Lectrosonics UCR195 - Manual (Page 7)

Compact UHF Receiver
The UCR195 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, producing 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 com­
presses 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.
The audio signal leaves the Detector circuit and is fed
through an amplifier to the 23 kHz Low Pass Filter
where all the high frequency noise (including the 32 kHz
pilot tone) is filtered out. After the 23 kHz low pass
filter, the signal is split into two parts via a 1 kHz low
pass filter and a 1 kHz high pass filter. The separated
signals are then processed in separate channels of the
NE572 2:1 Expander. Each channel of the 2:1 Expander
is optimized for its respective frequency band. The two
outputs of the 2:1 Expander are then summed in an op­
amp summer and sent to the output amplifier as one
signal.
PILOT TONE MUTE (SQUELCH)
The 195 system utilizes a separate ultrasonic tone
modulation of the basic carrier to operate the receiver
squelch. In the transmitter, a 32kHz tone is injected into
the audio signal path just after the compandor. The
supersonic pilot tone is filtered out of the audio signal
immediately after the detector in the receiver so that it
does not influence the compandor or various gain stages.
The basic benefit of the pilot tone squelch system is that
the receiver will remain squelched (muted) until it
receives the pilot tone from the matching transmitter,
even if a strong RF signal is present on the carrier
frequency of the system. Once a pilot tone is received,
the receiver will remain open during all signal condi­
tions.
The mute circuit drives a relay which physically discon­
nects the output amplifier from the output. This provides
complete muting of the audio and the noise. The pilot
tone function may be bypassed with the Pilot Tone
Disable switch (located in the battery compartment.)
When the pilot tone has been disabled with this switch,
the MOD function of the LED bargraph meter on the
front panel is also disabled. The Pilot LED on the front
panel will glow when the pilot tone has enabled the
receiver audio output.
OUTPUT LEVEL ADJUST AND RANGE SWITCH
The front panel Output control will adjust the audio
output within the range set by the Low/High/Fixed range
switch (located inside the battery compartment.) In the
Low position (toward the front panel) the adjustment
range is from –50dBm to –20dBm, the High position
(center) allows an adjustment from –30dBm to 0dBm,
and the Fixed position (toward the rear panel) sets the
audio output to a fixed +8dBm with no front panel
control.
POWER SUPPLY
The UCR195 may be operated from internal 9 Volt
batteries, from the supplied CH12 adapter, or from an
external 12 to 18 VDC source. The power supply has
protection circuits which prevent damage to the receiver
if a positive ground voltage source is used. The power
supply also has a built in Poly-Fuse to protect the unit.
This fuse is self healing by simply turning off the
receiver for about 15 seconds.
Rio Rancho, NM – USA 7