3
rd
Generation Rack-
Mount RDMS™
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Quasonix, Inc.
channels (about a quarter mile of free-space propagation). Similar to phase alignment, time alignment is dynamic,
accommodating changes in relative target antenna positions over time.
When enabled, the Time Aligner continuously measures skew between channels but remains in a “monitor” state
(with no timing correction) as long as the skew remains below a predefined threshold. When the skew exceeds the
threshold, the Time Aligner switches to a “run” state (with full timing correction) as long as the signal quality is
sufficient for it to continue to track timing skew.
If the propagation delay between channels is well-controlled and small, the Time Aligner may be disabled to
guarantee minimal timing jitter.
4.4.2.20 AFC Mode
AFC (Automatic Frequency Control) Mode, shown in Figure 102, compensates for frequency offset in the received
signal relative to the expected carrier frequency. Demodulators for all modes in the RDMS™ contain frequency
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tracking loops that can accommodate some amount of frequency offset. The amount of offset that can be tolerated
depends on the mode and is generally a small percentage of the bit rate. If the input frequency offset is greater than
this amount, then AFC is needed to make up the difference. Therefore, AFC is particularly applicable to lower bit
rates and may be automatically overridden to Off at higher bit rates, regardless of the user specified AFC mode.
The two main sources of offset are (1) reference oscillator frequency differences between the transmitter and the
receiver, and (2) Doppler shift. Reference oscillator differences are constant or very slowly time-varying. Doppler
shift, by its nature, tends to be dynamic. The optimal AFC mode depends on the source and magnitude of the
frequency offset. There are three possible settings: Off, Track, or Hold. In general, Quasonix recommends setting
the AFC Mode to Off if possible.
The user may toggle the AFC mode by pressing the Enter key on the front panel keypad until the desired value
displays.
Figure 102: Main Menu, AFC Mode Selection
Note that the AFC is automatically overridden (Off) if the demodulator can natively tolerate at least 50 kHz of
frequency offset. This prevents the AFC from potentially interfering with frequency tracking if AFC is unlikely to
be needed. Override may be disabled, and many other detailed AFC parameters may be controlled, via the command
line interface. Refer to the RDMS
™
Access via Telnet and Serial Control Protocol Technical Guide for AFC
command details.
