ITEM # 602100023500
DOCUMENT TYPE:
Operation and Installation Manual
REV:
A
DOC #:
602100023
PAGE 9 OF 16
TITLE:
Bi-Directional Amplifier System
Intermodulation, Power, and AGC
FCC requires that spurious emissions be less than {43+ 10 Log 10 (Power Out Watts)} dB below the
carrier Power Out (dB) level. This is always equivalent to -13 dBm. The primary contributor to
spurious emissions is multiple signal intermodulation. As multiple signals are amplified, they generate
intermodulation products (IM). The level of IM is a factor of the amplifiers linearity or 3
rd
Order
Intercept Point (IP3), and the number and power of signals being amplified (Pout).
IM = 3xPout - 2xIP3
The 48900 signal boosters use a combination of low distortion amplifiers and automatic gain control
(AGC) to achieve maximum output while automatically limiting spurious intermodulation levels to -13
dBm. A detector on the output of the amplifier provides a DC voltage proportional to the output
power. This voltage is compared to a factory setting. As long as this voltage is less than the setting,
no action is required. However, when the combination of signal level and or number of signals
causes the composite power to exceed the safe level, the AGC's comparator generates a DC voltage
to a pin diode attenuator that increases attenuation in proportion to the level of the DC voltage.
The attenuation reduces the output power until the detector voltage is at a safe level again. If the
number or power of the signals is reduced, the DC offset voltage will reduce the level of attenuation.
In typical operation, the AGC is only active when needed to prevent overdrive. If the AGC is
constantly activating, RFS suggests that you reduce the gain via the manual attenuator and verify
that an oscillation between the base and service antenna systems does not exist.
AGC Set Point Adjustment
A small hole (close to the RF connectors) in the top of the amplifier enclosure (DL and UL
respectively) provides access to the AGC set point. A potentiometer adjustment tool can be used to
increase the set point for approximately 5 dBm higher output power. Remove power and turn the pot
clockwise slowly until the stop is reached. This will increase the set point but retain the AGC to
protect the FCC certification and the BDA itself from damage due to runaway oscillation.
T
ABLE
1: T
YPICAL OUTPUT POWER PER SIGNAL
Table 1 lists the typical
output power per signal,
which can be expected
from the 48900 for a
given number of active
signals operating at
equal input levels. For a
donor site that has 8 signals, the power per signal would
be +7 dBm, assuming the incoming signal is sufficient
so that that level will be achieved. Output power is
never greater then input power + gain of the BDA.
CAUTION
Remove power to the BDA. Increasing the set point of the AGC may result in interference. Use only
a non-conductive potentiometer tool to prevent damage to surrounding circuitry.
iDEN signals
dBm
1
19.0
2
16.0
4
12.0
8
9.0
16
6.0
