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 PU  display. 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 dB . 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. The   mean 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 graph. 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 unusable. BER.—  This is the bit error rate per thousand. The incidence of bit errors increases as the signal-to- noise  ratio  decreases. 3-16