Automatic Gain Control (Agc); Operation - Comtech EF Data KST-2000L Installation And Operation Manual

Satellite Terminal System

Operation

4.5

Automatic Gain Control (AGC)

The KST-2000L incorporates a closed-loop Automatic Gain Control (AGC) function that
maintains the system gain, as measured from the TX IF input to the Ku-Band output of
the SSPA, at the user's preset value despite the effects of aging, operating temperature, or
cabling loss. This is not a Automatic Level Control (ALC) function, but a true AGC that
maintains the gain of the system constant independent of input and output absolute levels.
This is important to multicarrier operation, when individual carriers turn On/Off and the
level of the remaining carriers must remain unaffected. The transceiver can be set to op-
erate in either the AGC, non-AGC, or MANUAL gain mode.
4.5.1 Operation
The AGC function is implemented by using two calibrated RF detectors.
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The calibration data is stored in a non-volatile memory within each SSPA making all
SSPAs interchangeable without loss of system gain accuracy. The estimate of output
power corresponds to the detector voltage linearly interpolated between nearby frequency
and power steps stored in memory. The main processor reads the estimated output power
from the SSPA and computes an error function as follows:
Where Gain_Max is the maximum specified gain of the entire transceiver (converter unit
plus SSPA) and UCA is the value of the up converter attenuator and is set by:
The main processor processes this data and generates an analog voltage that adjusts the
up converter attenuator to drive the error function to zero.
When the uplink AGC is enabled (<add/UAGC_ON) the display value of UCA will in-
clude a decimal point. Attenuation is adjustable over a range of 0 to 20 dB in 1 dB steps.
When the uplink AGC is disabled (<add/UAGC_OFF) the displayed value of UCA does
not include the decimal point.
The first detector monitors the TX input (70 or 140 MHz; amplitude range of
–25 to –45 dBm). The DC voltage from the detector is converted to a digital
word in an A/D converter and read by the main processor.
The second detector monitors the output signal of the SSPA. This detector is cali-
brated for five frequencies over the output frequency range. Additionally, the
second detector calibration covers output power from the saturation point of the
amplifier down to 30 dB (approximately) below saturation.
Gain Error = SSPA Output Power – Input Power – Gain_Max + UCA
<add/UCA_xx.x (Appendix B)
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Revision 2
MN/KST2000L.IOM