Linx Technologies TRM-xxx-LT - Manual (Page 9)

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Using LADJ
The Level Adjust (LADJ) line allows the transceiver’s output power to be
easily adjusted for range control, lower power consumption, or to meet
legal requirements. This is done by placing a resistor between V
CC
and
LADJ. The value of the resistor determines the output power level. When
LADJ is connected to V
CC
, the output power and current consumption
are the highest. Figure 5 shows a graph of the output power vs. LADJ
resistance.
This line is very useful during FCC testing to compensate for antenna
gain or other product-specific issues that may cause the output power
to exceed legal limits. A variable resistor can be temporarily used so that
the test lab can precisely adjust the output power to the maximum level
allowed by law. The variable resistor’s value can be noted and a fixed
resistor substituted for final testing. Even in designs where attenuation is
not anticipated, it is a good idea to place a resistor pad connected to LADJ
and V
CC
so that it can be used if needed. For more sophisticated designs,
LADJ can also be controlled by a digital potentiometer to allow precise and
digitally-variable output power control.
Using the RSSI Line
The transceiver’s Received Signal Strength Indicator (RSSI) line serves a
variety of functions. This line has a dynamic range of 80dB (typical) and
outputs a voltage proportional to the incoming signal strength. The RSSI
levels and dynamic range vary slightly from part to part. It is important
to remember that the RSSI output indicates the strength of any in-band
RF energy and not necessarily just that from the intended transmitter;
therefore, it should be used only to qualify the level and presence
of a signal. Using RSSI to determine distance or data validity is not
recommended.
The RSSI output can be utilized during testing, or even as a product
feature, to assess interference and channel quality by looking at the RSSI
level with all intended transmitters shut off. RSSI can also be used in
direction-finding applications, although there are many potential perils to
consider in such systems. Finally, it can be used to save system power by
“waking up” external circuitry when a transmission is received or crosses a
certain threshold. The RSSI output feature adds tremendous versatility for
the creative designer.
Using the PDN Line
The Power Down (PDN) line can be used to power down the transceiver
without the need for an external switch. This line has an internal pull-down,
so when it is held low or simply left floating, the module is inactive.
When the PDN line is pulled to ground, the transceiver enters into a
low-current (~20µA) power-down mode. During this time the transceiver
is off and cannot perform any function. It may be useful to note that the
startup time from power-down is slightly less than when applying V
CC
.
The PDN line allows easy control of the receiver state from external
components, such as a microcontroller. By periodically activating the
transceiver, sending data, then powering down, the transceiver’s average
current consumption can be greatly reduced, saving power in battery-
operated applications.
ESD Concerns
The module has basic ESD protection built in, but in cases where the
antenna connection is exposed to the user it is a good idea to add
additional protection. A Transient Voltage Suppressor (TVS) diode, varistor
or similar component can be added to the antenna line. These should have
low capacitance and be designed for use on antennas. Protection on the
supply line is a good idea in designs that have a user-accessible power
port.
Note: If the T/R_SEL line is toggled when the transceiver is powered
down, internal logic wakes up and increases the current consumption to
approximately 350µA. When high, the T/R_SEL line sinks approximately
15µA, so the lowest current consumption is obtained by placing the LT
into receive mode before powering down.