Tait TB9100, Service Manual, Page: 29 / 132

TB9100 Reciter Service Manual Reference Switch 29
© Tait Electronics Limited January 2006
4 Reference Switch
Refer to Figure 4.1 on page 31 .
4.1 Synthesizer
The external reference synthesizer consists of a programmable frequency
synthesizer IC, a 12.8MHz VCXO, and a stable 10MHz or 12.8 MHz
reference frequency supplied to the reciter externally via a BNC connector
on the rear panel.
The synthesizer uses a phase-locked loop to lock the 12.8 MHz VCXO to
the external reference frequency. The synthesizer IC receives the divider and
control information from the RISC processor via a 3-wire serial bus (clock,
data and enable). When the data bits are latched in, the synthesizer processes
incoming signals from the 12.8 MHz VCXO feedback buffer (f
vcxofb
) and
the external reference buffer (f
ref
).
A transistor is used as a unity gain 12.8MHz VCXO feedback buffer for the
prescaler within the synthesizer IC.
The 10MHz or 12.8MHz externally supplied reference is detected, buffered
and divided down to the 100 kHz divider reference within the synthesizer
IC. The same divider reference is maintained by dividing the 12.8MHz
VCXO feedback buffered signal using the programmable dividers of the
synthesizer IC. Phase lock is achieved when both divider references have the
same phase and frequency content (i.e. their difference is zero or DC).
This is achieved by the digital phase detector (part of the synthesizer IC),
which compares both divider references and delivers an error signal.
A
±
1mA charge pump circuit (also part of the synthesizer IC) and the
passive loop filter circuit convert this error signal to a DC voltage (0 to 3V)
to tune the 12.8MHz VCXO for correction. A loop filter with a bandwidth
of 180 Hz further filters the VCO control line reference side bands and
spurious signals.
Note
The VCXO frequency increases as the control line voltage
increases.
4.2 VCXO
The VCXO is implemented by using a varactor to linearly tune a 12.8 MHz
crystal unit over a specified frequency range. The frequency range provided
will cover frequency drifts due to calibration, the temperature tolerance of