Lectrosonics UCR210d - Manual (Page 3)

UHF Wireless Diversity Receiver
GENERAL TECHNICAL DESCRIPTION
The UCR210D is a portable, high performance, triple-conversion,
frequency synthesized, UHF receiver. The RF performance is
extremely stable over a very wide temperature range, making the
UCR210D perfectly suited to the rough environmental conditions
found in the field. The proprietary audio processing includes a
dual-band compandor for very low distortion and a superior signal
to noise ratio. The Smart Squelch system is operated by a separate
pilot tone and mutes the audio output directly at the output connec­
tor.
DIVERSITY RECEPTION
The antenna phase SMART switching diversity technique was
chosen in order to keep the receiver compact enough for camera
mounted or shoulder bag applications. This diversity reception
technique effectively minimizes dropouts in short range situations
where multi-path reflections can cause serious problems. The
optimum diversity reception is realized with the diversity antenna
placed away from the receiver, however, dropouts are significantly
reduced even if the two antennas are mounted directly on the
receiver.
RF SECTION
The problem posed to the design staff was to retain the RF reliabil­
ity of the Lectrosonics’ fixed frequency designs but add the
flexibility of a frequency agile design. The universal (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 pro­
gram. Basically, the show must go on. In multi-channel
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 consists of a total of
4 transmission line resonators with variable capacitance applied to
each resonator by the hexadecimal switches.
This sophistication produces a front end that is as selective as fixed
frequency designs. The next step to improve the front end is to use
good old fashioned “brute force.”
HIGH CURRENT LOW NOISE AMPLIFIERS
The gain stage in the front end uses a rather special transistor in a
feedback regulated high current circuit that combines three param­
eters 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 consider­
ation 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 stage with the tracking front end
produces a receiver that is unusually immune to single and multiple
interfering signals close to the operating frequency and in addition
strongly rejects signals that are much farther away.
FILTER
AMP
1ST
LOCAL
OSCILLATOR
2:1
EXPANDER
TREBLE
2:1
EXPANDER
BASS
23 KHZ
LP
FILTER
OUTPUT
LEVEL
ADJUST
XLR
OUT
PILOT
TONE
MUTE
FILTER
uP
HI-LEVEL
DIODE MIXER
RF MODULE
3RD MIXER
AND
IF AMP
50KHz
LP FILTER
XTAL
CONTROLLED
3rd
OSCILLATOR
RF LEVEL
LEDs SAW
FILTER
71 MHz
IF AMP COUNTING
DETECTOR
AUDIO
AMP
uP
UCR210D
BLOCK DIAGRAM
uP
VARIABLE
CUT-OFF
LP FILTER
TO DATA
DISPLAY
ANTENNA
SWITCHING
FREQ
SWITCHES
FILTER
uP
E PROM 2
FILTER
2nd
VCO
Smart Diversity
Smart Squelch
2ND
MIXER
10.7 MHZ
SYNTHESIZER
Rio Rancho, NM – USA 3