HP 5065A Operating And Service Manual page 226

Model 5065A
Circuit Diagrams, Theory, and Maintenance
60 MHz FREQUENCY MULTIPLIER A3 THEORY
Frequency Multiplier A3 provides a stable, spectrally
clean, 300 milliwatt, 60 MHz signal which connects to
harmonic generator/step recovery diode in A12 RVFR
Assembly. Another output is the isolated 5 MHz signal
to A13 Buffer Amplifier.
The 5 MHz signal from A10 Quartz Oscillator connects
through input jack J2 to Q1 buffer amplifier; then
through an isolating network to first multiplier stage
Q2. Q1 output couples through T1 and through J3 to
A13 Buffer Amplifier. R8, R9, R10, and R12 form a
10-dB pad to prevent feedback from Q2 to A10 Quartz
Oscillator and output circuits.
Q2 modulator doubler stage doubles 5 MHz to 10 MHz
and phase modulates the 5 MHz input at a 137 Hz rate.
The primary in Q2 collector circuit is tuned to 5 MHz by
C19 producing a strong 10 MHz second harmonic in the
rectified output of CR1 and CR2. Phase Modulation
is accomplished by varactor diodes CR3 through CR6.
Varactor capacitance is varied at 137 Hz by the modu­
lation signal from J1 through R11. This 137 Hz modu­
lation signal is generated in the modulation oscillator
portion of A8 Phase Detector Assembly.
Q6 doubles 10 MHz with CR7 and CR8 producing a 20
MHz second harmonic full-wave rectified output. T3
primary is tuned to 10 MHz by C30. Q8 triples with
squaring diodes CR11 through CR14 producing strong
odd harmonics. C49 tunes L20 to the input 20 MHz
and C52 tunes L24 to the 60 MHz output.
Q3 is a MOS FET which furnishes high input impedance
for isolation between Q8 tripler stage and the output
amplifiers. Q3 voltage gain is about 20. Class A rf
amplifiers Q5 and Q7 drive Q9 output amplifier. Q9
output connects through a 3-dB pad (C53, R38, R39,
and R41) to J8. J8 output connects through J7 to TP1
and through the pi matching network of C61, C57,
C59, and L25 to J4 output jack. This pi network
matches the 50 ohm output impedance at J8 with the
step-recovery diode in A12 RVFR Assembly. 5.315...
MHz connects to the pi matching network through J5
from A1 Synthesizer Assembly. R3 sets the amount of
5.315...MHz that adds to the 60 MHz output. R40 con­
trols dc bias for the step-recovery diode in A12 RVFR
Assembly.
Voltage divider R28 and R34 biases feedback diodes
CR9 and CR10. These diodes rectifiy the 60 MHz out­
put and produce a dc feedback signal to control Q3
bias with feedback amplifier Q4. In this manner, the
60 MHz output is amplitude stabilized.
A3 MAINTENANCE
NORMAL OPERATION
a. J4 output is 60 MHz, phase modulated at 137
Hz, plus 5.315...MHz that comes from the A1 Synthe­
sizer Assembly.
8-22
b. J3 output is 5 MHz, 1 volt into 50 ohms.
c. TP2 is the AGC voltage test point. When Multi­
plier is operating normally, this voltage will be 4.5 Vdc
or greater.
OPERATIONAL CHECK
a
N
Measure dc AGC voltage at TP2; should be 4.5
V or greater. If less, complete loop alignment pro- ~
cedure, including realignment of the Multiplier pi
matching network of Section 5-19, LOOP ALIGN­
MENT PROCEDURE. _
b. Remove the short cable connecting to J8. Using
the Micon-to-BNC test cable provided, connect a 50-
ohm coaxial load to J8. Connect an RF Voltmeter to —
this load. The RF meter should read 2.7 to 3.0 V rms.
Excessive voltage at this point means that the AGC
circuit is not functioning properly.
c. Reconnect P8 and disconnect cable from J4.
Connect the test cable with 50-ohm load to J4 and
connect the RF voltmeter to the 50-ohm load. The _
voltmeter should indicate the presence of 60 MHz.
Generally this signal will be 1 volt or greater. How­
ever, this voltage level will vary a great deal from
unit to unit depending on the tuning of the pi matching _
network in the A3 Multiplier. Reconnect the 60
MHz output cable to A3J4.
d. Remove the cable from J2. This removes the —
5 MHz input from A3 leaving only the 5.315...MHz
signal on output jack J4. The RF voltmeter should
read about 20 mV (this voltage depends on the setting
of R3). As a further check, R3 may be adjusted and
the change in voltage noted.
NOTE _
R3 should be returned to its original setting.
e. Remove the P5 connector. This removes the __
5.315...MHz signal. The output voltage should drop to
near zero. Replace connections to J5 and J2.
f. Remove cable from J3. Connect test cable and —
50-ohm load to J3. Connect RF voltmeter to 50-ohm
load. RF voltmeter should read approximately 1 V rms.
g. Remove RF voltmeter and test cable. Recon-
—
nect cable to J3. This completes the test. If output
signal levels measured above are correct, but there
does not appear to be any RF excitation on the A12 _
RVFR Assembly, the A3 Assembly should be realigned
per LOOP ALIGNMENT PROCEDURE, Section 5-19.
TROUBLESHOOTING AND REPAIR —
a. REALIGNMENT AFTER REPAIR. When repairs
are made within the A3 Assembly, the repaired cir­
cuit must be retuned. The following paragraphs give