BL34018
As can be seen from the truth table, the Tx-Rx comparator dominates. The Transmit Detector
Comparator is effective only in the transmit mode, and the Volume Control is effective only in the
receive mode.
The Tx-Rx comparator is in the transmit position when there is sufficient transmit signal
present over and above any receive signal. The Transmit Detector Comparator then determines
whether the transmit signal is a result of background noise ( a relatively stable signal) or speech
which consists of bursts. If the signal is due to background noise, the attenuators will be put into
the idle mode (∆Vacf = 75 mV). If the signal consists of speech, the attenuators will be switched
to the transmit mode (∆Vacf = 6.0 mV.) A further explanation of this function will be found in the
section on the transmit detector circuit.
The Tx-Rx comparator is in the receive position when there is sufficient receive signal to
overcome the background noise AND any speech signals. The
∆Vacf
will now be 150 mV IF the
volume control is at the maximum position, i.e. VLC (Pin 24) = VB. If VLC is less than VB, the
gain of the receive attenuator, and the attenuation of the transmit attenuator, will vary in a
complementary manner. At the minimum recommended operating level (VLC = 0.55VB) the gain
of the tranmit attenuator is actually greater than that of the receive atttenuator, although it is at
receive mode. The effect of varying VLC is to vary
∆Vacf,
with a resulting variation in the gains
of the attenuators.
The capacitor at ACF (Pin 25) smooths the transition between operating modes. This keeps
down any “clicks” in the speaker or transmit signal when the ACF voltage switches.
The gain separation of the two attenuators can be reduced from the typical 45 dB by adding a
resistor between Pins 20(VCC ) and 25(ACF). The effect is a reduction of the maximum
∆Vacf
voltage in the receive mode, while not affecting
∆Vacf
in the transmit mode.
LOGAMPS
(Transmit and Receive Level Detectors)
The log amps monitor the levels of the transmit and receive signals, so as to tell the Tx-Rx
comparator which mode should be in effect. The input signals are applied to the amplifiers (at TLI
and RLI) through AC coupling capacitors and current limiting resistors. The value of these
components determines the sensitivity of the respective amplifiers, and has an effect on the
switching times between tranmit and receive modes. The feedback elements for the amplifiers are
back-to-back diodes which provide a logarithmic gain curve. The outputs of the amplifiers are
rectified, having a quick rise and a slow decay time. The rise time is determined primarily by the
external capacitor and an internal 500 ohm resistor, and is on the order of a fraction of 1 ms. The
decay time is determined by the external resistor and capacitor, and is on the order of a fraction
of 1 s. The switching time is not fixed, but depends on the relative values of the transmit and
receive signals, as well as these external components.
The Tx-Rx comparator responds to the voltages at TLO and RLO, which in turn are functions
of the currents sourced out of TLI and RLI, respectively. If an offset at the comparator input is
desired, e.g., to prevent noise from switching the system, or to give preference to either the
transmit or receive channel, this may be achieved by biasing the appropriate input (TLI or RLI). A
resistor to ground will cause a DC current to flow out of that input, thus forcing the output of that
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Total
16 Pages
8/15/2006
Wrote by 2006
BELLING [ SHANGHAI BELLING CO., LTD. ]
