ComStream CM601, Installation And Operation Manual

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01-0922-401 Rev. A 4/03
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1/Normal or Initial Acquisition. If operator invoked, causes the demodulator to enter a standard
sweep pattern beginning at the value of RS + the value of AO. The standard sweep pattern ensures
that each bin will be searched for the carrier to the limits of the overall acquisition search range
(typically +/-30KHz). Any time the power cycles, the modem resets or the fade acquisition fails to
find the carrier, the modem automatically reverts to this normal or initial acquisition state.
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2/Fade or Fast Acquisition. This state assumes the modem has already been synchronized to the
carrier at least one time and the receive offset frequency has been stored to AO. In this state, the
demodulator spends ½ the time in each bin as compared to normal acquisition. However, in this fade
state, the demodulator sweep has been programmed to “revisit” the last known acquired frequency (as
measured by the receive offset) every other bin. This simply means in the event of a carrier fade or
power cycle (for whatever reason) the modem will reacquire any carrier within seconds, regardless of
the magnitude of the carrier offset.
AR Acquisition Range (Config)(NV)
This command defines the overall search range of initial and fade acquisition over which the demodulator
will search for the carrier. The acquisition range is expressed in hertz.
The range is 0 to 32000 (+/- 32KHz). The default value is 30000 (30KHz). It is strongly recommended
that this value not be changed.
RO Receive Offset (Monitor)(Query)
Use this command to view or query the frequency offset (difference) between the demodulator
synthesizer frequency setting (RS command or front panel Rx Freq) and the actual demodulator RX IF
carrier frequency.
After carrier successful carrier acquisition, RO will display the difference (in hertz) between the
programmed and actual measured carrier frequency. This difference in frequency is normal and is due to
small differences in the local oscillators of the HPAs, satellite up and downconverters, LNBs and modems
in the satellite network. Once the modem’s carrier tracking circuit stabilizes itself with the incoming
carrier, (typically within 3 minutes) the value of RO will be programmed to AO to facilitate any
subsequent initial or fade acquisitions.
Mode of Operation
Initial Acquisition
Any modem that comes right out of the box has no idea where the actual carrier center frequency
might be located within its search range. The operator programs the modem configuration with the
proper (IF) frequency, data rate, code rate and modulation type to match the characteristics of the receive
carrier. Once the entire configuration programming has been completed and the input power level and
carrier to noise level has been determined to be adequate, the operator issues a command to begin initial
acquisition (AQ 1). The modem then begins at the center frequency (for this example 70MHz will be
used) plus any operator programmed offset frequency (which would be 0 if the modem was set to factory