Research Concepts RC2500 - Manual (Page 43)

36 RC2500 Antenna Controller Chapter 4 Inclined Orbit Satellites
Research Concepts, Inc. • 5420 Martindale Road • Shawnee, Kansas • 66218-9680 • USA www.researchconcepts.com
2. Similar to step 2 above except that both C and Ku (or L) band AGC voltages are obtained when the
antenna is looking at nothing. The installer should then choose the smallest of the two (or three)
levels as the “Off Satellite” voltage.
3. Follow steps 3 through 17 above.
4.4.1.3 Using Two AGC Channels
Connecting a second receiver to the controller is straightforward. The controller AGC input channel GAIN
and OFFSET adjustments have enough range to handle any possible receiver AGC scheme, as long as
the AGC level is between –15 VDC and +15 VDC. In step 7 of the adjustment procedure, the installer is
instructed to select the controller AGC input channel based on the range of the receiver's AGC output.
AGC input channel 1 is designed to handle 'small' AGC signal swings, and AGC input channel 2 is
designed for 'large' AGC signal swings. There is enough adjustment range, however, so that either
channel may be connected to any receiver AGC output.
If two receivers are used, both must have the same AGC polarity, and the GAIN and OFFSET pots of
each channel must be adjusted so that the same threshold values are appropriate for each channel. The
controller will use as its AGC input the stronger of the two signals. All pot adjustments will have to take
place in MANUAL mode with the scroll keys setting a single displayed AGC channel. (SCROLL UP or
SCROLL DOWN until the channel number is displayed i.e. 2: )
Currently the AGC Channel Scroll feature is not supported
4.4.1.4 Amplifier Gain vs. Frequency Characteristics
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 AGC 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 AGC input is above the threshold level for whichever band - C or Ku or L - has been
selected. When the receiver is tuned to various transponders, gain flatness problems could cause the
AGC 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 AGC 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 test the gain flatness of the satellite receiver, perform the following procedure. The procedure
assumes a single receiver, single frequency band system.
• Position the antenna well off of any satellite. Tune the receiver to each transponder.
Make sure that the signal strength reading is below the threshold assigned via CONFIG
mode.
• Align the antenna with a strong satellite. Tune the receiver to each active transponder on
the satellite. Make sure that the signal strength is well above the threshold and below
999.
If the system fails either of the tests above, then the user can either attempt to correct the problem by
readjusting the GAIN and OFFSET pots (using the procedure outlined earlier), or by correcting the gain
flatness problem. Most gain flatness problems can be traced to a problem with the coaxial cable
connecting the antenna to the receiver (sometimes called a 'suck-out') or to bullet amplifiers. Some bullet
amplifiers have been observed to have really terrible gain flatness characteristics.