Orban OPTIMOD 5750 Operating Manual page 34

Orban 5750 Technical Manual
Each output's level is independently adjustable from –12 dBu to +16.0 dBu.
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 daughterboard). 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 performance deteriorates significantly.
Connect the 5750'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 5750 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 example, by connecting the 5750'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.
•
Even when its composite limiter is being used heavily, the 5750 will always 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.)
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 subcarriers) above 23 kHz. Below 5 kHz,
sensitivity rolls off at 6 dB/octave to suppress hum that might otherwise be introduced into the subcarrier
inputs, which are unbalanced.
•
The subcarrier inputs are mixed into the 5750's composite output in the analog domain, after D/A conversion
of the 5750 stereo encoder's output. Rear-panel accessible PC-board-mounted trim pots allow the user to adjust
the sensitivities of the two SCA inputs from <100 mV p-p to >10 V p-p to produce 10% injection with respect to
100% modulation = 4 V p-p at the 5750's composite outputs. (The factory setting is 4 V p-p to produce 10%
injection.)
2-17
Installation