Lectrosonics UT200c - Manual (Page 3)

Frequency Agile Handheld Transmitter
GENERAL TECHNICAL DESCRIPTION
The UT200C 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
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.
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 noticeably 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 essentially provides a
dynamic pre-emphasis/de-emphasis function with extremely low distortion.
MIC
PWR MUTE
+9VDC
TRANSMITTER
Vref
BASS
TREBLE
LP FILTER
HP FILTER
SET
LED
LIMIT
LED
COMPANDOR
Vreg
Vreg
+5VDC
+3.6VDC
SHUNT
LIMITER
INPUT
AMP
AUDIO
LEVEL
LP
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
PREAMP ELEMENT
Rio Rancho, NM - USA 3