Tait TM8235, Руководство по обслуживанию

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52 Description TM8100/TM8200 Service Manual
© Tait Electronics Limited June 2006
2.5.1 RF Hardware
PIN Switch
The RF PIN switch circuitry selects the RF path to and from the antenna
to either the Tx or Rx circuitry of the radio. In addition to the switching
functionality, the PIN switch is used to provide attenuation to the Rx front
end in high signal-strength locations.
Front End and
First IF
The front-end hardware amplifies and image-filters the received RF
spectrum, then down-converts the desired channel frequency to a first
intermediate frequency (IF1) of 21.4MHz (VHF) or 45.1MHz (UHF)
where coarse channel filtering is performed. The first LO signal is obtained
from the frequency synthesizer and is injected on the low side of the desired
channel frequency for all bands. In receive mode, the modulation to the
frequency synthesizer is muted. See
“Frequency Synthesizer” on page 57 for
a description of the frequency synthesizer. The output of the first IF is then
down-converted using an image-reject mixer to a low IF of 64kHz.
Quadrature
Demodulator
The LO for the image-reject mixer (quadrature demodulator) is synthesized
and uses the TCXO as a reference. This ensures good centring of the IF
filters and more consistent group-delay performance. The quadrature
demodulator device has an internal frequency division of 2 so the second
LO operates at 2 x (IF1+64kHz). The quadrature output from this mixer is
fed to a pair of ADCs with high dynamic range where it is oversampled at
256kHz and fed to the custom logic device.
Automatic Gain
Control
The AGC is used to limit the maximum signal level applied to the image-
reject mixer and ADCs in order to meet the requirements for
intermodulation and selectivity performance. Hardware gain control is
performed by a variable gain amplifier within the quadrature demodulator
device driven by a 10-bit DAC. Information about the signal level is
obtained from the IQ data output stream from the ADCs. The control loop
is completed within the custom logic. The AGC will begin to reduce gain
when the combined signal power of the wanted signal and first adjacent
channels is greater than about -70dBm. In the presence of a strong adjacent-
channel signal it is therefore possible that the AGC may start acting when
the wanted signal is well below -70dBm.
Noise Blanking
(A4, B1 bands only)
With frequency bands 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.