Wavetek 3000 Instruction Manual page 21

LEVEL CONTROL AND AM
The circuitry for controlling the
RF
output level is directly related to the
above leveling system
because changing
the DC level reference changes
the RF
output level .
Of the 150 dB output range, 130 dB is
passive attenuation .
The remaining 20
dB is controlled
by changing the level
reference .
The VERNIER output control
has a 10 dB range .
The remaining 10 dB
is provided by switching the level ref-
erence range .
This range switch is pro-
vided so that when AM is not required the
output amp can provide a carrier at the
highest possible power .
Since the
RF level can be voltage con-
trolled, AM can be accomplished by ap-
plying the modulating signal to the
VERNIER level control . This causes the
reference
voltage to the Error Amp to
/ KHz
X7-AL
REF
change at the frequency of the modulat-
ing signal . The modulating signal is
from one of two internal oscillators or
from an external source .
3 .2 .2 Phase-Locked Loops
The basic signal generator discussed in
Section 3 .2 .1 has a frequency range of
1 to 520 MHz, has an output which is
leveled and adjustable and has the abi-
lity to
be amplitude modulated . With
the above circuitry, however, the ac-
curacy is only 3 MHz with 1 MHz resolu-
tion .
To achieve the desired 1 kHz re-
solution and .001% accuracy, the instru-
ment includes four phase-locked loops .
Phase-locked loops (PLL) #1, #2 and #4
are used to stabilize the Wide Oscillator
and tune it in 1 kHz steps . See Figure
3-1 . The Narrow Oscillator is included
in PLL #3 which provides stabilization
and allows FM operation .
Figure 3-2 . PLL #1
THEORY OF OPERATION
OUroar
3-3
lIvAVr
AROrfRAM
I I
OUTPUT
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/0.000 4A/Z
.9,999 9.999
r -
.999
9,00/ 9.00/