RFL 9780 RFL Electronics Inc.
September 8, 2001 18-21 (973) 334-3100
18.4.3.1 SELECTING THE BALANCING NETWORK
Jumper J1 is used to select an internal (resistive) balancing impedance or an external, user supplied,
balancing impedance. Placing the jumper in position A selects the internal impedance. Placing the jumper in
position B selects the external impedance. The internal impedance has a course adjustment using SW1 and a
fine adjustment using potentiometer R4.
18.4.3.2 SELECTING RECEIVE PORT PROTECTION AND TERMINATION
The receive port of the hybrid contains protection circuitry for the receiver’s front-end, and line
termination resistance. These features are similar to those used in the RF Line I/O, and may be
individually selected by using jumpers J2 and J3 as described in Table 18-5. Transient suppressor CR1
limits the voltage to the receiver and resistor R1 limits the current. Jumper J2 is used to bypass R1
when current limiting is not desired. J3 is used to enable the 50 Ohm termination resistance. The power
dissipated by the termination should be limited to 1W.
The protection circuit should be placed as close as possible to the receiver and the termination should
be placed at the end of the signal path. In a chassis with a single rear UHF connector, the termination
and protection should be enabled (“IN”) on the hybrid. The received signal is then wired directly to the
receiver circuits.
If the received signal is sent to a UHF connector on the line I/O (e.g. to go to another receiver), both the
termination and protection should be omitted from the hybrid (“OUT”). The received signal out of the
hybrid is sent to the line I/O’s UHF connector. Following the connector is the termination resistor. If
this is the last, or only receiver using this signal, the termination on the I/O should be enabled. The
protection circuit is always enabled on the I/O when connected to the local receiver, to protect the local
receiver. This configuration protects the receiver from line disturbances passed through the hybrid and
also protects against any transients applied to the UHF connector.
The termination must never follow any current limit resistance. The signal should be wired directly
from the protection circuit to the receiver, and each receiver should have protection.
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