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AISG Integrated Transceiver
External Termination and
AC-Coupling to Feeder Cable
The MAX9947 transceiver works in conjunction with an
external 50 I termination. The termination is connected
serially between TXOUT and the feeder cable. It acts as
series termination for the transmitting path (data flowing
from TXIN to TXOUT) and acts as parallel termination
when data is being received on RXIN.
The output of the transmitter is biased at 1.5V to maxi-
mize the power-supply rejection ratio and minimize
the emission. It is recommended that the device be
AC-coupled to the feeder cable through either an exter-
nal RF filter or a series 100nF capacitor.
Transmission Output Power
The MAX9947 output level at TXOUT can be set by
using two external resistors that connect at the RES and
BIAS pins as shown in the Typical Application Circuit
(Connectivity at the Base) and Typical Application
Circuit (Connectivity at the Tower) . The maximum volt-
age at TXOUT is 2.52V P-P . Assuming that the feeder
cable is terminated into a 50 I impedance, the external
filter is lossless at 2.176MHz, and a series 50 I termina-
tion is being used as in the Typical Application Circuit
(Connectivity at the Base) and Typical Application Circuit
(Connectivity at the Tower) , the output level of 2.52V P-P
corresponds to +6dBm at the feeder cable.
The TXOUT voltage level can be varied according to the
following equations:
V TXOUT (V P-P ) = (2.52V P-P x V RES (V))/1.5V
V RES (V) = 1.5V x R2/(R1 + R2)
V TXOUT (V P-P ) = 2.52V P-P x R2/(R1 + R2)
Use R1 = 0 I for maximum voltage level of 2.52V P-P .
The voltage at the RES pin must be between 0.84V and
1.5V. It implies that the minimum voltage level at TXOUT
is approximately 1.41V that corresponds to +1dBm at the
feeder cable. It is recommended that a 1μF capacitor be
connected between the BIAS pin and ground.
To obtain the nominal power level of +3dBm at the
feeder cable as the AISG standard requires, use R1 =
4.1k I and R2 = 10k I that provide 1.78V P-P at TXOUT.
The MAX9947 can provide up to 2.52V P-P to compensate
for potential loss within the external filter, cable, connec-
tions, and termination.
Receiver-Input Range and Threshold
The maximum OOK input power at RXIN into the 50 I
external termination is +5dBm. For a single-tone signal
at 2.176MHz, 5dBm corresponds to 1.12V P-P .
The MAX9947 internal threshold is -15dBm (112.4mV P-P )
with Q 3dB accuracy in compliance with the AISG stan-
dard specifications. This threshold sets the minimum
input signal level that is recognized as OOK carrier
being present (level logic-low).
Consider a corner case where the OOK signal at
2.176MHz present at the RXIN pin is at the minimum level
of -15dBm Q 3dB. To avoid the saturation of the receiver
input stage, any other adjacent carrier with power-up to
+5dBm must be either below 1.1MHz or above 4.5MHz.
External Clock
The MAX9947 integrated AISG transceiver operates
with an external crystal at 4x the 2.176MHz frequency,
or 8.704MHz. The crystal is required to achieve the
Q 100ppm frequency stability specification of the AISG
standard. A crystal with Q 30ppm is recommended
along with two 40pF ( Q 10% tolerance) capacitors con-
nected to ground as shown in Typical Application Circuit
(Connectivity at the Base) and Typical Application Circuit
(Connectivity at the Tower) . The capacitors do not affect
the oscillation frequency.
Multiple MAX9947 devices can share the same crystal
by using the SYNCOUT pin. One device acts as a mas-
ter and provides the 8.704MHz clock signal to the slave
device(s) through such a pin. To configure a device as a
slave, XTAL2 should be connected to ground. The exter-
nal clock coming from the master device feeds the XTAL1
pin of the slave device through a series 10k I resistor.
Connect a 1k I pullup resistor to V CC from the SYNCOUT
pin of the master device.
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