HP 8340A Service Manual page 100

A32 M/N VOLTAGE-CONTROLLED OSCILLATOR
INTRODUCTION
The M/N vco
(voltage-controlled oscillator)
cavity oscillator.
resonator, a transistor circuit,
electrical tuning mechanism.
and A33 M/N Output Assembly,
A32 M/N VOLTAGE-CONTROLLED OSCILLATOR CIRCUIT DESCRIPTION
A32A2 Voltage-Controlled Oscillator
The emitter of oscillator transistor,
resistance because of the base inductance,
fixed length of wire surrounded by air.
together with L3, is to match the
the cavity resonator.
transistor is physically connected to the cavity center post
(inductive coupling)
and RS establish the de bias for Q2.
capacitors that also filter power supply noise.
The frequency of the oscillator can be tuned mechanically by
adjusting Cl, which is a tuning screw that capacitively loads the
cavity center post.
the electrical tuning range.
electrically by using reverse biased varactor diodes,
The reverse bias to the diodes is the TUNE voltage from A33 M/N
Output Board. The actual electrical tuning network components are
physically located inside the resonator housing.
C2 and C3 are soldered onto the cavity center post. The anodes of
varactor diodes CRl and CR2 are soldered to C2 and C3,
respectively, and their cathodes are soldered together to a post
contacting the outer wall of the cavity.
combinations of CR1/C2 and CR2/C3 provide capacitive loading to
the cavity resonator at the frequency of oscillaton.
TUNe voltage is brought to the varactor diodes through inductors
Ll and L2, which are also soldered to C2 and C3, respectively, and
to a post that is electrically insulated from the outer wall of
the cavity housing and is soldered to the A32Al PC board.
The output of the vco is taken directly from the cavity resonator
through CS, a capacitively coupled E-field probe.
level is adjusted by varying the length of the probe.
is applied to the Buffer Amplifier,
Model 8340A - Service
It consists of a fore-shortened coaxial cavity
and both a mechanical and an
Refer to Figure 8B-13,
Schematic Diagram.
(reflective)
C2 provides a de block required because the
that is at DC ground potential.
This is used to adjust the frequency to within
The frequency is adjusted
Scans by HB9HCA and HB9FSX
is a varactor-tuned
A
Q2, exhibits negative
L3, which consists of a
The function of ca and R9,
emitter of Q2 to
C3, CS, and C7 are bypass
Chip capacitors
The result is that the
The output power
Block B.
A32 M/N VCO
Rl, R4, R6,
CRl and CR2.
The (VCO)
This signal
8-135