If this condition is not met, there is a chance of oscillation
occurring in the leveling loop or of excessive RF power damaging the leveling
mount. In general, the operating point of the stabilizer should be chosen so
that somewhere between 10% and 90% of the DC bias power is replaced by RF.
This may be adjusted by changing the sampling directional coupling ratio, or by
placing a level-set attenuator in the sampling arm.
(4) If difficulty is experienced in using a ferrite modulator it may be
necessary to reverse the polarity of the connection to the modulator. It is
desirable to use a modulator which has its attenuation minimum at zero bias
current, independent of the RF frequency. If this is not the case, it may be
necessary to use a very small external bias magnet on the modulator.
(5) If a PIN diode modulator is being used, one may tell by the
indication on the output voltage meter if the stabilizing loop is operational.
Usually it will read a few tenths of a volt negative.
(6) For either a PIN diode or ferrite modulator, a video detector and
detector may be attached to the side-arm of a directional coupler located on
the load side of the modulator.
PRECISION 0-10 VOLT DC SOURCE (FIGURE 8).
a. The reference generator can be used as a precision 0-10 volt DC source.
The output impedance is almost entirely the contact resistance of the V-REM-V
switch (S702). This is typically 10-20 milliohms. If a connection is made to
the rear of the switch, an output impedance in the micro-ohm range can be
The unit will furnish a current which depends upon the voltage
setting, providing more current at lower voltages before errors due to loading
occur. In particular, a current may be drawn which is not more than given by
Iout = (10.4 - Eout)/10 amperes.
This output is short circuit proof. It is protected by the current limiting
provided by the +15 V power supply in the bridge unit.
b. Operating Procedure - 10 Volt Source.
(1) Since the reference generator obtains its operating voltages from
the bridge, both must be turned on. If it is desired to operate a bolometer
mount, it can be connected to the bridge as described in para. 4a. If not, set
the BARR-THER switch in the BARR position before turning the instrument on.
When turned on, the bridge current will remain at zero with nothing connected
to the bolometer terminals.