4
UM250C Block Diagram
GENERAL TECHNICAL DESCRIPTION
The transmitter is 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.
TRANSMITTER
Vref
BASS
TREBLE
LP FILTER
HP FILTER
SET
LED
COMPANDOR
Vreg
Vreg
+5VDC
+3.6VDC
SHUNT
LIMITER
PEAK AUDIO
INDICATOR &
LIMITER
DRIVER
PILOT
TONE
OSC
COMPANDED AUDIO
TO XMTR
COMPANDED
AUDIO
PHASE LOCKED LOOP
VOLTAGE
CONTROLLED
OSCILLATOR
FREQ
SWITCHES
DIVIDER
LOW PASS
FILTER
PRESCALER
INPUT
AMP
AUDIO
LEVEL
LP
FILTER
5
4
3
2
1
MIC
JACK
+5V BIAS
SUPPLY
LF
ROLL-OFF
BUFFER
PWR
DELAY
+9VDC
PWR
LED
LIMIT
LED
50 CIRCULAR
ISOLATOR