Lectrosonics UH200c - Manual (Page 3)

Frequency Agile Plug-on UHF Transmitter
GENERAL TECHNICAL DESCRIPTION
The UH200C transmitters are comprised of a number of func­
tional 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
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
SUPPLY
BUFFER
ling applications.
UH200C Block Diagram
BASS
TREBLE
Rio Rancho, NM – USA 3