66 Circuit Descriptions TM8100 Mobile Radio Service Manual
May 2004 © Tait Electronics Limited
Front-end Circuitry The front-end circuitry is a standard varicap-tuned singlet (band-pass
filter), followed by an LNA (low-noise amplifier), and then a varicap-
tuned doublet (image filter). The varicap tuning voltage
CDC RX FE TUNE
is provided by a DAC, with voltages calculated from a calibration table
stored in non-volatile memory. The two varicap-tuned filters need to be
calibrated to ensure that maximum sensitivity is achieved.
First Mixer The first mixer is a standard diode-ring mixer with SMD (surface-mount
device) baluns and a quadruple SMD diode. For the VHF band the
receiver includes a circuit for suppressing ignition noise. This circuit
momentarily removes the LO signal from the mixer when an ignition
noise pulse is detected. The ignition-noise suppressor is selectable on a
per-channel basis when the radio is programmed.
First IF Stage and
Second Mixer
The first IF stage consists of a crystal channel filter (BPF1), followed by
an IF amplifier, and then another crystal filter (BPF2). The second mixer
is an IC quadrature mixer with an internal AGC amplifier. This IC has
a divide-by-two function on the LO input in order to provide the
quadrature LO frequencies required internally. The second LO
frequency is synthesized by an integer PLL (IC403), which uses the
TCXO frequency
SYN RX OSC (13.0000 MHz) as its reference.
Frequencies
of IF Stages
The frequency of the first IF stage depends as follows on the frequency
band of the radio:
■ B1 band: 21.400 029 MHz
■ D1 band: 21.400 029 MHz
■ H5 band: 45.100 134 MHz
■ H6 band: 45.100 134 MHz
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 64.000 kHz
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 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.
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