whichever occurs first. When the net cycles are set to
zero, the display updates immediately after the
designated PU has transmitted. The PU display is
shown in figure 3-12.
The PU display is activated when the operator
presses the PU function key on the keyboard. The
information in the PU display is presented in two bar
graphs with additional amplifying information just
under the bar graphs. In the PU display header, the
operator enters the address of NCS (or 77), the
address of the unit to be evaluated, the sideband to be
evaluated (USB, LSB, or DIV), and the number of
cycles to summarize for the display. The following
paragraphs describe the information presented in the
RELATIVE POWER (dB). This bar graph
displays the relative power in each of the Link-11
tones. The relative power is calculated with respect to
the average of the data tones. The expected values
should be +6 dB for the 605-Hz tone (tone 5) and 0
dB for the data tones. The TADIL A specifications
allow for a difference of 1.5 dB between the
maximum and minimum power levels of the data
tones. A noisy signal may cause the power levels of
the data tones to deviate considerably from the
standard. The bar graph for relative power is also
color coded. When the relative power of a data tone
is ±1 dB, the bar is green. If the power level is in the
range of +1 to +2 dB or -1 to -2 dB, the bar will be
yellow. The bar is red if the power level is greater
than +2 dB or less than -2 dB. The length of the bars
plotted on the graph is rounded off to the nearest 1/2
PHASE ERROR (DEGREES). The phase
error (degrees) bar graph shows the mean and the
standard deviation of the Link-11 tones. The standard
deviation of a tone is plotted by a color bar on the
graph. The size of the color bars is plotted to the
nearest whole degree of deviation.
deviation of the tone is indicated by a small white
line, usually in the center of the standard deviation
color bar. The mean phase error should fall between
+45 degrees and -45 degrees. If the data is bad, the
mean phase error is set to -45 degrees and the
standard deviation is set to 90 degrees. This causes
the bar to be drawn across both quadrants of the
The standard deviation is represented by a color-
coded bar for each tone. A green bar is displayed if
the standard deviation is within 10 degrees.
Deviations between 10 degrees and 20 degrees are
represented by a yellow bar, and deviations greater
than 20 degrees are red. The standard deviation must
be a positive value that is less than 45 degrees. If the
standard deviation is out of range for a given tone, the
data is bad. This condition is indicated by the
LMS-11 by setting the mean deviation to 45 degrees
and the standard deviation to 90 degrees. As with the
mean deviation phase error, this causes the bar to be
painted in both quadrants of the graph.
Some causes of phase errors are noise,
simultaneous transmissions, poor framing, and errors
in Doppler correction due to noise on the preamble.
For example, a picket unit transmitting Net Sync
during Roll Call will cause an error condition. The
expected value of the mean deviation is 0 degrees
with a standard deviation of ±5 degrees. If only one
tone has a mean value that is greatly different from
the other tones, it maybe an indication of a frequency
error on that tone.
SIGNAL POWER. The signal power is part of
the amplifying information under the two bar graphs.
The signal power is the total signal strength in the 16
tones. It is measured in dBm. If no signal is received,
the default value of- 51 dBm is listed.
SNR. This is the signal-to-noise ratio. It is
measured in dB and calculated as the ratio of the
average power in the data tones to the average power
in the noise tones. If the SNR value is preceded by
the symbol >, it indicates that the average power in
the noise tones is below the measurable threshold and
the actual SNR is greater than the value indicated.
The maximum value that the LMS-11 can measure is
about 34 dB. An SNR that is greater than 30 dB is
excellent. If the SNR is less than 10 dB, the data is
BER. This is the bit error rate per thousand.
The incidence of bit errors increases as the signal-to-
noise ratio decreases.