Maintenance
M7/M7L/M7LT Modular Satellite Modem
3-80
M7/M7L/M7LT - Rev. 0.05
3.7.4.
Using the Internal or an External Reference
The modem contains an internal Temperature Compensated Crystal Oscillator (TCXO) reference
which determines the basic accuracy of all modem frequency and rate settings. This internal
reference is nominally 2.0 ppm stability over normal operating temperature, and exhibits aging less
than 1 ppm per year. If this accuracy is not sufficient, or the network operating mode dictates, an
external reference can be used.
The M7L/LT, uses an Oven Controlled Crystal Oscillator (OCXO) and the standard unit has a 1 x 10
-7
stability and 2 to 3 x 10
-7
aging rate per year. The increased stability is necessary because the
oscillator can be used as the reference for a BUC.
The external reference frequency is applied at the rear panel BNC connector, J1, at a frequency of 1,
5, 9 or 10 MHz. Selection of the external reference and the reference frequency are selected at the
front panel from the
<Unit: Reference - Source
=External>
, setting to external which then enables
the entry for
<Unit: Reference - Frequency>
.
The external reference input does not perform any clean-up of an input other than band-pass filtering
with a pass-band from approximately 1 to 12 MHz. The reference input should therefore be a low
phase noise source.
3.7.5.
Using the M7L/LT BUC Reference Control
The M7L/LT modem contains a high stability 10 MHz OCXO reference oscillator designed to provide
a suitable reference signal to the Block Up Converter (BUC). The application of the reference to the
transmit cable is under front panel or remote control, as required by the BUC. Some BUCs use the 10
MHz signal to control application of power to the final PA, removing it and going to a low power state
when the 10 MHz is absent. This reference is activated in the
<Mod: BUC
– 10 MHz Ref>
parameter.
NOTE: See the specifications in Appendix A for the exact reference stability, aging, phase noise and
level specifications.
3.7.6.
Using the M7L/LT LNB Reference Control
The M7L/LT modem contains a high stability 10 MHz OCXO reference oscillator designed to provide
a suitable reference signal to the Low Noise Block Down Converter (LNB). The application of the
reference to the receive cable is under front panel or remote control, as required by the LNB. This
reference is activated in the
<Demod: LNB
– 10 MHz Ref>
parameter.
NOTE: See the specifications in Appendix A for the exact reference stability, aging, phase noise and
level specifications.
3.7.7.
Using the M7L/LT Transmit RF Frequency Feature
The M7L/LT modem output can be tuned to any frequency between 950 to 2150 MHz in 1 Hz
increments. The modem can display the actual satellite RF frequency being transmitted at the BUC
output. To enable this feature simply supply the
<Mod: BUC
– LO Frequency>
parameter with a
value other than “0”. When the value set here is equal to the BUC’s LO frequency then the modem
can automatically compute the RF frequency at the BUC output.
The M7L/LT modem will also set the proper spectrum for high side or low side LO. If the BUC has a
Low side LO, the spectrum is not inverted and the
<Mod: BUC
– Xmt Spectrum >
setting would
typical
ly be set for “Non-Inverted”.
NOTE: After entering a new
<Mod: BUC
– LO Frequency>
parameter, the modem requires a new
Transmit IF frequency input to recalculate the proper output frequency setting.
To return to using the L-Band IF frequency setting, simpl
y enter a value of “0” into the
<Mod: BUC
–
LO Frequency>
parameter.
Common BUC LO frequency for the 5.925 to 6.425 GHz C-Band range is 4900 MHz (low side LO),
while a low side LO for the 14.0 to 14.5 GHz Ku Band range is 13050 MHz.
