david vanhorn wrote:
>
> Look at <
> http://www.repeater-builder.com/tech-info/effectivesens.html
> <http://www.repeater-builder.com/tech-info/effectivesens.html>> - a
> writeup
> from Chris Boone WB5ITT, who happens to be a pretty sharp guy.
>
>
> Very good article. I got a 50dB sample pad from Telewave, and have it
> installed in the system.
> Makes it very handy to look at the transmitted spectrum, or do receiver
> adjustments.
Actually I think it'd be a better article if it pointed out there are
better ways than an Iso-T to inject that signal that are give better
(more reproducible/consistent) results.
Examples might include the the Bird/Telewave injection ports on their
meters, or a directional coupler (which you might as well have to do
good quality return-loss measurements anyway).
The Iso-T works, but it's another variable in the circuit. Good
engineering reduces variables in measurements, whenever possible.
A fixed loss port on a meter or directional coupler has no "moving
parts" for a human to screw up when setting up the test environment. If
more than one person is doing the same measurement, and they both have
access to the same test gear, the Iso-T method won't produce as
consistent results as a fixed input port and variable/calibrated signal
generator will.
Just opinion, but I like having the variable part of the signal
generation/SINAD measurement done at a device that has a calibrated
output level, and no variability of an Iso-T.
Just my opinion. And Iso-T's are cheaper than the alternatives.
And either method ultimately works well enough to see a problem and get
measurements!
Everyone should know the effective receiver sensitivity of their system
when hooked to the antenna at the site. Bench numbers for receiver
sensitivity mean very little in the real world of noise, IM
interference, mixing, and "junk". ;-)
What's the quote? "The unexamined life is not worth living."
Perhaps better stated: "The unexamined repeater is not worth using."
:-)
Measurements... oooh yummy. Mmmm. Fun.
Nate WY0X
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