OPTIMOD-FM INSTALLATION
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Connect your subcarrier generator(s) to the 8400’s subcarrier input(s) with coaxial cable
terminated with BNC connectors.
The subcarrier inputs have 600Ω impedance and are unbalanced. To mini-
mize noise pickup and to be compatible with the output levels available from
many subcarrier generators, gain from the subcarrier input to the composite
output is fixed at –20 dB. Therefore, the gain is scaled so that 1.5V peak at
the subcarrier input produces 10% subcarrier injection with reference to
3.0Vp-p = 100% modulation of the FM carrier. (Because the signals at the
subcarrier inputs are not digitized, it was not practical to offer a software-
settable gain control for them.)
Grounding
Very often, grounding is approached in a “hit or miss” manner. But with care it is possi-
ble 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:
• 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 com-
mon-mode rejection and prevent ground loops—regardless of whether it is driven
from a balanced or unbalanced source.
• The 8400 has balanced inputs. Its subcarrier inputs are unbalanced, but frequency
response is rolled-off at low frequencies to reject hum.
• All equipment circuit grounds must be connected to each other; all equipment chassis
grounds must be connected together.
• In a low RF field, cable shields should be connected at one end only—preferably the
source (output) end.
• 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.
• Whenever coaxial cable is used, shields are automatically grounded at both ends
through the terminating BNC connectors.
Power Ground
• Ground the 8400 chassis through the third wire in the power cord. Proper grounding
techniques never leave equipment chassis unconnected to power/earth ground. A
proper power ground is essential for safe operation. Lifting a chassis from power
ground creates a potential safety hazard.
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