Re: 7912AD Repair


cmjones01
 

On Sun, Apr 9, 2017 at 7:02 PM, David davidwhess@gmail.com [TekScopes] <
TekScopes@yahoogroups.com> wrote:



On Sun, 9 Apr 2017 17:45:31 +0200, you wrote:

I haven't checked the risetime yet. Doing that requires getting the 7912AD
and my CSA803A within a couple of feet of each other, which is no small
feat considering their size and weight. I worry about creating a black
hole
due to the concentration of mass. And one's at home and the other's in the
office. I'll do it and report back though.

Chris
The distance restriction can be relaxed a little by using larger
diameter coaxial cable (1) so it might pay to make a custom mimimum
length RG-8 sized patch cable for this sort of thing. LMR-400 cable
might be the best commonly available cable for this but maybe someone
else has some experiece with it. I get by with some 1ns and 2ns
length but practially indestructable RG-400 (RG-58 size) cables.

My personal experience is that the performance limitations of
RG-174/RG-316 is very apparent at any useful length and RG-58/RG-400
slightly so. There are some examples here at the bottom of using
different types and length of transmission lines:

<http://www.amplifier.cd/Test_Equipment/Tektronix/Tektronix_other/284.html
(1) Up to the TEM cutoff frequency anyway which is 16 GHz for LMR-400
and 13.5 GHz for RG-8 so that is not a problem.

https://www.microwaves101.com/encyclopedias/coax#cutoff


Thank you, that's very interesting. Having next to no SMA or 3.5mm cables
in stock, I invested in an armful of the cheapo RG174 cables from Chinese
suppliers. They work amazingly well for the nearly-free price, but the
degradation to a 20ps risetime step over 1.5m of cable is quite visible.
Reading the article from microwaves101, it seems that thinner cables like
RG174 exhibit higher loss but a higher TEM cutoff frequency. Presumably
figuring out what a 6-foot length of cable will do to the risetime is
complicated: the lower loss is nice but the lower TEM cutoff will have a
negative effect. However, I guess the TEM cutoff is probably academic when
it comes to the 300ps-ish risetime of a 7A29.

Chris

Join TekScopes@groups.io to automatically receive all group messages.