3
rd
Generation Rack-
Mount RDMS™
61
Quasonix, Inc.
Figure 90: Advanced Menu, DC Antenna Selection
The downconverting antenna has an LO that is used to downconvert C band signals (4400 MHz
–
5150 MHz) to a
lower frequency range known as P band (400 MHz
–
1150 MHz) using an LO frequency of 5550 MHz. This results
in two issues that are addressed by the downconverting antenna control.
1.
Spectral Inversion
In a downconverting antenna, the LO is higher than the RF (high side injection) and the lower side
band result is selected
—
the spectrum is inverted. All C to P band downconverting antennas are
assumed to produce a spectrally inverted signal. The receiver automatically reinverts the signal before
it is demodulated. (This is done in the downconversion to 70 MHz IF.) If an actual P band signal is
received, it is NOT spectrally inverted and the automatic reinversion done by the receiver improperly
causes the signal to appear inverted to the demodulator.
The demodulator has a mechanism to invert the spectrum in the digital domain. The downconverting
antenna setting determines how the spectral inversion is handled for P band signals.
2.
C Band Frequency Specification Ambiguity
It is common to tune to the C to P band downconverted signal by specifying the C band frequency. In a
receiver that also has actual C band receiver capability, an ambiguity develops when a C band
frequency is specified since it can be applied to either a C or P band signal. The downconverting
antenna setting determines how a specified C band frequency is interpreted in a system where both C
and P bands are enabled.
If a C band frequency is specified and the downconverting antenna is
enabled
, it is assumed the signal
is a C to P downconverted signal. The receiver is tuned to the P band equivalent and the automatic
inversion is used. If the downconverting antenna is
disabled
, the receiver is tuned to the specified C
band frequency and spectral inversion is not an issue.
If a P band frequency is specified, it is assumed there is no downconverting antenna. If there is a
downconverting antenna, it is ignored. The receiver is tuned to the actual P band frequency and the
automatic spectral inversion is disabled.
4.4.2.19.7 Mod Scale Index
The Mod Scale Index option, shown in Figure 91, allows the operator to manually set the modulation scale index.
This enables the receiver to operate at the optimum range of modulation desired by the user.
When modulation scale index is set, the Modulation Scaling option is simultaneously changed to Hold. The Hold
notation includes the new index number that the operator has chosen.
If the held index number is to be retained following a power-off cycle of the rack, then turn on Mod Persist from the
Main Menu. Refer to section 4.4.2.13, Modulation Persistence.
