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wrote:

What do you mean by 2nd and 3rd order intercepts?

When allocating wireless, there are a few things to be aware of:

1) External interference - go to the FCC web site and obtain a list of
all UHF radio transmissions in your area. Be especially aware of DTV
transmitters, and those that are listed as coming online in the next
year or two. It's possible to sneak in with an analog TV station, but
DTV is spread-spectrum and will be more problematic if you're in the
same frequency range.

Optimally, if you have a frequency analyzer available, you can use that
to find open ranges in your wireless spectrum. These are pretty
expensive, and you need to know what you're doing. If you don't already
have one, the FCC database should be fine.

2) Frequency spacing - as Phil said, the Sennheiser EW units allow you
to tune at increments of 25kHz. It's probably not a good idea to put
two systems that close to each other. I don't know the IF filter
topology in a Senn receiver, but especially in the lower end units
(e.g. EW100) I don't imagine that they have an incredibly narrow band
pass. This means that if you have two transmitters operating that close
to each other, they will produce noise in the others' output.

3) Intermodulation - this is what Scott is talking about when he
mentions "2nd and 3rd order intercepts" I believe (though I haven't
heard the term "intercept" before). Basically, there are going to be
some nonlinearities involved in your wireless system that create sum
and difference terms of the frequencies involved. If you have one
wireless TX at A=600MHz and another at B=700MHz, then you need to worry
about not putting anything at A+B=1300MHz or B-A=100MHz. These are the
2nd order intermod terms. In practical wirelessapplication, 2nd order
rarely is important -- you need to worry more about 3rd order. Namely,
2A-B = 500MHz and 2B-A = 800MHz. There are various calculator programs
available that will test a set of operating frequencies for intermod
problems.

If all of this seems like too much to handle, I recommend consulting
the provider of your wireless equipment. They should have staff on hand
who understand this and can provide you with an optimally configured
system.

Hope that helps
-Todd