High-end Tektronix (series 7000/70000) have proprietary TekConnect HF inputs, which accept only 50 ohm inputs.
This limits usage of such scopes only to special (read - expensive) TekConnect active probes.
Also using DIY is not a simple forward path either, as even simple passive adapter from TekConnect to BNC or SMA (TCA-BNC, TNC-SMA) cost few hundreds USD on ebay easy!
If you want use 1 Meg input impedance options even more limited, with Tek's offering of TCA-1MEG amplifier, which costs ~500$ used.
Was lucky to get one of these relatively cheap, so here's a teardown (article per image link):
![](http://doc.xdevs.com/doc/Tektronix/TCA-1MEG/autopsy1_1.jpg)