AKG MP 40 User Manual download pdf (Page 45)

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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

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