difference in the average thicknesses is less than 25 μmand the
thickness variation within and between specimens is less than
5 % of the average.
6.3 Materials of the specimens may be either homogeneous
or inhomogeneous, single or multiple layered, and conducting
or insulating. Measured SE values of inhomogeneous materials
are dependent on geometry and orientation, and results are less
repeatable than for homogeneous materials.
6.4 Before tests, condition test specimens for 48 h at 23 6
2°Cand 50 6 5 % relative humidity. Tests shall be performed
immediately upon removal from the conditioning environment.
7. Preparation of Apparatus
7.1 An initial check of the specimen holder should be
performed with a time-domain re?ectometer or other suitable
instrument to ensure that a characteristic impedance of 50 6
0.5 ? has been achieved during construction and that this
impedance has not been degraded during shipment or handling.
A time-domain system can give location of a mismatch in
addition to its magnitude.
7.2 Each time the ancillary equipment is connected to the
specimen holder, good practice requires measurement of a
reference specimen to ensure the measurement system is in
proper working order.
7.2.1 The dynamic range (DR) of the system can be checked
by comparing the maximum signal level obtained with a
reference specimen to the minimum signal level obtained when
using a metallic load specimen. The lower limit of the
measurement system sensitivity is a function of the sensitivity
and bandwidth of the receiver. Narrowing the bandwidth of the
receiver lowers the detectable level but increases the measure-
ment time. Leakage causedby connectors or cables may reduce
the DR of the system by providing a parallel signal path that
does not pass through the specimen. If a step attenuator placed
in series with the specimen holder causes a change in the
minimum signal detected that corresponds to a change in
attenuator setting, and if the step attenuator itself does not
cause a leakage path, leakage is negligible and the DR
measured above is correct. If the levels do not correspond, the
attenuation should be increased until a one-to-one correspon-
dence is achieved to determine the DR. Since leakage from a
coaxial connector is determined not only by the quality of the
connector, but also by the amount of torque used in tightening
the connector, connections should be rechecked.
7.2.2 If a standard reference specimen such as gold ?lm
deposited on mylar is available, measurement of its SE value
can provide assurance that the entire system is working
properly. A specimen with the surface resistivity of 5 ?
commonly possess SE = – 326 3 dB. Any other known
specimen may be used to check setup-to-setup repeatability.
7.2.3 Careful handling of the specimen holder and speci-
mens is important.
7.3 Preparation of 7.2 should be run in accordance with
procedures of Section 8.
8. Procedure
8.1 Follow the preparation of apparatus in accordance with
7.2 whenever the measurement system has been recon?gured
or not used for several days.
8.2 Prepare two specimens in accordance with Section 6.
8.3 Determine all frequencies for which SE values are to be
measured. The specimen mounting procedure described in 8.4
requires more time and effort than changing frequency, so it is
more efficient to record values at all frequencies for the
reference specimen, change to the load specimen, and then
record load values at these same frequencies. This procedure
can be automated if a computer and ancillary equipment with
IEEE-488 bus capability are available.
8.4 The procedure for inserting the specimens is as follows:
Use a support structure (a large roll of tape or special stand) to
support the specimen holder in a vertical position. Remove two
nylon screws, turn the holder end for end, remove the other two
nylon screws, and carefully lift off the upper half of the holder.
An indented, soft foam pad is useful for holding this upper half
of the specimen holder while continuing the installation or
removal of specimens. Place the two pieces of the reference
specimen on the ?ange of the bottom half of the specimen
holder ensuring that the disk for the center conductor is aligned
correctly. Use small amounts of transparent tape as needed.
Replace the half of the specimen holder that had been removed
so that the holes for the nylon screws are aligned. Reinstall two
nylon screws. Turn the holder end for end and then reinstall the
other two nylon screws. Reconnect the coaxial cables.
8.5 Measure the received power (or voltage) while using the
reference specimen. Record the measured received values as
P
2
or V
2
values at each frequency.
8.6 Replace the reference specimen with the load specimen.
8.7 Measure the received power with the load specimen.
Record these measured values as P
1
or V
1
values at the same
frequencies used in 8.5. If this value is within 10 dB of the
smallest detectable signal of the measurement system, either
the receiver bandwidth shall be decreasedand the measurement
repeated, or the SE value is beyond the dynamic range of the
measurement system and hence the SE value is reported as
exceeding the DR of the system.
8.8 If the recorded units were watts, use the power ratio Eq
1 to calculate SE. If the recorded units were volts, use the
voltage ratio Eq 2 from 3.2.5 to calculate the SE.
FIG. 2 Illustration of Reference and Load Specimens
D4935 - 10
3
Copyright ASTM International
-
-
`
,
`
,
,
`
`
,
`
,
,
,
,
,
`
`
,
,
`
`
`
`
,
,
,
,
`
,
`
-
`
-
`
,
,
`
,
,
`
,
`
,
,
`
-
-
-