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Banggood 50 ohm BNC feed-through terminator - a quick review
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TopQuark:
I have the Rigol ADP0150BNC, it is quite nice.

They advertise it to be good to 1GHz. Indeed it measures well at that frequency, with 1.2 VSWR at roughly 1.4GHz. Although I don't think anyone should be using an external terminator at that frequency.  :P

tggzzz:

--- Quote from: TopQuark on April 21, 2024, 06:48:43 am ---Although I don't think anyone should be using an external terminator at that frequency.  :P

--- End quote ---

Ideally not, but a 3dB pad/attenuator can improve the effective VSWR.

Gain is cheap; flatness isn't :)
TopQuark:

--- Quote from: tggzzz on April 21, 2024, 08:46:36 am ---
Ideally not, but a 3dB pad/attenuator can improve the effective VSWR.

Gain is cheap; flatness isn't :)

--- End quote ---

Wise words. I measured the VSWR of various combinations of oscilloscope 1M input, oscilloscope 50R input, Rigol terminator, 3dB attenuator. The oscilloscope has 500MHz bandwidth, so realistically I'm only interested in that range.

Takeaways:
- Terminator by itself (S11 - term), and oscilloscope 50 Ohms input (S11 - scope 50) is good, not really a surprise.
- Oscilloscope with 1M input (S11 - scope 1M) is bad, as one would expect.
- Oscilloscope with 1M input, with external 50 Ohms terminator (S11 - scope 1M + term), is quite bad, as some members predicted in prior replies. The weird and wonderful 1M//16pF input channel network will cause significant mismatches. The terminator almost hurts as much as it helps at slightly higher frequencies.
- As suggested by tggzzz, placing a 3dB attenuator should add 6dB of return loss for signals looking into the scope, improving effective VSWR. This can indeed be observed with the terminator (S11 - scope 1M + term + pad) and without the terminator (S11 - scope 1M + pad).

TLDR:
- Scope built-in 50 ohm termination is the best.
- External 50 ohm in-line terminator only helps input matching at lower frequencies (< 100MHz), and it doesn't do a great job at that either. At higher frequencies even the best made terminators will not help oscilloscope input matching.
- Adding an attenuator will significantly improve input matching, probably should always be used with external 50 ohm in-line terminators.

TopQuark:

--- Quote from: mawyatt on September 07, 2023, 02:59:05 pm ---Somewhere I recall that using two 100 ohms chip resistors across for each other produced a better load than 3 or more resistors. Here a 2D field solver might be useful in getting the best configuration & layout.

--- End quote ---

The VNA bible agrees, two 100R 0603 resistors will work better than one 50R 0603.

I hope Joel Dunsmore doesn't mind me reproducing 3 pages of his book  :P
shapirus:

--- Quote from: TopQuark on April 21, 2024, 10:32:28 am ---- As suggested by tggzzz, placing a 3dB attenuator should add 6dB of return loss for signals looking into the scope, improving effective VSWR. This can indeed be observed with the terminator (S11 - scope 1M + term + pad) and without the terminator (S11 - scope 1M + pad).

--- End quote ---
This is interesting. We place the attenuator between the terminator and the scope input, right?

A handmade two-in-one terminator+attenuator feed-through adapter should be possible to make. It will require a PCB, though, I think. Hmm. I may want to try it. I'm puzzled by those overshoot and undershoot that I'm seeing (shown in the screenshots in my previous posts) and I can't yet find an answer to what exactly is causing them. I'm curious if the added attenuator will tame them without slowing down the rise and fall times (as slowing them down intentionally is a way to reduce or remove them).
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