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A2537-002/01 February 2020
6-16
SEMCO Proprietary Information
B.
Loop Bandwidth Divider
– the
Loop BW Divider
feature is also activated for certain demodulator
formats when
Fast Acquire
is enabled. This feature allows a wider loop for acquisition, and then
automatically tightens the loop once lock is declared. The
Loop BW Divider
value defines how much
the Loop Bandwidth is reduced once lock is detected.
The selectable values for reducing the bandwidth is 1, 1/2, 1/4 and 1/8, with a value of “1” keeping the
Loop Bandwidth the same. This feature is available for all demodulator modes that function with a
Loop Bandwidth (PM, BPSK, QPSK, AQPSK, UQPSK, AUQPSK and SOQPSK-TG).
C.
FM Sweep
– This feature is used when acquiring FM signals below approximately 20 Kbps. When this
feature is enabled, the demodulator’s center frequency is swept using an AFC circuit. This ensures
that the desired signal is centered in the selected IF FIR filter bandpass for demodulating narrow FM
signals.
When narrow FM signals are off frequency, the desired signal may not be in the narrow IF FIR filter
and therefore will not be detectable. This setting is used to sweep the frequency offset of the
demodulator to give the best possible opportunity to recover the signal.
D.
Low Pass Filter
– This applies to NTSC Video, and is turned ON whenever NTSC Video is used.
E.
De-Emphasis
– This feature is also used as required when receiving NTSC Video.
F.
DQM
– The Data Quality Metric (DQM) feature provides for support of Best Source Selectors that are
used at several Flight Test Ranges. DQM is a means to measure and select data quality from multiple
signal sources (antennas). Embedded in the baseband video signal as it is processed by the
receiver’s demodulator, This DQM data is presented to a Best Source Selector (BSS), which then
selects the best signal over time based on a signal quality algorithm.
DQM Frame Size (Payload Length) is user selectable DQM parameter.
G.
LDPC
- Low Density Parity Check (LDPC) is a Forward Error Correction (FEC) block code, meaning
that a block of information bits has parity added to them in order to correct for errors in the information
bits. The term “low-density” stems from the parity check matrix containing mostly 0’s and relatively few
1’s.
There are six different LDPC codes with different coding rates (rate 1/2, 2/3, 4/5) and information block
sizes (1024 and 4096). LDPC only applies to SOQPSK-TG modulation. Both CCDS and IRIG De-
randomization is also applicable to LDPC FEC and is user-selectable.
When using LDPC, the operator must be cognizant of the bandwidth expansion factor associated with
each LDPC coding rate. 4/5 rate has an approximate 1.3 bandwidth expansion factor, with 2/3 rate at
approximately 1.5 and 1/2 at approximately 2 (twice the data rate).
H.
AE
-
The Adaptive Equalization (AE) feature mitigates RF multi-path that is present at the receiver’s
input and automatically adapts to time-varying properties of the RF signal. It is used in FM, PSK,
QPSK and SOQPSK-TG modes and mitigates the effects of multipath propagation and Doppler
spreading.
I.
STC
- Space Time Coding (STC) uses space diversity and time diversity to overcome the two-antenna
problem, which is characterized by large variances in the antenna gain pattern from a test article
caused by transmitting the same telemetry signal time through two transmit antennas. These signals
are typically delayed in time and have differing amplitudes.
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