Tait TM8000 Series, Руководство по обслуживанию

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56 General Description TM8100 Mobile Radio Service Manual
May 2004 © Tait Electronics Limited
Received Signal
Strength Indication
Received signal strength is measured by a process resident in the DSP.
This process obtains its input from the demodulator (value of RF signal
magnitude) and from the AGC (value of present gain). With these two
inputs and a calibration factor, the RF signal strength at the antenna can
be accurately calculated.
Calibration
The following items within the receiver path are calibrated in the
Factory:
■
front-end tuning
■
AGC
■
noise squelch
■
RSSI (received signal strength indication)
Information on the calibration of these items is given in the on-line help
facility of the calibration application.
Audio Processing and Signalling
Audio Processing
Raw demodulated data from the receiver is processed within the DSP.
The sample rate at this point is 48 kHz with signal bandwidth limited
only by the IF filtering. Scaling (dependent on the bandwidth of the RF
channel) is then applied to normalise the signal level for the remaining
audio processing. The sample rate is decimated to 8 kHz and bandpass
audio filtering (0.3 to 3 kHz) is applied. De-emphasis is then applied to
cancel out the receive signals pre-emphasised response and improve
signal-to-noise performance. Optional processing such as decryption or
companding is then applied if applicable.
Data and Signalling
Decoders
The data and signalling decoders obtain their signals from various points
within the audio processing chain. The point used depends on the
bandwidth of the decoders and whether de-emphasis is required. Several
decoders may be active simultaneously.
Side Tones
Side tones are summed in at the end of the audio-processing chain.
These are tones that provide some form of alert or give the user
confidence an action has been performed. The confidence tones may be
generated in receive or transmit mode. The side-tone level is a fixed
proportion (in the order of –10 dB) relative to full scale in the receive
path.
CODEC
The combined audio and side-tone signal is converted to analogue form
by a 16-bit DAC with integral anti-alias filtering. This is followed by a
programmable-gain amplifier with a range of 45 dB in 1.5 dB steps. The
amplifier performs volume control and muting. The DAC and volume
control are part of the same CODEC device (AD6521).