Tait TM8000 Series Applications Manual page 81

Figure 3.13 Connecting audio tap out and audio tap in
radio
600
buffer
TM8000 3DK Hardware Developer's Kit Application Manual
September 2003 © Tait Electronics Ltd
page 20). The buffer amplifier has an output impedance of nominally 600Ω
that is constant across frequency.
The audio tap in line is also DC-coupled. A DC bias network provides a bias
of nominally 1.5V. The valid DC input signal range is 0.5 to 2.5V nominally
regardless of bias voltage. Therefore, to avoid asymmetrical clipping and
reduced dynamic range, it is important that the input bias voltage is
preserved when driving the input. This can be achieved by simply AC-
coupling the drive signal. For data applications, DC-coupling may be
desirable so, in this case, the driver must provide a DC bias signal as close as
possible to 1.5V. After input biasing, the AUD_TAP_IN signal is fed to a
switched capacitor high-pass filter with a cut frequency of 3.7Hz.
This prevents the DC bias affecting the transmitter carrier frequency.
The high-pass-filtered signal is then low-pass-filtered to prevent aliasing,
and sampled by an analogue-to-digital converter at 48kHz. The analogue-
to-digital converter output is then fed to the DSP audio path (refer to
Figure 3.14 for details)
For some applications, such as a crossband link or fitting an encryption
module, it is necessary to connect the audio tap out line to the audio tap in
line. The two are not directly compatible but can be made so using a simple
external coupling network as shown in
external
coupling
network
C
AUD_TAP_OUT
600
The 600Ω shunt resistor reduces the maximum level of audio tap out to
nominally 2V to match the maximum input level of audio tap in.
p-p
The coupling capacitor removes the DC offset. For voice applications,
C should be at least 100nF. If high-speed baseband data modulation
C
throughput is required, C
capacitor should be a non-polarised type.
Figure
3.3V
220k
c
AUD_TAP_IN
180k
of at least 4.7µF is recommended. The C
C
3.13.
radio
3.7Hz
HPF
Programmable I/O Lines
C
81