Lectrosonics UT195 - Manual (Page 4)

GENERAL TECHNICAL DESCRIPTION - UR195 RECEIVER
The UR195 is a high performance, dual-conversion, UHF receiver.
The RF performance is extremely stable over a very wide tempera­
ture range, making the UR195 perfectly suited to the rough environ­
mental 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 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.
SIX-POLE HELICAL RESONATOR FRONT-END
The UR195 utilizes a six-pole helical resonator for front-end
filtering. The helical resonators are custom manufactured in-house
to provide the high performance needed, yet still fit into the small
UR195 package. This outstanding front-end keeps the UR195 from
being affected by high power, adjacent RF signals and also provides
extremely high image rejection.
GaAs FET FRONT-END FILTER COUPLING
The UR195 utilizes an ultra low noise GaAs FET amplifier in the
front-end section to compensate for the required losses between
filter stages. The GaAs FET devices are extremely quiet, especially
at the higher frequencies in the UHF band.
DOUBLE BALANCED DIODE MIXERS
A double balanced diode mixer is used in the UR195 to produce the
10.7 MHz IF signal. The mixer produces output at only the sum and
difference signals, with minimal spurious signals. This mixer offers
a very high overload threshold and a high degree of isolation
between ports. This translates to the ability of the receiver to accept
higher input signals without overloading and causing distortion and
less cross talk between receivers in multiple system installations.
10 POLE LINEAR PHASE FILTER
The 1st IF amplifier is a 4 stage amp with 2 poles of filtering after
each stage. The filters are high quality, low distortion, constant
group delay ceramic filters. This special type of filter is needed to
accommodate the wide deviation of the 195 system. The 2nd IF
incorporates 2 more poles of filtering.
DIGITAL PULSE COUNTING DETECTOR
The UR195 receiver uses an advanced digital pulse counting
detector to demodulate the FM signal, rather than a conventional
quadrature detector. The most common problem with quadrature
detectors is thermal drift, particularly those that operate at higher
frequencies like 10.7 MHz. The UR195 design presents an elegantly
simple, yet highly effective solution to this age old problem.
The UR195 detector basically works like this: A stream of DC
pulses is generated at 455kHz. The pulse width is constant, but the
timing between pulses varies with the frequency shift of the FM
signal. The pulse stream is controlled by the FM signal coming
from the IF section which has been heavily limited. The average
voltage of the pulses within any given time interval varies in direct
proportion to the frequency modulation of the radio signal, produc­
ing the audio signal.
This type of detector eliminates the traditional problems with
quadrature detectors and provides very low audio distortion and no
thermal drift. The counting detector also adds additional AM
rejection, in addition to the limiting in the IF section. The amplitude
of the pulses is constant, so level differences in the IF signal do not
affect the pulse.
2:1 EXPANDER (Dual–Band Compandor)
This circuit is the other half of the dual-band compandor in a 195
system. “Dual-band Companding” is a closed loop system, that is,
whatever is done in the transmitter must be mirrored in the receiver.
The transmitter compresses the audio signal in two separate audio
bands using two separate time constants in order to avoid the
inevitable trade-offs in a single-band compandor. The companion
circuit in the receiver then re-expands this compressed signal
restoring the original dynamic range and frequency characteristics of
the signal.
HELICAL
RESONATOR
GaAS
FET
470 - 608
MHz
XTAL
CONTROLLED
1ST
OSCILLATOR
8 POLE
LINEAR PHASE
FILTER
455KHZ
BP
FILTER
10.7MHz IF
AMP
2ND MIXER
&
IF AMP
COUNTING
DETECTOR
32 kHZ
BLOCKING
FILTER
HI-LEVEL
DIODE MIXER
XTAL
CONTROLLED
2ND
OSCILLATOR
2:1
EXPANDER
TREBLE
2:1
EXPANDER
BASS
AUDIO
AMP
MONITOR
OUT
OUTPUT LEVEL ADJUST
AND
RANGE SWITCH
XLR
OUT
PILOT
TONE
MUTE
HELICAL
RESONATOR
AMP
DISABLE
PILOT
TONE
MOD AUDIO
LED BARGRAPH METER
AMP
PILOT
LED
AMP ENABLE
32kHZ
V+
MONITOR
LEVEL
3
2
1
METER
MODE
HI
FIXED
HI
FIXED
Figure 1 - UR195 Block Diagram
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