Comtech EF Data DST - Manual (Page 55)

Digital Satellite Terminal System Revision 1
Operation MN/DST.IOM
5–3
5.2 LO, Mix and Spectrum (Inversion) Settings
The SDM-300L3 / CiM-300L Satellite Modem permits programming of terminal
(satellite) frequencies instead of the L-Band frequencies. This is useful because it allows
direct entry of the assigned TX and RX frequencies. Three parameters are adjusted to
setup the modem with the BUC/LNB. These are the:
♦ LO: Local Oscillator Frequency.
♦ Mix Sign: + or -, determines whether the L-Band carrier is added or
subtracted from the LO to translate to the satellite frequency.
♦ Mod Spectrum and Demod Spectrum (Inversion); needed to correct for any
inversion caused by the frequency translation.
• For BUC only. Refer to Chapter 3, for the LO frequencies for BUCs
provided by Comtech EF Data, along with the MIX(+ or -) and
Spectrum (Inversion) for several common frequency bands.
• For LNB only. Refer to Chapter 4, for the LO frequencies for LNBs
provided by Comtech EF Data, along with the MIX(+ or -) and
Spectrum (Inversion) for several common frequency bands
The LO and MIX are entered into the modem to program the satellite frequency for the
terminal. Frequencies other than those shown are possible as long as the LO and MIX are
known.
♦ MIX = “+” when LO < Satellite Operating Frequency
♦ MIX = “-” when LO > Satellite Operating Frequency
Whenever MIX = - the spectrum of the carrier is inverted. The SDM-300L3 / CiM-300L
easily corrects for this with the Normal and Inverted selections located under the
Utility: Modulator and Utility: Demodulator menus.
Match the modulator inversion to the MIX in the BUC and the demodulator inversion to
the MIX in the LNB using the following:
♦ Spectrum = Normal when MIX = +
♦ Spectrum = Invert when MIX = –
As a general rule it is best to radiate the spectrum toward the satellite in the Normal
spectral sense. Then transmission links with any other corresponding sites are always
consistent at the satellite interface.
When doing RF loopback testing over the satellite or end-to-end the settings are correct
with this setup. However, if there is a single inversion in the modem for operation over