_____
From: [email protected]
[mailto:[EMAIL PROTECTED] On Behalf Of Jeff DePolo
> This leads to another subject, duplexer isolation. Lots of
> people are under
> the impression that if a duplexer that is adequate with 70 db
> of isolation
> and provides no desense then using one with 100 db of
> isolation will be much
> better. In this case the 100 db duplexer is only better at
> emptying your
> wallet. Once you have reached the needed isolation, more does
> nothing for
> you.
Except you have to take into account external influences that may affect Z,
phase, noise, etc., including those caused by changes in temperature and
humidity. Just squeaking by with 70 dB under perfect conditions doesn't
mean that you'll still have no desense when there's ice on the antenna or
the building tempature spikes at 120 when the HVAC fails.
Agreed, you need some margin but not a ton. My point was that some believe
that extra margin in itself makes for better performance.
> It is all about how
> much total power
> a mixer can handle before being overloaded. The narrower the front end
> filter the less total signal power reaches the mixer. All
> signals received
> add together in the mixer and are a determining factor in
> overload not just
> the strongest signals.
Well, sure, but if you take 99% of the out-of-band signals out of the mix
via tight filtering, you've eliminated the potential for that many mixes
from occurring.
Agreed, but my main point was that newer receivers even though wide band may
be as good as older narrow band receivers. No question that additional
filtering makes them even better.
> This is particularly true of analyzers that are
> incorporated within a service monitor. Most suffer badly from lack of
> dynamic range.
Yes. Even good SM's like 8920's are easily overloaded, often making
harmonics appear grossly exaggerated when transmitting into the RF in/out
port. If you can reduce the input level by adding X dB of attenuation, and
you see the harmonic drop by greater than X dB, you know that you're
overloading the instrument...
> Using a sharp notch tuned to the strong signal
> will often
> allow you to look much further down to reliably see other
> noise and spurs.
Not really practical when using the RF in/out port on a service monitor.
You'd need to use a lightly-coupled sampler or directional coupler, with the
transmitter operating into the antenna or dummy load, otherwise the
transmitter would be looking into a very bad (and potentially damaging) load
Z.
When looking for spurious products on a transmitter one should never feed
the transmitter directly into a service monitor. The transmitter should be
feed to a directional coupler and dummy load or a high power attenuator and
coupled to the monitor. It is too easy to overload the monitor. Even stray
coupling paths within the monitor when using its internal load can cause
problems.
73
Gary K4FMX
--- Jeff
