Lectrosonics UDR195 - Manual (Page 7)

Wireless Diversity Receiver
DIGITAL PULSE COUNTING DETECTOR
The DR195 receiver uses an advanced digital pulse detector to
demodulate the FM signal, rather than a conventional quadra­
ture detector. The most common problem with quadrature
detectors is thermal drift, particularly those that operate at
higher frequencies like 10.7 MHz. The DR195 design
presents an elegantly simple, yet highly effective solution to
this age old problem.
The DR195 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,
high temperature stability 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.
RATIO COMBINING DIVERSITY WITH OPTI-BLEND
The audio outputs of the receivers are mixed together in a ratio
controlled by the RF level of the received signal.
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 transmiter 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.
The mixed audio signal leaves the Opti-Blend circuit and is
fed through a 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 NE575 2:1 Expander. Each channel of the 2:1
Expander is optimized for it's respective frequency band. The
two outputs of the 2:1 Expander are then summed in an op­
amp summer and sent to the Variable Cut-off Low Pass Filter
as one signal.
VARIABLE CUTOFF LOW PASS FILTER
After being processed in the 2:1 expander, the audio signal is
passed through a “dynamic noise reduction circuit”. The
cutoff frequency of this filter is varied automatically by
sampling the audio signal. The audio bandwidth is held only
to that point necessary to pass the highest frequency audio
signal present at the time. This results in a dramatic reduction
of “hiss” during pauses in voice or music content. During
passages with a large high frequency content, this filter gets
completely “out of the way” and passes the signal with no
decrease in high-frequency response.
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 after
the microphone preamp, just after the compandor. The
supersonic pilot tone is filtered out of the audio signal imme­
diately 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 conditions. If the transmitter signal degrades to the
point where hiss and noise may become objectionable, the
Opti-Blend and Variable Cutoff Low Pass Filter will work to
reduce the unwanted noise to an unnoticeable level. Since the
pilot tone is still keeping the receiver open, as soon as the
transmitter signal returns to normal, the audio signal is
instantly available with no delays.
The mute circuit drives a relay which physically disconnects
the output amplifier from the output audio transformer. The
relay then connnects the transformer primary to ground to
prevent hum pickup in the transformer due to an open primary
winding. This provides complete muting of the audio and the
noise. The pilot tone function may be bypassed with a rear
panel push button. Once pushed, the pilot tone mute is
“latched” in a disabled condition until the receiver is powered
off then back on. When the pilot tone is disabled, there is still
a “squelching” function provided by the Opti-Blend and
Variable Cutoff Low Pass Filter. These circuits can provide
approximately 50 dB of muting during weak or no signal
conditions when the pilot tone is disabled.
Rio Rancho, NM – USA 7