HP 608E Operating And Service Manual page 26

Section IV
4-29. The secondary windings in the oscillator turret
assembly couple RF energy from RF Oscillator
V6
through a coaxial cable to Buffer Amplifier
V22.
Re-
sistors R240, R242, R244, and R245 located in the
secondary windings of coils L2 through L5 damp un-
desirable resonance in the cathode of Buffer Amplifier
V22.
Resistors 8241 and R243 connected across the
secondary Windings of coils L4 and L5limit the RFdrive
at lower frequencies. Resistors R250, R251, and
capacitor C21B forma take-off network for an uncali-
bratedRFoutput signal that is applied through coaxial
cable WI to UNCAL RF OUTPUT connector J5. Re-
sistor R251 in the take-oU network is 301 ohms in
the Model 60BE and 200 ohms
in
the Model 60BF.
This configuration
is
due to the difference
in
RF out-
put level between instruments. The RF signal at
connector J5 can be used with a frequency counter or
other external measuring equipment. When the Model
60BF is used with a Model B70BA synchronizer dur-
ing
phase-lock operation, the signal is used by the
synchronizer.
4-30. Buffer Amplifier V22 is a type EC-8010 triode
connected as a grounded-grid amplifier. The stage
isolates RF OscUlator V6 and Power Amplifier
va,
and reduces any incidental frequency modulation to a
minimum level. Cathode resistors R255 and R253
operate with resistors R254 and R256 in the plate cir-
cuit to limit plate current to a safe value. Power to
the stage is supplied through RFl filters FL2, FL4,
and FL5, with the plate, cathode and mament supplies
being decouple<! by capacitors C222, C220, and C221
respectively. The plate circuit of Buffer Amplifier
V22 is coupled to the cathode circuit of Power Ampli-
fier
va
through a wide-band network consisting of dc
blocking capacitor C224 and coil L22. Coil L20 in the
plate circuit of V22 serves as an RFchoke, while iso-
lation network Lll blocks an RF signal path to the out-
put circuit of the Amplitude Modulation Section.
4-31. Power Ampli!1er VB is a grounded-grid,
cathode-modulated ampllfier that delivers the modu-
lated or CW output signal to the Attenuator Section.
The plate circuit of
va
contains the amplifier turret
assembly which houses inductors L12 through L16.
These inductors are placed into the output circuit of
VB by operation of the FREQUENCY RANGE switch
discussed in paragraph 4-26. The second segment of
Frequency Control capacitor Cl7 is also located
in
the plate circuit of Power Amplifier VB. Linkage
between capacitor C17B and C17A in the RF Oscilla-
tor Stage permit tuning to a frequency within a range
as discussed in paragraph 4-26. A second linkage
between capacitor C17B and the AMPL TRIMMER
control permit the capacitor to be tuned separately or
in conjunction with push-to-operate switch S6 in the
Amplitude Modulation Section for peakIng the RFout-
put signal. This function is discussed further in para-
graph 4-45.
4-32. When the instrument is set to provide an amp-
litude modulated output, the modulating signal and an
RF leveling signal are received at the cathode
of
Power
Amplifier VB from the Amplitude Modulation Section
through isolation network LII. During CW operation,
only the RF leveling signal
is
received from the
4-4
Model608E/F
Amplitude Modulation Section. Diode CR7 connected
in the signal path prevents damage to Power Amplifier
V8 by limiting the negative swing ri. the cathode. Ca-
pacitor C32 is connected in the plate circuit of VB in
the Model 608F only, and is adjusted for improved
tracking characteristics
in
the power amplifier stage
at higher frequencies. Capacitors C30, C229, C230,
and C231 connected across coils L16, L15, L14, and
L13 respectively are also installed in the Model 60BF
only. These capacitors are selected
in
test to improve
tracking between the power amplifier and oscUlator
stages. Nominal values for the capacitors are illus-
trated in Figure 7-4. The RF output signal from Power
Amplifier VB is inductively coupled through the ampli-
fier turret assembly coil and applied to the Attenuator
Section. This section of the instrument
is
discussed
in paragraph 4-51.
4-33. RF GENERATOR SECTION, MODEL 6OBF.
4-34. The RF Generator Section in the .Model 606 F is
essentiall y the same as the generator section in the
Model 608E, except for the addition of circuits to per-
mit the unit to be used with a Model 8708A Synchroni-
zer. These circuit differences occur in the tank cir-
cuit of RF OscUlator
V6
and the output stage of Power
Amplifier
V8.
4-35. Two varactor diodes, CR31 and CR32, are con-
nected across the oscillator tank circuit in the Model
6OBF. The varactors are reverse-biased junctions
that act as variable capacitors, changing capacitance
with a change in the applied reverse bias.
In
quiescent
corrlition the diodes are biased by a voltage divider
network comprised of resistorS R271, R267, andR268.
A change
in
bias, and the \ ;,;31.11t...lli .,;r.<u.ge in capaci-
tance, is produced by a dc control voltage received at
FREQ CONTROL INPUT connectorJ6. Duringopera-
lion with a Model B70BA Synchronizer, a connection is
made between UNCAL RF OUTPUT connector J5 and
the synchronizer to provide a sample oftheRFoutput
from the Model 608F. A second connection is made
between the synchronizer and FREQ CONTROL INPUT
connector J6 to receive thedc control voltage from the
synchronizer. The frequency sample sent to the syn-
chronizer is compared to a reference frequency deve-
loped in the synchronizer. A drilt tendency in R F
Oscillator V6 is compared with the reference fre-
quency, and translated into a dc control voltage for
application to varactors CR31 and CR32 in the Model
608F. The varactors change capacitance in direct
proportion to the detected drilt holding RF Oscillator
V6
on frequency.
4-36. Potentiometer R269 is installed in the Model
60SF only, and is mechanically linked to Frequency
Control ca,acitor C17B in the output circuit of Power
Amplifier VB. The potentiometer output resistance
varies as the Frequency Control adjustment is tuned,
presenting 4K ohms at the low-frequency end of the
capacitor tuning range and zero ohms at the high-
frequency end. The resistance is applied through
FREQ ANALOG OUTPUT connector J7 to the dc con-
trol voltage circuit in the synchronizer. This loop
arrangement controls the voltage level applied to
varactors CR31 and CR32, thereby assuring correct
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