CRYSTAL-CONTROLLED VS. FREQUENCY SYNTHESIS
Crystal controlled wireless units can be designed with
wide audio frequency response, low noise, low distortion,
and relatively long battery life. They are the most cost-
effective choice for fixed frequency applications involving a
moderate number of simultaneous systems. One
limitation inherent to a crystal controlled transmitter is the
generation of spurious emissions due to output multiplier
stages, though these can generally be kept to a minimum
with careful design.
For tuneable systems, frequency synthesis is the most
practical technique. The absence of spurious emissions
from the transmitter also simplifies coordination of multiple
systems. However, it is more difficult (and more
expensive) to design equally low noise, low distortion
frequency synthesized systems. A limitation inherent to
the audio frequency response of this type of transmitter
results from the use of a sharp lo-cut filter to prevent very
low audio frequencies from interfering with the PLL control
circuit. This places a lower limit on the audio frequency
range that may be transmitted. Special techniques are
required to achieve extended low frequency response in
frequency synthesized systems. In addition, the greater
power consumption of frequency synthesized transmitters
reduces battery operating time.
As in the choice of other wireless microphone system
characteristics above, it is necessary to evaluate the
application to determine which frequency generation
method is preferable. For most fixed applications, crystal
controlled systems are suitable. Frequency synthesized
systems should be considered when frequency agility is a
primary requirement or if there are other included features
that are desirable for the application.
Once the wireless microphone system(s) choice is
made, careful setup and proper use are necessary to
obtain the best performance.
SYSTEM SETUP: TRANSMITTER
Transmitter setup first requires optimizing the source-
to-transmitter interface. Sources include dynamic and
condenser microphones, electronic musical instruments
and general audio sources such as mixer outputs,
playback devices, etc. The output signal of each of these
sources is characterized by its level, impedance and
configuration (balanced or unbalanced). For sources
such as condenser microphones, some type of power
(phantom or bias) may be required.
The transmitter may be a bodypack, plug-on or hand-
held type and its input will also have a characteristic level,
impedance and configuration (balanced or unbalanced).
It may be capable of supplying power to the source. The
interface can consist of some type of connector or it may be
hard-wired, either internally or externally. (See Figure 4-1.)
The simplest interface is the handheld transmitter. This
design should insure that the microphone element is
already optimally integrated (electrically and mechanically)
with the transmitter. The only choice involves systems that
offer a selection of microphone elements. If each is
equipped for proper interface the decision should be
made based on the performance characteristics of the
microphone element for the intended application.
The plug-on transmitter offers a range of interface
possibilities. Mechanically, the 3-pin XLR type connector is
standard but the electrical characteristics of the chosen
microphone and transmitter combination must be
considered. The input impedance of the transmitter
should be higher than the microphone output impedance.
All transmitters of this type will work with typical low-
impedance dynamic microphones. If the transmitter input
impedance is high enough (>10,000 ohms) a high
impedance microphone may also be used. Most plug-on
transmitters will work with either balanced or unbalanced
microphone outputs.
Some plug-on transmitters are also capable of
supplying "phantom power" to a condenser microphone.
This is only possible with a balanced transmitter input and
a balanced microphone output. Even then, the transmitter
must supply at least the minimum phantom voltage
37
Selection
and Operation
of Wireless Microphone Systems
CHAPTER 4
Figure 4-1:
examples of transmitters (left to right: handheld, bodypack, plug-on)