Demodulation; Channel Filtering; Received Signal Strength Indication; Front-End Agc Control - Tait TM9100 Service Manual

Demodulation takes place within the FPGA. Demodulated audio is passed

Demodulation

to the DSP of the digital board for processing of the receiver audio signal.
Raw demodulated audio can be tapped out from the DSP for use with an
external modem. The modem may be connected to the auxiliary connector
or to the external options connector when an internal options board is
fitted.
The receiver has an AGC circuit to enable it to cover a large signal range.
Automatic Gain
Control
Most of the circuit functions are implemented in the FPGA. The FPGA
passes the AGC signal to the CODEC IC204 for output from pin 14
(
IDACOUT
quadrature mixer IC400. As the antenna signal increases, the AGC voltage
decreases.
The channel filtering is split between the first and third IF stages.

Channel Filtering

The channel filtering circuit in the first IF stage comprises a pair of two-pole
crystal filters. The first filter has a 3dB bandwidth of 12kHz, and the second
a 3dB bandwidth of 15kHz. Most of the channel filtering, however, is
implemented in the FPGA. When the radio is programmed, the different
filters are selected as assigned by the channel programming. The selectable
filters plus the fixed crystal filters result in the following total IF 3dB
bandwidths:
(The FPGA runs from the
12.288MHz.) The receiver requires the TCXO calibration to be completed
to ensure that the channel filtering is centered, thereby minimizing
distortion.
The RSSI is calculated in the FPGA and DSP, and can be passed as an analog
Received Signal
Strength Indication
voltage to the internal options interface and the external auxiliary interface.
To obtain an accurate estimate of the RSSI (over the signal level and
frequency), it is necessary to calibrate the AGC characteristic of the receiver
and the front-end gain versus the receive frequency.
The receiver has an FE AGC circuit to enable it to handle large receiver
Front-End AGC
Control
signals with minimal receiver distortion. This is very important for the
correct operation of the C4FM modem (P25 modulation). FE AGC is
controlled by an algorithm which monitors the RSSI and configures the
DAC to turn on the FE attenuation via the receive pin diode of the PIN
switch.
70 Circuit Descriptions
) and thence via IC201 as the signal
wide channel spacing
medium channel spacing: 12.0kHz
narrow channel spacing : 7.8kHz
DIG SYS CLK
CDC RX AGC
: 12.6kHz
signal, which has a frequency of
© Tait Electronics Limited August 2005
to pin 23 of the
TM9100 Service Manual