User Manual

VCom Inc.

AP5857 Manual; ml_ap5857_08 (Oct 2005)

Approved: R.B.

3.2.2 Upstream Gain

The upstream gain setting is used both to compensate for the link distances and to enable the upstream

signal to meet the required level into the tap. Note that the broadband noise level in the 18 to 42 MHz

upstream band is directly proportional to the upstream gain setting, so in general it is desirable to operate with

the lowest practical gain setting to minimize the noise level.

 On the setup of at least the first RF link to the AP5857, the setup should be performed

with the Upstream Mute set to “Unmute”, and the Output Alarm Threshold set to “Disable” so that

the upstream signal is continuously on. This will simplify the setup.

It is recommended that the upstream gain be set to approximately 50 dB for initial setup. Since some taps

may require higher upstream levels than this will produce, the CM may not be able to register with the initial

gain settings. If that is the case, the upstream gain can be increased in 10 dB increments until the subscriber

CM registers, allowing time for registration at each gain.

When registration is achieved, the upstream gain should be optimized by setting it to a value that has the

subscriber CM transmitting between 40 dBmV and 50 dBmV. The optimal gain setting for a specific link will

depend on both the link distance and the upstream level required at the tap. Lower upstream gain settings

will cause the CM to transmit harder to maintain the same level at the tap. This may improve the

signal-to-noise ratio (SNR), but will also reduce margin to compensate for gain variations in the path between

the CM and the CMTS. Conversely, higher upstream gains will cause the subscriber CM to reduce it’s

transmit power to maintain the same level at the tap, and this may degrade the upstream SNR.

3.2.3 Upstream Muting

The upstream muting has three modes, which include Unmuted, Muted and Automatic.

The Unmuted setting should be used for initial setup and may also be used for normal operation. In this

mode, the upstream output is always on.

The Muted setting is used to manually turn off the AP5857 upstream signal. This may be used for example if

the unit is put out of service. Manually muting the upstream output does not affect the internal cable modem,

so communications and control are unaffected.

The Automatic Mute setting may be used to reduce the impact of the upstream noise on the cable system.

In the automatic mute mode, the upstream output is muted unless there is a signal above the Output Auto

Mute Threshold level. When a signal above the threshold is detected, the upstream output is turned on only

for the duration of the upstream burst. The Output Level reading in the Upstream Settings group indicates the

approximate measured level of the upstream burst signal. The threshold is typically set near that level or up

to a few dB lower. The threshold setting is important since if the threshold is set too low, noise may keep the

output unmuted, while setting the threshold too high will not allow the signal to turn on the output.

Note that there is more latitude in setting the threshold if the AP5857 upstream output is higher. The range

between highest and lowest threshold setting at +50 dBmV output is approximately 7 dB, but only 3 dB at +35

dBmV. The automatic muting is not recommended for upstream output levels below 35 dBmV.