2-20 Installation
To permit daisy-chaining sync signals, the input impedance is greater than 1 K:. If the 5500 is the last device
driven by the sync coaxial cable, you should terminate it by using a BNC Tee connector and a 75: BNC terminator.
This will prevent performance-degrading reflections in the cable. This is required for both wordclock and AES11id
operation.
WARNING: Do NOT apply an AES3 or AES3id signal to this input. Doing so will eventually cause your OPTIMOD to
suffer a “communications board error.”
Audio Grounding
Very often, grounding is approached in a “hit or miss” manner. However, with care it is possible to wire an audio
studio so that it provides maximum protection from power faults and is free from ground loops (which induce hum
and can cause oscillation).
In an ideal system:
x All units in the system should have balanced inputs. In a modern system with low output impedances and high
input impedances, a balanced input will provide common-mode rejection and prevent ground loops,
regardless of whether it is driven from a balanced or unbalanced source.
x The 5950 has balanced inputs. Its subcarrier inputs are unbalanced, but frequency response is rolled off at low
frequencies to reject hum.
x All equipment circuit grounds must be connected to each other; all equipment chassis grounds must be
connected together.
x In a low RF field, cable shields should be connected at one end only — preferably the source (output) end.
x In a high RF field, audio cable shields should be connected to a solid earth ground at both ends to achieve best
shielding against RFI.
x Whenever coaxial cable is used, shields are automatically grounded at both ends through the terminating BNC
connectors.
Audio Circuit Grounds
To maintain the same potential in all equipment, the circuit (audio) grounds must be connected together:
x When the 5950’s stereo encoder is driving an unbalanced exciter input, you may encounter a ground loop.
(Some older exciters have unbalanced inputs.) Unlike some older Orban FM processors, the 5950 does not
have a ground lift switch. If you cannot reconfigure your grounding scheme to eliminate such a loop, you can
balance and float the exciter input with a Jensen JT-123-BMCF transformer.
x In high RF fields, the system is usually grounded through the equipment rack in which the 5950 is mounted.
The rack should be connected to a solid earth ground by a wide copper strap. Wire is completely ineffective at
VHF because of the wire’s self-inductance.
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