HP 5065A Operating And Service Manual page 286

Model 5065A
Circuit Diagrams, Theory, and Maintenance
QUARTZ CRYSTAL OSCILLATOR ASSEMBLY
A10 THEORY
GENERAL
The voltage controlled 5 MHz signal is generated by the
circuits in Quartz Oscillator Assembly A10. This
assembly is composed of 4 major sections:
a. Temperature control circuits.
b. 5 MHz quartz oscillator circuits.
c. Automatic gain control circuits.
d. Power amplifier circuits.
The quartz oscillator generates the 5 MHz signal. The
AGC amplifier provides feedback to stabilize oscillator
crystal current. Power amplifier circuits and a crystal
filter isolate the quartz oscillator from external vari­
ations and supply the buffered 5 MHz to Frequency
Divider Assembly A6 and Multiplier Assembly A3.
Oven control circuits maintain the factory set temper­
ature through proportional control of the internal
heater.
The oven temperature is set at the factory to operate the
crystal at a temperature where changes in crystal
temperature have the smallest effect on oscillator fre­
quency. Placing the quartz oscillator and AGC com­
ponents inside the oven further improves the oscillator
temperature-vs-frequency stability. Shielding and de­
coupling networks in all leads, except the 5 MHz out­
put, reduce the Radio Frequency Interference radiated
or received by the oscillator assembly. The assembly is
sealed at the factory; the only adjustments accessible
are the Oscillator Frequency FINE knob adjustment, the
Oscillator Frequency COARSE screwdriver adjustment,
and the Filter crystal adjustment.
TEMPERATURE CONTROL CIRCUITS
PROPORTIONAL CONTROL. Two heaters maintain
oven temperature: HR1, which is proportionally con­
trolled to provide continuous oven temperature control;
and HR2, which is thermostatically controlled, and
provides fast warm-up. The heater current in a pro­
portional control circuit is a continuous function of oven
temperature. The heater current in a thermostat control
circuit is either "on" or "off", depending on oven
temperature setting. Both control circuits contain a
thermal fuse to prevent damage to components within
the oven, if the assembly overheats.
NOTE
Reference designators in the following
paragraphs are abbreviated; for complete
reference designators, add prefix "A10" to
reference designators used.
PROPORTIONAL TEMPERATURE CONTROL CIRCUITS
Ac Controller A1 is a Wien bridge oscillator with emitter
follower and detector providing a dc signal voltage,
proportional to oven temperature, for the dc controller.
The Wien bridge oscillator frequency (about 3 kHz) is ^
determined by a phase-shifting network in the bridge
A1R1, A1R2, A1C1, and A1C2.
The oscillator amplitude is determined by the degen­
erative feedback through A1R3, A1R4, and RT1 (inside
the oven). Since thermistor RT1 is within the oven, _
Wien bridge coscillator output level is determined by
the oven temperature. The thermistor has a negative
temperature coefficient so a decrease in oven temp­
erature causes thermistor resistance to increase, in- _
creasing oscillator amplitude. Diodes A1CR1, A1CR2,
and capacitor A1C10 translate this to a negative dc
level applied to the dc controller circuit in A3.
Dc Controller A3Q2, A3Q3, A3Q4, and associated
components receives the dc signal (proportional to
Wien bridge oscillator amplitude) from ac controller
circuits and controls current through HR1. The HR1 "
—
heater current is controlled by A3Q4 which is driven
through A3Q3 by amplifier A3Q2. Diodes A3CR1 and
A3CR2 develop 1.2 volts. The voltage across A3R8 _
depends on heater current through it, and is added to
the voltage across the diodes. This forms a degenerative
bias signal for A3Q2. Thus, an increase in current
through A3R8 increases A3Q2 current, decreases A3Q4 _
current and decreases current through A3R8 and HR1.
5 MHz OSCILLATOR AND AGC CIRCUITS
GENERAL. The 5 MHz signal is generated by an elec- _
trically controlled quartz crystal oscillator within the
oven. An AGC circuit, also in the oven, provides some
amplification for the 5 MHz signal and AGC feedback
to prevent mechanical vibrations from causing crystal —
damage. The oscillator frequency is coarse tuned by
A2A1C7 which is connected to the front panel Oscillator
Frequency COARSE adjustment knob and fine tuned by
R3 (front panel Oscillator Frequency FINE adjustment). —
The only other adjustment that can be performed in the
field is the 5 MHz Filter adjustment, A3C10. For access
to A3C10, A10 Assembly must be detached from the
main chassis. In operation, with the FUNCTION switch
at OPER, the oscillator frequency is electrically tuned by
the dc error signal from A9 Integrator Amplifier
Assembly. This dc error signal connects through _
A10J1, marked (-EFC) to frequency-controlling Varactor
A1A1CR1 to correct the 5 MHz output frequency.
In the oscillator circuit, oscillator stage A1A1Q1 d r i v e s -
resonant circuit A2A1L2, A2A1C6, A2A1C7, A2A1C9,
and A2A1CR1. Oscillator feedback is through
A1A1C10, A2A1C11, A2A1C12, and A2A1L3 to
A2A1Q1 base. Capacitor A2A1C8 serves to bypass~
A2A1Q1 emitter resistance, A2A1R2. Bias control for
oscillator stage A2A1Q1 is provided by the AGC circuit
in A2A2 subassembly; as the oscillator output level in-
creases, the AGC circuit decreases A2A1Q1 bias, and
decreases oscillator output amplitude.
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