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.
MISSED CODES PERCENTAGE. This is a
percentage of each type of code that is missed. The
number of codes (start, stop, and address call-ups)
missed and received is tabulated and the percentage of
each type missed is calculated.
FRAMES. This is the total number of data
frames received, including the phase reference and
control code frames.
CS. This field displays the carrier suppression
value of the upper and lower sidebands as a ratio of
the power in the 605-Hz to the power of the carrier
frequency. The value display is measured in dB.
RFE/DS. The radio frequency error or Doppler
shift of the received signal in Hertz. The display is
color-coded cyan if frequency correction was disabled
during LMS-11 initialization.
The spectrum display graphically shows the power
levels of all the Link-11 tones and the noise tones that
are the odd harmonics of 55 Hertz. The spectrum
display screen is shown in figure 5-13. The x-axis of
the bar graph is numbered from 1 to 30 to represent 30
tones. Tone 05 is the 605-Hz Doppler tone. Tones 8
through 21 and tone 26 are the data tones. The
remaining tones are not used by the Link-11 system
but are sampled and displayed to give the operator an
indication of the noise level.
The y-axis of the bar graph displays the relative
power of each tone in dB. The highest value of the
scale is 0 dB and decreases to -40 dB. The tone with
the greatest amount of power is set to 0 dB on the
scale. This should be the 605-Hz tone. The
remaining tones are measured relative to the tone with
the greatest power. A single blue line is drawn
horizontally across the screen at the -6 dB level.
Ideally, all data tones should extend up to this line.
The 605-Hz tone and the data tones are displayed
by solid green vertical lines. If the power of a data
tone is greater than -6 dB with respect to the 605-Hz
tone, the area above the -6 dB line is indicated by an
open yellow bar on top of the green bar. If the power
level of a data tone is below the -6 dB threshold, an
open yellow bar is used to fill in the remaining
distance. This allows the operator to view the effects
of the noise. The power of the noise tones is also
indicated by open yellow bars.
To enter the spectrum display, depress the SPECT
key on the keyboard. Several options are available to
the operator by entering data into the header fields of
the spectrum display. The operator may designate the
address of the NCS. The default address is 77. The
operator can also select a particular sideband (USB,
LSB, or DIV) for display. By using the RESTRICT
field, the operator can restrict the display to only data
frames or only preamble frames, or choose no
restrictions. The PU field allows the operator to
designate a particular PU for display. If 00 is entered
into the PU field, then the data display is continuously
updated with samples from the entire net.
Carrier Suppression Display
The carrier suppression display measures how
successfully the carrier frequency is suppressed. The
carrier suppression measurements can only be made
during Net Sync. To measure the carrier suppression,
the radio must be off-tuned by -500 Hz for the upper
sideband and +500 Hz for the lower sideband. This
off-tuning allows the program to measure and
compare the relative power of the carrier frequency
and the 605-Hz tone of the preamble.