Comtech EF Data DST - Manual (Page 61)

Digital Satellite Terminal System Revision 1
Operation MN/DST-IOM
A–3
A.1.3 Frequency Uncertainty Budget
An example of a typical frequency uncertainty budget for a Ku-Band link with a 3 ppm
LNB is expressed by:
± 309 Hz BUC with LO = 15,450 MHz, ±0.02 ppm
± 32,250 Hz LNB with LO = 10,750 MHz, ±3 ppm
± 58 Hz Modulator + Demodulator at 1,450 MHz, ±0.02 ppm
± 10,000 Hz Satellite Uncertainty
± 42,617 Hz ± Uncertainty
x 2 Convert ± to total End to End
85,234 Hz End to End Uncertainty, fv
÷ 2.6 2 * k, where k = 1.3
32,782 sps SR ≥ fv /(2 * 1.3), minimum symbol rate for 1.3 carrier spacing
Data rate and symbol rate are related by:
DR = SR * m * CRv * CRrs
where,
DR = Data Rate
m = Modulation Index (m = 1 BPSK, 2 QPSK, 3 8PSK, 4 16QAM)
CRv = Viterbi or Turbo Code Rate, 1/2, 3/4 etc.
CRrs = Reed Solomon Code Rate, 201/219 etc. or 1/1 when there is no RS
For the example above, with a total frequency uncertainty of 85.2 kHz and minimum
symbol rate of 32.8 ksps, the minimum data rate for QPSK 3/4 Turbo is 49.2 kbps.
A.1.3.1 Uncertainty For 0.02 ppm Modem and 0.02 ppm BUC
Depending upon the satellite, the uncertainty will vary so a plot of the minimum symbol
rate versus satellite frequency uncertainty is convenient as shown in Figure A-2. Cases
are presented for both Ku-Band and C-Band where the modem supplies a 0.02 ppm
reference to the BUC and the LNB is either uses the 0.2 ppm from the modem or is a 3
ppm internally referenced unit. The LO frequency for the Ku-Band LNB is 10,750 MHz
and 5,150 MHz for the C-Band LNB.
The first and second curves at the top of Figure A-2 show a 3 ppm LNB at Ku-Band and
C-Band, respectively. The bottom line in the figure is actually two lines plotted for
Ku-Band and C-Band for an externally referenced LNB and 0.02 ppm. However, the
curves are virtually on top of each other because the modem / BUC / LNB combination
has negligible contribution to the frequency uncertainty, only 582 Hz for Ku-Band
frequencies and 309 Hz for C-Band. In this configuration, the satellite stability dominates
the frequency uncertainty budget.