BNC 50ohm Feed Thru

[Randy222]

I am just a hobby ham / IoT person, so kHz MHz and GHz are in my toolset, to some extent.

I am evaluating some 50 ohm feed thru items for my scope that only has 1meg Z on inputs.

Looking to see if my testing is valid when evaluating VSWR into the scope.

I opted to not evaluate with scope attached, but put a 1meg terminator on end of the BNC feed thru to simulate being attached to the scope.

I created criteria to mark a usable bandwidth, using VSWR 1.25 as the threshold.

Using my LiteVNA64 I get bandwidth of about 650kHz to 300MHz (VSWR <= 1.25). Beyond those ends the VSWR starts to rise quickly.

However, in observation, with an open ended feed thru the VSWR was fairly flat and low from about 10kHz out to near 3GHz.

Is my terminated method valid for evaluating VSWR between a transmitter (VNA) and receiver (simulated scope 1meg)?

[rf-messkopf]

I did a similar measurement some time ago and posted the results in a German electronics forum. See also the attachment.

The feedthrough termination is model J01006A0013 made by Telegärtner, click here for a datasheet.

The traces in telegaertner.png mean the following:

-red: measured when the scope side of the termination is open.
-green: measured when attached to a vintage HP 54503A scope with vertical scale set to 500 mV/div (front panel says 7 pF input capacitance).
-blue: measured between two ports of the VNA, where the resulting S-matrix was transformed such that port 2 (scope side of the termination) has a virtual impedance of 1 MΩ, with an additional 5Ω series resistor and a 7 pF capacitor from that resistor to ground.

As you can see, things start to become really ugly at frequencies beyond 500 MHz (the specification says more than 15 dB return loss at frequencies between 200 and 500 MHz, with the scope end open). Nevertheless, the termination does meet its specification.

For comparison, I also measured a BNC tee with a 50Ω BNC termination on one end (bnc-tee.png, same meaning of the traces colors as above). Actually, that one turns out to be better than the feedthrough termination. 

[TimFox]

Out of curiosity, what is the measured DC resistance of your 50 ohm terminators?

[rf-messkopf]

Out of curiosity, what is the measured DC resistance of your 50 ohm terminators?

I grabbed two Telegärtner model J01006A0013 feedthrough terminations from my adapters drawer. They measure 49.711 ohms and 49.879 ohms. I can't tell which one I used in the return loss measurements over a year ago.

[joeqsmith]

I tried to modify one that had been damaged.   Being dead, I had no reference but my results were poor.   

https://www.eevblog.com/forum/testgear/pasternack-pe6008-50-thru-term/msg947602/#msg947602

[Randy222]

The DUT's I have are no-name stuff, china made most likely.

I used three of my "best" non calibrated meters, one being a Fluke87

These results are same no matter what end I ohm at.

(tenths resolution)
BNC 1 - 50.4 50.3 50.3
BNC 2 - 50.2 50.1 50.1

[tszaboo]

For comparison, I also measured a BNC tee with a 50Ω BNC termination on one end (bnc-tee.png, same meaning of the traces colors as above). Actually, that one turns out to be better than the feedthrough termination. 

This is the second time I hear this. Why do you think this is happening?

[Bud]

Is my terminated method valid for evaluating VSWR between a transmitter (VNA) and receiver (simulated scope 1meg)?

No, you are missing an equivalent capacitor on the scope end of the adaptor. Refer to your scope what input capacitance it has.

[rf-messkopf]

For comparison, I also measured a BNC tee with a 50Ω BNC termination on one end (bnc-tee.png, same meaning of the traces colors as above). Actually, that one turns out to be better than the feedthrough termination. 

This is the second time I hear this. Why do you think this is happening?

Because the termination on the BNC tee is much better than the termination resistor in the feedthrough device. The manufacturers don't seem to spend much effort on the quality of the resistive element in feedthough terminations (often just a THT resistor, see Joe's posting above) as the resulting return loss is spoiled by the scope input anyway.

I noticed that in my measurements the return loss depends very sensitively on the set virtual scope input capacitance. And this capacitance depends on the individual scope.

These feedthrough devices are good up to a couple of MHz, but do not replace a scope with a dedicated 50 ohms signal path.

[Randy222]

Is my terminated method valid for evaluating VSWR between a transmitter (VNA) and receiver (simulated scope 1meg)?

No, you are missing an equivalent capacitor on the scope end of the adaptor. Refer to your scope what input capacitance it has.

Thanks for pointing that out, so I went and tested.
Scope is rated 15pF +- 3
Using my LCR meter my 1meg BNC terminator shows 13.2pF, although I suspect a few pF due to the proximity of the test leads used, I did try to separate them as much as I could, but the lead ports are spaced pretty close to each other.

Meter is not a calibrated unit, but it has given me matching results to OEM specs of parts I checked over the years.

I'll build another 1meg terminator using a different method to see if my VSWR results change much, or I buy one already made.
I'll also need to buy a name-brand feed thru, ThorLabs T4119 or the like.

[Randy222]

Won't need to hash out on this topic, there's plenty here on EEVb for me to digest.

This one is good
https://www.eevblog.com/forum/testgear/inexpenive-50-ohm-feed-through-terminations/msg2548299/#msg2548299

[joeqsmith]

I have the opposite problem in that my two higher speed scopes don't offer high impedance inputs.   They are 50 ohms only.  If you want to use a 10X probe, you must use some sort of buffer or I have higher speed active probes for them.

I assume your scope has much higher BW than what you have for 10X probes and are wanting to use resistive probes with it.  This is the reason for your interest in through terminators.   I normally use the 10X probes unless the loading it too much.   My normal use of the terminators is to provide a simple load for my signal generator before connecting it to a high impedance circuit.    Then again, we can say the scope is a high impedance... Which it is, until it's not. Like everything, those parasitics get in the way...   


**
Tried to make clearer but failed...  Easiest way to put it, the faster you go, the more you need to consider....

https://www.eevblog.com/forum/testgear/12-ghz-active-probe-project/

[joeqsmith]

I am thinking if the goal is to get a decent return loss with a short, or um scope attached, it could maybe be done if you wanted to give up some of the dynamic range.   

Using your criteria,  say 20dB return loss with a 3dB cutoff at 350MHz, DC coupled and 6dB loss with a 12-18pF load, uni-directional made with all passive parts.  If this seems reasonable,  I could try and model something up.  May get lucky....

[rf-messkopf]

You could build it like a X10 scope probe, i.e., a compensated voltage divider, with a 50 ohms input resistor. See the attachment for a quick simulation (the portion in the dashed box is the actual feedthrough device). That gives you 20 dB return loss up to 1 GHz if you are able to pull it off in practice. The 10 megohms resistor and the compensating capacitance (which must be variable in practice) will be a challenge at GHz frequencies. You could think of using other division ratios to end up with more reasonable component values.

No idea if such a device would be useful. Maybe it's better to just add a 3 dB attenuator to the 50 ohms side.

[joeqsmith]

I'm thinking something similar but using common values that could be realized in hardware.  With OPs scope being from 12-18pF, thinking I need at least one trimmer to compensate for it.   

A 3dB attenuator to a 50 ohm terminator attached to the scope will improve the return loss but the response will not be very flat.  18pF @ 300MHz is 30 ohms, in parallel with the 50.  I think your other idea is heading down the right path.

Still, if you are wanting to work at GHz with a scope and your scope doesn't support have internal termination, seems like it is time for a new scope.. 

[rf-messkopf]

Another run with a 2 megohns divider resistor and 18 pf scope input capacitance. Compensating capacitance is about 9 pF with 22 ohms in series, selected to give a good compromise between transient response and flatness. Flatness is okay up to 1 GHz, but the return loss is mediocre: 20 dB only up to about 100 MHz. It seems that some tradeoffs must be made with a purely passive device.

[tszaboo]

For comparison, I also measured a BNC tee with a 50Ω BNC termination on one end (bnc-tee.png, same meaning of the traces colors as above). Actually, that one turns out to be better than the feedthrough termination. 

This is the second time I hear this. Why do you think this is happening?

Because the termination on the BNC tee is much better than the termination resistor in the feedthrough device. The manufacturers don't seem to spend much effort on the quality of the resistive element in feedthough terminations (often just a THT resistor, see Joe's posting above) as the resulting return loss is spoiled by the scope input anyway.

I noticed that in my measurements the return loss depends very sensitively on the set virtual scope input capacitance. And this capacitance depends on the individual scope.

These feedthrough devices are good up to a couple of MHz, but do not replace a scope with a dedicated 50 ohms signal path.

Right, so the through terminations, that are designed to be used for oscilloscopes, they are low quality, and stop below 1GHz, while the regular terminators will go to GHz regions. Actually, we could compare a system if we use a simple T junction with a through termination.
I'm still thinking there is a bit more to this.

[joeqsmith]

I think with the OPs 300MHz, I assume 3dB flatness, you may be able to pull it off.   I tried using 6dB of attenuation and a network of RLCs.  It would need to be trimmed, just like a probe.   OP may not want to give up 6dB.  I figured I would wait for the constraints before diving in too deep. 

This would make for a fun little contest circuit.  It's something that someone may actually have a use for, unlike an oscillator on a breadboard.   

Right, so the through terminations, that are designed to be used for oscilloscopes, they are low quality, and stop below 1GHz, while the regular terminators will go to GHz regions. Actually, we could compare a system if we use a simple T junction with a through termination.
I'm still thinking there is a bit more to this.

Again, at least for the OP wanting 300MHz, their scopes input capacitance is 18p max, or 30 ohms at 300.   At one point, directly measuring the scope's input, I bet you get a decent match at 50ohms.   Adding R isn't going to give you a flat impedance, well, unless we can say use a 1ohm source rather than 50....

[Randy222]

My noted 650kHz to 300MHz was just width of VSWR <= 1.25
My hobby scope is a rigol dho804 opened up some to be an 814 where -3db is around 180MHz.

It's noted in the EEVb link I posted that the feed thru's act better when you front them with an attenuator.

[joeqsmith]

Another run with a 2 megohns divider resistor and 18 pf scope input capacitance. Compensating capacitance is about 9 pF with 22 ohms in series, selected to give a good compromise between transient response and flatness. Flatness is okay up to 1 GHz, but the return loss is mediocre: 20 dB only up to about 100 MHz. It seems that some tradeoffs must be made with a purely passive device.

Didn't want you to think I was a slacker.   I modified one that I had simulated in SPICE, without adding any attenuation.   From testing I could see the original Pasternak part out performed the two I had originally modified, so  I chose a simple approach by adding a bit of inductance.   Some small gains but really, the idea of working at high speeds with this sort of setup is flawed from the start.   

https://www.eevblog.com/forum/testgear/inexpenive-50-ohm-feed-through-terminations/msg5227260/#msg5227260

[tszaboo]

I think with the OPs 300MHz, I assume 3dB flatness, you may be able to pull it off.   I tried using 6dB of attenuation and a network of RLCs.  It would need to be trimmed, just like a probe.   OP may not want to give up 6dB.  I figured I would wait for the constraints before diving in too deep. 

This would make for a fun little contest circuit.  It's something that someone may actually have a use for, unlike an oscillator on a breadboard.   

Right, so the through terminations, that are designed to be used for oscilloscopes, they are low quality, and stop below 1GHz, while the regular terminators will go to GHz regions. Actually, we could compare a system if we use a simple T junction with a through termination.
I'm still thinking there is a bit more to this.

Again, at least for the OP wanting 300MHz, their scopes input capacitance is 18p max, or 30 ohms at 300.   At one point, directly measuring the scope's input, I bet you get a decent match at 50ohms.   Adding R isn't going to give you a flat impedance, well, unless we can say use a 1ohm source rather than 50....

I've just watched your video about the topic. So I actually wonder if there is a solution to matching a 18pF input to 50 Ohm.
If I approach it as an antenna matching problem, then adding L or C would only match it at one frequency. So I'm thinking a wideband matching with several networks could work better. For example 200 Ohm resistors with 4 different impedances, to compensate for the different frequency components?
But then again, are we looking for flatness in the frequency band, are we looking for keeping the shape of the signal intact? Are these two goals different? DC accuracy?

[joeqsmith]

If OP doesn't respond,  I would say a free for all on the requirements, goals...     rf-messkopf's simulation used values that may have made it difficult to construct.  I wanted to show something in physical hardware.  Adding a small bit of inductance seemed like something I could handle building...

[tszaboo]

If OP doesn't respond,  I would say a free for all on the requirements, goals...     rf-messkopf's simulation used values that may have made it difficult to construct.  I wanted to show something in physical hardware.  Adding a small bit of inductance seemed like something I could handle building...

I think for something more complicated, a small PCB would be better. What would be even better is ceramic boards.
I did a lot of search for housings for feedthrough terminations, or attenuators. The issue I see is the cost is around 40-50 USD for such a housing, or DIY solutions that show. I kind of feel like 80% of RF design or probe design is mechanical, and the last 20% is the electronics. Most connectors don't come with good dimensional drawing, and the ones that do are some specific imperial thread pitches that drive me crazy.

[G0HZU]

I've just watched your video about the topic. So I actually wonder if there is a solution to matching a 18pF input to 50 Ohm.
If I approach it as an antenna matching problem, then adding L or C would only match it at one frequency. So I'm thinking a wideband matching with several networks could work better. For example 200 Ohm resistors with 4 different impedances, to compensate for the different frequency components?
But then again, are we looking for flatness in the frequency band, are we looking for keeping the shape of the signal intact? Are these two goals different? DC accuracy?
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The (close to) ideal matching component would be a parallel negative capacitor with a capacitance of -18pF. As this component doesn't exist in a passive form, the next best thing is to add a small amount of series inductance inline with the 50R termination resistor.

A series RL can mimic a 50R resistor in parallel with -18pF over quite a large bandwidth.  See the second image below. You can see it produces a -18pF capacitance over a large bandwidth. However, 46nH is a lot of inductance to add, and so in practice, it can be risky to do this as it can cause a spike to appear on the leading edge of a fast pulse if a really fast pulse is then fed to the scope. This is because (when this network is added to the scope input) the input impedance of the scope will then have an upwards bump up at 100MHz-200MHz or so. So it's usually best to keep the series inductance value fairly low if you want to look at fast pulse edges.

Note that a typical scope input isn't as simple as 1Meg in parallel with 18pF. There will be some series resistance right at the scope input and this is typically in the range 20R to about 50R. This resistance often varies a bit depending on the attenuation setting. A typical scope input can be modelled reasonably well with the network given in the first image below, as long as the correct value for R1 and C1 are used.

[joeqsmith]

I think for something more complicated, a small PCB would be better. What would be even better is ceramic boards.
I did a lot of search for housings for feedthrough terminations, or attenuators. The issue I see is the cost is around 40-50 USD for such a housing, or DIY solutions that show. I kind of feel like 80% of RF design or probe design is mechanical, and the last 20% is the electronics. Most connectors don't come with good dimensional drawing, and the ones that do are some specific imperial thread pitches that drive me crazy.

Feel free to use any construction techniques and burn as much cash as you wish for your demonstration.   Personally, I don't see much value in it as the whole idea is flawed.   

I had ran some other tests that were not shown in the video.  Attached photo showing the scope simulator and terminator on FR4 using 2X1206 100.   The graph compares the custom thru terminator attached to the LeCroy 64xi waveblunder, compared with the simulated 64xi with both the custom terminator and the prototype built on FR4.   Sure, some slight differences but good enough.   

That 10x probe I show on this scope will out perform it to I think we said 60MHz, where the my resistive probe presents a higher impedance load. 
https://www.eevblog.com/forum/testgear/12-ghz-active-probe-project/msg4988716/#msg4988716

We can achieve higher BW using a homemade resistive probe and better scope.   
https://www.eevblog.com/forum/testgear/12-ghz-active-probe-project/msg5006290/#msg5006290