Tait TM8100 mobiles, Service Manual

Page: 74 / 620

74 Circuit Descriptions TM8100/TM8200 Service Manual
© Tait Electronics Limited November 2007
Received Signal
Strength Indication
The RSSI is calculated in the FPGA and DSP, and can be passed as an analog
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.
Front-End AGC
Control
The receiver has a front-end AGC circuit to enable it to handle large
receiver signals with minimal receiver distortion. This is very important for
the correct operation of the THSD modem (Tait High-Speed Data).
It enables THSD to maintain residual BER of < 10
-4
. The front-end AGC
is controlled by an algorithm which monitors the RSSI and configures the
DAC to turn on the front-end attenuation via the receive pin diode of the
PIN switch.
Noise Blanker (A4,
B1 bands only)
If the frequency band is between 66 and 174MHz, a noise blanker can be
selected to remove common sources of electrical interference such as vehicle
ignition noise. The noise blanker functions by sampling the RF input to the
receiver for impulse noise and momentarily disconnecting the first LO for
the duration of the impulse. The response time of the noise blanker is very
fast (tens of nanoseconds) and is quicker than the time taken for the RF
signal to pass through the front-end hardware, so that the LO is disabled
before the impulse reaches the IF stage where it could cause crystal
filter ring.