Paradise Datacom P300 Series Installation & Operating Handbook Download

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P300H

P300 Series Modem Installation and Operating Handbook

Page 97

6.7.14 Change, Rx, Demod SWEEP Menu

Demod sweep range: [Normal=

25kHz]

1=Normal 2=User

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 + f FEC + Modulation) or

25 kHz,

whichever is the lower.

Assuming closed network, QPSK rate 1/2 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 1/2 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.

ËÍÛÎ

is selected, then the following menu is displayed:

Set user demod sweep: [

16kHz]

(1-31kHz, normal=

25kHz)

??kHz (YES)

Change, Rx, Demod Sweep, User Menu

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

±1/2, ±1/4

, &

±1/8

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).