all Siglent 10X probes can handle at least 300Vrms you should be able to measure up to 1.8 kW – provided you have a suitable oil-cooled dummy load that doesn’t release the magic smoke when such power levels are thrown at it.
For even higher power levels, there would be 100X and even 1000X probes (not available from Siglent) that can handle even higher voltage (=power) levels.
As stated before, I would not recommend measuring across a dummy load using a passive high-Z probe, whenever alternative methods are feasible.
The BNC-T method is limited by the power rating of the connected dummy load. Since you need a dummy load that can be plugged directly on the BNC-T, the max. power rating will probably be limited to about 5 watts or so.
I have multiple 50-ohm fixed attenuators and a switched attenuator to bring the power levels down.
Yes, of course, the usual method is using a power attenuator, that can handle the required amount of power and outputs an attenuated signal of a few watts, which can be further attenuated to match the maximum power handling of the 50 Ω inputs or any alternative solutions.
The attenuators will naturally introduce inaccuracies as well, these will usually have even higher impact than a not ideal VSWR. But they can be eliminated by error analysis and -correction, whereas errors due to a bad VSWR are increasingly harder to handle with longer cable connections. Therefore, a suitable attenuator of at least 10 dB directly at the oscilloscope input will limit the possible errors (caused by VSWR) to the rather benign figures calculated a few paragraphs above.
How hard is it for manufacturers to include 50-ohm inputs?
There are several different approaches. In early times and with low bandwidth oscilloscopes, a simple 50 Ω resistor connected in parallel to the input path by means of a relay was quite obviously an acceptable solution, even though it could be hardly any better than the external solutions that I’ve shown in my posting.
Serious instruments make a little more effort to provide a proper 50 Ω input path without the input capacitance of the 1 MΩ inputs.
In any case there is an additional relay per input – something that can be significant for a low-cost instrument like the SDS800X HD – all the more so as Siglent doesn’t give up some minimal quality standards, like a two-stage input attenuator, which already requires an additional relay, when compared to a far inferior single stage attenuator design, which other manufacturers seem to prefer in their low-end instruments.
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