User's Manual

Figure 5-20 shows a quiet sector – no modems bursting on the 6.4 MHz IF upstream. The
solitary burst is on an adjacent sector centered at 9.6 MHz. This is evident as the modulation
envelopes of the ARCell upstream signal are 3.2 MHz wide. Thus the 6.4 MHz-centered
upstream has energy between 4.8 and 8 MHz. The solitary burst is greater than 8 MHz so it is
part of an upstream signal centered at 9.6 MHz with energy between 8 and 11.2 MHz. In this
system a physically adjacent sector is operating with a 9.6 MHz-centered upstream.
Note the rise in the noise floor of 6-8 dB centered on this trace. This is the result of a SAW filter
internal to the downconverter in the Hub Transceiver. The noise floor in this trace acts like a
tracking generator in that it shows a portion of the response (gain) increasing in the pass band
of the downconverter. Actually the cutoff on either side of the filter is much greater than is
shown here due to the noise floor of the particular spectrum analyzer. However, the nominal 5
MHz pass band of the filter admits the 3.2 MHz wide upstream signal, and in this case just
enough of the upstream signal from an adjacent upstream channel (centered on 9.6 MHz) is
present for the burst to be captured in the display.
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Spectrum Analyzer
BVT U1
Frequency (1.4 - 11.4 MHz)
Figure 5-20: Spectrum of quiet sector (no subscriber units).
A very useful check for receiver operation is to use the spectrum analyzer at the upstream 12
dB tap point to check for the characteristic shape of the rise of the noise floor centered on the
frequency of the particular upstream as shown in Figure 5-19. Occasionally (long IF cable runs)
this shape is not noticeable as it is lost in the spectrum analyzer noise floor. In these cases the
coaxial cable from the Hub Transceiver must be disconnected from the 12 dB tap and directly
connected to the input of the spectrum analyzer.
5.8 Hub Power Cable Convention
The standard power cable shipped with the Hub units is a 25 ft cable with a connector at one
end. The cable is made from Belden #8762 wire, which has one twisted pair, a shield, and a UV
rated PVC grey jacket.
June 2003 Page 5-23