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Technical Description
Hard Limit
A +13.5 dB gain hard limit follows the pre-emphasis block in the transmit audio
path in order to limit the maximum audio level out of the ASIC and into the modula-
tor block.
Transmit Lowpass Filter
A 3 kHz lowpass filter follows the hard limit to filter out the harmonic components
caused by clipping in the hard limit.
AUDIODEV Programmable Gain Stage
The AUDIODEV programmable gain stage follows the TX lowpass filter to allow
adjustment of the maximum transmit deviation. This stage is provided to trim out
tolerances in the following summing stage.
The nominal gain in the AUDIODEV stage is 0 dB with an adjustment range from
+3.2 to -2.8 dB in steps of 0.4 dB.
Test mode 14 can be used to adjust the AUDIODEV stage for TX audio.
The transmitted DTMF tone has a separate setting for AUDIODEV in EEPROM. It
needs to be adjusted to provide enough gain after the compressor so that the signal
does not clip in the compressor.
Summing Amplifier
The summing amplifier is provided to combine the TX audio / DTMF tone, SAT
tone and Manchester data into the modulator. The input resistors and feedback resis-
tor for this circuit are external to the ASIC to allow for gain adjustments.
TXSENSE Programmable Gain Stage
The TXSENSE programmable gain stage is an amplifier which is provided to trim
out tolerances in the modulator but is currently not used, however set to a default
value of 0 dB. This stage has an adjustment range from +3.2 to -2.8 dB with steps of
0.4 dB.
Low Voltage Detect Circuit
A comparator is included in ANTON to provide a reset signal to GUSTAV when the
regulated voltage drops below approximately 3.3 V. This circuit is enabled at power
up and can be disabled by software to save a small portion of current in standby
mode.
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