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TB9100 Reciter Service Manual Reference Switch 29

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 12.8MHz

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 12.8MHz 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 12.8MHz VCXO feedback buffer (f

vcxofb

) and

the external reference buffer (f

ref

A transistor is used as a unity gain 12.8 MHz 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 100kHz 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 180Hz 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 12.8MHz

crystal unit over a specified frequency range. The frequency range provided

will cover frequency drifts due to calibration, the temperature tolerance of

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