| At the ground station, the PCM stream, whether carried directly over
wire or fiber, or ingested via an antenna and RF telemetry receiver, is
reconstituted into the original raw measurands and data.
Because
transmission distorts data for both transmission mediums (wire versus "antenna"), the received PCM data signal must first be
reconstructed. Prior to transmission, the square wave PCM stream is filtered
to round the wave train, thus reducing the bandwidth required to carry
it and ensuring power is concentrated in the spectrum carrying the data.
The first signal processing function reconstructs the signal with a minimum
number of symbol errors. Then the synchronous timing information is derived.
This crucial signal processing function is called bit synchronization.
A bit synchronizer or "bit sync" is a device that establishes a series
of clock pulses that are synchronous to an incoming signal. The bit sync
then classifies the value of each bit in the stream.
Bit syncs are available in a number of form factors.
Heritage units typically occupied an entire 5.25-inch by 19-inch wide
rack-mount chassis with controls for setup and LEDs for status. More recent
technology, as used in L-3’s
EMR 832 or MBS 720 (shown below), supports up to four independent
streams, continuously tunable to 30 Mbps — all in the same rack space
as a heritage single channel unit. Similar technology is used in board-based
bit syncs, which come in a variety of form factors such as VME
and PC.
The bit
synchronizer includes multiple inputs and allows the ground station
to select the one of interest (e.g., telemetry receiver output, PCM
simulator,
or instrumentation tape recorder). The signal is amplified and an internal
oscillator phase-locks to incoming data. Each bit is reconstructed
by
a conventional "hard" decision (0 or 1) circuit that produces bit decision
accuracy within the unit’s theoretical "signal-to-noise ratio versus bit
errors" curve. If not originally in an NRZ-L format, the signal is converted
and output to the next step in data reduction — the decommutator. Data
is also output in any format for storage on an instrumentation tape recorder.
At playback, the signal is reintroduced into the bit sync and output
to
the decom. Often, the tape is sped up by factors of 2, 4, and even 8
to decrease processing time.
The most frequently cited
measurement of bit sync quality is "bit
error rate probability as a function of input signal-to-noise ratio with
respect to the theoretical." Typically, a goal of 1 dB of theoretical
over the entire range of operation is specified (see the figure below).
Bits syncs such as the EMR 832 achieve at least this result.
Other measures of performance
include susceptibility to signal flutter when played back from analog
tape recorders, the ability
to support the time codes, and short acquisition
time.
The next processing step is frame
synchronization … |
