General Technical Description - Lectrosonics UM300B Operating Instructions Manual

Frequency-agile uhf belt-pack transmitter

Table of Contents

The UM300B transmitters are comprised of a number of functional sub-systems as shown in the block diagram

below.

GENERAL

The 300 system uses 50kHz wide deviation for an extremely high signal to noise ratio. The transmitter circuits are all

regulated to allow full output power from the beginning (9 Volts) to the end (6.5 Volts) of battery life. The input ampli

fier uses a Motorola 33178 op amp for ultra low noise operation. It is gain controlled with a wide range input com

pressor which cleanly limits input signal peaks over 30dB above full modulation.

DUAL BAND COMPANDOR

Traditionally, compandors have been a source of distortion in wireless microphone systems. The basic problem with

conventional systems is that the attack and decay times are always a compromise. If the time constants are fast,

high frequency transients will not be distorted, but this will cause low frequency distortion. If the time constants are

slower, low frequency audio distortion will be low, but high frequency transients will then be distorted. The 300

system introduces an entirely new approach to solving this basic problem, called "dual-band companding."

There are actually two separate compandors in the 300 system, one for high frequencies and one for low frequen

cies. A crossover network separates the frequency bands at 1kHz with a 6dB per octave slope, followed by separate

high and low frequency compandors. The attack and release times in the high frequency compandor are fast enough

to keep high frequency transient distortion at a low level, and the low frequency compandor uses slower time con

stants, reducing low frequency distortion to well below that of a conventional compandor.

+5V BIAS

SUPPLY

MIC

JACK

BUFFER

SHUNT

LIMITER

LIMIT

LED

SYNTHESIZER

FREQ

SWITCHES