3
rd
Generation Rack-
Mount RDMS™
147
Quasonix, Inc.
Figure 250: Antenna Controls Window, AM Polarity Drop Down Menu Selections
4.5.3.6.11 AM Scale
To change the AM Scale value, type the desired value or use the up/down arrows to scroll to the appropriate value.
The AM Scale has a range from -128 to +128. At its default setting of 1, the response is 2V p-p, into a 50 ohm load
with a 50% AM.
4.5.3.6.12 Downconvert Antenna
The DC Antenna option is only available when using the 5-band downconverter AND P and C band are enabled.
•
When the downconverting antenna is not available, this command displays only an assumed value.
•
The downconverting antenna setting only applies to C band frequencies.
•
Click on the box in the DC Antenna field to check the box and enable the option, as shown in Figure 251.
Figure 251: Antenna Controls Window, D/C Antenna Selection Checked
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
