Iridium Pilot TPIM1301, Руководство по установке, Страница: 64 / 70

5: APPENDICES
Iridium Pilot Land Station Installation Guide
59
for slowly changing emissions. Interferers are classified as “in-band” and “out-of-band” and the signature can be
quite different.
1. “In-band” interferers may occupy a single channel or several channels clustered together. Since the
frequency band is regulated and only approved for Iridium, in-band interference is frequently a result of
inter-modulation distortion (IMD). IMD is caused by two or more interfering signals that combine or “mix”
in the ODU receiver. An example of this would be a transmitter at 1000 MHz combining with a 621 MHz to
form a product frequency at 1621 MHz at the center of the Iridium band. IMD can also be a subtraction of
two signals like 1721 MHz minus 100 MHz to achieve 1621 MHz. Other in-band sources can be unintended
emissions from equipment like computers or digital electronics with fast switching speeds. The last potential
for in-band emission is from harmonics of a device transmitting at ½ the Iridium band, 805 to 816 MHz,
or some other frequency that is an integer multiple of the 1616 to 1625.5 MHz band. An example of in-
band noise can be seen in Figure 28. It is caused by the harmonic or the ODU’s internal analog to digital
conversion frequency and appears as slightly elevated “mean” frequencies at channels 24/25, 125 and 226
This becomes apparent in Figure 28 given that frequencies mentioned above have an elevated “mean” that
has lasted for a significant portion of the 10 scans. In this case you can expect to see this “self-noise” in all
installations and it does not detract from normal operation.
Figure 28.
2. “Out-of-band” emissions will typically be a single emission close to the iridium band. They are characterized
as a broad-band noise that raises the “noise floor” of the receiver. Typically the noise floor should be below
-120 dBm. Figure 28 shows noise that was produced by placing a laptop computer directly on top of the
ODU. The antenna display shows a red peak on the vertical antenna number 7 where the noise floor was
raised.
3. In Figure 29, the center section of the lower scan was not affected because the computer was not present
during the time these channels were scanned.