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Technical Description
Audio Design
The baseband audio is processed by ANTON which is a mixed signal ASIC with a
compander, filters, limiters and programmable gain blocks.
Receive Audio Circuit Description
Receive Audio Interface
The receive audio is demodulated by a digital discriminator in BERTINDY and then
passed through a first order active bandpass filter which is comprised of an op-amp
located in BERTINDY. External resistors and capacitors set the gain and bandpass
of the filter and the output of this filter is then AC coupled through a capacitor to an
external de-emphasis network. The interface between the radio and the baseband
audio is between the discriminator's bandpass filter and the external de-emphasis
network.
De-Emphasis Network
The de-emphasis network is composed of two series resistors and a shunt capacitor.
The audio is routed through a resistor pair into the receive input port on ANTON.
The received SAT is tapped off between the resistor pair and routed to a port on
ANTON. The shunt capacitor provides the necessary de-emphasis slope to restore
the low frequency components of the audio that were attenuated by the pre-empha-
sis in the base station transmit path.
RXSENSE Programmable Gain Stage
The receive audio enters ANTON at the receive input port. The signal first passes
through an anti-aliasing filter and is then routed to the RXSENSE programmable
gain stage. This gain stage is provided to trim out the tolerances between the dis-
criminator output and the expander input. The RXSENSE gain stage has a nominal
gain of 8 dB and can be adjusted +6.4/-6.0 dB in steps of +0.4 dB.
Test mode 10 'RX Sensitivity' can be used to update this gain stage.
This stage is used to adjust the AFMS level to 25 mVrms with a 1 kHz tone at 2.3
kHz deviation and nominal volume.
Receive Bandpass Filter
The output of the RXSENSE stage is fed into a 300 Hz to 3 kHz switch-capacitor
bandpass filter.
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