Benefon DRAGON TDP70WN, Руководство по обслуживанию, Страница: 25 / 76

5.9.2000
4Z2200GB.fm 8
Product: TDP70WN / OC2285 Processor/Audio/RF
normal deviation gain control circuit to set the signal level. The P/E & LIMIT
pre-emphasis circuit and limiting circuit, emphasize the higher frequency
component of the signal in order to improve the signal-to-noise ratio of
modulated signal. This block includes a limiting circuit for signal amplitude in
order to confine the maximum deviation of the transmit modulated signal.
Before TXLPF is the FFSKTX switch. TXLPF is the low pass filter to reject the
higher frequency component on the transmit signal. VR3 is a maximum
deviation gain control circuit to set the transmit signal level. After the VR3
comes switch TXAUDON, which mutes the tx-audio signal using the TXMUTE
control. From the switch, the signal is fed to the summing junction (ADD3) of
the tx-audio signal and Fii-signal. Next comes VR4 gain control circuit to set.
SMF1 is smoothing filter for tx-audio signal. The tx-audio signal is then fed to
the RF-stages.
5.2.3 RX-Audio
The rx-audio signal coming from the receiver through the V103 pin 8 is fed to
the audio circuit I7 pin 23. Inside the circuit, the signal is fed to the operational
amplifier (AMP2) inverting input. The amplifier gain is set by resistors R85 and
R95. The signal is next passed through an anti-aliasign filter. VR5 is a gain
control circuit to set the rx-audio signal to the correct level. Next comes de-
emphasis (D/E) circuit. Equalize the pre-empassized rx-audio signal. The
signal passes from the D/E through the switch RXAUDON. RXBPF is the band-
pass filter for the rx-audio signal. RXDET is the rx-audio signal detection circuit.
This circuit works as a full wave rectifier. After the RXBPF comes I8 which is
the split-switch of audiosignal and then expander circuit (EXP). Expand the rx-
audio signal amplitude. It can be bypassed. The linearity is adjustable by the
control register EVR. VR6 is a gain control circuit to set the rx-audio signal to
the correct level. After VR6 comes the RXMUTE switch, which is operated by
the RXMUTE control. ADD4 is the summing junction of the rx-audio signal,
external signal (not used), DTMF signal and transmit signal. VR10 is the
volume control circuit to set the level of earphone and external earphones. The
scrambler circuit I9 is optional and it is controlled by the serial interface circuit.
The signal is then fed via R91 to the audio amplifier I10. The amplifier drives
the earphone A4 and by controlling the I10 SE/BTL pin, the signal can be
routed to the HF-connector.
5.2.4 FII Signal
The NMT system uses the FII signal to check the radio path quality. This
approx. 4 kHz signal is split from the rx-audio signal after the VR5 and is
filtered through the band-pass filter (FBPF). VR7 sets the FII signal to the
correct level. Switch FILOOPON can be operated by the FIION control, to be
summed with the tx-audio signal before the VR4.