Keithley 148 Service manual

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Kelthley~in&&nts, Inc. ~&arrantSth~s product to befree from defects

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;S~~~EM~E,N~T~OF~CAL1BRATION

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~Nationat ~Bureau of Standards through equipment which is cal~@reted at ~Nationat ~Bureau of Standards through equipment which is cal~@reted at

,~ fWrned ~Jnterv& by comparisqn to ,~ertlfii Standards maintained in

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the Laboratories of :Keithley lnstruments;~ Inc. the Laboratories of :Keithley Instruments;~ Inc.

INSTRUCTION MANUAL

Model 148

Nanovoltmeter

DOCUMENT #29029

CONTENTS

MOOEI, 148

ILlsJSTRATIONS

ILLUSTRATIONS

FIG. TITLE PACE

la Front Panel ................................

lb Front Panel With Model 1481 Input Cable 2

Front Panel Conrrole. ...........................

3 Rear Panel Conerols and Connections

4

Model 1481 Low-Thermal Input Cable. 5 Made! 1483 Low-Thermal Connection Kit 6 Normal Wave Form at Demodulator with Input Sbor’ted. 7 Wave Format Demodulator Sham with Some Pickup.

8

Wave Format Demodulator when Amplifier is Saturated

9 “sing Model 148 with 4-Terminal Connections 10 Exploded “few far Rack Mounting 11 Block Diagram of Model 148 Amplifier Circuits 12 13 Block Diagram of Model 148 Power Supplies 14 Model 148 Input Compartment 15 Correct Wave Form in dc-to-dc Inverter. 16 correct wave Form at Point F in Oscillator circuit. 17 Improper wave Form af point F in Oscillator Circuit 18 NOf Used.

19 Nof Used. .................................

20 Top View of Model 148 massis 21 Bottom View of Model 148 Chassis. 22 Transistor Locations on Printed Circuit 76. 23 Capacitor and Diode Locations on Printed Circuit 76 24 25 Resistor and Test Point Locations on Printed Circuit 76

26 27 Component Locations on Printed Circ”it 74, Top Face 28 Resistor and Test Point Locationa on Printed Circuit 75 29 Capacitor and Transistor Locations on Printed Circuit 75.

30 31 Resistor LOcationS on RANOE SWitCh (5102)

Model 148 Input Circuit

.................................

Resistor Locations on Printed Circuit 76. Resistor and Test Point Locations on Printed Circuit 74. Bottom Face.

Resistor Locations on RANGE Switch (S102)

.......................... 20

...................... 18

........................ 26

....................... 38

.................. 3

.................... 6

.................... 10

............ : ...... 10

............ 15

.............. 15

............ IS

................ 17

............... 19

................. 23

.................. 31

............ 32

..................... 39

................ 40

............ 40

................. 41

............ 42

................. 44

--

.......... 41

.......... 4,

.........

... 42

43

1 4

0375

SPECIFICATIONS

MODEL 148

SPECIFICATIONS

iv

0375

MODEL 148

GENERAL DESCRIPTION

SECTION 1.

GENERAL DESCRIPTION

l-l. GENERAL.

The Keithley Model 148 Nanovoltmeter conveniently measures dc potentials

a.

from 10 nanovolts (10 x 10-g

volts) to 100 millivolts full scale. It makes accurate and sensitive measurements without painstaking methods often previously required.

Meter accuracy is 2% of full scale on all ranges. than 1 nanovolt peak-to-peak on the lo-nanovolt range. than 10 nanovolts per 24 hours after warm-up.

On the three most sensitive

Zero drift is less

Noise is less

ranges, line-frequency rejection is greater than 1OOO:l.

For reliable and versatile use,

b.

design, except for the first two input stages.

the Nanovoltmeter is of solid-state

It has high line isolation

and battery or line operation.

0972

FIGURE la. Front panel.

GENERAL DESCRIPTION

MODEL 148

1-2.

ing many grounding problems.

FEATURES. Battery operation permits complete isolation from power line, eliminat-

a.

Battery operation also allows flexibility and

convenience in use. The Model 148 automatically recharges the battery if

needed when the ac power cord is connected.

The Nanovoltmeter has a 21 volt at 1 milliampere output at full-scale

b.

meter deflections for driving a recorder or oscilloscope.

Accuracy is 1% of

full scale for output.

c. A zero suppression circuit permits measuring small changes in a larger

dc signal.

l-3. APPLICATIONS.

The Model 148 Nanovoltmeter measures very small dc potentials or

a. small changes in dc potentials from low impedance sources. These are found in fundamental or applied research, laboratory standards work, cryogenic

experiments and instrument development for space research.

It can also

serve as an amplifier in these uses.

Typics.1 uses include measuring small temperature differences and

b.

small temperature changes indicated by thermocouple outputs, small changes

in conductance,

outputs used in narrowband spectrum analysis. the thermoelectric power of metals,

making Bolometer

super conductivity in the lo-6 ohm range, and thermopile

Other uses are determining

conducting Hall effect studies, and

measurements.

Also, the Model 148 is suited for use with potentiometers, ratio sets and resistance bridges, including Wenner, Wheatst and Kelvin Double bridges. It can be used to make 4-terminal resistance

measurements.

2

0972

MODEL 148 GENERAL DESCRIPTION

0972

FIGURE lb.

Front Panel With Model 1481 Input Cable.

3

GENERAL DESCRIPTION

ZERO s”PPmssIoN

POWER

SWITCH

5201 COARSE 5103

MODEL 148

*

FINE R168

4

FIGURE 2. Front Panel Controls.

0972

MODEL 148 NANOVOLTMETER

OPERATION

SECTION 2.

.2-l. FRONT PANEL CONTROLS.

a. AC CONNECTED Lamp.

The Lamp is lit whenever the unit is connected to

(See Figure 2.)

OPERATION

the ac line and the POWER SUPPLY Switch is in the AC or OFF position.

NOTE

The AC CONNECTED Lamp indicates only that the instrument is connected to the ac power line; it does not indicate that the Nanovoltmeter is operating.

Also, when the POWER SUPPLY Switch is turned from AC to OFF, a difference in Lamp brightness is normal.

BATTERY CHARGING Lamp. When lit,

b.

charging.

not lit,

POWER SUPPLY Switch.

c.

The charge current determines its brightness.

then the battery is charged.

The Switch controls the mode of operation for

this Lamp indicates the battery is

If the lamp is

the power supply.

AC position:

1.

The battery will be charged if needed; then,

The Nanovoltmeter will operate from the ac power lint.

the BATTERY CHARGING Lamp

will light.

OFF position: The Model 148 is not operating.

2.

will be charged,

BATTERY position:

3.

if needed and if the power cord is connected.

The Nanovoltmeter is operated from its battery.

The ac power line is internally disconnected;

However )

the AC CONNECTED Lamp is off;

tk! battery

the battery cannot be charged.

BATT.TEST position:

4.

When the POWER SUPPLY Switch is held in this

position, the Model 148 shows the state of the battery charge directly

on its meter. All circuits within the instrument are the same as for

battery operation except at the meter terminals.

Switch Settin

TABLE 1.

Indicating Lamps and POWER SUPPLY Switch Settings.

The table shows the relationship between the front panel

lamps,

the power cord and the POWER SUPPLY Switch setting.

OPERATION

MODEL 148 NANOVOLTKETER

RANGE Switch.

d.

The RANGE Switch selects the full-scale meter sensitivity

(either microvolts or millivolts) for one of nine ranges, from 0.01 to 100.

FUNCTION Switch.

e.

The FUNCTION Switch selects the function - MICROVOLTS

or MILLIVOLTS - which is to be measured.

ZERO SUPPRESS Controls.

f.

The COARSE Control disconnects the suppression circuit (in OFF position)

1.

or selects one of four suppression voltages in decade steps.

The FINE Control is a continuously variable adjustment for the suppression

2.

voltage set by the COARSE Control.

Two controls determine the amount of zero suppression.

Refer to Table 3.

It adjusts the range between the positive

and negative values of the maximum voltage set by the COARSE Control.

INPUT Receptacle.

g.

The INPUT Receptacle is of a special low-thermal design.

Use only the Models 1481, 1482 and 1486 for mating connectors.

5103:

FIGURE 3. Model 148 Rear Panel Controls and Connections. refer to Replaceable Parts List and schematic diagrams.

2-2.

REAR PANEL CONTROLS AND CONNECTIONS.

Line Voltage Switch. The screwdriver-operated slide switch sets the

a.

Model 148 for 117 or 234~volt ac power lines.

Fuse.

b.

For 117~volt operation,

1. For 234volt operation, use only a MDL Slow-Blow l/16-ampere fuse.

2.

Power Cord. The 3-wire power cord with the NEMA approved 3-prong plug

c.

provides a ground connection for the cabinet.

use a 3 AG or MDL Slow-Blow l/8-ampere fuse.

An adapter for operation from

2-terminal outlets is provided.

WER

RD

Circuit designations

6

0464

MODEL 148 NANOVOLTMETER

A note above the power cord shows the ac power line frequency

for which the rejection filter is adjusted.

will work at any line frequency from 50 to 1000 cps, but ac

rejection is best at the indicated frequency.

OPERATION

The instrument

DEMODULATOR TEST.

d.

A phone jack provides access to the demodulator for

test purposes.

e.

OUTPUT.

The OUTPUT Receptacle provides fl volt at one milliampere for

a full-scale meter deflection on any range.

GND and LO Terminals. The ground terminal (GND) is connected to the chassis

f. and the third wire of the power cord. The low terminal is connected to circuit ground and the low side of the input connection.

2-3.

or from its battery. For most uses,

operation, however,

problems.

MODE OF OPERATION. The Model 148 operates either from an ac power line

it functions well from ac. Use battery

if the ac power line will create ground loop or isolation

Isolation from low to ground is complete for battery operation when the power cord is disconnected; it is greater than 10' ohms with the power cord connected. Also use battery operation to reduce the 8-cps ripple which may appear at the output with the input shorted in ac operation.

See

paragraph Z-13.

NOTE

Before using the battery operation, thoroughly read paragraph 2-4.

Inproper to

inaccurate

battery

measurements.

operation can damage the

battery

pack and lead

2-4. BATTERY OPERATION.

The Model 148 is supplied w;th a rechargeable 6-volt, 4 ampere-hour

a.

nickel-cadmium battery pack. Re<,c~mmended:

than eight consecutive hours without recharging.

rate /

the battery should last about 1000 recharge cycles.

Do not use the battery more

At this discharge

NOTE

Permanent damage to the battery pack occurs if it is used

for more than 14 consecutive hours without recharging.

At this discharge rate, the recharge cycles are greatly

reduced.

Before using the Model 148, check the state

of the battery charge.

Check the battery charge before making a measurement. Hold the

b.

POWER SUPPLY Switch in the BATT. TEST position.

The minimum acceptable charge is a meter indication of +8; full charge is shown by the BATTERY CHARGING Lamp not being lit.

Recharge if needed. Otherwise, battery operation is the same as for the ac power line operating mode; refer to paragraph 2-5.

1067'

7

OPERATION

MODEL 148 NANOVOLTMETER

NOTE

When the battery is used beyond its capacity, two effects are seen.

There is a shift in zero offset from ac to battery operation. Also, the power supplies do not regulate and high ripple voltages appear at the supply outputs.

(See paragraph 4-7.)

To recharge the battery,

c.

come-t the power cord to an ac power line.

the POWER SUPPLY Switch to AC or OFF.

The BATTERY CHARGING lamp will light. The

Turn

battery will be charged only if needed, and the circuit automatically prevents it from being overcharged.

It is suggested that the battery be used during the day and be recharged

d.

at night.

Leave the instrument always connected to the ac power line; then turn the POWER SUPPLY Switch to OFF at night, After a fully charged battery is used for eight consecutive hours, it will recharge within 14 hours. the power cord connected has little effect on the isolation:

109 ohms with

Leaving the POWER SUPPLY Switch in BATTERY position and the low-ground link disconnected. 2-5. OPERATING PROCEDURES,

Set the front panel controls as follows:

a.

POWER SUPPLY Switch OFF FUNCTION Switch MILLIVOLTS RANGE Switch 100 ZERO SUPPRESS COARSE Control OFF

NOTE

Make sure the ZERO SUPPRESS COARSE Control is OFF. If it is not, a suppression voltage is introduced, causing an error in measurements.

Connect the unknown voltage source to the INPUT Receptacle.

b.

Refer to

paragraph 2-6 for suggestions.

Check the voltage shown on the rear panel Line Voltage Switch; connect the

c.

Model 148 to the ac power line. Make sure the frequency shown above the power

cord is the frequency of the ac power line. At this point, the AC CONNECTED Lamp

will light, as will the BATTERY CHARGING Lamp if the battery is being charged. If

the circuit low is to be at ground, put the low-ground link between the LO and GND terminals on the rear panel.

Turn the POWER SUPPLY Switch to the desired mode of operation, AC or BATTERY.

d.

Increase the sensitivity of the Model 148 until the

e.

meter

shows the greatest

on-scale deflection.

Check the source resistance to make sure it is within the maximum value

1. specified for the range being used. (See Table 2.) If the maximum resistance is exceeded, the Model 148 may not be within its specifications.

1067

MODEL 148 NANOVOLTMETER OPERATION

Zero offsets with the Zero Suppress Controls off will vary with the

2. quality of the circuit's thermal construction. See paragraph 2-14. a Model 1486 with a copper-wire short is on the Model 148 INPUT Receptacle, offset should be less than 0.2 microvolt.

Shifts in source resistance also affect the zero offset, if the

3. source resistance approaches the maximum value given in Table 2. effect is negligible for source resistances less than 10% of the

maxiwm value.

If the input is left completely open-circuit, the meter will drift

4.

off scale on any range.

Refer to Table 4 if problems exist during the measurement.

5.

Minimum

Input Resistance Maximum Source Line Frequent

Range Greater Than Resistance Rejection

when

This

0.01 microvolt

0.03 microvolt

0.1 microvolt

0.3 microvolt

1 microvolt

3 microvolts .O microvolts 300 kll i0 microvolts 300 kn 10 microvolts 300 krl

0.01 millivolt

0.03 millivolt

0.1 millivolt

0.3 millivolt 1 millivolt

3 millivolts .O millivolts IO millivolts 10 millivolts

TABLE 2. Model 148 Input Resistance, Maximum Source Resistance, and Minimum Line Frequency Rejection by Range. the ratio of impressed peak-to-peak line frequency (50 or 60 cps)

voltage at input to the indicated dc voltage.

1 kcl 3 kcl 30 n

10 kn

30 kn

100 lcrl 300 kn 3 kc1

1 I%,2 1lQ l?Sl 10 kn 1 Ma. 1Kl 3Kl 5 Ml 5 I%1 5 Nl

10 n

100 n 300 11

1 lu? 3 kn

3&l 3kn

10 kn 10 kcl

10 kn

10 kn 30 m 50 Icn

50 k!J 5:l 50 kn

The rejection is

3OOO:l 1OOO:l 1OOO:l

5OO:l 5OO:l 1OO:l

decreasing

to 5O:l

100: 1

5O:l 2O:l 2O:l 2O:l 1O:l

1O:l

5:l

Three millivolt and microvolt ranges overlap: 0.01, 0.03 and 0.1

f.

millivolts and 10, 30 and 100 microvolts.

source resistance is high or if large 60-cps fields are present.

volt ranges are more convenient to use if subsequent measurements require more

sensitive ranges. frequency rejection by range.

At low levels,

g.

between the input leads and the circuit under test.

leave the instrument connected, and adjust the zero after establishing

a zero reference in the apparatus under test.

1067

Refer to Table 2 for maximum source resistance and line

spurious emf's may be generated simply by contact

use the millivolt ranges when the

The micro-

If possible, always

For example,

in bridge measurements,

9

MODEL 148 NANOVOLTMETER

disconnect the bridge exciting voltage; or with a phototube, shield the tube from light.

2-6. ACCESSORIES FOR INPUT CONNECTIONS.

a. The easiest way to connect the

voltage source to the Model 148 input

is with the Model 1481 Low-Thermal

Input Cable supplied with the instru-

ment.

Use the Cable for temporary

setups, for measurements at several points, and when fast connections are needed. The Model 1481 connects directly to the INPUT Receptacle.

I

FIGURF: 4.

Model 1481 Low-Thermal

Input Cable.

possible or where very low thermal

Where more permanent setups are

b.

connections are needed, use the Model

1482 Low-Thermal Input Cable. It is similar to the Model 1481, except it

has bare copper leads instead of alligator clips.

Clean the bare wire with

a non-metallic abrasive, such as Scotch Brite or its equivalent, before making

the connection. Crimp connections to the voltage source, as possible with the Model 1483 Kit, provide the best low-thermal connections.

If cadmium solder is used for a connection, make sure the soldering

C.

iron used is clean and that it has not been used with regular solder before.

Use only rosin solder flux.

If possible, heat sink all cadmium-soldered

joints together to reduce generated thermal emf's.

Use crimp connections with copper

d.

wire and lugs for the best low-thermal

joints.

The Model 1483 Low-Thermal Connection Kit contains a crimp tool, shielded cable, an assortment of copper

lugs,

copper wire, cadmium solder and nylon bolts and nuts. It is a complete kit for making very low-thermal measur-

ing circuits. The Kit enables the user of the Model 148 to maintain the high

thermal stability of the Nanovoltmeter

in his own circuit.

The Model 1486 male low-thermal

e.

input connector is for connecting custommade circuits to the Model 148. It also makes a good low-thermal shorting plug

for testing the Nanovoltmeter: crimp a

short length of pure copper No. 18 or

'IGURE 5. Model 1483 Low-Thermal

Connection Kit.

No. 20 wire between the two pins of

the connector.

Other available accessories are:

f.

contains replacement parts for the Model 1483.

The Model 1484 Refill Kit, which

The Model 1485 female low-

thermal input connector to use with the Model 1481, 1482 or 1486 for building

shielded low-thermal circuits.

10

0464.

MODEL 148 NANOVOLTMETER

2-7. ZERO SUPPRESS OPERATION.

OPERATION

Purpose:

a.

order to use a more sensitive range to observe a superimposed signal.

100 times full scale may be suppressed on the ranges from 0.1 millivolt to

0.01 microvolt. For example, the Model 148 can measure changes of less than one microvolt in a lOO-microvolt steady signal on its l-microvolt range.

Suupression Voltaxes Available: The COARSE Control sets the suppression

b.

voltage to one of eight values, depending upon its setting and the FUNCTION

Switch setting. (Refer to Table 3.) The FINE Control continuously adjusts the voltage between tile positive and negative value of COARSE Control setting.

For example, if the COARSE Control is at 1 for a suppression voltage of

0.24 millivolt, +0.24 mv.

FUNCTION Switch

Setting

MICROVOLTS

MICROVOLTS MICROVOLTS MICROVOLTS

MILLIVOLTS 1 0.24 millivolt MILLIVOLTS 2 1.2 millivolts MILLIVOLTS 3 12 millivolts

MILLIVOLTS 4 120 millivolts

The zero suppression circuit cancels any constant voltage in

up to

the FINE Control adjustment span is from -0.24 mv to 0 to

Maximum

ZERO SUPPRESS COARSE

Control Setting

1 2 3 4

Suppression

Voltage

0.24 microvolt

1.2 microvolt6

12 microvolt6

120 microvolt6

TABLE 3.

voltage shown is the maximum value, ?15%, for each FUNCTION Switch and

COARSE Control setting.

c. Operation,

Keep the COARSE Control in OFF position.

1.

Switches for the most sensitive meter reading.

Completely turn the FINE Control in the direction opposite to the meter

2.

deflection (counterclockwise for positive deflections and clockwise for negative deflections).

Increase the COARSE Control setting until the meter needle passes

3.

through zero.

Set the RANGE Switch to a more sensitive range, up to 100 times more

4. sensitive than the original range (four RANGE Switch positions). the FINE Control to zero, if necessary.

Suppression Voltage by Control Settings. The zero suppression

Adjust the RANGE and FUNCTION

Adjust the FINE Control for zero deflection.

Readjust

0365

11

OPERATION

MODEL 148 NANOVOLTMETER

NOTE

On the highest zero suppression range - 120 millivolts maximum - a zero offset will be apparent when changing the RANGE Switch settings. On this zero suppres-

sion range, first set the RANGE Switch to the range intended to be used. Then zero the meter with the ZERO SUPPRESS FINE Control. This offset is introduced

only when the ZERO SUPPRESS COARSE Control is set to 4 and the FUNCTION Switch

is set to MILLIVOLTS. There is no significant offset on any other zero supprss-

sion range.

2-8. DIFFERENTIAL MEASUREMENTS.

The Model 148 will measure the difference between two voltages, neither or which is

a.

at power line ground. It can be floated up to 1-400 volts off ground in ac operatior

the Model 148 is battery operated it is completely isolated from line.

Unplug the Model 148 power co? and use battery operation before measuring a source which is more than f400 volts instantaneous off ground.

Damage to the

instrument can result if the power line is connected under these conditions.

When

CAUTION

The front panel controls are electrically connected to the case.

"cord is unplugged,

Use necessary safety precautions.

For best results in making differential measurements, follow the steps below:

b.

Remove the link from the LO or GND terminal on the rear panel.

1. Connect the voltage source to the Nanovoltmeter i,L,wt.

2.

scribed in paragraph 2-5.

urements.

Do not ground any recorders used with this operation, since the low of the

the case may be at a voltage equal to the off-ground voltage.

Make measurements as de-

The zero suppress controls may be used for differential meas-

If the power

Model 148 output is no longer grounded.

3. If power line frequency pickup is a problem, battery operation usually provides

better results.

2-9. RECORDER OUTPUT. on any range is *l volt at one milliampere.

tance is less than 5 ohms within the amplifier pass band.

ac and battery operation.

The output of the Nanovoltmeter for a full-scale meter deflection

Accuracy is 1% of full scale. Output resis-

Output may be used during both

If the Model 148 is used for differential meas?xements, do not

ground the recorder connected to the output.

When recording the Keithley Model 370 Recorder offers complete compatibility with

a.

the Model 148. The output is sufficient to drive the Model 370 without the use of any re-

corder preamplifiers. The Model 370 allows maximum capability of the Model 148.

1% linearity, 10 chart speeds and can float up to *500 volts off ground.

Using the Model

It has

370 with the Model 148 avoids interface problems which may be encountered between a meas-

uring instrument and a recorder.

The Model 370 is very easy to use with the Model 148. All that is necessary is con-

b.

necting the two units and adjusting an easily accessible control for full-scale recorder

12

1067

MODEL 148 NANOVOLTMETER

OPERATION

L

Trouble (seen on meter)

Change in offset between ac

Possible Cause

Low Battery

Refer to

paragraph 2-4

and battery operation Very slow response time

High source resistance paragraph 2-12

Improper shielding paragraph 2-13

Excessive drift

I I

Thermal emfs

paragraph 2-14

I

Improper connection to input paragraph 2-15

Excessive noise or needle

instability

High source resistance paragraphs 2-11,2-l improper shielding paragraph 2-13 Improper connection to input

paragraph 2-15

Thermal emfs paragraph 2-14

Excessive temperature sensitivity Thermal emfs

paragraph 2-14

presence of large, constant zero suppress Controls on paragraph 2-5 signal Thermal emfs paragraph 2-14

Improper connection to input paragraph 2-15

Excessive 8-cps beat at output

or meter magnetic shielding

I

TABLE 4.

Troubleshooting Operating Procedures. *he Table gives some possible sources of

Improper location or poor

I

paragraph 2-13

I

errors while using the Model 148 and refers to instructions to correct the situation.

deflection. The furnished Model 3701 Input Cable mates with the output connector on the

Model 148. On the most sensitive ranges of the Model 148, under some conditions, a" 8-

cps beat may appear. This condition can be eliminated by mounting a 100~microfarad capac-

itor across pins 14 and 17 in the back of the Model 370 Recorder.

2-10.

ACCURACY CONSIDERATIONS.

For sensitive measurements - 10 millivolts and below -

other considerations beside the voltmeter affect accuracy. The Model 148 reads only the

signal received at its input; therefore, it is important that this signal be properly transmitted from trle source. The following paragraphs indicate factors which affect ac-

CllrWy: thermal noise, loading, shielding, thermal emfs and circuit connections.

Table

4 also offers a quick reference to correct troubles which may occur. 2-11 THERMAL NOISE.

The lower limit in measuring small potentials occurs when the Johnson noise, or

a. thermal agitation, becomes evident. The amount of noise present in the source is show" in the following equations.

1. The thermal noise in any ideal resistance can be determined from the Johnson noise,

equation:

-As

=4kTRF Eq. 1

where Er,, is the rms noise voltage developed across the voltage source;

T is the temperature in degrees Kelvin;

1067

13

OPERATION MODEL 148 NANOVOI~'~ETEl?

R is the source resistance in ohms; F is the amplifier bandwidth in cps; k is the Boltzmann constant (1.38 x lo-23 joules/OK).

For an ideal resistance at room temperature (300°K), equation 1 simplifies to

rms = 1.29 x lo-lo (R F)112

Peak-to-peak meter indications are of more interest than the rms value.

2.

mentally,

E

the peak-to-peak Johnson noise is about five times the rms value.

Eq. 2

Experi-

At room

temperature, equation 2 becomes

= 6.45 x lo-10 (R F)li2

EPP

Eq. 3

where Epp is the peak-to-peak noise voltage developed across the voltage source.

The Model 148 bandwidth, F,

3.

can be estimated from the response speed, tr, by:

F = 0.35/tr Eq. 4

The response speed varies with the range used and the source resistance.

On the O.l-

microvolt range when the source resistance is less than 10 ohms, for example, the bandwidth is greater that 0.7-cps.

The maximum specified response speed for this situation

is 0.5 second, so the 0.7-cps bandwidth is a minimum value.

As an example, determine the Johnson noise of a lo-ohm ideal resistor. Measured

4-

with the Model 148 on the 0.1~microvolt range and using equation 3, this becomes

17.1 x lo-10 volts peak-to-peak or 1.71 nanovolts peak-to-peak minimum. In general, good wirewound or low-noise metal-film resistors approximate ideal resis-

b.

tors,

and equations 2 and 3 are nearly correct. If the source resistance is.composed of

other materials, it may be necessary to include other terms in the equations to account for flicker, l/f, and current noise over and above the thermal noise.

c. As seen in equations 2 and 3, ficant in the microvolt region. fore, keep the source resistance as low as possible.

the noise of even low resistance values becomes signi-

The noise in non-ideal resistors is even greater. There-

Other effects of high source resis-

tances are decreased response speed and added pickup of extraneous voltages.

2-12. INPUT RESISTANCE.

is obtained by using high feedback factors.

The Model 148 is a feedback amplifier, and its input resistance

When the source resistance exceeds its physi-

cal input resistance - the amplifier without feedback - the feedback is partially de-

stroyed.

higher resistances may be used, but noise, offset,

result.

Normally, do not exceed the maximum source resistance specified in Table 2;

slow response time and instability may

On the most sensitive ranges,

the maximum source resistance is consistent with

Johnson noise considerations. Z-13. SHIELDING.

Due to its narrow bandwidth, the Model 148 is somewhat insensitive to ac voltages

a.

superimposed upon a dc signal at the input terminals.

However,

ac voltages which are

large compared with the dc signal may drive the Model 148 ac amplifier into saturation,

erroneously producing a dc output

at

the demodulator.

Therefore, shield the circuit to

the Nanovoltmeter input, particularly for low-level sources.

14

1067

MODEL 148 NANOVOLTMETER OPERATION

Improper shielding can cause the Model 148 to react in one or more of

b.

the following ways:

Needle jitter or instability, from 10% to 20% of full scale.

1. High offset (dc bias). Changing the power cord polarity or the

2.

connection between the LO and GND terminals may affect the amount of offset.

Slow response time, sluggish action and/or inconsistent readings

3.

between ranges.

Amplifier saturation.

4.

Observe the wave form with an oscilloscope connected to the DEMODULATOR TEST Jack (Figure 3). With the input shorted, it should approximate the wave form shown in Figure 6. If excessive pickup occurs, the wave form will resemble that of Figure 7. The circuit

will operate reasonably well as long as the wave formis not clipped, as

shown in Figure 8.

---

FIGURE 6.

Normal Wave Form at Demodulator with Input Shorted. Scale is 0.1 v/cm vertical and 10 msec/cm horizontal.

At this point a dc offset is introduced.

FIGURE 7.

Wave Form at Demodulator

Shown with Some Pickup. are adequate; some noise.

there is no offset but

Scale is 0.5 v/cm vertical

Measurements

and 10 msec/cm ho,rizontal.

0365

FIGURE 8. Wave Form at Demodulator when

Amplifier is Saturated. DC offset is intro-

duced and there is greater noise. Note the

Nanovoltmeter still reacts to the input signal,

Scale is 5 v/cm vertical and 10 msecfcm horizontal.

15