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Signal Encoding and Pilot Tone
In addition to controlling the limiter, the DSP also en-
codes the digitized audio from the A/D converter and
adds an ultrasonic pilot tone to control the squelch in
the receiver. A pilot tone squelch system provides a reli-
able method of keeping a receiver output muted when
the squelch is active, even in the presence of signifi-
cant interference. When the system is operating in the
hybrid mode, a different pilot tone frequency is gen-
erated for each carrier frequency to prevent squelch
problems in multi-channel systems.
Microprocessor Control
A microprocessor monitors user command inputs from
the control panel buttons and numerous other internal
signals. It works intimately with the DSP to ensure the
audio is encoded according to the selected Compat-
ibility Mode and that the correct pilot tone is added to
the encoded signal.
Compatibility Modes
SMB transmitters are designed to operate with Lectro-
sonics Digital Hybrid receivers and will yield the best
performance when doing so, however, due to the flex-
ibility of digital signal processing, they can also operate
with Lectrosonics Euro version IFB receivers.
Control Panel
The control panel includes four membrane switches and
an LCD screen to adjust the operational settings. Multi-
color LEDs are used to indicate audio signal levels for
accurate gain adjustment and for battery status.
Rio Rancho, NM
Super-Minature Belt Pack Transmitters
Battery Options and Operating Time
Switching power supplies convert regulated battery
voltages to operate various circuit stages with maxi-
mum efficiency. With the variety of alkaline, lithium and
rechargeable NiMH batteries available today in the AA
format, there are many choices to maximize operating
time or minimize cost as needed for any application.
The firmware "remembers" the power status when a
battery fails or power is disconnected, so the trans-
mitter will be turned on automatically when power is
restored and previous settings will be enabled.
Frequency Blocks
Lectrosonics established a "block" numbering system
years ago to organize the range of frequencies that
are available. Legacy transmitters and receivers used
two binary switches, each with 16 positions, to set the
operating frequency. 16 x 16 = 256, which defined the
standard number of frequencies in each block to be
256. Since the steps between frequencies is 100 kHz,
this results in a switching range of 25.6 MHz.
The lowest frequency in the switching range divided by
25.6 yields the block number. For example, 640.000 di-
vided by 25.6 equals 25. In other words, block 25 starts
at 640.000 MHz.
To determine what block a particular frequency falls
into, divide the frequency and use the two significant
digits to the left of the decimal. For example, to cal-
culate the block for 580.500 MHz, divide 580 by 25.6,
which equals 22.656, which indicates block 22.
Circulator/Isolator
The RF output circuit includes a one way circulator/iso-
lator using a magnetically polarized ferrite. This device
greatly reduces RF intermodulation produced when
multiple transmitters are used in close proximity to one
another (a meter or two, or less). The isolator also pro-
tects the output amplifier against static shock.
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