Model 428B
4-43. NEGATIVE FEEDBACK CURRENT CIRCUIT.
4-35. In summary then, C and D are alternately grounded,
and the polarity of the signal across T2 changes as C and
D are switched to produce an output wherein the polarity
4-44. The negative feedback current path is shown in
is dependent on the phase of the input. Where C is in
Figure 4-8 . Current divider S1 A divides the feedback
phase with A, F will be negative when C and D are current in proportion to the dc current being measured*.
grounded. Where C is 180° out of phase with A, F will be
For a dc input of 10 A, approximately 50 mA feedback
positive when C and D are grounded.
current is fed to the probe head. Since an equal number
of ampere-turns are necessary for canceling the main dc
4-36. DC AMPLIFIER.
flux, the feedback coil inside the head requires
4-37. The dc amplifier supplies a negative dc feedback approximately 200 turns.
current to the probe proportional to the output of the
* Maintaining the current through meter M1 constant (5
synchronous detector. The polarity of the negative
mA maximum) for all current ranges. Inductance L6
feedback current changes if the polarity of the dc current
isolates the 40 kHz signal from the dc current circuit.
(measured in the probe) changes. In this way the
feedback of the system remains negative at all times thus
4-45. 40 kHz PHASE SHIFTER.
maintaining the stability of the instrument.
The output of the 40 kHz phase shifter is fed to the head
4-38. In addition, this local negative feedback loop
of the probe to cancel any residual 40 kHz output signal
stabilizes the gain of the DC Amplifier.
which exists when zero dc is being measured. The
4-39. Tube V6 is a differential amplifier in which a signal canceling signal is obtained by adding two voltages
of approximately 1 volt (for full-scale deflection) is fed to which are 90° out of phase and variable in amplitude.
pin 7 and compared with the signal on pin 2. The output Figure 4-9 shows the circuit and the idealized phase
of V6 is fed to the base of Q3. relationship of the two output voltages with respect to the
signal from the oscillator.
4-40. Transistor Q3 drives the current-amplifiers Q1 and
Q2 which are used as emitter-followers in a push-pull By adding the two output voltages (vector A and B) a 40
NPN-PNP pair combination. kHz signal is obtained, having phase angle and
amplitude to cancel exactly the residual 40 kHz signal
4-41. The output current from the complimentary pair, Q1
from the probe (vector C). Once the residual 40 kHz
and Q2, goes through the meter circuit to the current
signal of the probe has been canceled, the ZERO control
divider S1A which feeds a portion of this current,
compensates for any normal variations of zero shift. This
appropriate for the range this instrument is working on, to
control is necessary only on the lower ranges.
the probe head as negative current feedback.
4-42. After passing through S1A and the probe head, the
4-48. POWER SUPPLY.
combined current goes through the parallel resistor
4-49. A single series-regulated power supply of the
network R60-64. This develops a voltage at the junction
conventional type provides 280 volts regulated for the
of R61 and R62 which is proportional to the feedback
circuits of the instrument. Voltage reference tube V11
current. This voltage is applied to pin 2 of V6 to complete
provides a constant cathode potential at control tube V10,
the local feedback loop of the DC Amplifier. This circuit
and this is the reference potential for the control grid of
makes the output current of the DC Amplifier proportional
V10.
to the voltage applied to the input grid, pin 7, of V6.
Figure 4-8. Negative Feedback Figure 4-9. 90° Phase Shift
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ACEC
