User's Guide

99
Intermodulation, Power, and AGC
Per CFR Title 47, Part 90, there shall be no spurious emissions greater 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
‘Pout’ is the combined power of all the signals in the passband known as the composite power. Radio
Frequency Systems, Inc. has designed the 48900 signal boosters using a combination of low
distortion amplifiers and automatic gain control (AGC) to achieve maximum output while
automatically limiting spurious intermodulation levels to -13 dBm or less for any number/power
combination of signals.
The AGC circuitry is housed in each power amplifier stage. 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 in the uplink is rarely needed in modern radio installations. As a portable comes closer to the
internal antenna network, its signal becomes stronger at the base site. Typically, the dynamic power
control at the base will turn the portable's power down to reduce interference and conserve power.
Overdrive is more likely to occur on the downlink path. All active signals at the donor base site will be
amplified by the repeater regardless of whether or not they are in use in the service area. Ideally, the
donor antenna is directional. This limits the number of donor base sites the repeater recognizes and
reduces the potential for interference. RFS recommends that you observe the signal levels before
connecting the repeater. Out of band signals can also cause overdrive. The most likely cause of an
overdrive condition is oscillation from inadequate antenna isolation.
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 16 signals, the
power per signal would be +9 dBm, assuming the incoming signal is sufficient so that that level will be
achieved.
Number of signals dBm
1 22.0
2 19.0
4 15.0
8 12.0
16 9.0
32 6.0
64 2.0
TABLE 1: TYPICAL OUTPUT POWER PER SIGNAL