Lectrosonics UT700 - Manual (Page 6)

UDR700 / UM700 / UT700
The 244 MHz from the first IF is reduced to the second
IF of 10.7 MHz, and is then fed to the Quadrature
Detector. The first mixer is a GaAs MMIC device with a
rated IP3 (third order intercept) of +24 dBm to minimize
undesired IM products. Because the signal is digital,
thermal drift in the detector has little effect on the
signal’s content, unlike an analog receiver.
Digital Demodulator
The digital demodulator consists of a PLL clock recov­
ery circuit and a bit slicer. The recovered bit clock and
data stream are fed to the DSP. In addition, the recov­
ered bit clock is used to derive the timings for the
receiver’s digital audio circuits, so the receiver audio is
synchronous with the transmitter audio, sample for
sample.
Digital Signal Processing
The DSP uses the packet headers as a timing refer­
ence to recover the digital audio data. The data is then
decrypted and decoded to recover the original digitized
audio samples. Additionally, the packet headers also
contain transmitter battery status information, which is
extracted by the DSP.
Audio Output
The digital audio samples are sent to the AES-3id
transmitter for digital audio output, and to the D/A
converter for conversion to analog. The converter
output is filtered and amplified, and then fed to a
transformer, attenuator and XLR jack. The analog
output is also sent to a separate headphone amplifier
on the UCR700 front panel for monitoring purposes.
The signal at the headphone output is taken just ahead
of the output transformer, so it provides the same audio
quality as the XLR output and can also be used as a
second output or recorder feed.
UM700 Block Diagram
The main audio output amplifier is set for maximum
gain. This output passes through a passive attenuator
and phase switch on its way to the rear panel Audio
Output XLR jack. The passive attenuator reduces the
audio level in calibrated 5 dB steps, ensuring the signal-
to-noise ratio of the receiver remains the same at all
output levels selected by the rear panel control.
Encrypted Digital Transmitters
Two encrypted digital transmitters are offered, a belt
pack unit and a hand held unit. Although their physical
packages differ, internally, they are essentially the
same. The major difference is the microphone input
jack in the UM700 and the VariMic
™ used in the UT700
hand held transmitter.
The microphone input jack used in the UM700 is
configured so that virtually every lavaliere, hand-held or
shotgun mic can be used, regardless of whether they
use positive or negative bias. (See UM700 Controls
and Functions and 5-Pin Input Jack Wiring.)
The UT700 uses a built in microphone element. (See
UT700 Microphone Element.)
DSP-Cntrolled Dual Envelope Analog Limiter
In order to make the best use of the high quality A/D
converter, microphone audio is limited in the analog
domain first, before being sampled. The DSP controls
this process, but because the limiting is done in the
analog domain, levels near the converter’s maximum
may be used without fear of clipping. (The limiter has a
range of more than 30 dB for excellent overload protec­
tion.)
The limiter has a fast attack, but different release
characteristics, depending on the nature of the signal
that drove the input into limiting. Brief transients result
in a fast decay, to avoid “pumping” effects, while sus­
tained loud signals result in a slower decay, to keep
distortion to a minimum. The result is a transparent-
sounding limiter with excellent low distortion character­
istics.
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
Hi/Lo
Pass
Filter
Audio
Bicolor
Power
LED
Audio
Level
Input
Amp
LF
Rolloff
Digital
Modulator
RF
Amp
5
4
3
2
1
+3.3V Bias
Supply
Mic
Jack
Dual
Envelope
Limiter
Encode Encrypt
Voltage
Controlled
Oscillator
Encryption
Key Link
LECTROSONICS, INC. 6