AKG Acoustics WMS 40 - Manual (Page 45)

www.akg.com 43
Microphone A carrier frequency
Spectrum analyzer trace Intermodulation of carrier frequencies A and B
The trace clearly shows that third-order intermodulation
products are only 38 dB lower in level than the carrier
frequencies. Microphone B carrier frequency
Spectrum analyzer trace
Intermodulation trace on a spectrum analyzer display
Third-order intermodulation products from three carrier
frequencies.
The received signal level declines in pro-
portion to the square of the distance bet-
ween transmitter and receiver, and the
intermodulation level produced in the
receiver declines in proportion to the third
power of the received signal level. This
implies that intermodulation declines expo-
nentially, in proportion to the sixth power of
the distance between transmitter and recei-
ver. If the distance is longer than 66 feet
(20 m), receiver intermodulation is drow-
ned out by noise. What remains is another
important type of intermodulation distor-
tion that has not yet been mentioned: trans-
mitter intermodulation. In this case, the
intermodulation products are not generated
in the receiver, but in the transmitters, and
are radiated by them along with the desired carrier frequencies. This will only happen,
however, if there is enough crosstalk of car-
rier frequencies between two neighboring
transmitters that intermodulate with each
other. In this case, the antenna of one
transmitter receives the carrier signal of a
neighboring transmitter. If this signal
makes it into the non-linear output stage of
the transmitter, the first harmonic of the
desired signal will transform it into a signal
whose frequency is indistinguishable from
the receiver intermodulation. The same
happens in the other transmitter that will
generate a mirror-image intermodulation
product. Curiously, a love duet close to the
receiving antennas may lead to intermodu-
lation distortion caused by the nonlinear
receiver. If the two singers move away, the intermodulation remains unchanged, but is
now produced in the transmitters. In large
multichannel systems, reducing the RF
output of the transmitters is a way to bring
down transmitter intermodulation by mini-
mizing the nonlinear response of the trans-
mitter's output stage. The RF output of
WMS 4000 transmitters, for example, can
be reduced from 50 mW to 10 mW (ERP
–Equivalent Radiated Power).
Here are some hints on how to minimize
receiver intermodulation:
Always lay out the antenna system so as to
ensure reliable transmission from every
point on the stage. Moreover, be sure to use
only the types of cables recommended in
the user’s manual. The distance between
transmitters and active antennas should be
at least 15 feet (5 m) (see also Antenna
Position Check Applet on p. 45).
Increasing the input attenuation of the
antenna system helps, as does reducing the
transmitter RF output to 10 mW. The latter
has proved particularly useful for hand-held
transmitters in situations where range is
not an important consideration; generally,
the RF output level used should always be
just high enough to ensure adequate trans-
mission. Systems with a higher transmitter
RF output (ERP) are more prone to inter-
modulation problems, but this is compen-
sated for by their better resistance to elec-
trosmog.
When multichannel systems are used on
Broadway, for instance, only the strongest
will survive.
Intermodulation trace on a
spectrum analyzer display
Intermodulation products from 2
carrier frequencies, 3rd-order IMD
and 5th-order IMD.
AKG WMS MULTICHANNEL TECHNOLOGY