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Banggood 50 ohm BNC feed-through terminator - a quick review
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shabaz:
Advance warning: apologies for adding more options to the list of possibilities!:

If it's for a 'scope that doesn't have an in-built 50-ohm termination, then a BNC T-piece can be used (attached at the 'scope end of course), along with a 50-ohm terminator. It's not great, but it should be adequate for a couple of hundred MHz (I did ask someone at Tektronix once, and they stated that was an acceptable thing to do on their MSO 2k series, which didn't have 50 ohm termination built-in). (And beyond a couple of hundred MHz BW, most 'scopes will have built-in 50 ohm termination anyway).

However, regardless, here is where a decent brand purchase makes a lot of sense (and it still won't be excellent unless you're even more selective since even branded BNC ones are often made to the operation level of old LAN terminators, I think!). On the other hand, Minicircuits, in particular, will have as decent BNC terminators as one can get for a reasonable price, e.f. BTRM-50+ is 11 euro at Mouser. That plus a T-piece would be an option anyway.

Yet another option (if a nano-VNA or similar is on the purchasing horizon one day) is to get a little SMA 50-ohm terminator, and an SMA-to-BNC adapter piece, and a BNC T-piece. That way, you only have to spend the Minicircuits-level budget on one part (the 50-ohm terminator) and you can use it with both the 'scope, as well as with typical SMA cables with the Nano VNA.

(EDIT: and if you're making your own terminator (or feed-through termination), then as @mawyatt says, two SMD 100-ohm resistors in parallel (but at 180 degree positions, i.e. not placed together) is probably the best DIY way, soldered directly on the connector, no need for a PCB unless you can keep the inductance low for the termination, unlike that really bad PCB. Mechanically it could be an issue (e.g. cracking), so you'd have to consider how you achieve such an approach, but electrically, it's a very good way. The photo below shows the 180 degree placement as an example (ignore that they are 200 ohm resistors in the photo, they were used for a different purpose, they should be two 100 ohm resistors). The epoxy was the attempt to reduce cracking risk.

tggzzz:

--- Quote from: shabaz on March 20, 2024, 04:26:47 am ---If it's for a 'scope that doesn't have an in-built 50-ohm termination, then a BNC T-piece can be used (attached at the 'scope end of course), along with a 50-ohm terminator. It's not great, but it should be adequate for a couple of hundred MHz (I did ask someone at Tektronix once, and they stated that was an acceptable thing to do on their MSO 2k series, which didn't have 50 ohm termination built-in). (And beyond a couple of hundred MHz BW, most 'scopes will have built-in 50 ohm termination anyway).

--- End quote ---

If your scope has a 1Mohm 15pF input, then adding a 50ohm terminator in parallel will result in a 50ohm//15pF input. That will, of course, cause frequency-dependent reflections.

Whether that is acceptable depends on the length of the cable (which determines the frequencies) and your application (which determines what you can tolerate).
shapirus:

--- Quote from: shapirus on March 20, 2024, 02:34:15 am ---So this Chinese thingy has a nice enclosure, but a poor PCB.

--- End quote ---
...and that's a single-layer board, too.
shapirus:

--- Quote from: shapirus on March 20, 2024, 02:34:15 am ---I think I'm going to make a proper board for it and then do a before/after comparison.

--- End quote ---
Maybe not exactly "proper", and not the best looking, but it performs better.




Here's a waveform capture before and after. The cyan trace is a reference saved earlier, with the original PCB. The yellow trace is my current handmade one. The signal is coming from a 74LVC-based pulser. This overshoot becomes worse when the distance between the 50 ohm load and the scope input increases, e.g. if I add a length of coax cable between them. I will need to try different lengths btw -- with a 15 cm cable it is still seen as just an overshoot, and I wonder what will happen as the cable length is gradually increased.

shapirus:

--- Quote from: shapirus on March 21, 2024, 07:42:53 pm ---
--- Quote from: shapirus on March 20, 2024, 02:34:15 am ---I think I'm going to make a proper board for it and then do a before/after comparison.

--- End quote ---
Maybe not exactly "proper", and not the best looking, but it performs better.

--- End quote ---
For comparison, I made a couple more feed-through terminators out of PCB-mount BNC male and female connectors with SMD resistors soldered in star arrangement on the male one (which is the closest to the scope input possible), in this case 3 x 150 Ohm.

No need to post another screenshot of the waveform to compare it to the improved P57: it is exactly the same, ideal match. In this particular case they perform equally well (or equally poorly, I really don't know how to tell one from the other). Maybe a faster scope or a VNA (which I don't have) would show some difference.
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