Orban OPTIMOD-FM 5700i, Operating Manual

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OPTIMOD-FM DIGITAL INSTALLATION
2-9
Connect the 5700i’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 5700i close to the exciter and limit the length of the composite cable
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 1-16).
If the isolation transformer is in use, the
G
ROUND
L
IFT
switch will almost
always be set to
GROUND.
Even when its composite limiter is being used heavily, the 5700i will al-
ways protect the stereo pilot tone by at least 60 dB (
±
250Hz from 19 kHz)
and will protect the region from 55 kHz to 100 kHz by at least 75 dB (re
Figure 2-3: Separation vs. load capacitance