HP 11941A Quick start guide

11941A
CLOSE-FIELD PROBE
July 1987
Copyright © Hewlett-Packard Company
1212 Valley House Drive
Rohnert Park, CA 949284999 U.S.A

CERTIFICATION
Hewlett-Packard Company certifies that this product met its published specifications at the time of ship¬
ment from the factory. Hewlett-Packard further certifies that its calibration measurements are traceable
to the United States National Bureau of Standards, to the extent allowed bv the Bureau's calibration facili-
n. and to the calibration facilities of the other International Standards Organization members.
WARRANTY
This Hewlett-Packard instrument product is warranted against detects in material and workmanship for
a period of one year from date of shipment. During the warranty period, Hewlett-Packard Company will
at Its option, either repair or replace products which prove to be defective.
For warranty service or repair, this product must be returned to a service facility designated by HP. Buyer
shall prepay shipping charges to HP and HP shall pay shipping charges to returri the product to Buyer.
However, Buyer shall pay all shipping charges, duties, and taxes for products returned to HP from another
country.
HP warrants that its software and firmware designated by HP for use with an instrument will execute its
programming instructions when properly installed on that instrument. HP does not warrant that the opera¬
tion of the instrument, or software, or firmware will be uninterrupted or error-free.
LIMITATION OF WARRANTY
The foregoing warranty shall not apply to defects resulting from improper or inadequate maintenance by
Buyer, Buyer-supplied software or interfacing, unauthorized modification or misuse, operation outside
of the environmental spiecifications for the product, or improper site preparation or maintenance.
NO OTHER WARRANTY IS EXPRESSED OR IMPLIED. HP SPECIFICALLY DISCLAIMS THE
IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
PURPOSE.
EXCLUSIVE REMEDIES
THE REMEDIES PROVIDED HEREIN ARE BUYER’S SOLE AND EXCLUSIVE REMEDIES HP
liable FOR ANY DIRECT, INDIRECT, SPECIAL, INCIDENTAL, OR CONSE¬
QUENTIAL DAMAGES. WHETHER BASED ON CONTRACT, TORT. OR ANY OTHER LEGAL
THEORY.
ASSISTANCE
Product maintenance agreements and other customer assistance agreements are available for Hewlett-
Packard products.
For any assistance, contact your nearest Hewlett-Packard Sales and Service Office. Addresses are provid¬
ed at the hack of this manual.

CONTENTS
1- 1 . General Information ..* ^
1-2. Description ... ^
1-4. Accessories Supplied. ^
1-6. Options .. ^
1-11. Initiallnspection. ^
2- 1 . Specifications. ^
3- 1. Operation . ^
3-2. Theory of Operation.... ^
3-4. Antenna Factors. ^
3-7. Power Density and Electric Field Calculations.. 4
3-9. Localized Field Source. ^
A 3-11. Test Setups. ^
3- 1 6 . Measurement Techniques. 8
4- 1. Operation Verification..
4- 5. Calibration Cycle..
4-8. Output Impedance Measurement Test ..*.10
4-9. Insertion Loss Measurement Test .1^
5- 1 . Replaceable Parts . 1 ^
5- 3. SMA Adapter Replacement Procedure.lb
5-4. SMA Rotary Joint Installation Procedure (Option 001) .16
5-5. Ordering Information. 1 ^
5-7. Direct Mail-Order System.1^
6- 1 . Factory Returns . 1 ^

SAFETY SYMBOLS
The following safety symbols are used throughout this manual and in the instrument. Familiarize
yourself with each of the symbols and its meaning before operating this instrument.
A
Instrurtion manual symbol. The instrument will be marked with this symbol
when it is necessary for the user to refer to the instruction manual in order to
protect the instrument against damage. Location of pertinent information
within the manual is indicated by use of this symbol in the table of contents.
Indicates dangerous voltages are present. Be extremely careful.
^UTiOIM 1 CAUTION sign denotes a hazard. It calls attention to a procedure which, if
correctly performed or adhered to, could result in damage to or destruction
of the instrument. Do not proceed beyond a CAUTION sign until the indicated
conditions are fully understood and met.
rWARNING I WARNING sign denotes a hazard. It calls attention to a procedure which,
^. I if correaly performed or adhered to, could result in injury or loss of life.
Do not proceed beyond a WARNING sign until the indicated conditions are
fully understood and met.

HP 11941 OPERATION NOTE
1-1. GENERAL INFORMATION
1-2. Description
1-3. The HP 11941A Close-Field Probe is a hand-held electromagnetic field sensor designed to make
repeatable, calibrated magnetic field measurements from 9 kHz to 30 MHz. It is optimized for the location
and measurement of sources of electromagnetic interference (EMI) in electronic equipment The HP
11941A can also be used as a localized magnetic field source when operated in conjunction with an
appropriate signal generator, thereby allowing circuit-board-level susceptibility testing of instruments.
1-4. Accessories Supplied
1-5. The following three accessories are included with the probe:
• One 2m (6.6 ft.) double-shielded RF cable with two SMA (male) end connector, HP Part Number
8120-4811
• One Type N (male) to SMA (female) adapter, HP Part Number 1250-1250
• One BNC (male) to SMA (female) adapter, HP Part Number 1250-1700
The double-shielded cable (RG-223), reduces measurement errors in high field impedance environments by
minimizing current leakage through the braided coaxial outer conductors. For less demanding applications,
the use of a more flexible single-shielded cable type Is sufficient (e.g., RG-58, RG-174).
1-6. Options
1-7. Three options are available for the HP 11941 A.
1-8. Option 001 includes a factory-installed SMA RF Rotary Joint connector, HP Part Number 0955-0323,
Check Digit 5, (See figure 17.) The connector increases handling flexibility of the probe by removing any
rotational torque from the interconnect cable. The rotary joint is especially useful when making
measurements using the double-shielded cable. Section 5-4 includes instructions on installing the rotary joint
1-9. Option 003 consists of the HP 11941A Close-Field Probe without the double-shielded cable or the two
adapters.
1-10. Option 910 includes an additional Operation Note.
1-11. Initial Inspection
1-12. If the shipment contents are incomplete or damaged, notify the nearest Hewlett-Packard office. If the
shipping container is damaged, or the cushioning material shows signs of stress, notify the carrier as well as
the Hewlett-Packard office. Keep the shipping materials for the carrier’s inspection. The HP office will
arrange for repair or replacement without waiting for a claim settlement
1-13. See Paragraph 4-1 for Operation Verification.
1

HP 11941 OPERATION NOTE
2-1. SPECIFICATIONS
2>2. Specifications for th~e HP 11941A are listed in table 1. An asterisk (*) placed after an entry indicates a
supplemental characteristic. Supplemental characteristics are not specifications, but are characteristics
included as additional information for the user.
2-3. Figure 1 illustrates the characteristic frequency response for the HP 11941A Close-Field Probe. See
section 3-4, Antenna Factors, for additional information.
3-1. OPERATION
3-2. Theory of Operation
3-3. The HP 11941A is a balanced magnetic-field sensor that provides an output voltage proportional to the
strength of the magnetic field at its tip. Radiated magnetic fields couple directly to the HP 11941A and
produce an output signal; radiated electric fields also couple to the HP 1194 lA^ but do not produce an
output voltage due to the construction of the probe. The HP 11941A uses a dual-loop sensor and a balun to
reject electrically-coupled signals. These loops are illustrated in figure 2, which shows the probe with its
non-removable protective tip cover cut away for clarity.
The probe does not rectify the input signal; it provides both amplitude and frequency information. It is
ideally suited for use with a spectrum analyzer. The HP 11941A circuitry is passive and therefore only
contributes thermal noise to any measurement system. Inspect the probe tip for damage before using, and
do not use the probe if the protective tip cover is missing or damaged.
3-4. Antenna Factors
3-5. The specific antenna factors of each HP 11941A are measured to within ±2 dB at five frequencies: 9
kHz, 100 kHz, 1 MHz, 10 MHz, and 30 MHz. This calibration Information is listed on the bottom cover of
each probe. The antenna factors are given in the units of dB(uA/m/uV). To calculate the magnetic field
strength in dB(UA/m) at a specific frequency, add the amplitude of the measured voltage in dB{uV) (as
indicated on the spectrum analyzer) to the antenna factor for that specific frequency. To compensate for
cable losses and preamplifier gains, add the losses and subtract the gains (in dB). Refer to the following
equation: _
H-Field Strength (dB = Vsa + AF + K
V
m
where
VsA = voltage in dBuV measured on analyzer
AF = HP 11941A Antenna Factor in
K = loss or gain in dB between HP 11941A and spectrum analyzer
(Positive value if loss, negative value if gain.)
2

HP 11941 OPERATION NOTE
Table 1. HP 11941A Specifications
3

HP 11941 OPERATION NOTE
3-6. Use the data included in figure 1 for interpolating antenna factors between the five calibration
frequencies included in the probe’s calibration table.
3-7. Power Density and Electric Field Calculations
3-8. The HP 11941A measures magnetic field strength. An estimate of the magnitude of the associated
electric field can be made if a field impedance is assumed. An example is given below using a 377-ohm field
impedance. Radiated power density can also be estimated, assuming that the fields are in phase and that the
field impedance is 377 ohms.
E-Field Strength (dB Vsa + AF + K + 20 log(377)
” *** AF + K + 51.5
Power Density 7 ^
Vsa +AF + K -104.3
EXAMPLE:
\ cm^
where
VsA = voltage in dBfxV measured on analyzer
AF = HP 11941A Antenna Factor in
K * loss or gain in dB between HP 11941A and spectrum analyzer
(Positive value if loss, negative value if gain.)
At 10 MHz, measured Vsa “ 65.3 dB/itV. Let K * 0 and AF * 39
then
E 65.3 +39+ 51.5= 155.8 dB^V/m ^ 61.7 V/m
s(dB^)= 65.3 + 39- 104.3 = 0dB2^ = 1 ^
\ cmy cm^ cm*
3-9. Localized Field Source
3-10. The probe can also be used as a localized magnetic field source for locating susceptible components
on a circuit board. It is a reciprocal device: that is, a current fed into the probe will generate a magnetic
field at the dual-loop sensor. The specified antenna factors cannot be used to estimate the magnitude of the
output field. Table 2 lists characteristic field strengths developed 1 mm (0.040 in.) away from the probe tip,
using an incident power of 0 dBm into 50 ohms.
CAUTION
Power applied to the probe must not exceed the maximum Input power
listed In table 1 or damage to the probe will result.
4

HP 11941 OPERATION NOTE
DUAL LOOPS
Figure 2, Magnetic Field Orientation for Maximum Coupling
Table 2. Characteristic Magnetic Field Strengths
Frequency
(MHz)
Input Power
(Into son)
Magnetic Field Strength
(fi), dB (pA/m)
0.009
0 dBm
122
0.100
0 dBm 120
1 120
10
0 dBm
no
30
OdBm
101
3*11. Test Setups
3*12. The HP 11941A operates as either a localized magnetic field probe or localized magnetic field source.
The following paragraphs describe different applications and test setups for the probe. Table 3 lists the
recomrhended test setup equipment used in these examples. Any equipment that satisfies the critical
specifications listed in table 3 may be substituted for the recommended equipment When used as a probe,
be sure to add the antenna factors and cable losses to the signal levels indicated on the spectrum analyzer.
If a preamplifier is used to increase measurement sensitivity, subtract its gain from the displayed signal
level
I WARNING I
The dielectric material on the tip of the probe has a characteristic
breakdown voltage of 1 kV. To prevent damage to the probe and/or
personal Injury to the user, the probe should not be used where a
potential of over 1 kV Is available.
5

HP 11941 OPERATION NOTE
3-13. Localized Emissions Test Setup: figure 3 illustrates a typical configuration used for localizing EMI
emissions. See section 3-16 for proper probe placement
HP 8530A
SPECTRUM ANALYZER
DEVICE UNDER TEST
00^0 GO aoooD
■ ■■ Qoaoa nn
• “ • ■■ nt-ir-inn “ “ ♦
O B *0
HP 11941A
AMPLIFIER
(RECOMMENDED)
OUTPUT
Figure 3. Localized Emissions Test Setup
Table 3. Recommended Test Setup Equipment
instrument
Critical Specifications
Recommended Model
Spectrum Analyzer
Frequency Range:
9 kHz-30 MHz
HP 8562A (100 Hz-2.9 GHz)
HP 8566B (100 Hz-300 GHz)
HP 8567A (10 kHz-1.5 GHz)
HP 8568B (100 Hz-1.5 GHz)
HP 8590A (10 kHz-1.5 GHz)
Option H51, Close-Field EMC Feature Set
HP 71 lOOA (100 Hz-2.9 GHz)
Option 002
Tracking Generator
' Mt-.
Frequency Range:
9 kHz-30 MHz
HP 8444A (0.5-1500 MHz)
Option 059. For use with HP 8566B,
HP 8567A, and HP 8568B
HP 70300A (100 Hz-2.9 GHz)
For use with HP 71 lOOA
Signal Generator
Frequency Range:
9 kHz-30 MHz
HP 8660C (10 kHz-2.6 GHz)
HP 8662A (10 kHz-1280 MHz)
Preamplifier*
Frequency Range:
9 kHz-30 MHz
RF Gain:
>20 dB
HP 8447A (9 kHz-50 MHz)
Option H64
HP 8447F (9 kHz-1.3 GHz)
Option H64
* An HP 85685A RF Preselector may be substituted if an HP 8566B, HP 8567A, or HP 8568B spectrum analyzer
is used.
6

HP 11941 OPERATION NOTE
3-14. Stimulus-Response Test Setup: The relative shielding effectiveness of various enclosures and
shielded structures can be tested using stimulus-response techniques. See figure 4. The tracking generator
output signal radiates from an antenna placed inside the enclosure under test, while the probe and spectrum
analyzer provide frequency and relative amplitude information. See section 3-16 for proper probe
placement.
MODULAR SPECTRUM ANALYZER
3-15. Localized Susceptibility Test Setup: Used as a field source, the probe provides a magnetic field for
susceptibility testing of electronic devices. The distance between the probe tip and the Device Under Test
(DUT) should be less than 1 cm (0.394 in.). As the probe is moved closer to the DUT, the absolute magnetic
field strength applied to the DUT approaches the calculated value produced at the probe tip. For
broadband susceptibility testing, use the probe with a swept or tracking source. See figure 5 for an example
of the basic localized susceptibility test setup.
CAUTION
Power applied to the probe must not exceed the maximum input power
listed in table 1 or damage to the probe will result.
SIGNAL GENERATOR
P.C. BOARD
Figure 5. Localized Susceptibility Test Setup

HP 11941 OPERATION NOTE
3-16. Measurement Techniques
3-17. The HP 11941A measures the magnetic field strength at the center of its sense loop, not at the probe
tip. Figure 6 details the relationship between the probe tip cover and the probe loop centerline. The probe is
designed to be held very close to potential radiating points. The plane of the probe tip loops must be
perpendicular to the radiating magnetic field as illustrated in figure 2. For example, if the seam of an
enclosure is being tested for EMI, hold the probe as illustrated in figure 7. When measuring the radiation
from a coaxial cable, position the probe as in figure 8, which shows the sense loops. The side of the probe
with the antenna factor calibration label should be facing away from the cable being measured. Do not use
the probe if the protective tip cover is missing or damaged.
3-18. The probe may be used as a source in susceptibility testing, as described in section 3-9 and also in
paragraph 3-15. In this application, hold the probe within 1 cm (0.394 in.) of the electronic circuit
component being tested.
Figure 6. Probe Tip Detail

HP 11941 OPERATION NOTE
ANTENNA
FACTOR
LABEL
Figure 8. Testing a Coaxial Cable
9

HP 11941 OPERATION NOTE
4-1. OPERATION VERIFICATION
4-2. Operation Verification for the HP 11941A consists of two tests: an output impedance measurement test,
and an insertion loss measurement test. Each test is relative and designed to detect damage in specific parts
of the circuitry. These tests are not designed to verify antenna-factor calibration, which requires a
calibrated magnetic field.
4-3. Probes that fail these tests may still detect electromagnetic radiation. In this case, the probe circuitry
no longer rejects electric field coupling and the antenna factors listed on the unit are no longer valid.
4-4. In the test procedures, instrument softkeys will be represented by print. Softkeys are keys
whose function is defined by text on the display.
4-5. Calibration Cycle
4-6. Field-site recalibration of the HP 11941A antenna factors is not possible. However, the probe’s antenna
factors can be recalibrated at the factory. A three (3) year recertification cycle is* recommended. Refer to
section 6-1 for factory recalibration information.
4-7. The HP 11941A contains no internal user-accessible adjustments or parts. The SMA adapter (see figure
16) is the only external replaceable part
4-8. Output Impedance Measurement Test
Description
This test measures the impedance of the HP 11941A at the SMA port over a frequency range of 100 kHz
to 1 MHz, to detect any damage in the output circuitry. Any equipment that satisfies the following critical
specifications for the test setup may be substituted for the recommended equipment:
Frequency Range
Test Port Match
. 100 kHz to 1 MHz
>15 dB Return Loss
Recommended Equipment
Network Analyzer...HP 3577 A
S-Parameter Test Set.... ... HP 35677A
Adapters:
Type N (m) to APC 3.5 (m)...HP 1250-1743
Accessories:
50 ohm Load, APC 3.5 (f)
Coaxial Short, APC 3.5 (f)
Open, APC 3.5 (f).
HP 909D, Option on
.HP 1250-1769
.HP 1250-1767
10

HP 11941 OPERATION NOTE
Procedure
1. C onnect the equipmelit as shown in figure 9. Allow the HP 3577A 60 minutes of warm-up time.
HP 3577A
NETWORK ANALYZER
d
HP 35677A
S-PARAMETER
TEST SET
I
PROBE
fllSAlA n
mn-
OPEN SHORT REF
LOAD
Figure 9. Output Impedance Test Setup
2. S et the HP 3577A controls as follows:
TRACE 1
INPUT . . . S I I
DISPLAY FUNCTION....POLAR
SMITH CHART....ON
FULL SCALE...1 UNIT
SWEEP TYPE.
SWEEP MODE
START FREQ
STOP FREQ . .
Lin. Freq. Sweep
....... Cont.
. 100 kHz
. 1 MHz
amptd
+15 dBm
3. P erform the full one-port calibration by pressing the Measr Cal key and On& Cxxt, Follow
the directions on the display.
4. C onnect the probe to Port One of the HP 35677A. The impedance display should appear as shown in
figure 10. At 100 kHz the measured impedance should approximate that of a short circuit; at 1 MHz the
probe should measure inductively, with a maximum normalized impedance value of j0.50. The most
common failure mode for this test would be an open in the input circuitry, in which case the impedance
display would appear as shown in figure 11.
11

HP 11941 OPERATION NOTE
Figure 10. Impedance Plot of Functional HP 11941A
(100 kHz to I MHz)
Figure 11, Impedance Plot of HP 11941A with Open Circuit
(100 kHz to 1 MHz)
12

HP 11941 OPERATION NOTE
4-9. Insertion Loss IVleasurement Test
Description
The insertion loss test is a relative indicator of damage in the input circuitry. Using a second HP 11941A as
a magnetic field source, this test selectively injects a magnetic field into each loop of the HP 11941A being
tested. Damage is indicated by differences in the measured output voltages. It is important to orient the two
probes properly as shown in figures 12 through 15. To facilitate testing, the source probe should be fixed in
place, the tip flush with the work surface (preferably wooden). This can be accomplished with clamps or
tape, which should be no closer than 50.8 mm (2 in.) to the source tip. Other test equipment may be
substituted if it meets or exceeds the critical specifications for the instruments listed below. Throughout the
test procedure, the probe being tested will be referred to as the DUT (Device Under Test).
Recommended Equipment
Network Analyzer...t . ..HP 3577A
S-Parameter Test Set. . .. HP 35677A
Field-Source. HP 11941A
Adapters:
Type N (m) to APC 3.5 (m) (2 required).HP 1250-1743
Cables:
SMA (m) to SMA (m), 2m (2 required) . ..HP 8120-4811
Procedure
1. Connect the equipment as shown in figure 12. Allow the HP 3577A 60 minutes of warm-up time.
HP 3577A
NETWORK ANALYZER
iPiiini
_2_2
IN R A B
SMA
CABLE
ASSY
PORT
i1
HP 35677A
S-PARAMETER
TEST SET
PORT
#2
SMA CABLE ASSY
L_r
L
' ' ' . ' V
PROBES
i
] vttan
...
SOURCE
/
DUT
SEE DETAIL IN
FIGURE 14.
Figure 12. Insertion Loss Test Setup
13

HP II941 OPERATION NOTE
2. Set the HP 3577A as follows:
INST PRESET
TRACE 1
INPUT.. 521
DISPLAY FUNCTION.Log Mac
REF LEVEL..
... dB/Div
REF POS
100 %
SWEEP TYPE. Log Freq Sweep
SWEEP MODE
ConL
START FREQ . ,Oo kHz
CTrvp HP pr\
oi\Jr rKHy.. MHz
AMPTD .. dBm
3. Physically align the probes as shown in figures 13 and 14. The identification label of the DUT should
face upward This alignment illuminates one of the sense loops of the DUT with a magnetic field.
SOURCE
WORK SURFACE
{PREFERABLY WOODEN)
Figure 13. Probe Orientation for Insertion Loss Test
TOP VIEW
1941A
ae 9 KHz > ao KHz
ZHM OC • ZKH 6 38
SOURCE
Figure 14. Detail of Probe Orientation for Insertion Loss Test
(Measurement of Loop #1)
14

HP 1194! OPERATION NOTE
4. Press the STORE DATA key, then Store & Display.
5. Realign the DUT as shown in figure 15, The calibration label of the DUT should face upward. This
position illuminates the second sense loop of the DUT with the same magnetic field intensity as the first
loop.
6 . Press the STORE DATA key and Store_^J^g 01 Then press the INPUT key and Vqtcumg^
The two displayed traces will typically be <3 dB apart for properly functioning probes, and >6 dB apart for
damaged probes. If your measurements are repeatedly yielding results between 3 dB and 6 dB, then the test
setup should be checked
TOP VIEW
Figure 15. Detail of Probe Orientation for Insertion Loss Test
(Measurement of Loop #2)
5-1. REPLACEABLE PARTS
5-2. The SMA adapter located at the end of the probe is the only replaceable part on the HP 11941A (see
figure 16), It protects the probe’s integral connector from mechanical damage and should be left on. Should
the end of the adapter become worn or damaged through use, it can be replaced using the procedure
outlined in paragraph 5-3.
SMA Adapter.HP Part Number 1250-1158
Check Digit 3
Loctite 222®.HP Part Number 0470-0573
Check Digit 9
15

HP 11941 OPERATION NOTE
5-3. SMA Adapter Replacement Procedure
SMA
CONNECTOR
Figure 16. SMA Adapter Location
1. Use a 7/32-inch wrench to remove the old SMA adapter (see figure 16). Turn the adapter
counterclockwise as viewed from the rear end of the probe.
2. Place a small amount of Loctite 222® on two or three end threads of the new SMA adapter.
3. Screw the new adapter clockwise into the probe’s SMA connector.
4. Using a torque wrench, tighten the adapter to 24 inch-pounds.
5-4. SMA Rotary Joint Installation Procedure (Option 001)
ROTATING
Figure 17, RF Rotary Joint Mounted on Probe (Option 001)
1. Use a 7/32-inch wrench to remove the old SMA adapter (see figure 16). Turn the adapter
counterclockwise as viewed from the rear end of the probe.
2. The SMA Rotary Joint has an SMA connector located at each end. One connector rotates with respect
to the body of the rotary joint. The other connector is fixed to the body. Cover two or three end
threads of the rotating end connector with a small amount of Loctite 222®
16