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SDA-5500/SDA-5510 Sweep Configuration: Configuring Reverse Sweep

290

During reverse sweeping using only the SDA-5500 in the headend, the level

of the telemetry (the telemetry coming back upstream from the SDA-5000)

needs to hit the input of the SDA-5500 at 0 dBmV +/-10 dB. You can verify

this easily by checking the telemetry level in the lower right corner of the

display. Again, if additional attenuation is required, you can use an in-line

pad on the low side of the diplex filter. If multiple nodes are being combined,

a pre-amplifier may be required.

Take care not to place the telemetry too far into the edges of the reverse

path band. The reverse telemetry signal needs to have a S/N > 20 dB

(300kHz measurement bandwidth), which effectively limits the number of

return nodes that can be combined on the input of the SDA-5500. A diplex

filter is used instead of a splitter because of better isolation and impedance

matching.

Stealth headend transmitters control reverse telemetry frequency. During

reverse sweep, the headend transmitter sends the reverse telemetry

frequency and channel plan to the field unit via the forward telemetry

channel. This means that only the forward telemetry frequency must be set

before reverse sweeping starts.

Reverse Sweep Channel Plans

Reverse sweep plans are used to define the frequencies at which sweep

points will be inserted by SDA-5500 units with the Reverse Sweep option.

The following points will help you prepare to create a reverse channel plan:

• If the frequency bands of the active return services are known, the

reverse channel plan can achieve a 250 kHz resolution. Then the active

service frequencies can be quickly edited out. However, if you're

uncertain about the frequency bands occupied by the active services,

you can use the SDA-5500 or SDA-5510 to collect this information in

Spectrum Analyzer mode, with a 50 MHz span and a center frequency of

30 MHz. A “max-hold” spectrum over an extended period can be

analyzed to determine which frequencies in the reverse path contain

active services so you can avoid interference.

• The “max hold” spectrum scan shown in Figure 14-13 is an example of a

fully-loaded reverse band. Services carried include telephony, data

(cable modem), and PCS. You can also see a few CW signals at the high

and low ends of the band, providing a consistent signal for the reverse

laser. The spectrum scan is 45 MHz wide (5 to 50 MHz) or 4.5 MHz per

division. A look at the spectrum indicates that these frequencies and

bands should be omitted: 6, 15-16.5, 18.5-20, 22.5-27.5, 29-30.5, 31-

32.5, 33-36, 36.5-38, 40 and 41-42.5.

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