Orban OPTIMOD-FM 5500 Operation Manual page 56

2-8
INSTALLATION
The output impedance of composite 1 output and composite 2 output can be set to
0 or 75 via jumpers J7 and J8 respectively (located on the Composite/SCA daugh-
terboard). As shipped, the link is on pins 3 and 4, yielding 0  impedance. To reset a
given output to 75, 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 in parallel with 0.047F before perform-
ance deteriorates significantly (see Figure 2-3 on page 2-8).
Connect the 5500'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 5500 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 or 75
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 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
Figure 2-3: Separation vs. load capacitance
ORBAN MODEL 5500