Orban OPTIMOD-FM 8600Si, Operating Manual

Page: 65 / 356

OPTIMOD-FM DIGITAL INSTALLATION
2-9
given output to 75
Ω
, place the link on pins 1 and 2 of its associated jumper. (See the
schematic on page 6-38 and the parts locator diagram on page 6-34.)
Each output can drive up to 75
Ω
in parallel with 0.047
μ
F before perform-
ance deteriorates significantly (see Figure 2-3 on page 2-9).
Connect the 8600Si’s composite output to the exciter input with up to 100 feet
(30.5m) of RG-58/U or RG-59/U coaxial cable terminated in BNC connectors.
Longer runs of coax may increase problems with noise, hum, and RF
pickup at the exciter. In general, the least troublesome installations place
the 8600Si close to the exciter and limit the length of the composite ca-
ble to less than 6 feet (1.8m).
We do not recommend terminating the exciter input by 50
Ω
or 75
Ω
un-
less this is unavoidable. The frequencies in the stereo baseband are low
by comparison to RF and video, and the characteristic impedance of coax-
ial cable is not constant at very low frequencies. Therefore, the transmis-
sion system will usually have more accurate amplitude and phase re-
sponse (and thus, better stereo separation) if the coax is driven by a very
low impedance source and is terminated by greater than 1k
Ω
at the ex-
citer end. This also eases thermal stresses on the output amplifier in the
stereo encoder, and can thus extend equipment life.
Ground loops can occur if your exciter’s composite input is unbalanced,
although you can usually configure system grounding to break them (for
example, by connecting the 5518’s and exciter’s power cords to adjacent
sockets on an AC power strip). In difficult cases, you can always break a
ground loop by using a Jensen JT-123-BMCF transformer (see page 1-15).
Figure 2-3: Separation vs. load capacitance