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then leaves the RF module via the demodulator IC at about -16dBv (for a deviation of +/- 25kHz). From the RF module, the audio is
then fed into a buffer amplifier where the audio is lowpass filtered and directed to both an audio channel and a data channel. The
audio undergoes this split in directions after the buffer amplifier because both data and audio share the same circuits upto and
including the buffer amplifier. The filtering at the buffer amplifier provides some rejection at higher frequencies (> 40kHz).
The buffer amplifier output is connected to an active third order lowpass filter with a - 3dB cut-off set at about 5.5kHz. The filter has
unity gain in its passband. The filtered audio is then passed to an active de-emphasis filter where de-emphasis occurs across the entire
audio band (300Hz to 3,400kHz) at a rate of 6dB/octave or 20dB/decade. After de-emphasis, the audio undergoes the expansion
process and is passed through a transmit audio level control. The level control which has a range of about 12dB, is used to set the
transmit audio level at the tip and ring of the telephone line. The transmit level can vary from component tolerances and variations.
After the level control, the audio is passed to a final stage of amplification. The output of this amplifier can be taken differentially, but
is taken single ended because of the amplifier’s adequate power capacity. From this amplifier, the audio is coupled to the transistor
amplifier which leads to the rest of the speech circuit and telephone interface. The output of the DTMF generator circuit is also
coupled in at the input to this final stage of amplification.
The transmit audio chain can be disabled at the expander amplifier by a mute function on the compander IC. This function is used to
mute the transmit audio chain when data is being received from the handset so that data noise does not enter the telephone line. To
minimize costs, the transmit audio mute function also simultaneously disables some of the receive audio circuits. When transmitting
data from the base to the handset the mute function is used to disable the receive audio circuits.
3.4.2
Receive Direction (Telephone line to RF module)
The receive audio signal from the telephone line makes its way through the line interface and the speech circuit before reaching the
receive direction audio circuits. From the speech circuit, the audio undergoes a first stage of amplification and light lowpass filtering.
Following the amplifier, the receive audio is compressed and fed directly to the pre-emphasis stage. The compressor does a straight
2 to l conversion, the dynamic range is reduced by one half. The compressor amplifier is also used to sum in DTMF feedback so that
the tones can be heard from the handset’s receiver. Pulse dialing feedback is accomplished similarly by summing in the hook switch
signal level at the same location.
From the compressor output, the receive audio signal enters a pre-emphasis circuit with an integrated level control. The pre-emphasis
like the de-emphasis is set at a rate of 6dB/octave or 20dB/decade throughout the entire audio band (300Hz to 3,400Hz). The level
control , as in the transmit direction, has a range of about 12dB and is used to set the level applied to the RF module. This level control
thus sets the FM deviation and is necessary to compensate for component tolerances and variations in the sensitivity of the FM
circuits.
Transmit data is resistively combined in just before the level control from which point it shares the rest of the audio circuits with the
receive audio. However, either only audio or only data will be present at any given time to prevent corruption of the signals. To
further minimize the chance of data corruption, the receive audio circuits are disabled at the compressor using the mute function as
mentioned in the previous section. Muting this part of the receive audio chain ensures that any noise or signals on the telephone line
do not interfere with data transmissions to the handset.
Following the receive audio level control (or deviation adjust), the audio goes through another stage of amplification and light
lowpass filtering before being passed to an active 3rd order lowpass filter. The filter’s -3dB cut-off is set to approximately 6.7kHz
and has unity gain in the passband. The 3rd order lowpass filter’s output is then coupled to the RF module’s frequency modulator.
3.5 MCU Circuits
A relatively inexpensive CMOS 8K x 4-bit MCU (LSC442350DW) is used to control all the functions in the base. The MCU is clocked by
a 4MHz crystal and controls such functions as DTMF generation, data communications, telephone signaling detection and ATE interfacing.
3.5.1
DTMF Generation
To minimize costs, power consumption and space, the MCU is used to generate the DTMF tones in lieu of a dedicated DTMF
generator IC. The MCU generates the tone waveforms by using a 1% R-2R ladder network connected to six of its ports to produce a
6-bit D-to-A convener. The D-to-A converter’s output is then passed through an active 3rd order lowpass filter to clear the waveforms
of high frequency ripple caused by the digital-to-analog conversion. The -3dB cut-off for this filter is set at about 4.6kHz and has
unity gain in the passband.
As mentioned in previous sections, the DTMF tones are combined in at the input of the last amplifier stage in the transmit direction
for transmission onto the telephone line. The transmit audio chain is disabled at the expander during DTMF dialing to stop audio
from the RF module from entering the telephone line and interfering with the dialing. As well, the DTMF tones are summed in at the
compressor stage in the receive direction for audio feedback at the handset receiver.
During DTMF tone generation, the MCU’s modem function are disabled to ensure that the MCU has enough resources to produce
distortion free tones. If the modem functions are not disabled, the data communications associated with the modem functions in
conjunction with the DTMF sample generation could overload the MCU and result in missed samples.
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