RC4500 Antenna Controller Chapter 4 Software Configuration / Installation
76
4.3.2 Signal Strength Adjustment
The RC4500 can sense satellite signal strength via the signal strength input circuits (refer to Chapter 3).
This section describes how to configure the signal strength input circuits for use with the satellite receivers
used for a particular installation. Correct adjustment of the signal strength circuitry is required for tracking
operations to peak up on a satellite.
4.3.2.1 Signal Strength Channel
To implement tracking algorithms, the controller requires an input signal which indicates the strength of
the received signal. Such a signal is generated within a satellite receiver and is referred to as an AGC
signal (AGC is the abbreviation for Automatic Gain Control.) On satellite receivers, this signal may also be
referred to as a 'Signal Strength' or 'Tuning Meter' output. An AGC output typically varies in proportion to
the received power of the transponder to which the receiver is currently tuned.
Signal strength indications from receivers, modems, etc. vary widely. The RC4500 provides two circuits
that are designed to accommodate any signal strength input that varies between +/- 10 VDC. These
inputs are labeled “REC1” and “REC2”. The REC1 hardware input is routed to the microcontroller’s 12 bit
ADC and can be seen in the AD VOLTAGES Maintenance as source “SIG”. The REC2 hardware input is
routed to the microcontroller’s 10 bit ADC and can be seen in the AD VOLTAGES Maintenance as source
“4-REC2”.
Each receiver input is scaled for a -10 to +10VDC range. REC2 can be optionally setup on the circuit
board to accommodate other ranges of input voltages (this type of modification should only be performed
at the factory.)
4.3.2.2 Signal Strength Channel Configuration
The following are guidelines that should be followed when programming the EXT SIG FACTORS screens.
The “THRES” entry refers to the Minimum Signal Threshold. The minimum value refers to the lowest
signal strength above zero (0) which allows the antenna controller to confirm that it is pointing to a satellite
rather than pointing to open sky.
4.3.2.3 Amplifier Gain vs. Frequency Characterization
An amplifier's gain vs. frequency characteristic, or gain flatness, is the variation of the amplifier's gain with
changing frequency. The ideal response is to have a flat gain characteristic (the gain does not vary with
frequency). Amplifiers with poor gain flatness characteristics can cause problems for the tracker. This
section describes a simple test (no equipment required) that the user can carry out to check the receiving
system's gain flatness.
There are four places in a satellite receiving system that may have gain flatness problems that can affect
the operation of the tracker. These are the LNB, the coaxial cable connecting the LNB to the receiver, line
amps (or bullet amps) inserted in the 950 - 1750 MHz IF, and the receiver's IF or AGC stages. If a
spectrum analyzer is connected into the receiver's block IF line, the gain flatness of the LNB and any line
amps present can be observed. When the antenna is pointed away from any satellite, the spectrum
analyzer displays the received noise, which should be constant with frequency. If the display is not a
horizontal line then some gain variation with frequency is present.
To understand how a poor gain flatness characteristic can cause problems, remember that the purpose of
the signal strength input is to let the controller determine whether a satellite signal is present or not, and to
provide relative signal strength information when peaking the antenna. A signal is assumed to be present
whenever the input is above the threshold level for whichever band has been selected. When the receiver
is tuned to various transponders, gain flatness problems could cause the signal to be above the threshold,
when in fact no satellite signal is present. For certain transponders, gain flatness problems could also
cause the controller's input scaling network to be saturated when the antenna is aligned with a strong
satellite, making it impossible for the controller to detect changes in signal strength when attempting to
peak the antenna.
To Next Page
