78 Network Circuitry TB9100 Reciter Service Manual
© Tait Electronics Limited January 2006
The CODEC power supply rail of 3.3V limits the maximum output swing
on each of the EAR1OP and EAR1ON outputs to approximately 2.3Vpp.
Since this is not sufficient, if connected directly through a line transformer,
to provide the required output levels to line, an additional amplifier stage,
comprising dual op-amp U501 and its associated components, is added.
This differential input, differential output amplifier circuit boosts the
CODEC differential outputs by approximately 2dB, as determined by
resistors R521, R522, R530, R531, R533 and R535. As this is greater than
the required gain, the input signal is attenuated approximately 3.7dB by
resistors R502, R520, R516 and R517. Filter capacitor C520 removes any
residual high frequency noise on the CODEC outputs.
The two op-amp differential outputs connect to the line transformer T500
via sense resistors R532 and R534. The correct transformer drive resistance
is synthesized by taking positive feedback, through R533 and R535, and
negative feedback, through R530 and R531, from these sense resistors.
With the resistor values shown, the differential output resistance of the
amplifier is approximately 402Ω, the value for optimum return loss.
Capacitors C507 and C509 optimize the high-frequency return loss as for
the transmit side.
The synthesized output resistance enables a greater output swing from a
given power supply voltage than is possible using a real output resistor; the
amplifier is capable of driving in excess of +6dBm to a 600Ω line over the
300-3400Hz voice band. Due to limitations in the line transformer’s signal
handling capability at low frequencies, the allowable output level has to be
progressively reduced to a maximum of -14dBm at 67Hz. Higher output
levels at low frequencies result in excessive, mainly third harmonic,
distortion.
When connected to a telco line, the output level must be limited to the
maximum of 0dBm at all frequencies for compliance with the telco
regulations. Due to CODEC gain inaccuracies and external circuit
variations totalling ±1.5dB, the nominal maximum output level is
downgraded to -1.5dBm to ensure that the maximum allowable line level is
not exceeded.
Since the telco network can present large transient voltages to equipment,
two levels of overvoltage clamping are provided in the audio line interface.
Initially, transients are absorbed by bi-directional transient suppressors D504
and D503. Any residual transient signals that are coupled through the
transformers are further clamped to the supply and ground rails by diode
array D505 on the transmit channel and by Zener diodes D506 and D507
on the receive channel.
The level of protection provided by the transient suppressors and clamping
diodes is regarded as “secondary” protection only: if extreme conditions are
expected, “primary” overvoltage protection devices, such as arrestors, may
need to be fitted externally.
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