Orban OPTIMOD-FM 8500S, Operating Manual

Page: 65 / 335

OPTIMOD-FM DIGITAL INSTALLATION
2-9
the 8500S 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
unless this is unavoidable. The frequencies in the stereo baseband are
low by comparison to RF and video, and the characteristic impedance of
coaxial cable is not constant at very low frequencies. Therefore, the
transmission system will usually have more accurate amplitude and phase
response (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
exciter 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 8500S 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
100% modulation). See
Figure 5-1 on page 5-4.
The subcarrier (SCA) inputs are provided for convenience in summing subcarriers
into the baseband prior to their presentation to the FM exciter.
The subcarrier inputs will accept any subcarrier (or combinations of sub-
Figure 2-3: Separation vs. load capacitance