Tait T830 Series, Руководство по обслуживанию

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Copyright TEL 31/01/96
2.9 Synthesised Local Oscillator
Figure 2.5 T835 Synthesiser Block Diagram
The synthesiser employs a phase-locked loop (PLL) to lock a VCO to a given reference
frequency.
A master oscillator at 12.8MHz (IC2) is buffered, divided by two and then divided
down to 6.25kHz or 5kHz within the synthesiser IC (IC3). A buffered output of the
VCO is fed to a programmable divider, comprising a VHF prescaler (IC1) and a divider
internal to IC3. These two signals are applied to the phase detectors in IC3. A digital
phase detector (PDB) provides rapid coarse tuning of the VCO until the phase error is
within the range of the high gain sample and hold detector (PDA). The phase detector
outputs are passed through an active loop filter (IC4a) which produces a DC voltage
between 0 and 20V to tune the VCO. This VCO control line is further filtered to attenu-
ate noise and spurs. As the control line voltage increases, the VCO frequency also
increases.
The division ratio of the programmable divider is stored within EPROM memory (IC1).
Up to 128 frequencies can be stored within the memory and are addressable using the
internal DIP switches. Three of the address lines are also available for external fre-
quency control via an extra D-range connector at the rear of the chassis. A change of
state of any of these three lines (CH SEL 0-2) commences a programming cycle during
which the frequency data in the EPROM is down-loaded to a divider within IC3. 32 bits
of data are loaded in eight 4-bit words.
The VCO transistor (Q1) operates in a common source configuration, with an LC tank
circuit coupled between its gate and drain to provide the feedback necessary for oscilla-
tion. The VCO control voltage from the loop filter (IC4) is applied to the varicaps
(D1-D4) to facilitate tuning within a 3MHz band of frequencies. A trimcap (VC1) is
used for coarse tuning of the VCO. The output from the oscillator circuit drives a cas-
40/41 /N
/R
EPROM
/2
12.8MHz
Reference
Oscillator
Fixed
Divider
Reference
Divider
Phase
Detector
Loop
Filter
VCO PCB
VCO Buffer
Output
Buffer
+22dBm
L.O.
Programmable
Divider
VHF
Prescaler
Buffer
Channel
Memory
Remote
Channel
Select
Channel
Select Switches
EPROM PCB
0
1
2