Lectrosonics UCR205d User Manual download pdf (Page 3)

UHF Wireless Diversity Receiver

GENERAL TECHNICAL DESCRIPTION

The UCR205D is a portable, high performance, dual-conver-

sion, frequency synthesized, UHF receiver. The RF performance

is extremely stable over a very wide temperature range, making

the UCR205D perfectly suited to the rough environmental con-

ditions 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 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

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

FILTER

2:1

EXPANDER

TREBLE

2:1

EXPANDER

BASS

23 KHZ

FILTER

HEADPHONE

OUT

OUTPUT

LEVEL

ADJUST

XLR

PILOT

TONE

MUTE

FILTER

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

UCR205D

BLOCK DIAGRAM

VARIABLE

CUT-OFF

LP FILTER

TO DATA

DISPLAY

ANTENNA

SWITCHING

FREQ

SWITCHES

FILTER

SYNTHESIZER

E PROM

VCO

Rio Rancho, NM – USA

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