A feed trough terminator is designed to be connected to the input of an oscilloscope or other instrument, not operated alone. The 8-20 pF input capacitance plus varying internal path length of an oscilloscope input will completely dominate the SWR. I did a video a few years ago where I showed this.
I like the way you added electric length to compensate for adapter, which is correct for approximate measurement. The absolute correct way is to cal it after the adapter, because the adapter itself has frequency dependent characteristics. The other side of the 50 Ohm should also be 1M ohm load ( the high impedance of oscilloscope ), so to make it look like how it used in real situation. Oscilloscope's high impedance is not any high impedance, it is exactly 1M Ohm, because we need to use 10X probe that assumes the scope has a 1M impedance. So connect the other side of 50 Ohm load to oscilloscope with good cable and set the scope to high impedance. You can also do s11, s12, s22, and s21 measurement and convert it to Z This way you can find out that how reciprocal is that. Also for being through, you need to test that over temperature. But hey less than 2 dB is good enough for many applications. Also, highest Power should be specified on that. That is this 50 Ohm can handle say 1dBm, 30dbm or 60dBm.
To distinguish my 75 Ohm atten/cables/terms I use a Purple(=7) paint-pen on the knurles of the BNC male end and then wipe off the excess. The paint stays in the grooves for decades. Everything else I have is assumed to be 50 Ohms.
When you added a 50 ohm termination you ended up with a 25 ohm feedthrough terminator. Try moving your system impedance point on the VNA from 50 to 25 ohms and you should see the SWR normalize.
I think you should load the other end with a typical oscilloscope input. Maybe the inductive one is better if loaded with about 10-20 pf as typical of a high impedance oscilloscope input. I would not be surprised if they are tuned to work best with a specific oscilloscope.
Even if your scope has a 50Ω input option you have to be careful as the input voltage level maximum will be very low, sometimes as low as 5VP-P. So if maintaining a 50Ω system is important using one of these feedthrough terminators for high signal inputs is important.
I used one of these when looking at the output of my Rubidium frequency reference on the ocilliscope. Unterminated the waveform was pretty funky, but with the 50 ohm passthrough terminator it was just fine.
I've only ever used these with current probes that need them when the scope does not have a 50 ohm input option. To the other commenter: some years ago, someone burned out one of the channels in a 1Ghz 4 channel lecroy at work by having it at 50 ohm and doing something dumb. That poor scope... It was decided too $$ to have repaired given its age (only maybe 10-12 yrs old at that time). A few years later I think it got tossed.
Most oscilloscopes look like 1 MOhm in parallel with 10-30 pF. Especially at rf, the capacitance matters. I would have soldered these onto a BNC plug for these tests.
Hi, I'd be very interested in how mine behaves, I don't have any throughs, so I use a T-piece and an end terminator, since I don't even have a spectrometer, so I won't find out. I did not expect such differences. Nice day 🙂 Tom
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A feed trough terminator is designed to be connected to the input of an oscilloscope or other instrument, not operated alone. The 8-20 pF input capacitance plus varying internal path length of an oscilloscope input will completely dominate the SWR. I did a video a few years ago where I showed this.
I like the way you added electric length to compensate for adapter, which is correct for approximate measurement.
The absolute correct way is to cal it after the adapter, because the adapter itself has frequency dependent characteristics.
The other side of the 50 Ohm should also be 1M ohm load ( the high impedance of oscilloscope ), so to make it look like how it used in real situation.
Oscilloscope's high impedance is not any high impedance, it is exactly 1M Ohm, because we need to use 10X probe that assumes the scope has a 1M impedance.
So connect the other side of 50 Ohm load to oscilloscope with good cable and set the scope to high impedance.
You can also do s11, s12, s22, and s21 measurement and convert it to Z
This way you can find out that how reciprocal is that.
Also for being through, you need to test that over temperature.
But hey less than 2 dB is good enough for many applications.
Also, highest Power should be specified on that. That is this 50 Ohm can handle say 1dBm, 30dbm or 60dBm.
To distinguish my 75 Ohm atten/cables/terms I use a Purple(=7) paint-pen on the knurles of the BNC male end and then wipe off the excess.
The paint stays in the grooves for decades.
Everything else I have is assumed to be 50 Ohms.
When you added a 50 ohm termination you ended up with a 25 ohm feedthrough terminator. Try moving your system impedance point on the VNA from 50 to 25 ohms and you should see the SWR normalize.
I think you should load the other end with a typical oscilloscope input. Maybe the inductive one is better if loaded with about 10-20 pf as typical of a high impedance oscilloscope input. I would not be surprised if they are tuned to work best with a specific oscilloscope.
I believe these 'plasticky' ones are shielded inside, isolated on the outside.. since people sometimes disconnect ground from their scope.
Even if your scope has a 50Ω input option you have to be careful as the input voltage level maximum will be very low, sometimes as low as 5VP-P. So if maintaining a 50Ω system is important using one of these feedthrough terminators for high signal inputs is important.
A power attenuator would seem like a better choice.
Interesting, i did not know those devices exists until now, usually i used a BNC T Connector together with a RF dummy load to do so...
This was very helpful to know what these impedance pads do, what the look like and how to use them. Thanks OM.
Back a few months ago I scored a Bird 500w 50ohm 30db attenuator for 60bucks!
That is a deal.
I used one of these when looking at the output of my Rubidium frequency reference on the ocilliscope. Unterminated the waveform was pretty funky, but with the 50 ohm passthrough terminator it was just fine.
Very NICE screen picture of the 3 responses - I can't wait to buy a Rigol SSA-3021 someday!
I also have similar looking series resistance feed-through terminators in 25, 50, 75, 93, and 600 ohms.
I've only ever used these with current probes that need them when the scope does not have a 50 ohm input option. To the other commenter: some years ago, someone burned out one of the channels in a 1Ghz 4 channel lecroy at work by having it at 50 ohm and doing something dumb. That poor scope... It was decided too $$ to have repaired given its age (only maybe 10-12 yrs old at that time). A few years later I think it got tossed.
would be nice to see what's inside.
Do you mean 'Suhner' ?
Most oscilloscopes look like 1 MOhm in parallel with 10-30 pF. Especially at rf, the capacitance matters. I would have soldered these onto a BNC plug for these tests.
Hi, I'd be very interested in how mine behaves, I don't have any throughs, so I use a T-piece and an end terminator, since I don't even have a spectrometer, so I won't find out.
I did not expect such differences.
Nice day 🙂 Tom
I now want to check my bnc splitter that has auto termination.
I’d like to see what’s in a bnc 75 ohm to 50 ohm adapter.
ruclips.net/video/aYDh9Hd8tLA/видео.html
Sono sempre stato curioso del perché gli autisti di camion parlano così. C’è una scuola che fanno per parlare tutti uguali in questo modo sgradevole? Un po’ come per i rapper
No comments from me 😀
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