USER’S MANUAL
SERIES 48710 RF REPEATER
The primary contributor to spurious emission is multiple signal intermodulation. As multiple signals are
amplified, intermodulation products (IM) are generated. The level of the 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 pass band known as the “composite power”. Radio
Frequency Systems has designed the 48710 repeaters 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 amplifer
provides a DC voltage proportional to the output power. This voltage is compared to a factory setting.
As long as this voltage is less then 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 increase attenuation in proportion to the
level of the DC voltage.
The attenuation reduces the output power until the detector voltage is at the safe level again. If the
number or power of the signals is reduced the DC offset voltage will reduce level of attenuation. In
typical operation, the AGC is only active when needed to prevent overdrive. If the AGC is constantly
activating it is suggested that you reduce the gain via the manual attenuator and check to make sure that
an oscillation between the base and service area 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 to 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 they are in use in the service area or not. Ideally, the
donor antenna is directional. This limits the number of donor base sites the repeater sees and reduces the
potential for interference. Observing the signal levels before connecting the repeater is reccomended.
Out of band signals can also cause overdrive. The most likely occurance of overdrive is oscilation from
inadaquate antenna isolation.
Table 4 lists the typical output power per signal which can be expected from the 48710 for a given
number of active signal operating at equal input levels. For a donor site that has 16 signals, the full power
per signal would be +14 dBm. Assuming the incoming signal is sufficient such that that level will be
achieved.
TABLE 1. TYPICAL OUTPUT POWER PER SIGNAL
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