Tait TB8100, Service Manual

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36 Reciter Circuit Description TB8100 Service Manual
© Tait Electronics Limited September 2006
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.
2.2.2 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
the crystal unit, and the frequency stability of the externally supplied
reference.
2.2.3 Reference Switch
The reference switch consists of the external reference detector, the
hardware switch, and the digital switch.
External Reference
Detector A discrete NPN dual transistor pair is used as a low level signal detector. The
Syn_Ref_Det signal has a high logic level when the externally supplied
signal has the correct level.
Hardware Switch
The hardware switch is implemented using a discrete dual transistor pair.
When the switch is off (default), it powers up the internal reference
12.8MHz TCXO and shuts down the external reference 12.8MHz VCXO.
When the switch is on, it powers up the external reference 12.8MHz
VCXO and shuts down the internal reference 12.8MHz TCXO.
Digital Switch
The digital switch is controlled by the RISC, which processes the
Syn_Ref_Det and Lock Detect signals from the external reference
synthesizer. The RISC controls the hardware switch using the
Syn_Ref_Ctrl signal. The hardware switch is on if the Syn_Ref_Det
and
Lock Detect signals have a high logic level for a set time. In all other
conditions the hardware switch is off.
Internal/External
Reference Clock
Branch
A complementary emitter follower, using NPN/PNP dual transistors, forms
two clock buffer branches which distribute the internal or external
references to the rest of the system. The branches provide a reasonable drive
level at low impedance.