5-8
1. When the spectrum analyzer is at the TDMA frequency will the
carrier be there?
2. When a signal is seen in the spectrum graph the question of
origin remains. Was that a carrier or some ingress?
While installing and characterizing your system, you realized that
certain frequencies were going to be unusable because of the
frequent, uncontrolled ingress. The spectrum around these
frequencies is wasted. Why spend time measuring these frequencies
instead of looking at the frequency(s) the system is using? What you
really want to know: is there anything wrong with my modem
signals?
TDMA signals have two modulation techniques. One modulation is
QPSK or 16 QAM. This modulation includes overhead and data at
the approximate rate of the bandwidth allocated. The second
modulation is pulse modulation. Pulse modulation allows the modem
to turn off the signal when no transmission is required. This allows
multiple modems to transmit at the same frequency. There are two
states to these TDMA signals.
1. When all modems are idle, there should be low noise at the
modem’s frequency.
2. When a modem is active the power level should be at a
predetermined level.
Both of these states must be met For optimum operation of the
modem,. both of these states must be met Another state occurs in
some of the older technology. This is called a collision. Collisions
occur when two or more modems try to access the same frequency
at the same time. When a collision occurs all messages are lost and
must be retransmitted.
The best way to look at a TDMA signal is using a “ZERO SPAN” on a
spectrum analyzer. “ZERO SPAN” uses the spectrum analyzer’s
tuner in a different way. Instead of providing a frequency versus level
graph, “ZERO SPAN” provides a time versus level graph at a
particular frequency. “ZERO SPAN” can be thought of as tuned
oscilloscope, displaying only the frequency of interest’s level
fluctuations. It is very important to have rising edge triggering when
viewing TDMA signals. This will allow the capture of the TDMA
events for analysis.