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Model 428B
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
is dependent on the phase of the input. Where C is in
phase with A, F will be negative when C and D are
grounded. Where C is 180° out of phase with A, F will be
positive when C and D are grounded.
4-36. DC AMPLIFIER.
4-37. The dc amplifier supplies a negative dc feedback
current to the probe proportional to the output of the
synchronous detector. The polarity of the negative
feedback current changes if the polarity of the dc current
(measured in the probe) changes. In this way the
feedback of the system remains negative at all times thus
maintaining the stability of the instrument.
4-38. In addition, this local negative feedback loop
stabilizes the gain of the DC Amplifier.
4-39. Tube V6 is a differential amplifier in which a signal
of approximately 1 volt (for full-scale deflection) is fed to
pin 7 and compared with the signal on pin 2. The output
of V6 is fed to the base of Q3.
4-40. Transistor Q3 drives the current-amplifiers Q1 and
Q2 which are used as emitter-followers in a push-pull
NPN-PNP pair combination.
4-41. The output current from the complimentary pair, Q1
and Q2, goes through the meter circuit to the current
divider S1A which feeds a portion of this current,
appropriate for the range this instrument is working on, to
the probe head as negative current feedback.
4-42. After passing through S1A and the probe head, the
combined current goes through the parallel resistor
network R60-64. This develops a voltage at the junction
of R61 and R62 which is proportional to the feedback
current. This voltage is applied to pin 2 of V6 to complete
the local feedback loop of the DC Amplifier. This circuit
makes the output current of the DC Amplifier proportional
to the voltage applied to the input grid, pin 7, of V6.
Figure 4-8. Negative Feedback
12
4-43. NEGATIVE FEEDBACK CURRENT CIRCUIT.
4-44. The negative feedback current path is shown in
Figure 4-8 . Current divider S1 A divides the feedback
current in proportion to the dc current being measured*.
For a dc input of 10 A, approximately 50 mA feedback
current is fed to the probe head. Since an equal number
of ampere-turns are necessary for canceling the main dc
flux, the feedback coil inside the head requires
approximately 200 turns.
* Maintaining the current through meter M1 constant (5
mA maximum) for all current ranges. Inductance L6
isolates the 40 kHz signal from the dc current circuit.
4-45. 40 kHz PHASE SHIFTER.
The output of the 40 kHz phase shifter is fed to the head
of the probe to cancel any residual 40 kHz output signal
which exists when zero dc is being measured. The
canceling signal is obtained by adding two voltages
which are 90° out of phase and variable in amplitude.
Figure 4-9 shows the circuit and the idealized phase
relationship of the two output voltages with respect to the
signal from the oscillator.
By adding the two output voltages (vector A and B) a 40
kHz signal is obtained, having phase angle and
amplitude to cancel exactly the residual 40 kHz signal
from the probe (vector C). Once the residual 40 kHz
signal of the probe has been canceled, the ZERO control
compensates for any normal variations of zero shift. This
control is necessary only on the lower ranges.
4-48. POWER SUPPLY.
4-49. A single series-regulated power supply of the
conventional type provides 280 volts regulated for the
circuits of the instrument. Voltage reference tube V11
provides a constant cathode potential at control tube V10,
and this is the reference potential for the control grid of
V10.
Figure 4-9. 90° Phase Shift
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