Lectrosonics UM250c User Manual download pdf (Page 4)

UM250C

GENERAL TECHNICAL DESCRIPTION

GENERAL

The 200 series uses 75kHz wide deviation for an ex-

tremely 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 distor-

tion 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 dis-

torted. 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 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 constants, reducing low frequency distortion

to well below that of a conventional compandor.

NO PRE-EMPHASIS/DE-EMPHASIS

The signal to noise ratio of the 200 system is high enough

to preclude the need for conventional pre-emphasis (HF

boost) in the transmitter and de-emphasis (HF roll off) in

the receiver. Pre-emphasis and de-emphasis in an FM

radio system usually provides about a 10dB improvement

in the signal to noise ratio of the system, but the high

frequency boost in the transmitter must be removed in a

purely complementary manner or else the frequency

response of the original audio signal will be altered.

Pre-emphasis can also cause distortion in the receiver.

As this signal is passed through the IF filters in the

receiver, distortion can be produced, most noticeable at

full modulation. De-emphasis cannot be applied until the

signal is converted into audio, so there is no way around

this problem short of eliminating pre-emphasis altogether.

Neither of these problems occur in the 200 system. The

dual-band compandor in the 200 Series system essen-

tially provides a dynamic pre-emphasis/de-emphasis

function with extremely low distortion.

PILOT TONE SQUELCH

The 200 system utilizes an ultrasonic tone modulation of

the carrier to operate the receiver squelch. This “pilot

tone” consists of a 32kHz signal mixed with the audio

Vref

BASS

TREBLE

LP FILTER

HP FILTER

SET

LED

COMPANDOR

Vreg

+5VDC

+3.6VDC

SHUNT

LIMITER

PEAK AUDIO

INDICATOR &

LIMITER

DRIVER

PILOT

TONE

OSC

COMPANDED AUDIO

TO XMTR

COMPANDED

AUDIO

VOLTAGE

CONTROLLED

OSCILLATOR

FREQ

SWITCHES

DIVIDER

LOW PASS

FILTER

PRESCALER

INPUT

AMP

AUDIO

LEVEL

FILTER

MIC

JACK

+5V BIAS

SUPPLY

ROLL-OFF

BUFFER

PWR

DELAY

+9VDC

PWR

LED

LIMIT

LED

CIRCULAR

ISOLATOR

TRANSMITTER

PHASE LOCKED LOOP

UM250C Block Diagram

1 2 3 4 5 6 7 8 9 ... 15 16

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Lectrosonics UM250c User Manual Page 4

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