HHS Publication FDA 88-8264 
Guide for Establishing and Maintaining 
A Calibration Constancy Intercomparison System 
for Microwave Oven 
Compliance Survey Instruments 
Revised March 1988 
US. DEPARTMENT OF HEALTH AND HUMAN SERVICES 
Public Health Service 
Food and Drug Administration 
Center for Devices and Radiological Health 
Rockville, Maryland 20857  PREFACE 
Public Law 90-602,  the Radiation  Control for Health and Safety Act of  1968 (the 
Act),  directs the Department  of  Health  and  Human  Services to evaluate production 
testing and quality control programs carried out by the .industry to assure adequacy of 
safegaurds  against  hazardous  electronic  product  radiation  and  to assure  that  the 
products comply with performance standards. 
Under the Act,  manufacturers of  microwave ovens, a product listed under 21 CFR 
1002.61,  are required to certify that their microwave ovens are in compliance with all 
of the applicable provisions of the Federal Performance Standard for Microwave Ovens, 
21 CFR 1030.10.  In order to comply with microwave emission level provisions of  the 
performance standard,  manufacturers must use properly calibrated microwave leakage 
measurement instruments in their production and quality control testing programs. 
This document has been prepared  in order to assist the microwave oven .manufac- 
turers in establishing and maintaining a calibration constancy intercomparison system 
for  compliance  survey  instruments  and  replaces  guidance  previously  issued  by  the 
Center for  Devices  and  Radiological  Health  (the  Center).  The guide  also  contains 
sample documentation forms for recording calibration data. 
Checks on the calibration constancy and use of  microwave survey instruments are 
two vital parts of  an adequate quality control and testing program.  Please read  the 
guide  carefully.  If  your  instrumentation  program  does  not  include  all  the  aspects 
covered in  the guide, you  should modify  your  program.  Any changes to your quality 
control program  must be reported to the Center using the instructions found in Part 8.0 
of  the Guide  for Preparing  Reports of  Radiation Safety of  Microwave  Ovens,  March 
1985. 
Walter E.  Gundaker 
Director 
Office of Compliance  FOREWORD 
In  October  1982,  the  Food  and  Drug  Administration  established  the Center for 
Devices and Radiological Health (CDRH) by merging the Bureau of Medical Devices and 
the Bureau of Radiological Health. 
The Center develops and implements national programs to protect the public health 
in the fields of medical devices and radiological health.  These programs are intended to 
assure  the  safety,  effectiveness and  proper  labeling of  medical  devices,  to control 
unnecessary human exposure to potentially hazardous ionizing and nonionizing radiation, 
and to ensure the safe, efficacious use of such radiation. 
The  Center publishes  the results of  its work  in  scientific journals  and  in  its own 
technical  reports.  These  reports  provide  a  mechanism  for disseminating 'results  of 
CDRH and contractor projects.  They are sold by the Government Printlng Office and/or 
the National Technical Information Service. 
Also, CDRH technical reports in radiological health are made available to the World 
Health Organization (WHO) under a  memorandum of  agreement between WHO  and the 
Department  of  Health  and  Human  Services.  Three  WHO Collaborating  Centers, 
established under the Bureau of  Radiological Health, continue to function under CDRH: 
WHO  Collaborating Center for Standardization of  Protection Against Nonionizing 
Radiations; 
WHO  Collaborating Center for Training and General Tasks in Radiation Medicine; 
and 
WHO  Collaborating Center for Nuclear Medicine. 
We  welcome your comments and requests for further information. 
/ 
john C.  Villforth 
irector 
for Devices and 
Radiological Health  CONTENTS 
Page 
.................................................... 
Foreword  i 
...................................................... 
Preface  ii 
................................................ 
1.0  Introduction  1 
................................................. 
2.0  Procedures  2 
................................... 
2.1  Preoperational Procedures  2 
............................................ 
2.2  DailyChecks  2 
.... ... 
2.3  Thirty-Day Check: Initial Establishment of the Reference FieId  ;  4 
2.4  Thirty-Day Check: Initial Polarization Response Check 
.............................. 
of the Compliance Instruments  6 
2.5  Thirty-Day Check: Constancy of the Reference Field 
................................ 
and Compliance Instruments 
........................................ 
2.6  Annual Calibration 
...................................... 
2.7  Periodic Recalibrations 
................................................. 
2.8  Repair 
Appendixes 
.  .................... 
1  Calibration Constancy Intercomparison System  11 
.  ................................. 
2  Block Diagram of the System  13 
.  ....................................... 
3  Daily Check Record  14 
.  ....................................... 
4  Renewal of Records  15 
.................................. 
. 
5  12-Month Historical Record  16 
.  ................................ 
6  Nonref erence Field Constancy  17 
.  ............................ 
7  30-Day Instrument Comparison Log  18 
.  ................................ 
8  2 dB Comparison Check Record  19 
.  ............... 
9  Flowchart for 30-Day Calibration Constancy Program  2? 
.....  GUIDE FOR ESTABLISHING AND MAINTAINING 
A CALIBRATION CONSTANCY INTERCOMPARISON SYSTEM 
FOR MICROWAVE OVEN COMPLIANCE SURVEY INSTRUMENTS 
1.0  INTRODUCTION 
Manufacturers of  microwave ovens must use properly calibrated microwave leakage 
measurement instruments in  their production testing and quality control programs to 
assure compliance with  the Federal Performance Standard for  Microwave Ovens (21 
CFR 1030.10). 
As  an alternative to frequent  recalibration  against absolute standards, the Center 
for Devices and  Radiological Health (CDRH)w ill  allow  manufacturers to establish  a 
system  that  demonstrates  that  the  calibration  of  each  instrument  remains  within 
prescribed  limits,  over  time,  subsequent  to  absolute calibration*  by  the  instrument 
manufacturer or other qualified calibration facility. 
- 
This document outlines procedures for  establishing and  maintaining a "calibration 
constancy  intercomparison  system''  (referred  to  as  the  "system"  throughout  this 
document) capable of  assuring adequate constancy of  calibration, while minimizing the 
necessity for absolute recalibration. 
It must be noted and understood that the system described herein is not intended for 
use in calibrating hand-held or automated scanning survey instruments, nor is it intended 
to be  used to transfer calibrations from one instrument to another.  It is only intended 
to provide a means to determine whether an instrument's calibration is changing with 
time, and whether it is continuing to function properly.  The underlying principle is that 
constancy of  calibration may be determined by comparing an instrument's reading with 
its  own  prior  readings  and  with  other  meters'  readings  when  it  measures  a  fixed 
parameter. 
The procedures for establishing and  maintaining a calibration constancy intercom- 
parison  system  consist  of  (1)  maintaining  an  apparatus  or  system  to  perform  a 
calibration constancy intercomparison, (2) documenting the results using a log or record 
to demonstrate  constancy over a period  of  time,  and  (3) adhering to  an  appropriate 
schedule of  checks to assure proper function and calibration constancy of  each instru- 
ment.  Such a schedule of  checks and a simple acceptable calibration constancy inter- 
comparison system are described below.  Briefly, the calibration constancy intercom- 
parison system consists mainly of: 
(1) a microwave source, 
(2) a radiated power monitor (RPM), 
(3) an anechoic environment with a radiator and probe-holding fixture, and 
(4) a local calibration reference (LCR). 
A  more  detailed description is given in Appendixes 1 and 2.  A  system using this 
apparatus has been demonstrated  to be  accurate and practical  to reproduce and use. 
Any  calibration  constancy  intercomparison  system  that  can  be  shown  to  provide 
comparable performance (at least within the same limits as the system described above) 
would be acceptable. 
*Absolute calibration (i.e.,  cafibration against absolute standards) means calibration of  a 
system in which all measurement parameters are traceable, with specified uncertainty, 
to the absolute standards maintained by the National Bureau of  Standards or equivalent. 
1  2.0  PROCEDURES 
2.1  PREOPERATIONAL PROCEDURES 
2.1.1  Requirement 
Preoperational procedures,  as  described  by  the  manufacturers  of  the calibration 
constancy intercomparison system and the survey instruments,  must be performed prior 
to use to assure that the system and survey instruments are functioning correctly and 
properly adjusted. 
2.1.2  Instrumentation and Method 
Various  preoperational  procedures  are described  in  the  manuals  provided  by  the 
manufacturers of the calibration constancy intercomparison system and survey instru- 
- 
ments.  For example:  - 
(1) a suitable period of time must be allowed after turning on the system and survey 
instruments for nwarm up1' or stabilization; 
(2) survey instruments' probe cones (spacers) should be checked daily and replaced if 
dirty or worn; 
(3) battery-operated  survey instruments must have their batteries checked thoughout 
the day; 
(4) charger covers must  be in place. on  the survey instruments  with  rechargeable 
batteries; 
(5) the  voltage  supply  for the AC-voltage  powered  instruments  must  be  checked 
periodically; 
(6) the survey instruments and system should be checked for any evidence of physical 
damage, and, if damaged, the entire instrument and/or system must .be checked 
prior to each oven survey; 
(7) baseline  or "zero1'  reading on  the survey  instruments  and/or  system  must  be 
checked prior to each oven survey; and 
(8) all other preoperational procedures specified  in  the owner's  survey  instrument 
and/or system manuals must be followed. 
2.2  DAILY CHECKS 
2.2.1  Requirement 
The polarization response of each survey instrument must be checked daily prior t.0 
its  use  to  determine  any  gross  shifts  in  response  which  could  indicate  instrumen'f 
damage. 
*..  2.2.2  Instrumentation and Method 
Make a check of  the survey instrument's polarization response using an adequately 
stable microwave field source.  This microwave field should be linearly polarized, have 
the  correct  frequency, and  cause at least  a  mid-scale  meter  reading on  the survey 
instrument.  A field setting (or power density) of approximately 1.0  mW/cm2 can be used 
for generating a mid-scale  meter reading.  However, different levels may be necessary 
for other specialized  meters  such  as probe/pre-amp  units from the automated  oven 
survey systems (scanners).  Specifications for performing polarization response measure- 
ments should be obtained from the survey instrument manufacturer.  A  probe-holding 
fixture must be provided to keep the probe on a constant axis while the probe is slowly 
rotated (approximately 2 revolutions per minute) about the axis normal to its detection 
plane (usually the handle axis).  Because unnecessary changes in the probe orientation or 
position may alter the measurement of  polarization ellipticity, the probe fixture should 
prevent  any unnecessary horizontal,  vertical, or transverse  motion while the probe  is 
being rotated (see Figure 1).  - 
Figure 1.  Probe rotation should prevent any horizontal, vertical, 
or transverse motion while the probe is being rotated 
.  Because each survey instrument has its own  ellipse of  polarization response  (also 
known as polarization  ellipticity error), the percentage  or polarization error of  each 
instrument must be calculated to determine whether each instrument is operating within 
the limit of  the manufacturer's specification. 
A common way to find the percentage of  polarization ellipticity error is as follows: 
(1) place the probe in  the fixture and  turn on  the microwave power  source so the 
survey meter reads a mid-scale value (for example 1.0  m W/cm2); 
(2) rotate the probe 360 degrees and note the minimum and maximum readings and 
calculate the mean (see Equation 1); and 
(3) calculate the percentage of  polarization ellipticity error using Equation 2.  The 
result  must be equal to or less than the percentage  error limit of  polarization 
ellipticity specified by  the instrument manufacturer (e.g.,  +5  percent, or a total 
of  10 percent).  Any instrument with a polarization ellipticity error greater than 
the allowed tolerance must be repaired or discarded. 
Ano:her  way to find the percentage polarization ellipticity error is as follows: 
(1) place the probe in the fixture and turn the power on so the survey meter .reads a' 
mid-scale value of  1.0  mW/cm2; 
(2) rotate the probe 360 degrees and find the minimum reading only;  (3) change the power setting until the survey meter reads 1.0  mW/cm2 at the mini- 
mum polarization response; 
(4) rotate the probe again to obtain the highest polarization response reading; and 
(5) subtract  1.0  from  the highest  polarization  response reading  and  multiply  the 
result by  100.  The result  must  be equal to or less than  the total percentage 
polarization  ellipticity  error  specified  by  the  instrument  manufacturer  (see 
Equation 3 below). 
+ 
Maximum  Minimum 
Mean = 
- 
Maximum  Minimum 
% Total Polarization Ellipticity Error = 
Mean 
Note: to get  percent, divide Total by 2 
_+ 
%Total Polariuztion Error = (Maximum - 1.011 00  (3) 
' 
For  example,  a  total  error for this procedure  of  10 percent  is  equivalent  to the 
survey  instrument  manufacturer's  specification  of  f5 percent  polarization  ellipticity 
error.  The daily  check  method  applies  to  instruments  employing planar  detectors. 
Alternate methods for other types of detectors may need to be developed. 
2.2.3  Documentation 
A separate "daily check" record should be kept for every survey instrument for com- 
pliance testing on the production line and audit.  The record should include:  the model 
and  serial  number  of  the  instrument  (probe  modevserial  number  if  necessary),  the 
percentage error in  polarization ellipticity specified by the instrument  manufacturer, 
the results of the test, and the name of the person who performed the test.  If  an instru- 
ment fails the test, the disposition of  the instrument should be clearly indicated in the 
"re marks" column.  Following the repair, the instrument's record must be renewed (e.g., 
started over).  See Appendix  3 for an example of  the documentation; any equivalent 
chart is acceptable.  Appendix 4 of  this guide contains a chart which describes briefly 
when and under what circumstances certain records must be renewed. 
2.3  THIRTY-DAY  CHECK:  INITIAL ESTABLISHMENT OF THE REFERENCE FIELD 
2.3.1  Requirement 
A reference field must be established prior to checking the 30-day  calibration con- 
stancy of  every instrument (including spare scanner probes or hand-held  survey meters) 
used for microwave oven compliance determinations. 
2.3.2  Instrumentation and Method of Establishing the Reference Field Using the LCR 
One instrument suitable for compliance determinations is set aside and not used for 
production or quality control t,esting.  It is retained as the local calibration reference 
(LCR), having been calibrated by its manufacturer or other suitable source.  This instru- 
ment  is  set aside  to minimize  the probability  of  damage  and/or  loss  of  calibration  through accident or the wear and tear of normal use.  It is recommended that the instru- 
ment selected as the LCR have a power density calibration which is near the geometric 
mean  of  the  power  density  calibrations  of  all  instruments used  for  compliance 
determination (e.g.,  if  the spread of  calibration factors is .98  to 1.08,  then the LCR 
should have a factor near 1.03).  It is also recommended that the LCR be of the same 
brandmake as the survey instruments being checked. 
An LCR is not to be misconstrued as a transfer standard.  If its calibration is 41 dB, 
it cannot be used as a transfer standard for instruments whose accuracy is also fl dB. 
Using the calibration constancy intercomparison  system,  an initial  reference field  is 
established with the probe of the LCR positioned at the mean of its polarization ellipse. 
Set the field at a power density of approximately 1.0  mW/cm"s  read on the LCR.  The 
mean of the polarization ellipse of the LCR must be located by finding the mean of the 
maximum and minimum readings when the LCR probe is slowly rotated 360 degrees in 
the calibration constancy intercomparison  system  chamber.  When  the LCR  probe  is 
positioned in the system at the mean of its polarization ellipse, the power density can be 
adjusted (if  necessary) to the appropriate reference field  (usually  1.0  m W/cm2).  The 
values of  the reference field should  be specified  by  the instrument  manufacturer or 
calibration facility.  After setting the reference field,  the percentage of  polarization 
error of the LCR must be checked to verify that it meets the instrument manufacturer's 
specification.  When the reference field has been set up using the LCR,  it is important 
that the readings on the radiation power monitor (RPM), (e.g.,  Forward, Reflected, and 
Difference (Net)) or from another power meter are recorded and noted on the log record 
(Appendix 5). 
2.3.3  Instrumentation and Method of Establishing the Reference Field Using the RPM 
(Optional) 
An alternative method for establishing the reference field can be performed by using 
the radiated power monitor (RPM).  After the LCR  has been calibrated by  its manu- 
facturer or other suitable source, use the LCR to find the mid-scale value (e.g.,  obtain a 
1.0  mW/cm2 midpoint scale reading on the LCR, then record the RPM readings).  For the 
subsequent  monthly checks, the reference field can be set by using the RPM to repeat 
the same reading obtained the first month.  When the reference field has been set up 
using the RPM, it is important that the readings from the LCR are recorded and noted 
on the log record (Appendix 5). 
2.3.4  Documentation 
A "12-month  historical record"  should be kept on the reference field  instruments. 
Both the LCR readings (Minimum,  Maximum,  and  Mean readings) and RPM  net power 
readings (Forward, Reflected, and Difference) are to be recorded and noted as reference 
readings.  Other items such as ambient temperature and the name of  the person who 
performed the tests should also be recorded.  If  the LCR fails the polarization ellipticity 
check, the disposition of the LCR should be clearly indicated in the "remarks"  column. 
Following the repair of  the LCR,  the 12-month historical record and all other records 
must be renewed (see Appendix 4).  See Appendixes 5 and 6 for example::  of  the docu- 
mentation; any equivalent charts are acceptable.  2.4  THIRTY-DAY  CHECK:  INITIAL POLARIZATION RESPONSE CHECK OF THE 
COMPLIANCE INSTRUMENTS 
2.4.1  Requirement 
After the reference  field  has been  established at a specified  power density (for 
example 1.0  mW/cm"  as described in Part 2.3.2  or 2.3.3,  the polarization response of 
each survey instrument to be used for compliance testing must be initially checked.  The 
polarization  ellipticity  error  of  each  survey  compliance  instrument  must  meet  the 
instrument  manufacturer's specification. This initial check must be performed prior to 
initial use on the production line,  when it is taken out of  spare stock, or after it has 
been returned from repair. 
2.4.2  Instrumentation and Method 
Follow the same procedures outlined in Part 2.2.2  above.  - 
- 
2.4.3  Documentation 
For each instrument used  in ~roductiono r in audit,  a separate "30-day  instrument 
de 
comparison log"  record  should  maintained.  This log should include the following. 
model  and  serial number  of  each instrument  (and  probe  model  and serial  number  if 
necessary);  date;  the  minimum,  maximum,  and  mean  of  the  polarization  response 
readings;  percentage  of  polarization  ellipticity  error;  the  highest  and  lowest  mean 
readings since the last calibrationhepair; and verification that each instrument met the 
polarization  ellipticity  error limit specified by  the instrument  manufacturer.  If  the 
instrument's polarization ellipticity error is outside the specification, the disposition of 
the instrument should be clearly indicated on the record.  See Appendix 7 for an exam- 
ple of the documentation; any equivalent chart is acceptable. 
2.5  THIRTY-DAY CHECK:  CONSTANCY OF REFERENCE FIELD AND COMPLIANCE 
INSTRUMENTS 
2.5.1  Requirement for the Reference Field 
The reference field must be established for the initial setup of the system and survey 
instruments (see Part 2.3).  For the subsequent monthly checks, the power density of the 
reference  field  in  the  calibration  constancy  intercomparison  system  should  be  re- 
established by adjusting the microwave power source to repeat either the previous LCR 
or RPM reference reading.  After the reference field is reset using one of  these two 
readings, the other reading (from the RPM or LCR) must agree with all of its preceeding 
readings within 10 percent.  That is,  no two of  these nonreference readings,  from one 
annual calibration or repair to the next, may disagree by more than 10 percent. 
For example,  if  the reference field is set using the reading of  the LCR,  then the 
monthly RPM (Diff) nonreference readings (ail readings accumulated since the last LCR 
calibration) must be verified to be within 10 percent of  each other.  If  the reference 
field  is set  using  readings  of  the  RPM,  then  the  monthly  LCR  nonreference  mean 
readings (all mean readings accumulated since the last LCR calibration) of  the polari: 
zation  ellipse  must  be  verified  to be  within  10  percent  of  each  other.  If  the' 
nonreference readings disagreed by more than 10 percent, then the nonreference  instru- 
ment must be repaired (or discarded).  See Appendix 6 for an example of the documen- 
tati on.  Following these act ions,  new 12-month historical records (Appendixes 5 and 6), 
and all other records, must be renewed (see Appendix 4).  2-5.5  Requirement for Each Compliance Instrument 
After each compliance survey instrument  receives its initial check of  the polari- 
zation  ellipse  (see  Part  2.4.1),  the  mean  reading of  each  instrument  must  then  be 
compared with all of  its own previous monthly mean readings for constancy within 10 
percent.  For the comparison,  use only those  mean readings taken since the survey 
instrument was last calibrated (or repaired),  or from when the LCR (or RPM) was last 
calibrated, whichever is most current. 
For those survey instruments falling outside the 10 percent limit, possible sources of 
the failure must be traced.  First,  the survey instrument itself  should be checked for 
obvious problems such as  weak  batteries.  Second,  the system  should  be  examined. 
Problems with equipment must be adequately resolved before proceeding further.  If  the 
problem  was traced  to the survey  instrument,  then  the  survey  instrument  must  be 
repaired (or discarded) and the 30-day  instrument comparison log (Appendix 7) and daily 
check record (Appendix 3) for that compliance survey instruments must be renewed.  If 
the problem was traced to the system, then the system must be repaiped and all records 
must be renewed.  See Appendix 4 for further explanation regarding reiiewal of records. 
2.5.3  Requirement for Calibration Data Constancy 
The least sensitive and most sensitive survey instruments (including the LCR) must 
not  disagree  by  more  than  2  dB,  including the extremes of  polarization  ellipse.  A 
monthly.2 dB check must be performed by comparing the ratio of  the highest polariza- 
tion  ellipse  reading from  all of  the instruments  (including  the  LCR)  to  the lowest 
polarization ellipse reading from all of  the instruments (including the LCR).  This ratio 
must not exceed  1.59  (2 dB).  Do not  include those instruments that did not pass the 
daily check  or the 10 percent  comparison' in the 30-day  check.  The 2-dB  check of 
instruments each month is done on a cumulative baiis; that is, the comparison period is 
the time between LCR calibrations (or repairs). 
If,  during the comparison check,  the ratio exceeded 1.59  (or 2 dB), then the entire 
system,  including  the  LCR and  individual  compliance  survey  instruments,  must  be 
examined and repaired as necessary.  Repeat the 10 percent comparison check of  the 
individual survey instruments (see Part 2.5.2).  Problems with all of the equipment must 
be adequately resolved  before proceeding further.  Then repeat the 2 dB  comparison 
check to determine that  the ratio is less than or equal to 1.59  and  ensure  that  any 
remaining defective instruments are found.  Following these actions, then a new 2 dB 
comparison check record (Appendix 8) and most or all other records must be renewed 
(see Appendix 4).  Appendix 9 illustrates the proper 30-day constancy check sequence in 
the form of a flowchart. 
2.5.4  Documentation 
Two separate records  must  be  maintained.  The first record,  "30-Day  Instrument 
Comparison Log" (Appendix 7), should be kept for every instrument used on the produc- 
tion line and  in  audit.  The 30-day  instrument comparison log should verify  that the 
highest and lowest mean readings of the individual instrument are within 10 percent of 
each other.  If  not, the disposition of the instrument should be clearly indicated on the 
records.  Following these actions, two new records, one for recording results of the daily) 
check (Appendix 3) and a second, the 30-day  instrument comparison log (Appendix 7), 
must be started (see Appendix 4).  The second separate record, "2  dB Comparison Checktt (Appendix 8),  should be kept 
to monitor the calibration data constancy of all the instruments used on the production 
line and in audit since the last LCR calibration.  Exclude those instruments that did not 
pass the daily check, and/or the 10 percent comparison in the 30-day check.  This record 
should include the highest and lowest polarization response readings taken from all of 
the instruments (including the LCR).  These values should be compared and the ratio of 
the highest polarization ellipse reading to the lowest polarization ellipse reading during 
the entire period should be less than 1.59 (2 dB). 
If the ratio exceeds 1.59,  make a complete check of the entire system and record the 
reason for failure.  If  the failure is traced to a defective individual survey instrument, 
the survey instrument must be taken out of production or audit facility for repair.  Then 
continue to use the same 2 dB comparison check log.  When the repaired survey instru- 
ment comes back,  renew the 30-day  instrument comparison log (Appendix 7) and daily 
check record (Appendix 3) for the survey instrument.  If  the problem  is traced to the 
system, then a new 2 dB comparison check record and all other records must be started 
following correction of  the problems (see Appendix 4).  See Appendix 8 for an example 
of the documentation; any equivalent chart is acceptable. 
2.6  ANNUAL CALIBRATION 
2.6.1  Requirement 
Absolute calibration of the LCR must be performed annually.  There'may be a need 
for annual absolute calibration of  compliance survey  instruments,  scanner probe/pre- 
amp, radiated power monitor (RPM), power meter, etc.,  depending upon the recommen- 
dations of the instrument manufacturer. 
It is important that the microwave oven manufacturer check with the instrumenta- 
tion manufacturer for specifications or recommendations concerning annual calibration. 
Whenever the LCR has returned after being calibrated and/or repaired, it is important 
to immediately determine its percentage of polarization ellipticity error and record the 
results (see Part 2.3). 
2.6.2  Instrumentation and Method 
The LCR  must be returned to the instrument manufacturer or other qualified cali- 
bration facility for absolute calibration (12-month  intervals).  Some compliance survey 
instruments or other electronic components of  the system also may  require  absolute 
calibration annually. 
2.6.3  Documentation 
A separate record of  the LCR calibration and repair should be kept.  This log should 
include model and serial number, calibration date,  and a list of  any repairs and their 
dates.  The 12-month historical record (Appendix 5) of the LCR is renewed every time 
the LCR hrs been returned from annual calibration.  The 30-day  instrument comparison 
log of  each compliance survey instrument (as well as scanner probe/pre-amp  units)  is 
renewed when the LCR has been returned from annual calibration.  2.7  PERIODIC RECALIBRATIONS 
2.7.1  Requirement 
The absolute .calibration of all instruments should be performed periodically. 
2.7.2  Instrumentation and Method 
In accordance with good engineering and manufacturing practice, it is recommended 
that all instruments used  to make compliance determination measurements (with the 
exception of  LCR and  RPM) be  returned to the instrument  manufacturer  factory or 
other qualified calibration facility for absolute calibration at least once every 3 years. 
2.7.3  Documentation 
See Parts 2.2.3 and 2.3.4  - 
- 
2.8  REPAIR 
2.8.1  Requirement 
Any instrument which has been found to.be damaged or failed to remain within its 
established constancy limits must not be used in the quality control and testing program 
until  it  has been  repaired  and  recalibrated  by  an instrument  manufacturer or other 
qualified calibration and repair facility.  Before repaired instruments are used  again, 
their operational functions and their polarization respons6s must be checked (see Part 
2.2).  Following these actions, the instrument's records must be renewed (see Appendix 
4).  It is recommended  that repairs be performed only by the instrument manufacturer 
or other qualified calibration facility unless the microwave oven manufacturer has in 
place an adequate repair facility to perform certain minor repairs (such as broken probe 
cables,  defective  meters,  etc.)  and  has  the  capability  of  checking  their  results. 
Microwave  oven  manufacturers  will  be  required  to submit  specific  technical  repair 
procedures to CDRH for review before implementing them. 
2.8.2  Instrumentation and Method 
Whenever the compliance survey meters,  LCR,  or scanner probe/pre-amp  units are 
. 
returned from the instrument manufacturer after repair, their polarization ellipse must 
be  checked  against  the  instrument  manufacturersf specifications and  verified  on the 
record prior to their use. 
2.8.3  Documentation 
The disposition of the defective instrument can be recorded in the "remarks" column.   APPENDIX 1 
CALIBRATION CONSTANCY INTERCOMPARISON SYSTEM 
A block diagram of  the calibration constancy intercomparison system is provided in 
Appendix 2.  The parenthetical numbers preceding each paragraph below and used within 
those paragraphs refer to the numbers in the Appendix 2 diagram. 
(1)  SOURCE 
The source is a microwave generator with (a) an appropriate power output (probably 
approximately 0.5  to 50 Watts maximum output) that may be variable (either internally 
or by some external attenuator) and (b) a frequency in the relevant ISM  band (915 5 25 
+ 
MHz or 2450  50 MHz).  Depending on the radiator or anechoic enclosure and on the 
inherent  frequency stability of  the source,  some control  over  the source output fre- 
quency may be necessary.  The source might be a magnetron or klystron oscillator, or 
might  be  a  low-powered  source amplified,  e.g.,  by  a traveling wave  tube  amplifier. 
However,  any type of  source with adequately stable or controllable output parameters 
could be used. 
(2), (3), (4)  DIRECTIONAL COUPLERS 
The directional couplers are used to sample a fixed fraction of  the power flowing 
through them in either the forward 'or reverse direction.  This fraction should be chosen 
so as to deliver a signal to items (5),  (6),  or (7) which is of a magnitude appropriate to 
the  specific  device.  Furthermore,  this  fraction  (or  ffcoupling factor")  must  be 
adequately  invariant  with  frequency  over  the  range  of- frequency  variations  of  the 
source.  Two of  these couplers might be replaced  by  a single bi-directional  coupler if 
desired.  Furthermore, if  the source has some built-in  frequency monitor, (4) might be 
eliminated.  Couplers (2) and (3) should be adequately insensitive to power flowing in a 
direction opposite  to that  intended  to be  measured  (i.e.,  these  couplers  should have 
adequate "directivityf').  It is not absolutely necessary that directional couplers be used, 
however some means of  differentiating between forward and reflected power (to allow 
determination of net radiated power) must be included. 
(S), (6)  POWER METERS 
These power  meters must provide repeatable, adequately sensitive, and reliable re- 
sponses to both absolute levels and relative variation of  power.  They should be  rela- 
tively  insensitive to frequency variation.  It  may  be  necessary  to insert  attenuators 
between the directional couplers and the power meters to bring the input to the power 
meters into an appropriate range.  It is not absolutely necessary that power  meters be 
used for these applications.  Whatever type of detector is used, some reliable means of 
calibration must be employed in an annual calibration.  In the system illustrated, if  the 
source can  be demonstrated to be  adequately stable,  a single pqwer  meter  might  be 
alternatively switched to the output of  the couplers (2) and (3) by  means of  a coaxial 
switch.  The power meter or meters and directional couplers form the radiated power 
monitor (RPM). 
(7)  FREQUENCY MEASURING DEVICE 
Some means must be  employed to verify that the frequency of the radiated signal 
remains within appropriate limits.  These limits are closely related to the quality of  the  anechoic environment (9).  If standing waves of considerable magnitude exist within this 
environment,  a change in the frequency of the radiated signal can cause a variation in 
the spatial distribution of the field and have a significant  effect on the reading of  a 
stationary probe in the fixture (10).  Thus,  the choice of  both source (1) and anechoic 
environment  (9)  must  be  made  with  this phenomenon  in  mind.  Although  frequency 
counters are often available in microwave laboratories, any of several other possibilities 
might be employed (e.g.,  a tunable cavity might be used with a crystal or power meter 
monitoring the output). 
Clearly, the use of  an anechoic environment of  appropriate characteristics with an 
adequately stable source can eliminate the necessity to employ the frequency measuring 
device shown in the illustration. 
(8)  RADIATOR 
Some means of radiating the generated power into space must be provided. :A mong a 
multitude of  possible devices that  might  be 'employed  for this purpose  are horn,  log 
periodic and dipole antennas, as well as flared or simply open-ended  waveguide sections. 
Considerations in the choice include directivity of the radiator vs.  the reflectivity and 
absorption of the anechoic environment, as well as the frequency response of  the radi- 
ator vs.  the source frequency stability.  If  a stable source is to be employed, a wave- 
guide or coaxial tuner may be employed to improve the system match to the radiator to 
minimize reflected power. 
(9)  ANECHOIC ENCLOSURE 
An anechoic environment should be provided to minimize the effects of  the labora- 
tory environment on the measurement.  The quality and size of  this anechoic environ- 
ment will determine the magnitude of the standing wave in the region of  the instrument 
under comparison. 
(10)  PROBE-HOLDING  FIXTURE 
A stable fixture to hold the instrument being evaluated in a rigid, repeatable position 
must  be  employed.  This  fixture  should  be  made  of  an  electrically  non-conductive 
material to minimize standing waves and perturbations of  the fields.  In  general,  the 
greater the magnitude of the standing wave and the closer the probe to the radiator, the 
more critical are the repositioning requirements of this fixture.    APPENDIX 4 
IZENDWAL OF RECORDS 
Documentation Forms 
Appcndix 3  Appendix 5  Appendix 6  Appendix 7  Appendix 8 
Daily Check  12-Month  Nonreference  30-Day Instrument  2 dB Comparison 
Status  Record  Historical Record  Constancy  Comparison Log  Check 
Daily Check (2.2) 
Failed survey instruments  Renew  Same  Same  Same  Same 
, 
- 
LCRIRPM Monthly (2.3.2  2.3.3) 
Polarization failure of LCR  Renew  Renew  Renew  Renew  Renew 
Failed RPM  Renew  Renew  Renew  Renew  Renew 
30-Day Check of Individual 
Survey Meters (2.4.1,2.5.2) 
Polarization failure  Renew  Same  Same  Renew  Same 
10% comparison failure  Renew  Same  Same  Renew  Same 
P 
VL 
30-Day Check of 
Nonreference Meter (2.6.1) 
10% comparison failure  Renew  Renew  Renew  Renew  Renew 
2 dB Comparison (2.5.3) 
System or LCR failure  Renew  Renew  Renew  Renew  Renew 
Survey meter failure  Renew  Same  Same  Renew  Same 
LCR Calibration (2.6) 
Annual calibration  Renew  Renew  Renew  , ,  Renew  Renew 
Numbers in parentheses refer to sections of this report.  APPENDIX 5 
12-MONTH HISTORICAL RECORD 
Name of constancy system:  Make of LCWRPM: 
Date of last LCRIRPM calibration:  Model number of LCWRPM: 
Serial number of LCR: 
Probe model number on LCR: 
Probe serial number on LCR: 
LCR Polarization Ellipticity Readings 
RPM  RPM  RPM  LCR  LCR  LCR  LCR  Ambient 
Date ,  Fwd  Rev  Diff  Max  Min  Mean  % Ellip  Temp  Remarks 
NOTES:  (1) The LCR must be positioned at its mean polarization response prior to setting the reference field. 
'  (2) If the reference field is set using the LCR readings, then the monthly RPM (Diffl readings must be within 10%. 
,(3) If the reference field is set using the RPM readings, then the monthly LCR (Mean) readings must be within 10% 
(4) If the readings taken from the nonreference instrument disagreed by more than lo%, then this instrument must be repaired 
(or discarded). Following these actions, a new Daily Check Record (Appendix 3) and all other records, must be renewed (2.5.1). 
+  - 
Maximum  Minimum  ~aiirnum Minimum 
Mean =  % Tota 1 Pola rizatwn Ellipticity Error = 
2  Mean 
% Total ellipticity error limit specified  by LCR instrument manufacturer is:  APPENDIX 6 
NONREPERENCE FIELD CONSTANCY 
The monthly reference field is set by (check one only):  LCR  RPM 
If you selected the LCR to set the monthly reference field, use column 1 only. 
If you selected the RPM to set the monthly reference field, use column 2 only. 
Column 1  Column 2 
Monthly  (A) Highest  (B) Lowest  (A) Highest  (B) Lowest  Less than 1.l o? 
Date  RPM Diff (or P)  RPM Diff (or P)  LCR Mean  LCR Mean  A/B  (YesINol* 
* NOTES:  If the reference field is set using the reading of the LCR, then the monthly RPM (Dim nonreference readings (all readings 
accumulated since the last LCR calibration) must be verified to be within 10 percept. of each other. If the reference field is set 
*:  using readings of the RPM, then the monthly LCR mean nonreference readings (all mean readings accumulated since last 
LCR calibration) of the polarization ellipse must be verified to be within 10 percent of each other. If the nonreference readings 
failed the 10 percent comparison test, then the nonreference instrument must be repaired and recalibrated.  Following this 
action, all records must be renewed.  .paMauaJ aq ?snw pJoaaJ s!q? uaqJ  '(pap.ms!p JO) pa?e.lq!leaa.r put? pa.I!odaJ aq Isnu ?!  'a~g~ajsa!p Ja ?aur X a ~ ~anqs?j I (E) 
'~a~n?38jnu?~uuarur nJ?su! aq? ;Cq pag!mds ?!ur![ Joua X?!x?d!lla uo!?t?z!~elodl elo?% aq? waur ?snw luaurn.wu! qxr3 (z) 
.uo!?e~qge3a 37 pnuuw uaam?aq lt?,walu! ay? Bu!.Inp (IsaMolo?? saq8!q) a8ue.1 %OI u!q?!~aJ r, s8u!pt?a~u vaw ay? ~ eLyp ~a (~ I)  :S~J,ON   APPENDIX 9 
FLOWCHART FOR 30-DAY CALIBRATION C.O NSTANCY PROGRAM 
. 
Start new records 
I  1 
Check polarization ellipse of LCR 
+ 
> 
, 
Correct or repair 
Polarization OK? 
No  & recalibrate 
t  I 
yes 
LCR probe positioned to read  - 
--  - 
Set field inside system (usually 1 m~lcrn'). 
Record LCR and RPM settings. 
I 
C 
or RPM reference  RPM 
I 
All previous RPM DlFF 
readingswithin lo%? 
Yes  Yes 
Set field (usually 1 m~lcm~C)h.e ck polarization of each 
compliance meter. Record MAX, MIN. & MEAN. 
I 
Calculate % ellipticity error.  I 
I 
, 
- 
No 
Repa~or r replace 
instruments 
C 
rument mean reading within 10%  No 
of own highest reading since  I 
last LCWRPM calibration? 
Yes 
If problem found 
All instrument readings, including  No  within system or 
LCR. within 2 dB since last LCR  )  reference instru-  j individual 
calibration?  ment, have it 
repaired or replaced 
<  . 
Yes  L 
1 
Has LCR been calibrated 
in last 12 months? 
\ 
Yes