Banggood 50 ohm BNC feed-through terminator - a quick review

[MF-jockey]

I received the full refund without having to return the parts and then opened the parts.
There was nothing in it but a wire, see the pictures.
Speak about this:  https://www.eevblog.com/forum/testgear/banggood-50-ohm-bnc-feed-through-terminator-a-quick-review/msg2078935/#msg2078935

Now I ordered 2 of the P57 should be good enough for use with my 100 MHz oscilloscope.

[kerouanton]

Reviving this old thread !

I've been using the good old BNC-T + 50ohm terminator when feeding RF signals to my Hi-Z scope, but was wondering about changing to more convenient R&S HZ22 or Aliexpress P57 units.

Wondering if anyone has done a VSWR measurement of those "new" P57 cheap terminators up to 1GHz, as they claim it's rated.

And by the way, I'd be curious to see the same measurement for the R&S HZ22 terminators for comparison, as they also claim 1GHz too. Upon searching on Internet I found a long thread on a german forum with measurements and so, but I had to resort to deepl automatic translation. Anyway this thread looks interesting 

https://www.mikrocontroller.net/topic/204806

[TERRA Operative]

Looks like a prime candidate for a replacement PCB with a greatly optimised layout..

Something like a continuous central 50ohm microstrip or similar with 4x 200ohm 1% decent wattage resistors, two hanging off each side tied to a bottom layer ground plane.

[mawyatt]

Looks as if the PCB can be utilized as an attenuator as well as a 50 ohm thru.

Might be a good project for someone to roll a PCB and 3D printed case. 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.

Best

[shapirus]

Another variant of the same "P57" thing from Aliexpress:



I have no idea how to measure VSWR, and very likely no means either, but I think a comparative estimate can be made by looking at the overshoot at the rising edge and the undershoot at the falling edge of a pulse with fast edges (presumably 300-500 ps) when the terminator is connected between a pulse generator and a 1M scope input. With this one the over/under -shoots were significantly higher than with a handmade terminator that I made from two BNC connectors of opposite genders. No screenshots, unfortunately, as I didn't make them before I broke my DIY terminator: two of the four 200-ohm SMD resistors cracked from mechanical stress, as there was nothing but them that would be holding the center conductor in place, axially, against the outer shell (and the plastic was not going a good job at it, because I overheated it too much during soldering). So I now have a useless 100 ohm terminator.

So this Chinese thingy has a nice enclosure, but a poor PCB. And an unforgivably high price of $6-7 currently. I think I'm going to make a proper board for it and then do a before/after comparison.

[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]

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).

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]

So this Chinese thingy has a nice enclosure, but a poor PCB.

...and that's a single-layer board, too.

[shapirus]

I think I'm going to make a proper board for it and then do a before/after comparison.

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]

I think I'm going to make a proper board for it and then do a before/after comparison.

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

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.

[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. 

[tggzzz]

Although I don't think anyone should be using an external terminator at that frequency. 

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

Gain is cheap; flatness isn't

[TopQuark]

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

Gain is cheap; flatness isn't

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]

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.

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 

[shapirus]

- 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).

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).

[tggzzz]

- 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).

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).

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]

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!)

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]

Not sure if it helps

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