24 Functional Description TB7100 Installation and Operation Manual
© Tait Electronics Limited December 2005
Quadrature
Demodulator
The LO for the image-reject mixer (quadrature demodulator) is synthesized
and uses the TCXO (temperature-compensated crystal oscillator) as a
reference. This ensures good centring of the IF filters and more consistent
group-delay performance. The quadrature demodulator device has an
internal frequency division of 2 so the second LO operates at
2 x (IF1 + 64kHz). The quadrature output from this mixer is fed to a pair
of ADCs (analog-to-digital converters) with high dynamic range where it is
oversampled at 256kHz and fed to the custom logic device.
Automatic Gain
Control
The AGC (automatic gain control) is used to limit the maximum signal level
applied to the image-reject mixer and ADCs in order to meet the
requirements for intermodulation and selectivity performance. Hardware
gain control is performed by a variable-gain amplifier within the quadrature
demodulator device driven by a 10-bit DAC (digital-to-analog converter).
Information about the signal level is obtained from the IQ (in-phase and
quadrature) data output stream from the ADCs. The control loop is
completed within custom logic. The AGC will begin to reduce gain when
the combined signal power of the wanted signal and first adjacent channels
is greater than about –70dBm. In the presence of a strong adjacent-channel
signal it is therefore possible that the AGC may start acting when the wanted
signal is well below –70dBm.
3.1.2 Digital Baseband Processing
Custom Logic The remainder of the receiver processing up to demodulation is performed
by custom logic. The digitised quadrature signal from the RF hardware is
digitally down-converted to a zero IF, and channel filtering is performed at
base-band. Different filter shapes are possible to accommodate the various
channel spacings and data requirements. These filters provide the bulk of
adjacent channel selectivity for narrow-band operation. The filters have
linear phase response so that good group-delay performance for data is
achieved. The filters also decimate the sample rate down to 48kHz. Custom
logic also performs demodulation, which is multiplexed along with AGC
and amplitude data, and fed via a single synchronous serial port to the DSP.
The stream is demultiplexed and the demodulation data used as an input for
further audio processing.
Noise Squelch The noise squelch process resides in the DSP. The noise content above and
adjacent to the voice band is measured and compared with a preset
threshold. When a wanted signal is present, out-of-band noise content is
reduced and, if below the preset threshold, is indicated as a valid wanted
signal.
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.
To Next Page
Loading...
