Products > Test Equipment
Banggood 50 ohm BNC feed-through terminator - a quick review
tggzzz:
--- Quote from: shapirus on April 21, 2024, 11:57:02 am ---
--- 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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The attenuator/pad requires 50ohms on both sides. Therefore it should be cable->pad->terminator->scope.
The reflected signal traverses the pad twice, hence a 3dB pad will improve the VSWR by 6dB (you know what I mean!)
shapirus:
--- Quote from: tggzzz on April 21, 2024, 01:48:16 pm ---The attenuator/pad requires 50ohms on both sides. Therefore it should be cable->pad->terminator->scope.
The reflected signal traverses the pad twice, hence a 3dB pad will improve the VSWR by 6dB (you know what I mean!)
--- End quote ---
I've just tried a 10dB feed-through attenuator (don't have a 3dB one) between the signal source (pulse generator) and the feed-through terminator. No change in the overshoot/undershoot amount. The shape of the waveform remains the same too: near perfect match (when vertical scale is set accordingly) with a previously saved reference of the same signal without an attenuator, so no change in rise/fall times either, at least nothing that can be detected with this scope (Rigol DHO800 with BW somewhere around 250..300 MHz).
Connection is like so: generator's SMA output -> 10dB SMA-to-SMA attenuator -> SMA-to-BNC adapter -> 50 Ohm BNC-to-BNC terminator -> 1M scope input.
Probably this overshoot/undershoot has nothing to do with VSWR, I have no idea at this point, and we may be talking about different things. I will create a separate topic about this, as I'm observing some curious reflection effects as well, just need to prepare and describe some test scenarios, make screenshots etc.
I do know, though, that the amount of overshoot/undershoot increases when the terminator is not made right (see e.g. the original PCB of the P57 shown in previous posts) or the distance between the terminator and the scope input is increased with e.g. a piece of coax cable or just some BNC-to-BNC adapters.
joeqsmith:
Not sure if it helps but we talked about compensating such a terminator for a scope. I made a short video going over the different terminators, SPICE models and my attempt to realize the compensation in physical hardware.
https://www.eevblog.com/forum/rf-microwave/bnc-50ohm-feed-thru/
shapirus:
--- Quote from: joeqsmith on April 21, 2024, 02:29:26 pm ---Not sure if it helps
--- End quote ---
It does!
It did not give me direct answers, as it was on a somewhat different topic, but instead it gave me an idea and suggested what I was previously missing to simulate what I'm seeing on the scope screen.
The overshoot and undershoot that I'm seeing seem to be a result of the combination of the 50 Ohm shunt in the terminator and the parasitic inductance between the scope input BNC connector and the 15 pF capacitance. As this inductance is increased, the edge ringing becomes worse -- just what I observed increasing the length of the line between the terminator and the scope input.
The simulation also agrees with what I'm seeing when the terminator is not used at all and the pulse generator is plugged directly into the scope input: the ringing is significantly reduced, but the rise/fall times become slower (in reality so much so that it can be seen directly on the scope screen).
joeqsmith:
Glad to hear it had some use. Teach a man to fish...
That video had a link to Lasmux's thread where I was showing them some data using some home made probes. Parasitic's hurt the performance (lots of ringing) and I made one using a thick ground strap to try and mitigate the problem (like the professionals). If you read though it, you can get a feel for what effect it had.
https://www.eevblog.com/forum/testgear/12-ghz-active-probe-project/msg5005309/#msg5005309
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