Tait TM8100 mobiles Service Manual page 73

Frequencies
of IF Stages
Demodulation
Automatic Gain
Control
Channel Filtering
TM8100/TM8200 Service Manual
© Tait Electronics Limited November 2007
The frequency of the first IF stage depends as follows on the frequency band
of the radio:
VHF bands: 21.400029MHz
■
UHF and K5 bands: 45.100134MHz.
■
The above are nominal values; the actual frequency will differ by a small
amount depending on the exact initial frequency of the TCXO.
The frequency of the second IF stage will always be precisely 64.000kHz
once the TCXO calibration has been completed. (The TCXO calibration
does not adjust the TCXO frequency, but instead adjusts the VCXO
frequency, which in turn adjusts the VCO or first LO frequency as well as
the frequency of the first IF stage. The second LO frequency remains fixed.)
The third IF stage is completely within the FPGA and is not accessible.
Demodulation takes place within the FPGA. Demodulated audio is passed
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.
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
( ) and then via IC201 as the signal
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.
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 in Table 3.1:
Table 3.1 Total IF 3 dB bandwidths
Channel spacing
Wide
Medium
Narrow
(The FPGA runs from the
12.288MHz.) The receiver requires the TCXO calibration to be completed
to ensure that the channel filtering is centred, thereby minimizing
distortion.
CDC RX AGC
All bands except K5
12.6kHz
12.0kHz
7.8kHz
signal, which has a frequency of
DIG SYS CLK
to pin 23 of the
K5 band
12.00kHz
9.0kHz
7.6kHz
Circuit Descriptions 73