Model 618B
Sect. 111 Page 10
.
A 'similar gap exists between the center conductor ployed so that the pick-up robe and indicating
and the plunger. However, the length of this gap dial can be moved by a simple gear train and the
is such that no resonances occur in the frequency dial may be calibrated directly in decibels.
range of the oscillator.
A cross-section of the attenuator and rf pick-up
Compensation is applied to control resonance of loop is shown in Figure 3-17. The rf pick-up loop
is terminated by a special resistor, R203, whichis
the line formed by the peripheral plunger gap in
the resonator. made by coating platinum on a glass bead. This
resistor is used to match the attenuator to the
output cable. The dc resistance of R203 is approxi-
D. RF POWER MONITOR AND ATTENUATOR mately 60 ohms.
Two pickup loops are located in the resonator
The polyiron section on the outsidebf the probe is
to collect rf power. The first is the output at-
designed to absorb power that may leak past the
tenuator loop which couples the power to be sup-
probe in the space between the outer conductorand
plied by the generator to the load throughan output
the waveguide walls.
connector on the panel. The position of this loop
is adjustable so that the output power level may
The power monitor rf probe, shown in Figure 3- 18,
be varied as desired. The second is the power
is constructed similarly to the attenuator probe.
monitoring loop which monitors the power level
in the oscillating circuit and establishes a reference
point to calibrate the output power.
POLYIRON FILLED BEAD THERMISTOR
Power is coupled to the load from the rf oscil- INSULATING BEAD
lator by a coupling loop located at a suitable point n
in the resonant line. This loop slides in a circular
waveguide section (Figure 3-13). The cross-
section of the waveguide is very small in relation
to the frequencies of operation, and normal prop-
agation down the waveguide will not take place.
However, some limited propagation does take
place, and the power level decreases exponentially
as the distance from the resonant line increases.
Figure 3-18. Cross-Section View of
Thus it is possible, by moving a pickup loop lin-
Power Monitor Probe
early in the waveguide to secure an output that
varies in decibels in proportion to the linear travel.
This type of attenuator is known as an attenuator
The power generated by the klystron varies with
of the cut-off type and its characteristics are em-
frequency, Therefore, a power monitor circuit
is included to indicate the level of the field at the
attenuator input.
-.
The power in the resonator is monitored by the
OUTER CONDUCTOR
thermistor bridge circuit shown in Figure 3- 19.
The bead thermistor R202 which is one arm of the
bridge, is located in the piston type probe. The
probe couples a small amount of the power from
POLYRON FILL the resonator and the resistance of the bead therm-
no istor varies from the heat developed.
SUPPORTING BLOCK
The bridge, essentially a Wheatstone bridge cir-
cuit, is balanced for dc conditions. When rf power
from the resonator is applied to the thermistor,
Figure 3-17. Cross-Section View of the bridge becomes unbalanced. The balance in-
Attenuator Probe dicating device is M101, a 200microamperemeter, |
ACEC
HEWLETT-PACKARD
