Lectrosonics UM250b User Manual download pdf (Page 4)

GENERAL TECHNICAL DESCRIPTION

The UM250B transmitters are comprised of a number of functional sub-systems as shown in the block diagram below.

GENERAL

The 200 series uses 75kHz 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 (7 Volts) of battery life. The input amplifier

uses a Motorola 33078 op amp for ultra low noise operation. It is gain controlled with a wide range input compressor

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 200 system

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

There are actually two separate compandors in the 200 system, one for high frequencies and one for low frequencies.

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 constants,

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

+5V BIAS

SUPPLY

PWR

+9VDC

TRANSMITTER

Vref

BASS

TREBLE

LP FILTER

HP FILTER

SET

LED

LIMIT

LED

COMPANDOR

Vreg

+5VDC

+3.6VDC

SHUNT

LIMITER

INPUT

AMP

AUDIO

LEVEL

FILTER

PEAK AUDIO

INDICATOR &

LIMITER

DRIVER

PILOT

TONE

OSC

MIC

JACK

ROLL-OFF

COMPANDED AUDIO

TO XMTR

PWR

LED

COMPANDED

AUDIO

PHASE LOCKED LOOP

VOLTAGE

CONTROLLED

OSCILLATOR

FREQ

SWITCHES

DIVIDER

LOW PASS

FILTER

PRESCALER

DELAY

UM250B Block Diagram

1 2 3 4 5 6 7 8 9 ... 17 18

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Lectrosonics UM250b User Manual Page 4

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