Tait TB9100, Service Manual, Page: 89 / 132

TB9100 Reciter Service Manual RF Circuitry 89
© Tait Electronics Limited January 2006
signal to a DC voltage (0 to 22V
1
) to tune the VCO for correction.
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.
6.1.5 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 emitter, Colpitts oscillator
configuration (virtual ground at the base). The LC resonator is coupled
between the collector and base. A high Q trimmer is coupled to the base
end, and varactor diodes are coupled to the collector end. 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
A BJT cascode amplifier is used as a broad band isolator and low noise
amplifier. This has internal self-bias circuitry, and the output provides
enough RF power to drive the following stages.
Harmonic Filter
The VCO has a relatively high harmonic content. A third order low pass
elliptic filter is used to attenuate this content.
Fixed Slope
Attenuator and
Final Driver
A silicon BJT 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 output
level, a fixed slope attenuator is introduced at the input to the final driver so
that the attenuation rate (slope) increases with an increase in frequency.
The B band VCO frequency spans from either 152.9MHz to 164.9 MHz,
or 164.9 MHz to 190.9 MHz, according to the product type. The VCO is
tuned to 16.9MHz above (high side injection) to produce the 16.9 MHz IF
signal at the output of the mixer.
1. The normal lock range is between 3V and 16V.