Model 618B Sect. 111 Page 11
connected across the bridge to indicate circuit The power monitor probe is moved within its wave-
balance. R201, is a disc-type thermistor connected guide in a manner similar to the power attenuator
in parallel with the meter and it acts as a sensi- pickup probe. Its position is controlled by the
tivity regulator to counteract the tendency of the POWER SET control on the front panel and is in-
bridge to decrease in sensitivity as the temperature dicated by the position of the transparent index
applied to the bridge increases. scale over the calibrated OUTPUT ATTEN. (at-
tenuator) dial.
The power level existing at the location of the
probes in the resonator varies as the signal gen-
erator is tuned over its frequency range. To com-
pensate for this, the position of the power monitor
probe is moved so that it picks up the amount of
power necessary to balance the bridge. This
establishes the basic reference level for 1 milli-
watt and positions the transparent index scale over
the attenuator dial. The attenuator control is then
adjusted with reference to this index scale to pro-
vide the desired attenuation below 1 milliwatt as
read on the calibrated dial.
R-202 RO
The OUTPUT ATTENUATOR control moves the
output attenuator probe mechanically and also
moves the calibrated dial. Thus when any cali-
Figure 3-19. 'Schematic of Power Level
Monitor Circuit bration point on the dial is under the index line, the
position of the output probe in its waveguide is
proportional to that of the power indicating probe
A second disc thermistor, R200 is shunted across (which has been set to the proper reference level)
the bridge in series with limiting resistor R183. and the power output may be read directly on the dial.
This acts as a balance regulating device to compen-
sate for unbalance due to resistance changes in Movement of the output probe in its waveguide will
R202 caused by variations in ambient temperature. cause attenuation as described in an earlier sec-
An ambient temperature increase will cause the
resistance of R202 to decrease causing more cur- tion. The power indicating dial is calibrated in
rent to flow through its leg of the bridge, increas- decibels below one milliwatt when coupled to a
52-ohm load.
ing the meter reading by unbalancing the bridge.
R200 will also decrease in resistance, causing a
drop in the total voltage across the bridge and a
decrease in current flow through its various legs, E. REPELLER VOLTAGE CONTROL
thereby decreasing the meter reading. The operating characteristics of a reflex kly-
stron are such that an optimum value of repeller
voltage exists for each operating frequency. This
Since the static resistance' of R202 is determined voltage is the value that will cause the bunched
by both the ambient temperature and the static electrons to return to the resonator grids at the
bridge current flowing through it, the decrease proper time. Figure 3-20 shows the repeller volt-
in bridge current caused by the actions of R200 age characteristics for the type 6236 klystron over
will act to decrease the temperature of R202 and the range employed in the signal generator.
tend to restore the static balance of the bridge.
The two disc thermistors are mounted directly on
the resonator so that they are immediately sen- The repeller voltage characteristic shown in Fig-
sitive to temperature changes. ure 3-20 provides for operation in the 2-3/4 re-
peller mode. The required voltage for optimum
RF power in the coupling loop is isolated from the operation is essentially linear with frequency for
bridge since sufficient stray capacity exists in the the frequency range above 4400 mc. In the fre-
probe and line to effectively bypass it to ground. quency range from 3800 to 4400 mc the required
This is indicated by the dotted capacity shown in voltage is not a linear function of frequency but is
Figure '3- 19. slightly curved as shown. |
ACEC
HEWLETT-PACKARD
