Lectrosonics UH200 User Manual download pdf (Page 3)

Frequency Agile Plug-on UHF Transmitter

GENERAL TECHNICAL DESCRIPTION

The UH200 transmitters are comprised of a number of functional

sub-systems as shown in the block diagram below.

GENERAL

The 200 system uses 75kHz wide deviation for an extremely

high signal to noise ratio. The transmitter circuits are all regu-

lated to allow full output power from the beginning (9 Volts) to

the end (6.5 Volts) of battery life. The input amplifier uses an

ultra low noise op amp. 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 conven-

tional 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 cross-

over 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 fre-

quency 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.

MIC

+5V BIAS

JACK

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 eliminat-

ing pre-emphasis altogether. Neither of these problems occur

in the 200 system. The dual-band compandor in the 200 Series

system essentially provides a dynamic pre-emphasis/de-em-

phasis 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” con-

sists of a 32kHz signal mixed with the audio signal after the

compandor, to control the audio output muting of the receiver.

The pilot tone is filtered out of the audio signal immediately after

the detector in the receiver so that it does not influence the

compandor or various gain stages. The basic benefit of the pilot

tone squelch system is that the receiver will remain muted until it

receives the pilot tone from the matching transmitter, even if a

strong RF signal is present on the carrier frequency of the

system. This is extremely important in applications that include

an automatic microphone mixer.

WIDE-BAND DEVIATION

±75kHz deviation improves the capture ratio, signal to noise

ratio and AM rejection of a wireless system dramatically, com-

pared to the more commonly used ±15kHz deviation.

LONG BATTERY LIFE

High efficiency circuits throughout the design allow over 4.5

hours of operation using a single 9 Volt alkaline battery. (A 9V

lithium battery will provide over 12 hours of operation.) The

battery compartment is a unique mechanical design which auto-

matically adjusts to fit any brand of battery. The battery contacts

are spring loaded to prevent “rattle” as the unit is handled.

FREQUENCY AGILITY

The transmitter section uses a synthesized, frequency select-

able main oscillator. The frequency is extremely stable over a

wide temperature range and over time.

Two rotary switches, located on the side panel of the unit,

provide 256 frequencies in 100kHz steps over a 25.5MHz range.

This alleviates carrier interference problems in mobile or travel-

PWR

MUTE

+9VDC

TRANSMITTER

Vref

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

COMPANDED AUDIO

TO XMTR

PWR

LED

COMPANDED

AUDIO

PHASE LOCKED LOOP

VOLTAGE

CONTROLLED

OSCILLATOR

FREQ

SWITCHES

DIVIDER

LOW PASS

FILTER

PRESCALER

SUPPLY

BUFFER

ling applications.

UH200 Block Diagram

BASS

TREBLE

Rio Rancho, NM – USA

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Lectrosonics UH200 User Manual Page 3

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