Model 8340A - Service
A32 M/N VOLTAGE-CONTROLLED OSCILLATOR
INTRODUCTION
The M/N
vco
(voltage-controlled oscillator) is a varactor-tuned
cavity oscillator. It consists of a fore-shortened coaxial cavity
resonator, a transistor circuit, and both a mechanical and an
electrical tuning mechanism. Refer to Figure 8B-13, A32 M/N VCO
and A33 M/N Output Assembly, Schematic Diagram.
A32 M/N VOLTAGE-CONTROLLED OSCILLATOR CIRCUIT DESCRIPTION
A32A2 Voltage-Controlled Oscillator
A
The emitter of oscillator transistor, Q2, exhibits negative
resistance because of the base inductance, L3, which consists of a
fixed length of wire surrounded by air. The function of
ca
and R9,
together with L3, is to match the (reflective) emitter of Q2 to
the cavity resonator. C2 provides a de block required because the
transistor is physically connected to the cavity center post
(inductive coupling) that is at DC ground potential. Rl, R4, R6,
and RS establish the de bias for Q2. C3, CS, and C7 are bypass
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. This is used to adjust the frequency to within
the electrical tuning range. The frequency is adjusted
electrically by using reverse biased varactor diodes, CRl and CR2.
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. Chip capacitors
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. The result is that the
combinations of CR1/C2 and CR2/C3 provide capacitive loading to
the cavity resonator at the frequency of oscillaton. The (VCO)
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. The output power
level is adjusted by varying the length of the probe. This signal
is applied to the Buffer Amplifier, Block
B.
8-135
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