Lectrosonics MM400c User Manual download pdf (Page 7)

Watertight Miniature UHF Belt-Pack Transmitter

Rio Rancho, NM

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 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

selected 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 be compati-

ble with Lectrosonics 400 Series receivers and will yield

the best performance when doing so. However, due to

theexibilityofdigitalsignalprocessing,theunit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 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

5.5hoursofoperationusingasingleAAlithiumbattery.

(AnalkalineAAbatterywillprovideabout1.5hours

anda2500mAhNiMHAAbatterywillprovideabout

4.5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 under a wa-

terproofcoveronthebottomoftheunit,provide256

frequenciesin100kHzstepsovera25.5MHzrange.

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 which is rarely seen in a wireless microphone

transmitter due to its high cost.

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