Keithley 151 Manuel utilisateur

INSTRUCTION MANUAL

MODEL 151

MICROVOLTMETER

CONTENTS

INTRODUCTION

SPJXIFICATIONS

Ranges

Accuracy

Input

Xesl3t3ncs

Overvoltage

Zero Adjust

Noise

Zero Drift

Response

Speed

Input Rejection

In-Phase Rejection

Impedance

output

Tube Complement

power

Connector3

Accessories Supplied

Dimension3

OPERATION

Operating Control3

Preliminary Set Up

Operation

CIRCUIT DESCRIPTION

Input Circuit

AC Amplifier

Demodulator Circuit

DC Amplifier

Loop Description

Power Supply

MAINTENANCE

General

Trouble Shooting Procedure

Replaceable Part3 List

Voltage Resistance Diagram

Circuit Schematic

SECTION

III

151

0763

SECTION 1 - INTRODUCTION

The Keithley Model 151 Null Detector is a stable, low-level DC

amolimwith high-~<~utesistance and low noise. Careful shielding.

fiitering,

and gu&di.nk permit floating operation, as in a bridge nuli-

detector, with excellent rejection to extraneous voltages.

With a power sensitivity of lo-l7 watt, a re3pon3e speed of ""3

second, lack of overshoot and a zero-centered meter, the

151

may be used

,;n applications Inhere a suspension galvanometer might be employed, or

in other applicaticns where a galvanometer is not sufficiently sensitive,

fast or rugged.

Five non-linear ranges are provided which have the same center-

scale sensitiv'ty as the linear ranges, but are compressed in a quasi-

logarithmi: way to three decade3 on each range. The3.e range3 permit

speedier null-searching th3.n the l~inear scales, without sncrlfizing

sensitivity at null.

Two zero controls for opera 2.r.d short-circuit inputs ai~low prop3r

zeroin,- for any Source resistance even in the presence of Bridge thermal

emfts eliminating the need for disconnecting bri~di=e voltage cc set zerr).

Although de?-:gned for nu?l detector applications, the Model

151

also useful as a dc voltmeter with 3 maximum full-scale sensi~isi~y ?f

100 microvolt3 and as an ,ammeter with a maxlm::m full-s,x1e ren:ii~tivL::.

of 10 micro-microamperes.

The 151 has i full-scale ouL.out of 10 volts at up to one mlliiamperc.

Thiv is sufficient Lo drive one milliar5~ere reco!.ders 'ii .well as ~iervo-

rebalance recorder3 and oscilloscopes.

I-l

0763

MODEL 151 ~fICROVOLTMETER

RANGE:

Linear: 0.1 millivolt full scale to 10 volts on zer*-center meter. ILL overLappin,:

ranges in lx and 3x steps.

Non Linear: Five ranges of 0.001, 0.01, 0.1, 1.0 and 10 volts full scal.c, eaci1 covering

3 decades.

ACCURACY:

Linear ranges: +3% of full scale exclusive OE noise and drift.

Non-linear ranges: ?lO% of input exclusive of noise and drift

ZERO DRIFT: Less than 10 microvolts per day after 30-minute warm-up.

INPUT NOISE (with input shorted): Less than 2% of fui.1 scale on ~11 r;,,r::cs

INPUT RESISTANCE: 10 megohms on all ranges.

LINK FREQUENCYREJECTION: 2,OOO:l

COMMONMODEREJECTION: With l-megohm source resistance. dc:5,000,000:1. l.i~,le rriqut,ncv:

500,000:1.

ISOLATION: Circuit ground to chassis ground: Approximately 109 ohins ijllillted 5'; ll.i!L

microfarad. Circuit ground may be floated up to ?500 volts dc or ~p~!ak:<i!:il rcspcc: KC,

chassis ground.

RISE TIME (10% to 90%): Approximately L second on all ranges.

OVERVOLTAGE: 200 volts steady, 400 volts transient

ZERO ADJUST: For both open and short-circuit inputs.

RECORDEROUTPUT:

Output: ?lO volts dc at up to 1 milliampere for full-scale meter deflection.

Resistance: Less than 50 ohms within the amplifier pass band.

Gain on linear ranges: 10 volts

Range setting in volts

LINE STABILITY: A 10% change in line voltage will cause less than a 2,X of full-scale

shift on all ranges.

CONNECTORS: Input: Binding posts. output: Amphenol 80-PC2F.

POWER: LOS-125 or 210-250 volts, 60 cps, 50 watts. 50-cps models available.

DIMENSIONS, WEIGHT: 10 inches high x 6-l/2 inches wide x 8-l/2 inches deep; net wcighc,

11-l/2 pounds. I

ACCESSORIES SUPPLIED: Mating output connector.

0166R II-1

SECTION III -- OPBlATION

A. OPERATINGCONTROLS

The operating controls of the Model 151 are listed below:

ON - OFF SWITCH;

RANGESWITCH: Selects both the linear and the logarithmic ranges.

SHORTCIRCUIT ZERO: Sets the meter zero ,with the input shorted.

This control compensates for thermal EMF's in

the connected circuit and the amplifier input.

OPENCIRCUIT ZERO: Sets the meter zero with open or high resistance

input. This control compensates for an:, leakage

currents present in the external circuit.

B. PRELIMINARY SET UP

Connectto the power line. Unless otherwise marked, the unit is wired Co.-

117 v.,

cps power. To convert to 220 volt and/or 50 cps operation,

consult MAINTENANCEsection. A three-wire line

cord

is furnished which

grounds the cabinet. If a three-wire receptacle is not available, use

the two pin adapter furnished, and ground the third lead to an external

ground for the best operation. If open circuit unbalance is encountered,

reverse line cord.

Set the range switch to the 10 volt linear range. Xrn on the power.

In about

seconds the meter will zero. Short the

input and

turn to the

.1 millivolt range. Zero the meter with the SHORTCIXCUIT ZERO. Open

the input

and

zero the meter with the OPEN CIRCUIT ZERO. In the first

30 minutes of operation some zero drift may occur and it may

desirable

to reset the zeros.

C. USE OF ZEROCONTROLS

----

The stability of the Model 151 is such that, after

30 minutes of operation,

only infrequent attention need be given to either zero co::trol except

occasionally on the most sensitive ranges. The twc zero controls, however,

allow the user to maintain a constant zero with varying input resistances.

The short circuit zero

bucks

ou,t low impedance voltages such as generated

by thermal EMF's. The open circuit zero is a current buckout control for

balancing any current generated by

chemical

effects or leakage. In normal

practice, if zeroing is required, the SHOR

T CIRCUIT ZERO is sufficient.

However, in the case of a critical null balance application where it is

necessaw to eliminate zero shifi; over a range of inuut re4istances, this

can be assured by balancing the null detectcr using i,ne SHORTCIRCIJIT

ZiTiO at the lowest impedance involved and the OPEN CIHCIIIT ZEROat. the

hiphest impedance involved.

0763

III - 1

D. INPUT RFSISTAlsCEAND SOURCEIMPEDANCERIWTRICTIONS

The input resistance of the Model 151 is 10 megohms, within 5% on the

most sensitive range and within 1% on all ether ranges. Other than the

consideration of circL.'.t loading and speed of respcnse (See F.), there

are no source impedance restrictions.

E. CURRENTMEASUREMENT

Since the input resistance is a constant 10 megohms, ibe Model 151

may

bt- used '.? measure current with a maximum sensitivity of lo-11 amperes

full scale. Since the tolerance cn input resistance is 5% on the mcst

sensitive range, an accuracy of about 7% can be expected there. On

all other ranpes an accuracy of k% is possible.

F. SPEEDOF RESPONSE

The specification in Section II is for low impedance input. With an

open-circuit input, speed is about 5 seconds to 90% of final value.

G. LINE FREQUENCYREJECTION

The specification for 60 cps rejection are given in Section II. The

rejection is high enough so that usually no precautions are necessary

with regard to 60 cps pick-up. While the input filter is specially

peaked at 60 cps for maximum rejection, there is sufficient rejection

at other harmonics to make pickup troubles unlikely.

H. FLOATING OPERATION

The common mode rejection to DC as well as 60 cps is extremely high in

the Model151 as spwified in Section II. The instrument will operate

with no difficulty up to 500 volts from pround. For floating cperation,

remove the shorting link between the LO and G terminals at the front of

the instrument.

I. RECORDING

The output at the recorder terminals for ful; scale is 10 ~clts at up to

1 milliampere. Since the output has

common ground with the input, when

recording, either the Model 151 must be grounded or the recorder input

capable of being floated. It shoud be remembered that, with a recorder

COMected

to the output, the system rejection will be no better than that

of the recorder.

III - 2

SECTION IV - CIRCUIT DESCRIPTION

The Model 151 is a narrow-band chopper amplifier employing negative feed-

back to stabilize the gain and increase the input resistance.

A. INPUT CIRCUIT

---

The input circuit contains the dc to ac modulator, the range Switch, and

input filter to reject spurious ac Signals, and the zero controls.

The modulator used in the Model 151 employs two photoconductive cells PD 101

and PD 102 (refer to circuit schematic diagram at the rear of this manual)

which are alternately switched by two neon lamps NE 101 and NE 102 operated

from the ac line. The action is similar to a Single-pole double-throw

mechanical chopper with the result that the dc input is converted into an

ac signal.

Spurious ac signals are prevented from entering the input by meanS of a

low-pass filter consisting o,f RlO8 and ClOL and a "twin-tee" filter con-

sisting of RlOl +,;lrough R105 and Cl01 through Cl03 which is tuned to the

line frequency. RlOh and R102 are set at the factory for maximum rejection

to line frequenc,v.

Below 10 millivolts, the sensitivity is changed by altering the feedback

factor. Above 10 millivolts an input divider is used. The l~nput divider

is formed by

R153

through Rl59. The total resistance of this divider is

10 megohms which is always across the input.

The zeroing circuits consist of batteries MO1 and MO2 and resistors

Rl38

through Rlir8. R139 together with Rl38 and the bias batteries form

a current source which bucks out any spurious currents appearing at the

invut. Rlk8 and either R1117or Rlb8, depending on range, place a voltage

in series with the low side of the modulator and buck-out any spurious

EMF's appearing at the ingut.

As indicated on the circuit schematic, critical parts of the input circuit

are surrounded by a Separate shield connected to the LO terminal. This

shield largely accounts for the high in-phase rejection.

AC AMPLIFIER

The AC amplifier consists of Vl and V2. The total gsin is approximately

500,000. The first stage tube is not Specially selected, although a tube

may have to be rejected for excessive hum pick-up which results in exces-

sive zero offset. Due to the excellent internal shielding of the first

stage tube, it is not necessary to use dc on the first stage filament.

The time constants of the amplifier are selected to give as narrow a

paSs-band as possible around the carrier frequency. ,

IV- 1

C. DEMODUIAMR CIRCUIT

The demodulator circuit employs a four-diode bridge circuit with silicon

diodes. A balanced configuration is used so that careful balance of the

transformer secondary is not necessary.

The demodulator is driven synchronously with the neon lamps which witch

the input modulator. The modulator output is a pulsating dc signal

which is fed through R119 to the input grid of the dc amplifier.

D. DC AMPLIFIER

The dc amplifier is required for two reasons. At the carrier frequency,

the dc amplifier is a feed back integrator. The integrating capacitor,

C115, is in a local feedback loop from output cathods to input grid.

The gain of the dc amplifier is about 500 so that the value of Cl15 is

effectively multiplied by 500,

eliminating

the need for a large value

of capacity for filtering. At dc, the demodulated signal is amplified,

increasing the loop gain by about 500. This additional feedback results

in a ver.v high input resistance for the null-detector. Exclusive of the

divider, the input resistance is in excess of 300 megohms on most ranges.

The dc amplifier circuit is conventional and consists of V3 connected as

a differential amplifier, Vb used as a dc an.7lifier, and the output cathode-

follow@r. Rl28, DC AMP BAL, adjusts the balance of the dc amplifier.

Once set, this control requires very infrequent adjustment. A misadjust-

ment is evident if, on the 10~ log range the meter is not exactly on zero.

E. THE WHOLELOOP DESCRIPTION

---

Sections of the amplifier are combined. The input fil.ter re-

moves any high frequency components from the input signal.

The modulator converts the filtered dc signal to ac, which is amplified

by the ac amplifier. The output of the ac amplifier is converted into a

pulsating dc signal filtered by the dc amplifier acting as an integrator,

and further amplified by the dc amplifier.

The output signal is fed back to the input by means of ~160 through R16h

and Rlh9 through Rl52. The feedback is applied to the low end of PD102,

the modulator diode. Applying feedback at this point not only stabilizes

the gain, but raises the input resistance Vera substantially.

The sensitivity is changed from .l millivolt to 10 millivolts by changing

the feedback factor. An input divider is used above 10 millivolts.

The ~*logarithmicfl ranges are obtained through the use of a non-linear

"thyrite" resistor Pi166 in the feedback loop. ?

IV - 2 0763

On some ranges, when the feedback factor becomes too large, it is necessary

to decrease the gain of the ac amplifier. This is done by shunting pin

7 of Q2 to ground with a small value of resistance, ~167.

F. POWERSUPPLY

Despite the high sensitivity, no regulation is employed in the power supply,

The plus and minus supplies are derived from standard rectifier-filter

transformer ccmbinst!.cn. Points 'X nnd V on the eame transformer

winding

are used to drive the modulator and demodulator.

IV - 3

SECTION V -- MAINTENANCE

A. GENERAL

The only periodic maintenance the Model 151 requires is replacement OC

EKDl and BAXZ, the zero set batteries, at inter-&Is of about two or three

years. If the zeroing controls become inoperative, the batteries need

replacement.

The modulator and other components have an indefinite life and should

not be tampered with unless there is a failure.

B. TROUBLE-SHOOTINGPROCEDLIRE

If the instrument is inoperative but replacement of tubes doe:: not cure

the trouble, the following step-by-step procedure is indicated.

1. Check the S-plus and B-minus voltages at the junctions of R170,

C120-B and R171,

C119B.

These should agree within about

of the voltages indirated on the schematic. If these arelOg

markedly different, check the fuse and 'hen the ac voltage frcm

the transformer (about '23i.~v to ground). Then check the selenium

rectifiers and the filter capacitors.

2. If the proper voltages are present in the power supply

Remove V-h, the 12AT7, from its socket, sh.crt the clltcut

terminals and the input termi.nals.

5. Connec' an oscillosope to the plate of W-B (pin

6).

Rotate the SHORTCIRCLTIT ZERO to one end of its travels.

If the ac amplifier is functioning and the bias batteries

BAl and BA2 are providing voltage, a 60 cps carrier signal,

proportional Ian amplitudeto the rotati.on of the potentic-

meter (until the amplifier saturates), should be seen.

By returning the short circcit zero con'~.rcl tr midymsition,

it should be possible to reduce the signal ir i.he point

where it disappears 51; the fi~rst stage tube n, i~se. If

th4.s occurs, the ac amplifier is functioning. Proceed

:n the instructions in paragraph D.

If there is an ac signal present at the output which is

not affected by the short-circuit zero control, short

of V-l to ground. You will have to remove the in-

put stage shield to do this. Replace it when 0bserv:r.g

the output with the oscilloscope cn pin 6 of V-2. If the

signal is no longer present and just background tube noise

is seen, either the modulator is defective yr the input cir-

cuit is open somewhere between Cl05 and the input terminals.

V-l

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