Lectrosonics UCR300 - Manual (Page 3)

UHF Wireless Diversity Receiver
GENERAL TECHNICAL DESCRIPTION
The UCR300 is a portable, high performance, dual-conversion,
frequency synthesized, UHF receiver. The RF performance is
extremely stable over a very wide temperature range, making
the UCR300 perfectly suited to the rough environmental condi­
tions 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 squelch system is operated
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 a ten LED bar graph meter.
DIVERSITY RECEPTION
The antenna phase switching diversity technique was chosen in
order to keep the receiver compact enough for camera mounted
or shoulder bag applications. This diversity reception tech­
nique effectively minimizes dropouts in short range situations
where multi-path reflections can cause serious problems. The
optimum diversity reception is realized with the diversity an­
tenna placed away from the receiver, however, dropouts are
significantly reduced with two antennas mounted directly on
the receiver.
RF SECTION
The problem posed to the design staff was to retain the RF
reliability 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 fre­
quency 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 sud­
denly an interference problem with a system in use, on stage for
instance, a frequency change cannot be made without inter­
rupting the program. Basically, the show must go on. In
multi-channel applications, changing the frequency of one sys­
tem 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 re­
ceiver 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 6 transmission line resonators with
variable capacitance applied to each resonator by the hexadeci­
mal switches.
This sophistication produced a front end that was as selective as
fixed frequency designs. The next step to improve the front end
was to use good old fashioned “brute force.”
HIGH CURRENT LOW NOISE AMPLIFIERS
The gain stages in the front end use some rather special transis­
tors 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 head­
room,” 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 frequency
and in addition strongly rejects signals that are much farther
away.
FILTER
AMP
1ST
LOCAL
OSCILLATOR
455KHZ
BP
FILTER
2:1
EXPANDER
TREBLE
2:1
EXPANDER
BASS
23 KHZ
LP
FILTER
HEADPHONE
OUT
OUTPUT
LEVEL
ADJUST
XLR
OUT
PILOT
TONE
MUTE
FILTER
uP
AMP
FILTER
HI-LEVEL
DIODE MIXER
RF MODULE
2ND MIXER
&
IF AMP
50KHz
LP FILTER
XTAL
CONTROLLED
2ND
OSCILLATOR
RF LEVEL
LEDs SAW
FILTER
70 MHz
IF AMP COUNTING
DETECTOR
AUDIO
AMP
uP
UCR300
BLOCK DIAGRAM
uP VARIABLE
CUT-OFF
LP FILTER
TO DATA
DISPLAY
ANTENNA
SWITCHING
FREQ
SWITCHES
FILTER
SYNTHESIZER
uP E PROM 2
VCO
Rio Rancho, NM – USA 3