Lectrosonics UDR200c - Manual (Page 4)

4
The UDR200C consists of two high performance, triple-conver-
sion receivers operating simultaneously. The audio outputs of
the receivers are blended in a ratio controlled by the compara-
tive RF levels in the receivers. The multistage RF front end is
a unique design that is tuned by a microprocessor to the
selected frequency. The RF and audio performance is ex-
tremely stable over a very wide temperature range, making the
UDR200C perfectly suited to mounting in studio equipment
racks. The proprietary audio processing includes a dual-band
compandor and dynamic noise filter for very low distortion and
a superior signal to noise ratio. The squelch system is oper-
ated by a separate pilot tone and mutes the audio output
directly at the output connector. The audio output is calibrated
for exact level matching, with wide range, peak responding
LED indicators.
RF SECTION
The problem posed to the design staff was to retain the RF
reliability of the Lectrosonics’ fixed frequency designs but add
the frequency flexibility of a frequency agile design. The uni-
versal (and poor) way to build frequency agile systems is to
design a wide open front end that will pass any frequency
within the tuning range of the system. This leads to very poor
RF performance with lots of interference, driving the user to
switch frequencies in an attempt to sidestep the interference.
This makes frequency agile receivers a self fulfilling system;
you have to use the frequency agility to get away from the
problems caused by the frequency agile design compromises.
The problem of frequency agility is further compounded when
you realize that frequency changes “on the fly” cannot be
made on any type of wireless system. For example, if there is
suddenly an interference problem with a system in use, on
stage for instance, a frequency change cannot be made
without interrupting the program. Basically, the show must go
on. In multichannel applications, changing the frequency of
one system will usually produce all kinds of new
intermodulation problems with the other systems operating in
the same location. Frequency agility is not the universal
panacea for interference problems. It is only another tool and
a limited tool at that. The first line of defense must be the
system’s basic immunity to interference. That required a new
look at frequency agile receiver design.
FREQUENCY TRACKING FRONT-END
Our solution to the wide open front end problem was to design
a selective front end that can be tuned to the frequency in use.
Since we wanted this front end to be equivalent to our fixed
frequency front ends, this was a daunting task. Lectrosonics
has always used front ends with more sections and much
more selectivity than any other wireless manufacturer. The
final design consisted of a total of 12 transmission line resona-
tors with variable capacitance applied to each resonator by a
microprocessor. This allows each resonator to be individually
tuned by the microprocessor for any user selected frequency
in a 25 MHz band. This sophistication produced a front end
that was as selective as fixed frequency designs, yet could
cover the entire 25 MHz range.
HIGH CURRENT LOW NOISE AMPLIFIERS
The gain stages in the front end use some rather special
transistors in a feedback regulated high current circuit that
combine three parameters that are generally at odds with one
another. These are: low noise, low gain and relatively high
power. It is easy to understand the advantages of low noise
and high power capability but why is low gain desirable? The
answer is that in a receiver, low gain allows the front end to
handle stronger RF signals without output overload, which is
“increased headroom,” so to speak. The result of a design that
takes all three of these parameters into consideration at once,
is a low noise RF amplifier with a sensitivity rating equal or
better than the best conventional design with a hundred times
less susceptibility to intermodulation interference.
Combining the high power gain stages with the tracking front
end produces a receiver that is unusually immune to single
and multiple interfering signals close to the operating fre-
quency and in addition strongly rejects signals that are much
farther away.
1st Local
Oscillator Ratio
Combiner &
Opti-Blend
2:1
Expander
Treble
2:1
Expander
Bass
23 KHZ
LP
Filter
Headphone
Out
Output
Level
Adjust
XLR
Out
Pilot
Tone
Mute
uP
Hi-Level
Diode Mixer
HI-LEVEL
DIODE MIXER
RF Module
2nd Mixer
10.7 MHz
50KHz
LP Filter
50KHz
LP Filter
2nd Local
Oscillator
RF Level
LEDs
RF Level
LEDs
Saw
Filter
71 MHz
IF Amp
Counting
Detector
Counting
Detector
Filter
Amp
Filter
Amp
Filter
Center
Frequency
Detector
Center
Frequency
Detector
Audio
Amp
uP
UDR200C
BLOCK DIAGRAM
RF
Only Tri-Mode
Dynamic
uP
uP
uP
uP uP
Variable
Cut-Off
LP Filter
To Data
Display
Filter
Synthesizer
Filter
Freq
Select uP
E PROM 2
3rd Mixer
& IF Amp
300kHz
2nd Mixer
10.7 MHz 71 MHz
IF Amp
3rd Mixer
& IF Amp
300kHz
Xtal
Controlled
3rd
Oscillator
Filter
Amp
Filter
Amp
Filter
Saw
Filter
GENERAL TECHNICAL DESCRIPTION