Tait TM8235, Руководство по обслуживанию

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68 Circuit Descriptions TM8100/TM8200 Service Manual
© Tait Electronics Limited June 2006
3.2 Receiver Circuitry
Introduction
For a block diagram of the receiver circuitry, refer to Figure 3.3 (B1, H5,
H6 bands, 05 issue PCBs) and Figure 3.4 (other bands and later issue PCBs).
The receiver is of the triple-conversion superheterodyne type. The first two
IF stages are implemented in hardware; the third stage is implemented in the
FPGA (Field-Programmable Gate Array) of the digital board. The FPGA
also carries out the demodulation of the received signals.
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 bands between 66 and
174MHz, 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:
■
VHF bands: 21.400029MHz
■
UHF 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
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.