MM0474
will generally be less than 3 ppm. At the same time, the closed loop gain of
the voltage follower is very nearly equal to 1.
Thus, the voltage which
appears at the slider of R703 is presented at terminal 22, but whereas the
impedance of slider R703 may be several thousand ohms, the action of the unity
gain voltage follower reduces the effective impedance at terminal 22 to a very
small fraction of an ohm.
Useful currents may be draws from this terminal
without serious loading errors. The actual output impedance is on the order of
20 milliohms and consists primarily of the switch and binding post resistances.
It can be seen, therefore, that the voltage measuring instrument used to
measure V need not have a very high input impedance because it is measuring
the voltage between two zero impedance sources. This also minimizes the errors
due to source resistance unbalance which appear in certain types of digital
(7) The operational amplifier, connected as a unity gain voltage
follower, requires AC stabilizing. This is accomplished by R711 and C702, by
R737 and C710, and by R731 and C706.
The output voltage meter M701 again
indicates the output of the reference voltage generator.
(8) The maximum current which may be drawn from terminal 22 is given by
the equation in the specifications Table 1B. In any case, the output current
is limited by the current limiting circuitry in the +15 volt power supply to
protect the components in the instrument.
(9) In the third (STAB) mode of operation, the instrument may be used as
a precision power stabilizer.
This configuration resembles that of the all
mode except that the output of the operational amplifier is used to drive a
modulator in the microwave transmission line. The voltage at the top of the
bridge is compared with the voltage at the slider of R703 and the difference is
amplified by the operational amplifier to drive a PIN diode modulator, or,
through a unity gain power amplifier, a ferrite modulator. The output for the
PIN modulator is takes directly from Q713 in order to protect the modulator.
This output current is limited to approximately 5 mA or less. The output for a
ferrite modulator is taken from the emitter of Q714. Both signals go negative
when higher attenuation is required by the servo loop. The current available
from Q714 is limited by R734 and by the current limiting circuitry in the -15
volt power supply. In this mode of operation, the RF level is set with divider
R703.
(10) The operational amplifier in the stabilizer mode requires AC
compensation.
This is afforded by the networks switched by Q711, Q712, and
C701.
The appropriate network of the two switched by the field effect
transistors is selected by the barretter-thermistor switch on the bridge unite
with the switching signal carried to the reference generator unit through the
rear connecting cable.
It should be noted that if the reference voltage
generator is to be used as a stabilizer with external 15 volt power supplies
instead of a bridge unit, the appropriate control signals for these analog
switches must be provided to insure stable operation. Because of the location
of the switching transistors Q711
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