User's Manual

Watertight Miniature UHF Belt-Pack Transmitter
Rio Rancho, NM
7
Input Limiter
The MM400C transmitters employ a DSP-controlled an-
alog audio limiter just before the analog-to-digital con-
verter.Thelimiterhasarangeofmorethan30dBfor
excellentoverloadprotection.Adualreleaseenvelope
makes the limiter acoustically transparent while main-
taining low distortion. It can be thought of as two limit-
ers in series, connected as a fast attack and release
limiter followed by a slow attack and release limiter. The
limiter recovers quickly from brief transients, so that its
action is hidden from the listener, but recovers slowly
from sustained high levels, to both keep audio distortion
low and preserve short term dynamic changes.
Two bicolor LEDs indicate limiter activity. (See Operat-
ing Instructions, Adjusting Audio Levels.) Generally
speaking, some limiting is desirable in normal operation
to improve the signal to noise ratio of the system. The
limiting action is not audible and does not create distor-
tion.
Digital Signal Processor
TheDSPconstructstheoriginaldigitizedaudiofromthe
A-D Converter, adds an ultrasonic Pilot Tone to con-
trol the receiver’s squelch (only in 400 Series and 200
Series Compatibility Mode with the Pilot Tone enabled
– see Pilot Tone Squelch), and implements the user
defined Compatibility Mode.
Microprocessor, PLL and VCO Circuits
An 8-bit microprocessor monitors user command inputs
and numerous other internal signals. It also drives the
Modulation LEDs, controls the Pilot Tone and operates
thePLL/VCOcircuits.
Compatibility Modes
The MM400C transmitter was designed to compatible
with Lectrosonics 400 Series receivers and will yield the
best performance when doing so. However, due to the
exibilityofdigitalsignalprocessing,theMM400Cis
also able to operate with Lectrosonics 200 Series, Lec-
trosonics100Series,IFBandcertainnon-Lectrosonics
analog receivers in special compatibility modes. (Con-
tact the Lectrosonics Sales Department for a complete
list of compatible transmitters.)
Pilot Tone Squelch
The 400 Series wireless system uses an ultrasonic tone
between25and32kHztooperatethereceiversquelch.
The pilot tone squelch system keeps the receiver muted
until it receives the pilot tone from the matching trans-
mitter, even if a strong RF signal is present on the car-
rier frequency of the system. The “pilot tone” frequency
isdifferentforeachofthe256availablecarrierfrequen-
cies to prevent the pilot tone from being transferred to
the wrong receiver via an intermodulation product.
Wide-Band Deviation
A±75kHzdeviationimprovesthecaptureratio,signal
to noise ratio and AM rejection of a wireless system
dramatically, compared to other designs that use 30
kHzto40kHzdeviation.Thiscombinedwithafull100
mWofpoweroutputmakesasignicantimprovementin
signaltonoiseratioandmaximumoperatingrange.
Magnetic Power/Mute Switch
A magnetic switch is used to control the application of
power to the MM400C circuits and as an audio mute
switch. The function of this switch can be set by the
user. (See Controls and Functions, Magnetic Power
ON/OFF Switch.)
Battery Life
Switching power supplies throughout the design allow
over 6 hours of operation using a single AA lithium
battery.(AnalkalineAAbatterywillprovideabout1.75
hours and a 2200 mAh NiMH AA battery will provide
about 4 hours of operating time.) The battery contacts
are spring loaded in order to prevent “rattle” as the unit
is handled.
Frequency Agility
Thetransmittersectionusesasynthesized,frequency
selectablemainoscillator.Thefrequencyisextremely
stable over a wide temperature range and over time.
Two 16-position rotary switches, located on the under a
waterproofcoveronthebottomoftheunit,provide256
frequenciesin100kHzstepsovera25.5MHzrange.
This alleviates carrier interference problems, a definite
problem in mobile or traveling applications.
Circulator/Isolator
TheRFoutputcircuitincludesaonewaycirculator/iso-
latorusingamagneticallypolarizedferrite.Thisdevice
greatly reduces RF intermodulation produced when
multiple transmitters are used within a few feet of each
other by blocking RF from returning back into the output
stage. It also provides additional RF output stage
protection but is rarely seen in a wireless microphone
transmitter due to its high cost.