Hybrid Coupler Installation and Operations Manual Page 11
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capacitance between the two wires. This distributed inductance, resistance and
capacitance causes a loss at the higher audio frequencies. These two deficiencies
in tandem limit the frequency response of the telephone line to a band that
extends from 300–3,300Hz. Since capacitive and inductive reactance both
change with frequency, the complex impedance of the telephone line will be
different at every frequency in the voice band. The purpose of a telephone hybrid
is to separate the full-duplex audio of a balanced phone circuit into separate send
and receive audio ports.
This separation is very important to preserve the acoustic quality of the send
audio. When audio is sent to a caller, it is transmitted on the telephone line. The
acoustics of this audio are altered by the telephone line’s frequency response. If
some of this altered send audio gets mixed with the caller audio, the audio will
sound hollow.
The measurement of a telephone hybrid’s performance is how well it isolates, or
attenuates, the send audio input port from the caller audio output port. This
isolation can be optimized by adjusting the hybrid’s null controls. When the
impedance of the null circuit matches the impedance of the telephone line
connected to the hybrid, a perfect null or attenuation of the send audio will result.
However, as discussed earlier, the impedance of the telephone line is different for
every frequency in the voice band, thus, making the perfect null impossible to
achieve. Even though a perfect null is impossible, very satisfactory results can be
obtained using this method for most applications.
The Hybrid Coupler is a simple device, both technically and in operation. Please
refer to Appendix C: Schematics (next page) during the following discussion.
Telephone line connection to the unit is made via two RJ11 modular connectors.
Pins 2 and 3 of these two connectors are tied together to allow simultaneous use
of the unit and an attendant telephone set. The varistor (V1) is placed across the
telephone line inside the unit for surge protection.
The Hybrid Coupler uses a PREM SPT-183 telephone coil to provide termination
to the phone line. This coil is wound to provide a receive and send port with
approximately 6dB of isolation. The components R6, the front panel 1kOhm
trim pot, and C5, the .01uF null capacitor, allow for further impedance matching.
RFI protection is provided via L1, L2, FB1 and FB2 for both the send and receive
ports of the T1 coil.
The resistors, R1–R4, are all 301 Ohm 1% components. These provide
connection to the SEND and RECEIVE jacks and proper impedance matching
for the PREM coil.
The send and receive limiters are constructed of back-to-back 1N4148 diodes,
CR1 through CR8. Both of these limiters can be removed from the circuit via
JPl, JP2, JP3 and JP4 placement of a berg shorting post.
Appendix B: Theory of
Operation
