View Single Post
Old 12th Aug 2018, 2:42 pm   #59
G0HZU_JMR
Dekatron
 
Join Date: Sep 2010
Location: Cheltenham, Gloucestershire, UK.
Posts: 3,077
Default Re: Back terminated cable?

It's really just a small fudge capacitor to flatten the response. There are several strays involved and the little cap helps offset the (tiny) internal capacitance of the 1500R resistor for one thing. I chose a resistor with very low internal capacitance despite the large package size but the shunt cap is still needed. I think my little shuntcap is 2 or 3pF but I can't remember now.

The reason I used an SMA end launcher was because it allows me to use several versions of these probes as probe tips and I can swap between them as required with a very good quality SMA RF coax cable (18GHz) into my HP8566B spectrum analyser or into a network analyser. So the far end of the coax is always terminated correctly in 50R with this setup. The probe isn't meant to be used with a 1Meg scope although you could use it if you selected (or fitted) a 50R input termination on the scope.

For more bandwidth I also have versions that either use Sucoform 141 or 086 RF cable and I use a 470R chip resistor for a -20dB version. The ground spike connection can also be much shorter with this setup. It also doesn't need the shunt cap. But this is so fragile I only use it when absolutely necessary. Even with care I often end up replacing the chip resistor more than once in a busy session. But it is cheap and gives good RF performance
__________________
Regards, Jeremy G0HZU
G0HZU_JMR is offline