Lectrosonics UDR200b User Manual download pdf (Page 4)

GENERAL TECHNICAL DESCRIPTION

The UDR200B consists of two high performance, dual-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

UDR200B 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 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 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.

FILTER

AMP

1ST

LOCAL

OSCILLATOR

455KHZ

FILTER

RATIO

COMBINER

OPTI-BLEND

455KHZ

FILTER

2:1

EXPANDER

TREBLE

2:1

EXPANDER

BASS

23 KHZ

FILTER

HEADPHONE

OUT

OUTPUT

LEVEL

ADJUST

XLR

OUT

PILOT

TONE

MUTE

FILTER

AMP

FILTER

HI-LEVEL

DIODE MIXER

HI-LEVEL

DIODE MIXER

RF MODULE

2ND MIXER

IF AMP

2ND MIXER

IF AMP

50KHz

LP FILTER

50KHz

LP FILTER

XTAL

CONTROLLED

2ND

OSCILLATOR

RF LEVEL

LEDs

RF LEVEL

LEDs

SAW

FILTER

SAW

FILTER

70 MHz

IF AMP

70 MHz

IF AMP

COUNTING

DETECTOR

COUNTING

DETECTOR

FILTER

AMP

FILTER

AMP

FILTER

CENTER

FREQUENCY

DETECTOR

CENTER

FREQUENCY

DETECTOR

AUDIO

AMP

UDR200B

BLOCK DIAGRAM

ONLY

TRI-MODE

DYNAMIC

uP uP

VARIABLE

CUT-OFF

LP FILTER

TO DATA

DISPLAY

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Lectrosonics UDR200b User Manual Page 4

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