Wegener UNITY 550, User Manual

Page: 36 / 58

Chapter 4: Search Functions
30 800072-01 Rev. A www.wegener.com
Table 4.4: Signal Quality Information
4.6 Frequency Tagging
General
Rules
Under normal operation, the IRD can only acquire carriers that contain frequency tags.
These tags are inserted in the
COMPEL control stream at the uplink and identify the carrier's
frequency. Tagging was mandated so IRDs will not lock on an adjacent carrier while looking
for the intended one. Traditionally, these tags have been sent at a rate of every 100 or 125
mS. The IRD supports both single and multiple tags. The "tag site", which is part of the
settings, tells the receiver which of the possible tags to use (for multi-hop carriers). This IRD
allows for Tag Site designations from 0 to 15 inclusive, with 15 specially reserved to mean
"do not require a valid ID tag to lock on this carrier". To reduce processing overhead, the
unit only looks for tags while it in the Get Tag sub-mode.
Parameter Description
Eb/No These are internal estimates* of normalized received signal-to-noise ratio. The "quick"
Eb/No is updated approximately every 500 mS, and the average is a running 10-
second average of these quick estimates. The Eb/No alarm is generated when the
average Eb/No is found to be lower than the SNR Alarm Level. A Margin warning is
generated when the average Eb/No is not yet low enough to be in full alarm, but it fails
to exceed that alarm threshold by the Margin Offset (in dB).
* The accuracy of these estimates is affected by any of a number of cumulative link
imperfections, such as converter-LO phase jitter, amplitude/group delay distortion, and
external impedance mismatches. Also, at high signal-to-noise ratios, the accuracy of
the estimate is reduced. [This is why the estimate will "top out" as > xx dB at a high
enough ratio.] Because of differing sensitivities to these imperfections, the user may
expect different IRDs to show diverging estimates, especially as the Eb/No gets higher.
RF Level Unit will show WARNING if the RF level is detected to be above -20 dBm or below -60
dBm.
Note: This is not to be confused with the Signal Strength OSD screen. (See
Signal Strength on page 20 and Figure 3.6 on page 20)
Signal Fades A counter is maintained for the transitions from Tracking mode to Fade mode.
RF Glitches A counter is maintained for carrier "hits". A "hit" is when the receiver board detects one
or more uncorrectable Reed-Solomon frames, yet small enough not to cause a
transition from Tracking to Fade mode. Note that an RF hit will induce a hit on the unit's
video/audio.
Availability Shows the percentage of time that the unit has been locked on a carrier since that
carrier was first acquired (timer.locked / timer.since_first_acquired).