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

[tggzzz]

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.

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.

That's all the calibration I did too. I had NanoVMA, N-SMA "saver", maybe 20-30cm cable, then SMA-BNC adaptor which I regarded as my reference point.

My graph for my 485 is shown above, and you can see it is always better than -30dB, with a wide -5dB dip at ~250MHz. Perhaps the cable was responsible for "smoothing out" that dip; I haven't tested that.

[David Hess]

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

Not all Tektronix BNC feedthrough terminations are the same.  The larger 5 watt ones were manufactured by someone else and Tektronix just put their name on them; I have one marked Tektronix and one marked Probe Masters.

011-0049-01   <1.1 DC to 250 MHz and <1.2 DC to 500 MHz   2? Watts   (small white one)
011-0099-00   <1.1 DC to 100 MHz   5 Watts   (long black anodized body with fins)

I also have a commonly available square plastic cased "1 GHz" feedthrough terminator which gets remarked by companies like Rigol.  When tested on my 300 MHz oscilloscope, all of the above perform practically identically, and the same as the internal switched termination.

[wb0gaz]

I glanced away from this discussion and just came back to see it's carried on further...

I received a couple of the pass-through terminators that were originally suggested early in the thread (amazon link, although I found the same thing on ebay for about $5 each); they're adequate for the immediate need (and roughly equivalent in behavior to a TEE connector and a minicircuits BNC terminator), however, I'll keep an eye out for 7A24 plug-ins (preferred) or suitable Tektronix type pass-through terminators.

Thanks again for the interesting discussion!

[David Hess]

7A24s are all old enough now that they will certainly need cleaning of the mechanical switches and poteniometers, but this is very feasible to do.

[wb0gaz]

Thanks, good to know that!

Dave