Re: 7912AD Repair


On Sun, Apr 9, 2017 at 7:02 PM, David [TekScopes] <> 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
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.

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:

(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.

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.


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