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Need 50 ohm input on an (old) oscilloscope that has only 1M ohm inputs: BNC TEE?

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bdunham7:

--- Quote from: tggzzz on October 24, 2022, 04:46:31 pm ---
* 485 internal attenuator: -35dB (VSWR <1.07)
* 2465 internal attenuator: -18dB (VSWR <1.28, spec 1.3)
* 485 with inline terminator: -8dB
--- End quote ---

So out of curiosity I tried this with a NanoVNA and I got something similar, but I don't think one number can tell a complete story.  Here's the whole picture. 

In order, Tek 485, Tek 2465B, Siglent SDS2354X+.  My battery ran out before I could get a photo of the inline terminator, but it was slightly worse than the 2465B and the Smith chart trace wandered off down and right a lot more.

tggzzz:

--- Quote from: bdunham7 on October 25, 2022, 09:08:01 pm ---
--- Quote from: tggzzz on October 24, 2022, 04:46:31 pm ---
* 485 internal attenuator: -35dB (VSWR <1.07)
* 2465 internal attenuator: -18dB (VSWR <1.28, spec 1.3)
* 485 with inline terminator: -8dB
--- End quote ---

So out of curiosity I tried this with a NanoVNA and I got something similar, but I don't think one number can tell a complete story.  Here's the whole picture. 

--- End quote ---

Yes, and very true.


--- Quote ---In order, Tek 485, Tek 2465B, Siglent SDS2354X+.  My battery ran out before I could get a photo of the inline terminator, but it was slightly worse than the 2465B and the Smith chart trace wandered off down and right a lot more.

--- End quote ---

Ah, someone else with good taste in scopes :) (And a NanoVNA!)

Your 485 trace looks noticeably worse than mine, and the S11 dip at @200MHz looks suspiciously like a cable effect. Did you do the short, open, load, though calibration with those at the end of the cable? I found the SOLT calibration made a significant difference to the results I observed.

Parenthetically, my NanoVNA's calibration has to be redone every time I turn it on or change the stimulus in any way, e.g. changing the frequency range. Most irritating.

bdunham7:

--- Quote from: tggzzz on October 25, 2022, 09:26:20 pm ---Your 485 trace looks noticeably worse than mine, and the S11 dip at @200MHz looks suspiciously like a cable effect. Did you do the short, open, load, though calibration with those at the end of the cable? I found the SOLT calibration made a significant difference to the results I observed.

--- End quote ---

I didn't use a cable, just an SMA male-to-male and SMA-BNC adapters.  Yes, I calibrated it but that's a bit tricky because I don't really have a super precision set of standards in BNC, so I just left it open for the open CAL and then took the adapter off and used the 50R and short that came with the NanoVNA, attached to an SMA female-to-female adapter that approximated the same length.  I think it turned out OK because the open readings were completely flat and perfect, the Smith charts looked good (and very tight on the 485).  Did yours just look less lumpy or were the values significantly different?  I actually did the 485 twice as I wasn't happy with the first calibration, it changed a bit but had the same overall look.

Siglent fans will note that the SDS2354X+ was a little better than the 2465B and really wasn't that much worse than the 485 at ~350MHz.

David Hess:

--- Quote from: wb0gaz on October 24, 2022, 02:26:23 am ---Can I use a BNC TEE (male-female-female), male to the scope input, terminate one of the two female sides in a (known good) 50 ohm terminator, and use the other female side as a 50-ohm input?
--- End quote ---

I have found that using a BNC-T with BNC termination only performs well to 10s of MHz.  The quality of BNC terminators is often questionable so maybe a better one is called for.

I have had good results using a feedthrough termination to 100s of MHz.

jonpaul:
Tektronix BNC term feedthu are fine, vintage US made are usually OK

any cable or adapters add impedance and dégradation SWR.

All network Analyzers required a CAL before each use.

We use Leo Bodnar 40ps pulser to check Trise, BNC is on the pulser direct to scope.

BW is not the most important scope spec.

Jon

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