9.2.5.3
THEORY OF OPERATION
For the following discussion, refer to the schematic diagram of the Line Board in Section 9 .The RF
Line board connects the RFL 9508 to the line coupling equipment. It contains two hybrid transformers,
a receive attenuator, a complex balance network, two impedance matching transformers, and two surge
arrestors. A block diagram of the Line Board appears in Figure 9-10.
9.2.5.3.1
HYBRID TRANSFORMERS
The RFL 9508 Line Board contains two hybrid transformers, T3 and T4. The primary winding of T3
accepts the transmit input signal, and the primary winding of T4 develops the receive output signal that
is passed to the Rx Filter.
Both hybrid transformers have two secondary windings, and are interconnected so that one secondary
of T3 is in series with a secondary of T4. One set of secondaries is connected across the complex
balance network, and the other set of secondaries is connected to the line coupling equipment through
line matching transformer T2. Fuses F1 and F2 provide overcurrent protection, and surge arrestors E1
and E2, protect the equipment against an overvoltage condition. For four wire applications, the signal
going to the line coupling equipment can be monitored at TP2, and the signal coming from the line
coupling equipment can be monitored at TP3. For two wire applications the signal going to or coming
from the line coupling equipment can be monitored at TP2.
9.2.5.3.2
RECEIVE ATTENUATOR
Resistors R9, R11, R12 and dual-section Receive Level potentiometer R10 are connected across the
primary of T4 to form an attenuator. The amount of signal sent to the Rx Filter is controlled by the
setting of R10. Zener diodes CR1 and CR2 clamp the signal to a safe level. The signal being sent to the
RX Filter can be monitored at test points TP6 and TP7.
9.2.5.3.3
COMPLEX BALANCE NETWORK
Inductors L1 through L8, capacitors C1 through C9, Coarse Switch S1, fine potentiometer R1 and
resistors R2 through R8 form an internal reactive balance network. This network is connected across
one set of secondaries of the hybrid transformers. Jumper J13 can be used to enable or disable the
internal balance network. The balance network is adjusted to match the line impedance. To adjust the
network, a frequency selective voltmeter (FSVM) is connected across test points TP6 and TP7 on the
Line Board, and S1 and R1 are adjusted to the lowest possible transmitter signal level. S1 provides a
rough adjustment of the resistance across the hybrid secondaries by determining how many resistors in
the network are connected in series. R1 provides a fine resistance adjustment that allows the resistive
balance to be precisely set.
Inductors L1 through L8 and capacitors C1 through C9 form the reactive portion of the balance
network. All components in the reactive portion can be jumpered in or out to balance out local line
reactance. L1 through L8 can be enabled or disabled by jumpers J13 through J19. Jumper J1 controls
C1, J2 controls C2, and so on. If no reactive balancing is required, J1 through J9, and J14 through J19
must be placed in the out position.
RFL 9785
RFL Electronics Inc.
May 16, 2011
9-12
(973) 334-3100
