Tait TB8100, Service Manual

Page: 46 / 306

46 Reciter Circuit Description TB8100 Service Manual
© Tait Electronics Limited September 2006
Harmonic Filter
The VCO has a high second harmonic content. A third order low pass
elliptic filter is used to attenuate this content.
Fixed Slope
Attenuator and
Final Driver
A silicon-based BJT transistor is used as a broad band matched Class A final
driver to drive the +20dBm local oscillator port of the mixer. To maintain
a fixed input level, a fixed slope attenuator is introduced at the input to the
final driver so that the attenuation rate (slope) decreases with an increase in
frequency.
The H-band VCO frequency spans the following frequency ranges,
according to the product type:
■
H1 – 470.1MHz to 510.1MHz (high side)
■
H2 – 369.9MHz to 409.9MHz (low side)
■
H3 – 399.9MHz to 449.9MHz (low side
■
H4 – 450.1MHz to 490.1MHz (high side).
The VCO is tuned to either 70.1MHz below (low side injection) or above
(high side injection) to produce the 70.1MHz IF signal at the output of the
mixer.
The K-band VCO frequency spans from 721.9MHz to 753.9MHz. The
VCO is tuned to 70.1MHz below (low side injection) to produce the
70.1MHz IF signal at the output of the mixer.
The L-band VCO frequency spans from 781.9MHz to 859.9MHz. The
VCO is tuned to 70.1MHz below (low side injection) to produce the
70.1MHz IF signal at the output of the mixer.
Refer to “Identifying the Reciter” on page 70 for more information on
reciter product types.
2.4.6 AGC (H Band Only)
The AGC is used to prevent the ADC from being overloaded by strong
interfering signals present at the receiver antenna port. The AGC loop
consists of a PIN diode attenuator, AGC buffer, and detector. The pick-off
point for the AGC is the output of the post-mixer buffer. The input signal
to the AGC is buffered, amplified and then detected. The detected DC
voltage is buffered and fed to PIN_CTRL to control the attenuation of the
PIN attenuator.
To prevent overload of the ADC, the peak level at its input should not
exceed 0dBm. This corresponds to –30dBm at the antenna connector, and
approximately –22dBm at the AGC pick-off point. The AGC operates over
a range of approximately 11dB.
The AGC circuit can be enabled or disabled using the Service Kit
(Configure > Base Station > Channel Profiles > General tab).