Tait TB9100, Service Manual, Page: 93 / 132

TB9100 Reciter Service Manual RF Circuitry 93
© Tait Electronics Limited January 2006
The loop filter has a bandwidth of 150 Hz and filters the VCO control lines,
reference side bands and spurious signals.
Note
The VCO frequency increases as the control line voltage increases.
6.2.5 VCO
The receiver VCO consists of a high Q VCO, low noise amplifier, harmonic
filter, fixed slope attenuator, and a final driver. Refer to
Figure 6.2 on
page 90 .
High Q VCO
The VCO BJT transistor operates in a common collector, Colpitts oscillator
configuration, and uses a shorted quarter-wave ceramic coaxial resonator.
The open end of the resonator is terminated by a combination of a high Q
trimmer and varactor diodes. This forms a high Q resonator which is both
mechanically and electronically tunable. Mechanical tuning is possible by
adjusting the trimmer. Changes in the control voltage from the loop filter
are applied to the varactors to facilitate electronic tuning.
Low Noise Amplifier
An N-channel dual gate MOSFET is used as a broad band matched Class A
low noise amplifier. It has internal self-bias circuitry, and the output provides
enough RF power to drive the following stages.
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 +20 dBm 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 from either 369.9MHz to 409.9MHz,
399.9MHz to 449.9MHz, or 470.1MHz or 510.1MHz, according to the
product type. The VCO is tuned to either 70.1 MHz below (low side
injection) or above (high side injection) to produce the 70.1 MHz IF signal
at the output of the mixer.
6.2.6 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.