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USING THE ES FOR DIGITAL APPLICATIONS
As previously discussed, it is important to note that this receiver does not provide
hysteresis or squelching of the DATA line. This means that in the absence of a
valid transmission or transitional data, the DATA line will switch randomly. In
many applications this hash will be ignored by the decoder or system software,
but, depending on your application, it may be useful to add an external circuit to
provide data squelching and hysteresis.
A squelch circuit will disable the DATA output when the RSSI voltage falls below
a reference level. Hysteresis will make the RSSI voltage have to fall lower than
the reference voltage before switching off, and to have to rise higher than the
reference voltage before switching on. This will prevent low amplitude noise from
causing the data line to switch, reducing hash during times that the transmitter is
off or during transmitter steady-state times exceeding 5mS. Strong signals can
still get through, so it is a good idea to have a noise tolerant protocol.
Creating a circuit that has additional hysteresis characteristics is very basic and
requires very few parts thanks to the A REF line. All you need is a couple of
resistors to provide some isolation for the AUDIO and A REF lines, a large
feedback resistor, a pull-up resistor, and an open collector comparator.
The RSSI and A REF lines allow a wide variety of squelch circuits to be
implemented. One such possibility is the circuit below, which is used on the ES
Series Master Development System, and may be employed for audio or data
squelching. It is ultimately the responsibility of the designer to determine what, if
any, circuit would be most appropriate for the needs of the product.
V C C
3 9 k
3 9 0k
V C C
V C C
10k
Qualified Data
5k
1
OUTA
VCC
8
2M
RSSI
3 9 k
0.01uF
3
4
INA-
INA+
GND
10k
AUDIO REF
10k
2M
LM3 9 3
AUDIO
10k
GND
GND
GND
GND
Figure 11: ES Series Receiver Squelch / Hysteresis Circuit
Data squelching in the circuit above is accomplished by comparing the RSSI
voltage to a voltage reference (typically a voltage divider) with an open collector
style comparator. When the voltage from the RSSI becomes lower than the
voltage reference, the comparator output is pulled to GND. This is useful
because this output can be used to disable the data-slicer circuit either when the
receiver is out of range or the transmitter is turned off.
The squelch threshold will normally be set as low as possible to ensure
maximum sensitivity and range. It is important to recognize that in many actual
use environments, ambient noise and interference may enter the receiver at
levels well above the squelch threshold. For this reason, it is always
recommended that the product’s protocol be structured to allow for the possibility
of hashing even when an external squelch circuit is employed.
Page 9
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