User's Manual

5.6 Installation Tuning
5.6.1 Downstream
The following diagrams showing the effects of overdriving the AR3155Hub Transceiver are for
illustrative purposes only and should not be used as a guide for setting the RF power output
level.
If the AR3155 RF power is set as described in Section 5.5.3 (above), transmitter linearity will be
maintained for proper operation in QPSK and 16QAM downstream modes. For operation at
64QAM the downstream level may have to be reduced by 1 dB to 3 dB to ensure optimal error
free performance.
Figure 5-13 illustrates a correct downstream signal at the subscriber location. This trace was
captured by using a two-way splitter on the wireless cable modem side of the subscriber
transceiver power inserter – one output to the wireless cable modem, the other to the spectrum
analyzer.
-50
-40
-30
-20
-10
0
10
20
30
40
435
440
445
450
455
460
465
470
475
480
485
Ref Level :
40.0 dBmV
dB / Div :
10.0 dB
Spectrum Analyzer
GOOD
Model: MS2711B Serial #: 00215050
Date: 05/23/2002 Time: 11:55:01
RBW: 1 MHz VBW: 30 kHz Detection: Average
CF: 459.0 MHz SPAN: 50.0 MHz Attenuation: 0 dB
dBmV
Frequency (434.0 - 484.0 MHz)
Square shoulder
459 MHz center frequency
Figure 5-13: Proper downstream IF signal at subscriber location.
Note that the “shoulders” of the digital signal at 455 and 463 MHz are square – there is no rise
in the noise base line adjacent to the signal.
Note also that the received IF center frequency is 52 MHz lower than the transmitted IF
(transmit centered on 511 MHz, receive centered on 459 MHz (Figure 5-13). This is the result
of the design of the Subscriber Transceiver. The subscriber’s transmit (upconverter) IF
frequencies and subscriber’s receive (modem) IF frequencies are detailed in the Table in
Frequency Planning section of this manual. The frequencies are chosen such that the receive
IF frequencies correspond to the center frequencies of the standard North American CATV
frequency plan. The Figure shows 459 MHz, which is the center of channel 53. One of the
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