–

–

–

–

8

9

To get a better idea of the circuit’s operation, clip an oscilloscope probe on

both RX DATA and SQ. DATA (Squelch Data). With the transmitter off, the

SQ. DATA line is high (which means that the output is squelched) while RX

DATA is switching randomly. Squelching is accomplished by comparing

RSSI with a voltage reference created by R17 (potentiometer), R18, and

R21. When the RSSI falls below the voltage set by this reference, the

output of the comparator (U6) is pulled to ground. This disables the data

slicer created with the additional comparator contained within U6. Figure

14 shows the schematic of this circuit.

Setting a higher squelch threshold reduces the random noise on the DATA

line but also reduces range. The squelch level affects only the threshold of

the data going to the RS-232 serial port and the SQ DATA line on TS2.

To set squelch, turn off the transmitter and turn on the receiver. Place an

oscilloscope probe on the SQ DATA line, and adjust R17 until SQ DATA

remains high. Two resistors (R27 and R28) are used to connect the module

to the squelch circuit. These may be removed to disconnect the squelch

circuit and prevent it from slightly loading the AUDIO and AUDIO REF lines.

Using the Encoder and Decoder

The MS Series encoder and decoder use a 24-bit address to provide

uniqueness to the transmission and to prevent unintended activation.

The development boards come with a default address. To create a new

address, press and hold the CREATE button on the transmitter board.

The address is randomized for as long as the button is held down. Once

released, the MODE_IND LED begins flashing to indicate that the encoder

is ready to accept Control Permissions. Press the Buzzer and/or Relay

buttons to tell the encoder that they will be used. Press the Create button

again to exit Create Mode, or let the encoder time out after 15 seconds.

On the decoder board, press the LEARN button and the MODE_IND

LED begins flashing to indicate that the decoder is ready to learn a new

address. Press one of the authorized buttons on the transmitter board to

send a signal. Press the LEARN button again to exit Learn Mode, or let the

decoder time out after 17 seconds and the system is ready for use.

The encoder and decoder operate on one of four different baud rates as

set by the baud rate selector switches. A faster baud rate gives a faster

response time. Please see the encoder or decoder data guide for the

settings. If the switch is up then it is connected to Vcc, if it is down then it

is connected to GND.

Range Testing

Several complex mathematical models exist for determining path loss in

many environments. These models vary as the transmitter and receiver are

moved from indoor operation to outdoor operation. Although these models

can provide an estimation of range performance in the field, the most

reliable method is to simply perform range tests using the transmitter and

receiver in the intended operational environment.

Simple range testing can be performed with the transmitter and receiver

development boards. Pressing S0 on the transmitter activates the buzzer

on the receiver board, while S1 activates the relay.

As the maximum range of the link in an area is approached, it is not

uncommon for the signal to cut in and out as the transmitter moves. This

is normal and can result from other interfering sources or fluctuating signal

levels due to multipath. Multipath results in cancellation of the transmitted

signal as direct and reflected signals arrive at the receiver at differing times

and phases. The areas in which this occurs are commonly called “nulls”

and simply walking a little further usually restores the signal. If this does not

restore the signal, then the maximum effective range of the link has been

reached.

Since the evaluation boards are intended for use by design engineers,

they are not FCC certified. The transmitter has been set to approximate

legal limits by resistor R29 so that the range test results will approximate

the results from a well-designed, certified product. For applications where

Part 15 limits are not applicable or output levels can be legally raised due

to protocol duty cycle, R29 can be changed according to the attenuation

graph in the ES Series Transmitter Data Guide.

To achieve maximum range, keep objects such as your hand away from

the antenna and ensure that the antenna on the transmitter has a clear and

unobstructed line-of-sight path to the receiver board. Range performance

is determined by many interdependent factors. If the range you are able to

achieve is significantly less than specified by Linx for the products you are

testing, then there is likely a problem with either the board or the ambient

RF environment in which the board is operating. First, check the battery,

switch positions, and antenna connection. Next, measure the receiver’s

RSSI voltage with the transmitter turned off to determine if ambient

interference is present. If this fails to resolve the issue, please contact Linx

technical support.