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P300H P300 Series Modem Installation and Operating Handbook Page 94
Demod sweep range: [Normal=±25kHz]
1=Normal 2=User
Change, Rx, Demod’ Sweep Menu
Set user demod sweep: [±16kHz]
(1-31kHz, normal=±25kHz) ±??kHz (YES)
Change, Rx, Demod’ Sweep, User Menu
6.7.14 Change, Rx, Demod’ SWEEP Menu
The operator has control of the sweep range of the Demodulator, but under normal circumstances
`Normal` should be selected. Normal is defined as:
BPSK, QPSK & 8PSK
±80% of `the recommended carrier spacing` or ±25 kHz, whichever is the lower. The
recommended carrier spacing is 1.4 times the symbol rate (ie data rate + framing + FEC +
Modulation). See note later when operating without FEC AND without Framing.
OQPSK
±25% of the receive symbol rate (ie data rate + framing + FEC + Modulation) or ±25 kHz,
whichever is the lower.
Assuming closed network, QPSK rate ½ coding, this means that for data rates down to 22 kbps the sweep
will be ±25kHz, reducing linearly below this rate to 10.74kHz for 9.6kbps QPSK rate ½ FEC, or 4.8kbps
BPSK. See note 2 in the following paragraph for an explanation of why the sweep width narrows at lower
data rates.
If USER is selected, then the following menu is displayed:
The operator can configure the Demodulator to sweep up to ±32kHz to compensate for errors introduced
in the Tx frequency conversion, satellite, and Rx frequency conversion equipment. Please note the
following:
1. Increased sweep width proportionally increases the carrier acquisition time.
2. With low data rates, and wide sweep widths (ie wider than the recommended carrier spacing), it
is perfectly feasible to acquire the wrong carrier. Typically low rate carriers are clustered together
on a transponder, and the modem has no way of telling which carrier is which other than it has to
be in the sweep range, and of the correct data rate.
Note
Applicable to operation without FEC AND without framing only
Part of the Rx carrier acquisition process when the Demod sweeps is for the Demod to temporarily lock
onto `false locks`, these are detected as false because although the Demod has locked, the FEC will
not lock correctly. This information is used to step the Demod directly onto the true carrier lock (this all
happens in the background without the operator being aware of the process).
If the Demod is operated without FEC (ie uncoded), then frame alignment is used to determine if the
lock is a false one and to push the Demod into acquiring the correct lock. HOWEVER, if the Demod
is operated without FEC AND without framing, it cannot distinguish a between a true lock and false
lock. False locks occur either side of the carrier at offsets of ±½, ±¼, & ±
th the symbol rate for BPSK,
QPSK, and 8PSK respectively. In such a case, the `Normal` sweep width is automatically narrowed to
within these limits, to make the sweeping Demod avoid any false locks (assuming the Rx carrier is
exactly on frequency).
When operating in this unusual situation without FEC AND without Framing, if the Rx carrier is
not exactly on frequency, then it is possible for the Demod to lock onto a `false lock` (and not be able
to detect it as false as there is no FEC or framing). In this situation you can either: