Model 405C Sect.111 Page 1
TIEORY OF
3-1 INTRODUCTION appropriate degree of attenuation for the displayed
The 405C is a voltage-to-time converter. It mea- range. Since the Low-Pass Filter introduces some
attenuation even to dc voltages, the output of the
sures the time required for an internally gener- Attenuator is less than *0.999 volt when the dis-
ated voltage (ramp) to increase from essentially played range is correct.
zero to the value the input voltage has after pass-
ing through an input filter and attenuator. Three
decade counters indicate elapsed time by totaliz-
ing the output of a 50-kc oscillator for the duration
of the time interval. Thus the readoutis a number 3 -3 VOLT AGE-TO-TI ME CO N V ER4 IO N
of time units or periods of the 50-kc signal. Ramp The 405C measures the time required for the
slope is such that the readout equals the input ramp voltage to increase from essentially zero to
voltage. the value of the attenuated input voltage. The ramp
voltage is the voltage developed across the ramp
The 405C requires about 0.2 second to make a capacitor as it charges through a series resistance
single measurement; it does not continuously moni- toward ~t 150 volts. The polarity of the 150 volts is
tor its input voltage as do meter-movement types the same as the polarity of the selected range. The
of voltmeters. During the measurement time, the CALIBRATE control permits some adjustment of
40% zero-sets itself, makes a measurement,. the series resistance to allow adjustment of the
checks range, and if necessary, changes range. ramp slope.
The SAMPLING control determines the rate at
which measurements are repeated except that if Ramp Gate Multivibrator V7 starts the time mea-
the instrument is on the wrong range, it will shift surement when it opens the Ramp Gate (CR5, CR6,
range and remeasure at its maximum rate until it CR7 and CR8) and triggers Oscillator Gate Multi-
finds the correct range. vibrator V5. The Oscillator Gate Multivibrator
opens the Oscillator Gate, V6B, and the counters
start totalizing the output of Oscillator V6A.
Figure 3-1 is an over-all block diagram of the
Model 405C.
A DC Differential Amplifier (Vl, V2 and V3) ends
the time measurement by producing a voltage step
when ramp voltage equals the attenuated input volt-
3-2 INPUT CIRCUIT age. The amplifier has two input grids and ampli-
The input circuit consists of a Low-Pass Filter fies the difference between the two applied voltages
and an Attenuator. The Low-Pass Filter attenu- a thousand times. Two crystal diodes limit the
voltage swing at the output of the amplifier to about
ates any ac voltage superimposed on the dc voltage 1 volt; thus there is no change in amplifier output
being measured. Cut-off characteristics of the until the ramp voltage is within about 0.5 millivolt
filter are such that a 60-cps signal with a peak of the attenuated input voltage. As the ramp voltage
value equal to full scale of the displayed range
I- causes a readout variation of about i4 counts, equals and becomes greater than the attenuated in-
put voltage, the amplifier output quickly goes from
one limit to the other, producing the voltage step.
Four series resistors make up the Attenuator and Amplifier Inverter V4 amplifies this voltage step
provide 1:1, 1O:l. 100:1, and 1OOO:l attenuation. and inverts it, if necessary, to provide a negacive-
Stepping switch K1, controlled automatically or going input to the Oscillator Gate Multivibrator.
by the front-panel RANGE switch, selects the This multivibrator then closes the Oscillator Gate,
005 18- 2 |
ACEC
