SRc-941 Dual Receiver
LECTROSONICS, INC.
4
IF Amplifiers and SAW Filters
The first IF stage employs two SAW (surface acoustic
wave) filters. The use of two filters significantly increas-
es the depth of filtering while preserving sharp skirts,
constant group delay, and narrow bandwidth. Though
expensive, this special type of filter allows primary filter-
ing as early as possible, at as high a frequency as pos-
sible, before high gain is applied, to deliver maximum
image rejection. These filters are made of quartz, and
they are very temperature stable.
In receiver 1, the signal is converted to 248.450 MHz
in the first mixer stage, then passed through two SAW
filters. After the SAW filters, the signal is converted to
350 kHz and then the majority of the gain is applied.
In receiver 2, the same conversions take place at differ-
ent frequencies: first to 243.950 MHz, then to 250 kHz.
Digital Pulse Counting Detector
Following the IF section, the receiver uses an elegantly
simple, yet highly effective digital pulse counting de-
tector to demodulate the FM signal and generate the
audio, rather than a conventional quadrature detector.
This unusual design eliminates thermal drift, improves
AM rejection, and provides very low audio distortion.
The output of the detector is fed to the microprocessor
where a window detector is employed as part of the
squelch system.
DSP-Based Pilot Tone
The Digital Hybrid system design uses a DSP generat-
ed ultrasonic pilot tone to reliably mute the audio when
no RF carrier is present. The pilot tone must be present
in conjunction with a usable RF signal before the audio
output will be enabled. A unique pilot tone is generated
every 100 kHz across the tuning ranges of the 941 MHz
system. This alleviates erroneous squelch activity in
multichannel systems where a pilot tone signal can ap-
pear in the wrong receiver via IM (intermodulation).
The 941 MHz models can tune in either 25 kHz or 100
kHz steps, and the pilot tones increment in 100 kHz
steps. The systems are never operated within 100kHz
spacing, so the pilot tone is the same for all four ad-
jacent frequencies in each 100 kHz increment. For
example, 959.300, 959.325, 959.350 and 959.375 all
have the same pilot tone.
SmartSquelch
™
A DSP-based algorithm called SmartSquelch
TM
optimizes
the receiver performance in very weak signal conditions.
The RF level and supersonic noise in the audio are con-
tinuously monitored to determine the appropriate noise
reduction needed and the point at which squelch (com-
plete muting of the audio) is necessary.
As the RF level decreases and supersonic noise in the
signal begins to increase, a variable knee, high frequency
roll-off filter is applied to suppress high frequency noise.
The filtering action moves in and out smoothly to avoid
abrupt changes that could be audible. When the RF signal
becomes so weak that the receiver can no longer deliver
usable audio, the squelch will activate.
SmartDiversity
™
Microprocessor controlled antenna phase combin-
ing is used for diversity reception. When the incoming
RF level drops to a certain point, the phase (polarity)
of one antenna is reversed and the resulting level is
compared with the one in the previous state. If the level
has increased, the phase is retained. If the level has
decreased, the phase is reversed back to the previous
state.
The logic behind this design is based upon the fact that
two antennas mixed in phase will deliver a stronger
signal than either antenna by itself. If the antennas are
spaced more than a few inches apart, the signals arriv-
ing at them will be non-correlated (diverse), to effec-
tively deal with multi-path dropouts.
Turn On and Turn Off Delays
A brief delay is applied when the receiver is powered up
or down to prevent audible noise such as a thump, pop,
click or other transient noise.
Test Tone
To assist in matching the audio levels of equipment con-
nected to the receiver, a 1 kHz audio test tone genera-
tor is provided, with an output level adjustable from -50
to +5 dBu in 1 dB increments.
The tone simulates the audio output with a steady sig-
nal at full modulation, making it easy to adjust the level
to precisely match the optimal level for the connected
device and maximize the signal to noise ratio of the
system.
