Sect. III Page 2 Model 405C
and the counters display thetotal number of pulses 3-5 THE MEASUREMENT PROCESS
which passed through the gate. .
Sampling Rate Thyratron V8 starts the measure-
The Ramp Gate Multivibrator returns to its stable ment process by triggering the Relay Control Multi-
condition, closing the Ramp Gate, about 30 milli- vibrator, V9. The Relay Control Multivibrator
seconds after it starts the ramp; and should there immediately triggers Photocell Control Multivi-
be no output from the amplifier, the Oscillator brator V18, and these two multivibrators zero-set
the ramp. After about 80 msec, the Photocell Con-
Gate Multivibrator returns to its stable condition trol Multivibrator returns to its stable condition,
at about the same time. Thus the instrument can darkening the photocell. Some 30 msec later the
count to about 1- 1/2 times full scale. Relay Control Multivibrator returns to its stable
condition, de-energizing the relay and triggei-ing
3-4 ZERO-SETTING THE VOLTMETER ' Delay Multivibrator V10. The Delay Multivibrator
To insure that the ramp always starts from the allows the relay to completely de-energize before
correct voltage, the ramp is zero-set prior to each the start of the ramp. The delay is about 4 msec.
measurement. Two multivibrators, Relay Control
Multivibrator V9 and Photocell Control Multivibra- When the Delay Multivibrator returns to its stable
condition, it triggers the Ramp Gate Multivibrator,
tor V18 zero-set the ramp. The Relay Control V7, to start the ramp and open the Oscillator Gate,
Multivibrator energizes a relay which applies a V6B. Oscillator V6A synchronizes the return of
voltage of approximately *l mv to the signal input the Delay Multivibrator to its stable condition. Thus
grid of the amplifier. The polarity of the voltage there is no count ambiguity at the start of the ramp.
is always opposite to the polarity of the displayed
range. The Photocell Control Multivibrator illu- When the ramp voltage equals the attenuated input
minates a photocell V23 in a feedback loop around voltage, the DC Differential Amplifier closes the
the amplifier. When the photocell is illuminated Oscillator Gate, and the readout indicates the volt-
its resistance drops to a low value, and full nega- age applied to the instrument.
tive feedback is applied to the amplifier. Thus
any voltage at the amplifier output is fully fed back 3-6 CHECKING RANGE
to the input and essentially cancelled. Because of During each measurement the 405C checks the
the internal unbalance and drift in the amplifier, range by monitoring the output of the Tens and
plus the *l mv applied to the signal input grid. Hundreds Counters. If there is no output from the
cancellation is not complete. This feedback brings
the amplifier output and the corresponding ramp Tens Counter, the range is too high, for the ramp
grid input down to less than *lo0 mv. The ramp voltage equals the attenuated input voltage before
is zero-set at this time, since the ramp capacitor the counters register 100 counts. If the Hundreds
is connected to the ramp grid of the amplifier and Counter produces an output, the range is too low,
charges to the feedback voltage. Amplifier drift or the displayed polarity is wrong, for the counters
is not fast enough to upset the balance in the time count off scale before the ramp voltage equals the
attenuated input voltage. Thus the range is correct
of a single measurement. Thus the ramp always only when the Tens Counter produces at least one
starts from the same zero-set value. output pulse, while the Hundreds Counters produce
none. However. on the most sensitive ranges,
The Photocell Control Multivibrator opens the feed- (+. 999 v), the Tens Counter is effectively bypassed.
back loop (by turning off the light shining on the This action permits the counters to display levels
photocell) before the Relay Control Multivibrator of less than 100 counts, during which time the Tens
de-energizes the relay. Thus the input signal Counter is unable to produce an output pulse.
voltage is applied to the amplifier after the feed-
back loop opens and cannot upset the ramp zero Tens Binary V13 and Hundreds Binary V14 do the
level. Amplifier output has been reduced to less monitoring. The Tens Binary is reset by Delay
than 5100 mv as explained in the previous para- Multivibrator V10 when that multivibrator triggers
graph. With no input the output remains at this the Ramp Gate Multivibrator. The Tens Binary in
level. However, the amplifier gain has gone up to turn resets the Hundreds Binary. (Binaries have
approximately 1000 when the photocell is not illu- two stable conditions. Hereafter, condition 0 is
minated. Since the signal at the output remains at the reset condition; condition 1, the non-reset con-
less than +lo0 mv, the effect of this additional dition. ) With no output from the Tens Counter,
gain is to reduce the apparent input to the amplifier both binaries remain in condition 0. The first out-
by the amount of the gain. Thus the apparent in- put pulse from the Tens Counter switches the Tens
put level has been reduced to less than tlOO pv. Binary to condition 1. and the Tens Binary immedi-
Therefore thevoltmeter is zero-set to within 100 pv ately switches the Hundreds Binary to condition 1
of zero. also. Succeeding pulses from the Tens Counter
00518- 1 |
ACEC
