Tait TB7100 Service Manual

46 Circuit Descriptions TB7100 Service Manual
© Tait Electronics Limited October 2005
Automatic Gain
Control
The receiver has an AGC circuit to enable it to cover a large signal range.
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) and then via IC201 as the signal CDC RX AGC to pin 23 of the
quadrature mixer IC400. As the antenna signal increases, the AGC voltage
decreases.
Channel Filtering The channel filtering is split between the first and third IF stages.
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 base station 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:
■ wide channel spacing : 12.6kHz
■ medium channel spacing: 12.0kHz
■ narrow channel spacing : 7.8kHz
(The FPGA runs from the
DIG SYS CLK signal, which has a frequency of
12.288MHz.) The receiver requires the TCXO calibration to be completed
to ensure that the channel filtering is centred, thereby minimizing
distortion.
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 C4FM modem (P25 modulation). 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
(B1 band only)
With the B1 band, 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.

Tait TB7100 Specifications

General

Brand	Tait
Model	TB7100
Category	Accessories
Language	English