Keithley 147 Service manual
INSTRUCTION MANUAL
MODEL
NANOVOLT NULL DETECTOR
147
OCOPYRIGHT
PRINTED, APRIL 1977, CLEVELAND, OHIO, U. S. A.
1972,
KEITHLEY INSTRUMENTS, INC.
CONTENTS
MODEL 147
CONTENTS
Section
GENERAL DESCRIPTZON---------------------------------------------- 1
1.
OPERATION-------------------------------------------------------- 5
2.
APPLICATIONS----------------------------------------------------- 21
3.
C1I<CUIT DESCRIPTION----------------------------------------------
4.
MhINTENANCE------------------------------------------------------ 31
5.
REPLACEABLE PARTS------------------------------------------------ 51
6.
Page
25
SCHE~T~CS------------------------------------------------__-----__-- 65
0477
ii
MODEL 147
igure
NO.
la.
lb.
2.
a:
5.
6.
7.
a.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
25.
26.
27.
28.
29.
30.
31.
32.
33.
34.
35.
36.
37.
38.
39.
40.
ILLUSTMTIONS
ILLUSTNATIONS
Title
FrontPanel..
______-
.............................
Pag:e
1
Front Panel with Cable ......................... 3
Front Panel Controls .......................... 4
Model 147 Rear Pmel Controls & Connections. .............. 6
Model 1481 Low-Thermal Input Cable ................... 10
Model 1483 Low-Thermal Connection Kit. ................. 10
Bus System for Model 147 ........................ 12
Thermal Sink Construction.
.......................
15
Normal Wave Form at Demodulator with Input Shorted ........... 17
Wave Form at Demodulator Shown with Some Pickup. ............ 17
Wave Form at Demodulator when Amplifier is Saturated .......... 17
8-cps Filter Circuit for Recorder Output ................ 18
Using Model 147 with 4-Terminal Connections. .............. 18
Exploded View for Rack Mounting .................... 19
Circuit UsingGuildline9144 with Model 147 Null Detector ........ 21
Circuit Using Guildline 4363DL with Model 147 Null Detector. ...... 22
Circuit Using Guildline 9120 with Model 147 Null Detector. ....... 22
Circuit Using Biddle 605001 with Model 147 Null Detector ........ 23
Circuit Using Leeds & Northrup 7556. .................. 23
Circuit Using Model 147 Null Detector with ES1 240, 8OOR and KS 925. .. 24
Block Diagram of Model 147 Amplifier Circuits. ............. 25
Model 1.47 Tnput Circuit. ........................ 26
Diagranl of Power Supplies & Battery Charging Circuit .......... 29
Model 147 Input Compartment. ......................
32
Correct Wave Form in dc-to-dc Inverter ................. 36
Correct Wave Form in Oscillator Circuit. ................ 38
Improper Wave Forms in Oscillator Circuit. ............... 38
Correct Wave Form at Demodulator Test Jack ............... 41
Out-of-Phase Wave Form at Demodulator Test Jack. ............ 41
Top View of Model 147 Chassis ..................... 44
Bottom View of Model 1~47 Chassis .................... 45
Transistor Locations on Printed Circuit 76 ............... 46
Capacitor & Diode Locations on Printed Circuit 76. ........... 46
Resistor Locations on Printed Circuit 76 , ............... 47
Resistor 6 Test Point
Locations
on Printed Circuit 76. ......... 47
Resistor b Test Point Locations on Printed Circuit 74, Bottom Face ... 48
Component Locations on Printed Circuit 74, Top Face. .......... 48
Resistor & Test Point Location on Printed Circuit 75
Capacitor & 'Transistor Locations on Printed Circuit 75
Resistor Locations on RANGE Switch (5102). ...............
..........
.........
49
49
50
Resistor Locations on RANGE Switch (S102). ............... 50
i
0477
iii
SPECIFICATIONS
MODEL 147
SPECIFICATIONS
iv
0477
MODEL 147 NULL DETECTOR
GE:NLSAL DESCRIPTIOX
SECTION 1. GENERAL DESCRIPTION
l-1.
electronic null detector.
x
source resistance and 10 nanovolts with a
a power sensitivity of 3 x lo-21 watt.
GENERAL.
The Keithley Model 147 is designed specificaLly RS a col,vcnicnt self-contained dc
a.
Its sensitivity is 0.6 x LOT3 microvolt per mi.llimrter or 0.03
10-e
microampere per millimeter.
Its resolution is better tllnn 3 nanovolts with a LO-oll:il
300-ohm
source resistance.
This corresponds Lo
Zero shift is less than 15 nanovolts for sourcf
resistance changes from 0 to 300 ohms. Line frequency r~!jcction is better than 5OOO:l 011
its most sensitive range.
The Model 147 has 16 ranges from 30 nanovolts full scale to 1.00 millivolts on a
b.
zero-center meter. Meter accuracy is f2% of full scale on ~11 ranges.
For reliable and versatile use, the Null Detector is of solid-state design, cxccpi
c.
for the first two input stages. It has high line isolation - lOl(J ohms - and battery
or ac power line operation.
0572
FIGUPJ? La.
Front ,>anel,.
I
GENERAL DESCRIPTION
MODEL 147
l-2.
grounding problems.
FEATURFS,
Battery operation permits complete isolation from ac power lines, eliminating many
a.
Battery operation also allows flexibility and convenience in use.
The Model 147 automatically recharges the battery if needed when the powercord is connected.
As an electronic null detector,
b.
the Model 147 is immune to mechanical vibrations.
It will also recover from a Z-volt overload on its inost sensitive range in less than 20
seconds.
c. Besides performing as a null detector, the Model 147 can also be used as a 2% direct-
reading nanovoltmeter.
The Null Detector has an output of ;tl volt at up to 1 milliampere for full-scale
d.
meter deflections to drive a recorder or oscilloscope. Output accuracy is ;tl% of full
SCSlS.
e. A zero suppression circuit,
furnishing up to 100 microvolts, permits measuring small
changes in a larger dc signal or compensating for thermal emf's.
l-3.
APPLICATIONS. (Also see Section 3.)
The Model 147 is designed specifically as a null detector. It has sufficient sensi-
a.
tivity to be used in most applications with all commercially available potentiometers, in-
cluding 6-dial models, ratio sets and resistance bridges, including Wenner, Wheatstone and
Kelvin Double Bridges. It can be used to make 4-terminal measurements.
Keithley's Model 147 is more sensitive than the finest galvanometer systems. It is
b.
also immune to mechanical vibrations,
thus eliminating the need for shock-free mountings.
Additional advantages over galvanometer systems'include the ability to recover from 2-volt
overloads in 20 seconds,
much less off-null loading, plus considerably faster speed of
response.
0572
MODEL 147
CKNERAL DESCKIPTION
0572
GENERAL DESCRIPTION MODEL 147
FIGURE 2. Front Panel Controls.
0572
MODEL 147 NULL DlXI:C'l'OR
SECTION 2.
2-l.
line and the POWER SUPPLY Switch is in tile AC or OFF position,
charge current determines its brightness.
charged.
FRONT PANEL CONTROLS.
a. AC CONNECTI?D Lamp. l:hc Lamp is lit whcncvcr the unit is connected t0 i.lll xi. pwur
BATTERY CHARGING Lamp. When lit,
b.
POGJIZR SUPPLY Switch.
c.
AC position: The Null Detector will opcratc from the nc power Lint.
1.
battery will be charged if needed; then,
(See Figure 2.)
this Lanq indicates the ibattery ins clrnr:ini:.
Ihe Switch controls tlrc mode of opcratiori for tile powel- suppI!:.
OPERATION
NOTE
If the lamp is noi l~it,
the BATTEQY CIIARGINC Lamp will lil:ht.
tllc" LllC lbilCLtdl.\ ii
'TIIC
p1,:
OFF position:
2.
charged,
3.
power line is internally disconnected, whether or nut tile power cord is coluieclcd;
the AC CONNECTiXD Lamp is off; the battery cannot be charged.
4.
the Model I.47 shows the state of the battery charge directly on its mctcr.
circuits within the instrument are the same as for battery operation exccpl at Lllc
meter terminals.
POWER SUPPLY
Switch Setting Connected
if nccdcd and if the power cord is connected.
BATI'ERY position: The Null Detector is operating from its battery.
BATT. TEST position:
AC
OFF
BATTERY
BATT TEST
The Model 147 is not operating. Howcvcr ,
When the POWER SUPPLY Switch is held in this posil:ioii,
Power Cord AC CONNKTED
I
Yes
YCS
NO
Yes
NO
Yes
NO
Lamp
0”
On Oil
Off
Off
Off
Off
Off
I
tlic battery wil~l lbc
'I‘liC ni'
Al I
BATTERY CIIARGINC La"{'
(If battery is charging)
on
Battery CllnnOt bc cli;lr::c!d
Battfry cannot IbC char;:ed
Battery cannot hc chnr~cd
Battery can,,ot bc cllar~:cd
LIattcry cannot be charged
TABLE 1.
relationship between the front panel lamps,
Switch setting.
0365
Indicating Lamps and POWER SUPPLY Switch Settings.
The Table SllOWS tile
the power cord and the I'Oli'ER SUl'1~'I.Y
OPERATION
MODEL 147 NULL DETECTOR
RANGE Switch.
d.
The RANGE Switch selects the full-scale meter sensitivity (either
microvolts or millivolts) for one of eight ranges, from 0.03
FUNCTION Switch.
e.
The FUNCTION Switch selects the function - MICROVOLTS or HILLI-
VOLTS - which is to be measured.
ZERO SUPPRESS Controls.
f.
The COARSE Control disconnects the suppression circuit (in OFF position) or
1.
Two controls determine
the
amount of zero suppression.
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.
It adjusts the range between the positive and
negative values of the maximum voltage set by the COARSE Control.
INPUT Receptacle. The INPUT Receptacle is of a special low-thermal design.
g.
only the Models 14.81, 1482, 1486 and 1488 for mating connectors.
to
100.
Refer to Table 3.
Use
FIGURE 3.
Model 147 Rear Panel Controls and
to Replaceable Parts List and schematic diagrams.
2-2.
RTtiR PANEL CONTROLS AND CONNECTIONS.
Line Voltage Switch. The screwdriver-operated slide switch sets the Model 147 for
a.
117 or 234-volt ac power lines.
Fuse.
b.
For 117-volt operation, use a 3 AG or MDL Slow-Blow l/@ampere fuse.
2.
For 234-volt operation,
2.
Power Cord. The 3-wire power cord with the NEMA approved 3-prong plug provides a
c.
ground connection for the cabinet.
use only a MDL Slow-Blow l/16-ampere fuse.
An adapter for operation from Z-terminal outlets is
provided.
6
Connections.
Circuit designations refer
0365
MODEL 1~47 NULL DETCC'TOR
A note above the power cord shows the ac power line i:rcqucncy for
which the rejection filter is ad.justcd. 'The instrument will work
at any line frequency from 50 t" 1000 cps,
at the indicated frequency.
filter circuit.
NOTE
but ac rejection is bcsc
Paragraph 2-18 dcscribcs adjustirl~; lilf
DEMODULATOR TEST Jack.
d.
A phone jack provides access to LllE denl"dLll~ator ior ti.5:
purposes.
f.
OUTPUT.
'The OUTPUT Rcceptaclc provides
iL
volt at
one! mill.iampcrc f:or a
iilll.-scale
meter deflection on any range.
GND and LO Tcrmiasls.
f.
third wire of the power cord.
'The ground terminal. (GND) i.s conncctcd to tllc chassis ;~II)~! ! 1)~.
The low terminal is connected t" circui.t ground and cl!&:
low side of the input connection.
2-3.
MODE 01: OPERATION.
battery. b-or illost "SfS,
the ac power line will create ground loop or isolation problems.
The Model 147 operates cithfr irom an ac power line or irom iis
it functions well from ac. Use battary operation, llo~ifver, i~1~
1solnfi"n irun low to
ground is complete for battery operation when tllc power cord is disconnccl:cd; i,t is
greater than lOlo ohms wit11 the power cord connected.
Al.so use battery operntiiln it>
reduce the 8-cps ripple which may appear at the output with i-11" input shorted in i&c
operation.
See paragraph Z-16.
NOTE
Before using the battery operation, tll"r"ugllly read paragraph 2-4.
Improper battery "pcration can damage the battery pack and lead to
inaccurate measurements.
2-4. UXTERY OPERATION.
The Model 1.47 is supplied with a rcchargcablc h-volt,
ba:tery pack (Model 1489).
Recommended:
secutive hours without recharging.
Do n"t usf the battery more tlliln cigirt COII-
At this discharge rate,
4 ampere-ilour Iliclr~l-~~l(llnilIr!l
the battery sl10u1d last abOUt
1000 recharge cycles.
NOTE
Permanent damage to the battery pack occurs if it is used for more
than 16 consecutive hours without recharging.
rate, the recharge cycles are greatly reduced.
Model 147,
Cheek the battery charge before making a measurement.
b.
in the BATT. TEST position.
check the state of the battery charge.
The minimum acceptable charge is a meter indi.cntion oi ,8;
full. charge is shown by the BATTERY CHARGING Lamp not being lit.
At this discharge
Bflorf usi~ng tile
Hold the POWER SUPl'LY Switch
Recharge il needed.
Otherwise, battery operation is the same as for the ac power line operating mode; rcI:cr
to paragraph 2-5.
0365
OPERATION
When the battery is used beyond its capacity, two effects aye seen.
There is a shift in zero offset from ac to battery operation.
the power supplies do not regulate and high ripple voltages appear
at the supply outputs.
MODEL 147 NULL DETECTOR
Also,
(See paragraph 5-8.)
To recharge the battery,
c.
POWER SUPPLY Switch to AC or OFF.
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 at night.
d.
Leave the instrument always connected to the ac power line; then turn the POWEK SUPPLY
Switch to OFF at night. After a fully charged battery is used for eight consecutive
hours ) it will recharge within 16 hours.
effect on the isolation: 1010 ohms with the POWER SUPPLY Switch in BATTERY position and
the shorting link between GND and LO Te?.minals disconnected.
2-5. OPERATING PROCEDURES,
Set the front panel controls as follows:
a.
POWER SUPPLY Switch
FUNCTION Switch
RANGE Switch
ZERO SUPPRESS COARSE Control
Make sure the ZERO SUPPRESS COARSE Control if OFF. If it is not, a
suppression voltage is introduced, causing an error in measurements.
connect the power cord to an ac power line.
The BATTERY CHARGING Lamp will light.
Leaving the power cord connected has little
OFF
MILLIVOLTS
100
OFF
NOTE
Turn the
The battery will
Connect the voltage swrce or null circuit to the INPUT Receptacle.
b.
graph 2-6 for suggestions.
Check the voltage shown on the rear panel Line Voltage Switch; connect the Model 147
c.
to the ac pcwer 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, asp will the
BATTERY CHARGING Lamp if the battery is being charged. If the circuit low is to be at
ground, put the shorting link between the LO and GND posts on the rear panel.
Turn the POWER SUPPLY Switch to the desired mode of operation, AC or BATTERY.
d.
Increase the Model 147 sensitivity until the meter shows the greatest on-scale de-
e.
flection.
1. Check the source resistance to make sure it is within the maximum value specified
for the range being used.
sitive ranges is exceeded, the Model 147 may not measure within its specifications.
2. Zero offsets seen when the Zero Suppress Controls are off will vary with the
quality of the circuit's thermal construction,
Shorting Plug is connected to the Model 147 INPUT Receptacle, offset should be less
than 0.3 microvolt.
8
(See Table 2.)
If the maximum resistance for the more sen-
See paragraph Z-14. When a Model 1488
Refer tO para-
046613
OP1:llATION MODEL I,47 NULL I1lYl'ECTOR
Make sure the signal is greater than Johnson noise in the source resistance (par-
1, ,
agraph 2-12 ).
2. Use materials which generate a low thermal emL (paragraph 2-14).
3. Mini.mize temperature changes and thermal gradients (paragraph Z-14).
4 Reduce magnetically induced signals by proper shielding and minimizing experimen-
tal layout area (paragraph 2-15).
5. Eliminate ground loops through proper grounding and connection to the signal cir-
cuit (paragraph 2-16).
2-6.
14~7 input is with the Model 1481 Low-Thermal Input Cable,
mcnt Use tile Cable for temporary setups,
for measurements at several points, and
when fast connections are needed.
del 148~1 connects directly to the INPUT
1~eceptacl.e.
sible or where very Sow thermal connections FIGUKC 4. Model 1481 Low-Thermal Input
arc needed, use the Model 1482 Low-Thermal Cable.
Input Cable.
1481 ,
stead of nl,ligator clips. Clean the bare
wire! width a non-metallic abrasive, such as
Scotch Isrite, before making the connection.
illc> Model 1483 Kit, is best.
iron used is clean and that it has not been used with regular solder before.
LOW-THEIMAL INPUT CONNECTIONS.
a Tlie easiest connection to the Model
supplied with the instru-
The Mo-
b. Where more permanent setups are pos-
'The Model 11+82 Low-Thermal Input
It is similar to the Model
except it leas bare copper leads in- leads instead of alligator clips.
('. Si cadmium solder (Model 1503) is used for a connection, make sure the soldering
Cable is similar except it has bare copper
Making crimp connections, as possible with
USC only
rosin solder flux. If possible, heat sink
all cadmium-soldered joints together to reduce generated thermal emf's.
techniques will keep thermal emf's below
0.1 microvolt.
Careful
I'LCUIW 5 .
K in t .
lieTcr to Section 6 for contents.
Model 1483 Low-Thermal Connecti
d. Use crimp connections with copper
wire and lugs for the best low-thermal
joints.
10 nanovolts or less using the copper wire,
sleeves and Lugs found in the Model 1483
Low-Thermal Connection Kit.
tains a crimp tool, shielded cable, an assortment of copper lugs, copper wire, cad-
mium solder and nylon bolts and nuts.
is a complete kit for making very low ther-
mal measuring circuits.
the user 011 the Model 147 to maintain the
Thermal emf's can be reduced to
The Kit con-
It
The Kit enables
0466K
OPERATION
MODEL 14,7 NULL DETECTOR
2-8.
line ground.
2-9.
range is $1 volt at 1 milliampere.
less than 5 ohms within the amplifier pass band.. Output may be used during both ac and
battery operation. If the Model 147 is used for differential measurements, do not ground
the recorder connected to the output.
FMATING OPERATION.
The Model 147 can be connected between two potentials,
a.
It can be floated up to ?4,00 volts off ground.
CAUTION
The front panel controls are electrically connected to the case.
power cord is unplugged,
ground voltage. use necessary e precautions.
For best results with floating operation, follow the steps below:
b.
Remove the shorting link from the LO or GND Post on the rear panel.
1.
Connect the input circuit to the Null Detector.
2.
2-5.
wfth this operation, since the low of the Model 14,7 output is no longer grounded.
better results.
The zero suppress controls may also be used.
If power line frequency pickup is a problem,
3.
KECORDER OUTPUT.
-
The Null Detector output for a full-scale meter deflection on any
the case may be at a voltage equal
Do not ground any recorders used
battery operation usually provides
Accuracy is ?I% of full scale. Output resistance is
neither of which is at power
If the
to
the off-
Operate as described in paragraph
Model 147
P
FlGURF 6. Synchronized Buss System for Model 14,7.
used in one system, an oscillator beat may occur; see paragraph 2-10.
instruments by connecting them as shown.
12
Model 147
V
Model 147 Model 147
v
LOW
When two or m"re Null Detectors are
See Figure 29 for point II.
b
4
Synchronize the
P
4
1
04,67R
MODEL 1.47 NULL DETECTOR
2-10. USING MOIW THAN ONE MODEL 147 IN A SYSTEM.
The Model 1.47 oscillator is adjusted for a nominal freqnency ol 94 cps. ,,Oi.!i~VC~1 ~
a.
slight variations in frcqucncy do occur between models.
tectors in one system,
an oscillator beat may occur.
Wl,en using two or ,morc Xill I ill
Synchronizing oscillators
b.
"ents together at the collector of transistor (219 (Figure 29, Ipoint II), using iii: O.'i"icro.farad myl.ar capacitor.
At times the system is suc11 that the Null Detector lows may not bc conncctc~! di~-~~~t-
c.
ly together.
instruments. A IlO-volt,
farad isolation capacitor in seri~es with iboth pri~mary and secondary wi~ndin~s o! C!IE i~~:i,i::former.
For several Null Detectors connected together, l,sc a synct,r"ni~zcil bliss :;ysli'T!, :iii
d.
shown in Figure 6.
2-11.
other considerations beside tile instrument affect accuracy.
working with higher voltages are very important with mi~crovolt signals.
reads only the signal received at its input; tllcrciorc, it is i~mportant tllat this signal
be properly transmitted from the source.
affect accuracy: thermaI noise
connections.
ACCURACY CONSIDERATIONS. For sensitive measurcmcnts -
Then,
'Table 4 also offers a quick rcfcrence to correct troubles wllich may occ11r.
use a 1:l transformer havi~ng a fairly higil impedance between tllc Cvil
low power isolation transformer works wit.
prevents an objectionable beat. Connect tllc! cwc in:,t iii-
USC n 0..5-:nicr<v
10 mi~llivolts and bt~1lbi.x -
i:l:Ifcts not Iroticcnblc ,.i'!len
Tl,c, ~lodel vi 7
The fol,i~owing paragraphs indi~cate iactor xiii c:.
, input resistance, thermal emi's,
shi~cldilll: iin<l (. il(.l.: *
1. The thermal noi~sc in any ideal resistance can bc dctarmi~ned Srom tiic Joi~nson !~rlis~'
equation:
EZ",, =
where Erms is the rms noise voltage developed across the voltage source;
T is tile temperature in degrees Kelvin;
R is the source resistance in ohms;
F is the ampl~ificr bandwidth in cps;
k is the Boltzmann constant (1.38 x 10e23 joules / OK).
yor an ideal resistance at room temperature (3OO*K), equation 1 simplilics to
Ii,",
Peak-to-peak meter indi.cations are of "ore interest than tlrc rms vaiuc.
2.
mentally,
temperature, equation 2 becomes
where 13
0477
the peak-to-peak Johnson noi.se is about live times the rms value. At r<10111
EPP
is the peak-to-peak noise voltage developed across tile voltag" source.
PP
4 I< T 11 I; 1:q I
= 1.29 x lo-lo (RF)1'2 xi,
i:xpcri -
= 6.45 x 10-l' (RF)"' I:<,
Very slow response time
MODEL 147 NULL DETECTOR
2-13. INPUT RESISTANCE.
obtained using high feedback factors.
physical input resistance is partially destroyed.
exceed the maximum source resistance listed in Table 2.
but noise, offsets,
ranges, the maximum specified source resistance is consistent with Johnson noise considcrations.
2-14.
a.
tween any two junctions of dissimilar metals.
which the Model 147 can measure
b.
Model 147 can have some offset (paragraph 2-5).
touching the circuit, by putting a heat source near the circuit, or by a regular piliter,,
of instability,
light,
THERMAL EMF'S.
Thermal emf's (thermo-electric potentials) are generated by thermal gradients Ihe-
Thermal emf's can cause the following problems:
Meter instability or zero offset much higher than expected.
1.
Meter is very sensitive to ambient temperature differences.
2.
slow response and instability may result.
corresponding to heating and air conditioning systems or changes in sun-
The Model 147 is a feedback amplifier;
When the source resistance exceeds the amplifier's
amplifier input resistance without feedback - the fecdbacl;
Then the instrument may not operate properly.
Iligher resistances can be used,
These can be large compared to the si!;nals
its input resia:nnc<. is
Normally, do not
On the most sensitive
Note, thougll, the
T%is is seen by
To minimize the drift caused by thermal emf's,
c.
the same thermo-electric powers in the input circuit.
have o thermo-electric power within about f0.25 ,rv/oC of copper.
inbalance of 1oC between these metals would generate a thermal emf of about 0.25 microvolt
At the other extreme, germanium has a thermoelectric power of about 320 ,iv/oC, and silicon
will develop about 420 jrv/% against copper.
Standard physical handbooks contain tables
of thermoelectric powers of materials.
Since the Model 147 input circuit is made
of pure copper, the best junction is copper
to copper. HOWeVer, copper oxide in the
junction will cause thermal emf's on the
order of 100 nanovolts per oC or less.
Also, differences in processing of two
pieces of copper can cause thermal emf's
of up to 0.2 microvolt per OC. The Model
1483 Kit contains all necessary equipment
to make very low thermal copper crimp joints.
See paragraph 2-6.
Besides using similar metals, thermal
d.
emf's can be reduced by maintaining constant
temperatures, Keep all circuits from open
windows, fans,
similar sources which vary temperature. Connect leads or lugs as close as possible.
Minimize thermal gradients by placing all Separate only with insulation of high heat
junctions physically close on a large heat conductivity.
sink.
fore making a connection.
Thoroughly clean all copper leads be-
air conditioning vents and
Crimp together
FIGURI? 7.
use the same metal or metals having
Gold, silver and low-thermal soldcr
This means a temperature
COPPER "AI"ERI
L Bw.II BOLT
Thermal Sink Construction.
1S
OPERATION
MODEL 147 NULL DETECTOR
the ends of each copper wire;
bolt the lugs for each connection point together; mount all
stacks of lugs on a thick metal plate having high thermal conductivity. Thermal conductivity between the junctions and the heat sink can be kept at a high level by using mica
washers or high conductivity ceramics for electrical insulation.
Several other techniques will reduce the effects of thermal emf's.
e.
suppression circuit to buckout constant voltages.
If connections must be soldered, use
only cadmium-tin low-thermal solder, such as supplied in the Model 1483 Kit.
metals -
including regular solder -
well-controlled oil bath or a good heat sink is used.
may be used and low thermal emf's obtained if a
Thermal voltages may be calculated
Use the zero
Unlike
from the thermoelectric power of the materials in the junction and the possible temperature difference between the junctions.
z-15.
conductor can produce large signals compared to the instrument's sensitivity.
MAGNETIC SHIELDING.
a. In the
Low
resistance
circuitry
used with the Model 147, magnetic
Lines
cutting a
The amount
of signal developed is proportional to the area enclosed by the circuit and the rate of
change of magnetic flux. For example, motion of a 3-inch diameter Loop in the earth's
magnetic field will induce a signal of several tenths of a microvolt.
size of the Loop or moving it more rapidly will increase the signal.
Increasing the
Magnetic fields
from ac power lines will cause even more difficulty.
To reduce the effect of magnetic fields, use magnetic shielding. Where high ac
b.
magnetic fields are present, it may be necessary to magnetically shield the measuring
circuit , the unknown emf circuit or auxiliary equipment in the circuit. Magnetic shielding is available from
several
companies in the form of plates, foil or cable.
Twist input leads to minimize the area enclosed by the circuit loop,
c.
experimental Layout for minimum enclosed area is also of particular
Z-16.
a.
AC SHIELDING.
Due to its narrow bandwidth, the Model 147 is somewhat insensitive to ac voltages
value.
Planning the
superimposed upon a dc signal at the input terminals. However, ac voltages which are
large compared with the dc signal may drive the Model 147 ac amplifier into saturation,
erroneously producing a dc output at the demodulator.
Usually it is sufficient to connect
the cases of all apparatus in the measurement circuit together and ground at one point.
This provides a "tree" configuration, which minimizes ground
which all shields are connected should be as near as possible
Loops.
to
The common point at
the
circuit
ground of the
Null Detector at its input.
Improper shielding can cause the Model 147 to react in one or more of the following
b.
ways :
Needle jitter or instability,
1.
High offset (dc bias). Changing the power cord polarity or the connection between
2.
from 10% to 20% of full scale.
the LO and GND Posts may affect the amount of offset.
Slow response time, sluggish action and/or inconsistent readings between ranges.
3.
Amplifier saturation. Observe the wave form with an oscilloscope connected to the
4.
DEMODULATOR TEST Jack (Figure 3).
With the input shorted, it should approximate the
16
0466R
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