Lectrosonics UM250c - Manual (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
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
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
TRANSMITTER PHASE LOCKED LOOP
UM250C Block Diagram
4