RFL Electronics 9780 - Page 383

RFL 9780 RFL Electronics Inc.
September 8, 2001 18-18 (973) 334-3100
18.4.2 THEORY OF OPERATION
For this discussion, refer to the schematic diagram in Figure 18-10.
The RFL 9780 Dual Hybrid Module uses four hybrid transformers to couple signals between a transmitter, a
receiver, and a line tuning unit. It provides impedance matching and signal separation, and prevents mutual
loading and interference. Signals can be accepted from amplifiers with a source impedance of 50 Ohms with
up to 10 Watts of output power.
The combined transmitted and received signal passes through a hybrid formed from transformers L1
and L2. These transformers each have two secondaries, interconnected so that the first secondary of one
transformer is in series with the second secondary of the other. One set of interconnected secondaries is
connected to the line tuning unit through edge connector A24/C24 (high) and A25/C25 (low). This
allows both the transmitters and the receiver to be coupled to the line tuning unit. The other set of
secondaries is connected across the internal balancing network formed by resistors R4 through R11.
The coarse balance switch SW1 and fine balance potentiometer R4 can be adjusted to vary the
resistance across the transformer secondaries, which determines the amount of transhybrid loss. If
additional transhybrid loss is required, the internal balancing network can be disabled by placing
jumper J1 in the “B” position and placing an external network across edge connector terminals
A18/C18 and A19/C19.
The combined transmitter output is connected across the primary of L1, and the receiver input is connected
across the primary of L2. Combined output signals are coupled through L1 to the line tuning unit, and
incoming signals detected by the line tuning unit are coupled through L2 to the receiver. Transmitted signals
are attenuated by about 3.8 dB total as they pass through the Dual Hybrid module, and received signals are
attenuated by about 12.5 dB. Because both the received signal and its noise content are attenuated equally,
signal-to-noise ratios are unaffected.
The transmitted signals pass through a transformer hybrid formed from transformers L3 and L4. These
transformers each have two secondaries, interconnected so that the first secondary of one is in series with
the the secondary of the other. One set of interconnected secondaries is the combined transmitted signal.
The other set of secondaries is connected across balancing resistors R15/R16 which determine the amount
of isolation between four-wire ports.