Lectrosonics UT700 - Manual (Page 7)

Encrypted Digital Wireless System
Digital Signal Processing and Modulation
The preamplified and limited audio signal is converted
to digital using a 24-bit A/D converter and fed to the
DSP. Within the DSP, the audio is encoded to reduce
the bit rate and increase entropy in the data stream
prior to encryption. The data stream is then encrypted
and apportioned into packets, delimited by packet
headers. The complete bit stream is modulated onto
the carrier using a modified pi/4 DQPSK (differential
quadrature phase shift keying) method. This modula­
tion method makes efficient use of the RF spectrum
and is easy to demodulate reliably.
Transmitter RF Output Section
Intermodulation (IM) is a problem that occurs in the final
amplifier stages of conventional transmitters when the
transmitters are within a few feet of each other. This
can create serious problems in multichannel wireless
systems when an IM signal falls in the same range as
the carriers, IF frequencies, local oscillator or image
frequencies of the systems being operated. The
UM700 and UT700 eliminate this problem by passing
the modulated radio signal through a circular isolator
before it enters the antenna. The circular isolator
functions like a “one-way check valve,” allowing the RF
signal to pass through to the antenna, but not allowing
spurious RF to pass back into the transmitter amplifier
stage. This provides excellent stability and eliminates
IM in the output stage of the transmitter.
Long Battery Life
The use of highly efficient circuits and switching power
supplies throughout the design allow over 4 hours of
operation using a single 9 volt alkaline battery. (A 9 V
lithium battery will provide over 7.5 hours of operation.)
The battery compartment is a unique mechanical
design which automatically adjusts to fit any brand of 9
volt battery, and the battery contacts are spring loaded
to prevent “rattle” as the unit is handled.
UT700 Block Diagram
Mic
Frequency Agility
700 Series wireless systems are currently available in
four different “blocks” of 256 frequencies each, from
562.200 to 665.500 MHz.
Note: Frequencies between 608.000 MHz to
614.000 MHz are reserved and not available for
commercial use.
Each of these blocks is preset at the factory and
provides 256 selectable frequencies in 100 kHz steps
over a 25.6 MHz bandwidth. This wide variety of
selectable frequencies alleviates carrier interference
problems in mobile or traveling applications. Two 16­
position rotary switches are used to select the fre­
quency.
The transmitter section uses a synthesized, frequency
selectable main oscillator. The frequency is extremely
stable over a wide temperature range and over time.
Antenna
At UHF, where wavelengths and antennas are shorter
than at VHF, a resonant length wire is preferred over
using the microphone cable as the antenna. The
antenna on the UM700 is a flexible 1/4 wavelength
insulated galvanized steel cable, detachable via an
SMA connector. The impedance of this connector is 50
Ohms.
The UT700 has a dipole antenna incorporated into the
circuit board.
UT700 Microphone Element
The UT700 includes the Lectrosonics VariMic TM mic
element. The VariMic TM is a cardioid condenser (back
electret) microphone that is adapted for the unique
circumstances of wireless microphones. It offers
excellent dynamic range while minimizing handling
noise and low frequency noise (rumble or wind).
In the VariMic TM , an unusual pumped source FET circuit
increases the usable dynamic range 12 dB and greatly
reduces distortion, just as if the FET were being
Phase
Locked
Loop
Freq
Switches
11001001
A-D
Converter
Digital Signal Processor
Shunt
Limiter
Bicolor
Modulation
LEDs
Microprocessor
9V
Battery
Switching
Power
Supply
+3.3v
+1.8v
+9v
-3v
50
Isolator
Audio
Bicolor
Power
LED
Audio
Level
Input
Amp
LF
Rolloff
Digital
Modulator
RF
Amp
Dual
Envelope
Limiter
Encode Encrypt
Voltage
Controlled
Oscillator
Encryption
Key Link
Preamp Element
Preamp
Level
Tone
Rio Rancho, NM 7