HP 608E Operating And Service Manual page 28

Section IV
through RFI filter FL6 and isolation network Lll to
the cathode of Power Amplifier VB, where modulation
and RF leveling occur.
4-45. A feedback path for the output of Modulators Qa
and Q9 is provided through filter network R235 and
capacitor C200 which removes the modulating signaL
The leveling signal is passed through MODULATION
switch-wafer 4F and applied to the normally-open
contacts of AMPL TRIMMER pushbutton switch S6
when the MODULATION switch is in positions I through
4. As previously mentioned, the AMPL TRIMMER
pushbutton is mechanically linked to AMPL TRIM-
MER capacitor C17B in the output circuit of Power
Amplifier V8. During normal operation the detected
feedback from diode CR2 in the Attenuator Section is
applied through resistor R20l, potentiometer R202,
and the normally-closed contacts of pushbutton switch
S6 to RF OUTPUT meter MI: therefore, the meter
normally displays the leveled RFoutput of the instru-
ment. Rotation of RF OUTPUT control R212 changes
the conduction level of Differential Amplifier Q2-Q3
arid the signal output level of Modulators Q8-Q9. This
change is reflected in the RF output of Power Amplifier
V8, and di;5played on RF OUTPUT meter Ml. Opera-
tion of AMPL TRIMMER control C17B without engag-
ingthe pushbutton provides a fine-tune adjustment for
use with the RF OUTPUT control to maintain a con-
stant, leveled RF output across a frequency range.
When AMPL TRIMMER pushbutton S6 is depressed the
leveling signal feedback from Mqdulators Q8-Q9'is
displayed on meter Ml, and operation of the control
permits a sharp peak to be observed on the meter.
Hence, the RFoutput of Power Amplifier va can be
peaked. During internal and external sine-wave opera-
tion, MODULATION control R210 controls the level
of the modulation and is used in conjunction with the
PERCENT MODULATION meter discussed in para-
graph 4-59.
4-46. EXTERNAL SINE-WAVE MODULATION. Oper-
ation of the instruments during external sine-wave
modulation is similar to the internal mode, and is
established by setting MODULATION switch S7 to the
AM position (switch poSition4). Modulation Oscillator
Y2 is disabled by removing the stage plate voltage at
MODULATION switch-wafer 2R, and a Signal path of
the external sine-wave signal is established through
MODULATION switch-wafer IF. A cable connection
is made between the external source and AM/PULSE
MOD INPUT connector J2, with the incoming signal
being routed to MODULATION control resistor R2l0.
The signal is then applied through the frequency-
compensation network and MODULATION switch -wafer
IR tothe base of transistor Q3 in the differential am-
plifier stage. Operation of the remaining circuits and
controls in the Amplitude Modulation Section is the
same as described during internal sine-wave modula-
tion.
4-47. EXTERNAL PULSE MODULATION. Duringthis
mode of operation MODULATION switch S7 is set
first to the PULSE/SET RF OUTPUT position (switch
position 5) to establish operating conditions in the out-
put circuit of the Amplitude Modulation Section, and
4-6
Model 608E/F
consequently the RF output level of the instrument.
The switch is then set to the PULSE position (switch
position 6) to pulse modulate the RF carrier. In both
positions Modulation Oscillator V2 is disabled and the
input signal path through MODULATION control re-
sistor R210 is interrupted. The position of MODU-
LATION switch-wafer 4F interrupts the leveling signal
path through the normally-open contacts of AMPL
TRIMMER pushbutton switch S6, and removes the
push-to-peak function of the AMPL TRIMMER control
during pulse operation. However, the detected signal
from diode CR2 in the Attenuator Section is received
through resistor R20l and potentiometer R202 for ap-
plication to RF OUTPUT meter Ml. Therefore, in
the PULSE/SET RF OUTPUT position, the AMPL
TRIMMER and R F OUTPUT controls can be used to
set the desired RFoutput level as indicated on the RF
OUTPUT meter. The two controls are operated only
in the PULSE/SET RF OUTPUT position.
4-48. Transistor Q3 in the differential amplifier stage
has an inhibiting voltage applied to the base through
resistor R211 and MODULATION switch-wafer lR.
With transistor Q3 cut off, transistor Q2 conducts
heavily and places a disabling voltage on the base of
Amplifier Q4. This condition interrupts to leveling
signal path through the Amplitude Modulation Section.
Resistor R212 is removed from the circuit by the posi-
tion of MODULATION switch-wafer lR; however, the
RF OUTPUT control is also connected to potentio-
meter R226 in the collector circuit of Amplifier Q7
for rf output level control. With the MODULATION
switch in position 5, a positive voltage is applied to
the base of Pulse Amplifier Q5 through resistors R220
and R225 to enable the amplifier. The position of
MODULATION switch-wafer 3R forms a voltage divider
network conSiSting of resistors R228, R226, and R270
in the collector circuit of Amplifier Q7 and the base
circuits of Modulators Qa and Q9. Operation of RF
OUTPUT potentiometer R226 controls the conduction
level of transistors Q7, Q8, and Q9, and the output
level of ?ower Amplifier va.
4-49. While the MODULATION switch is still in the
PULSE/SET RF OUTPUT position, the RF OUTPUT
control is adjusted for an indication on RF OUTPUT
meter Ml. AMPL TRIMMER control C17B in the out-
put stage of Power Amplifier va is then peaked for
maximum RFoutput. The RF OUTPUT is readjusted
for an indication of +7 dBm (Attenuator Calibrated) on
RF OUTPUT meter Ml. With the preliminary adjust-
ments completed, the external modulating equipment
is connected to AM/PULSE MOD INPUT connector J2,
and MODULATION switch S7 is set to the PULSE posi-
tion (switch position 6). The incoming pulse train is
passed through MODULATION switch-wafer 3F to the
base of Pulse Amplifier Q5, and then routed through
Amplifier Q6 and the remaining circuits of the Ampli-
tude Modulation Section as previously described.
4-50. CW OPERATION. Tosetthe equipment for CW
operation, MODULATION switch S7 is placed to the
CW position (switch position 1). Modulation Oscillator
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