Cal-Chip
Electronics, Incorporated
CHQ S
ERIES
Surface Mount Chip Capacitors:
Ultra High Frequency
High Frequency Measurement and
Performance of High ‘Q’ Multilayer
Ceramic Capacitors
Introduction
Capacitors used in High Frequency applications are generally
used in two particular circuit applications:
• As a DC block providing an AC coupling path between other
components.
• As a shunt path to ground for AC voltages thus providing a
decoupling path.
At very high frequencies much more capacitor design data is
needed by a circuit designer. As well as the normal data relating
to Capacitance and Tan
δ,
‘Q’ and E.S.R. are required. If
RF/Microwave Circuit Simulation aids are being used, then the
designer will require information relating to the 1 Port and 2 Port
parameters, the ‘S’ parameters denoted by S11, S21, S12, S22.
The measurement problem becomes complex because the
resultant measurements should properly describe the parameters
of the multilayer capacitor but be totally uninfluenced by any test
jigs used in the measurement.
The first and extensive part of this measurement sequence
involves the calibration (otherwise known as “de-embedding’) of
all the test jigs. The information on Cal-Chip High ‘Q’ Capacitors
contained in this catalogue has been produced utilizing a Hewlett
Packard Network Analyzer - HP8753A, together with the Hewlett
Packard ‘S’ Parameter Test Set - HP85046A.
Accuracy of capacitor placement relative to the calibration plane
is also critical. For instance, measurements of a capacitor having
a ‘Q’ of approximately 3000 and thus a Tan
δ
of 0.00035 will mean
the phase loss angle will be of the order of 0.02 or restated -89.98
of phase or further restated, real and imaginary ratios approach-
ing 1:3000.
To achieve measurement accuracy, the connections to the
capacitor under test should operate to at least one order better
than this phase angle value. In jigging or mechanical terms
1.00mm of displacement from the correct or calibration plane, rep-
resents 0.1 of phase angle, thus the phase angle errors due to the
jigging etc., should be less than 0.02mm (0.0008”). These calcu-
lations assume a dielectric constant of 1 and a frequency of
100MHz.
Measurement Techniques
Three different Measurement Jig methods have been used:
• The H.P.16091A Co-Axial Test Jig was used to determine:
Capacitance
Tan
δ
‘Q’
E.S.R.
• To stimulate the DC block mode and shunt or decoupling mode,
special Micro-Strip Line Test Jigs were designed and made.
Equipment
The measurement system used comprises a HP8753A Vector
Network Analyzer, HP85046A ‘S’ Parameter Test Set and
HP16091A Test Jig together with the relevant specialist cables,
connectors and Micro-Strip Line Test Jigs.
Measurement Theory
At frequencies above 30MHz, the measurements from convention-
al capacitor bridges become invalid because it is not possible to
maintain a true four-terminal connection to the capacitor under
test, hence phase errors occur and this prohibits the separation of
the resistive and reactive components which need to be measured.
In addition the ‘open’ circuits and ‘short’ circuits used to calibrate
the bridge become degraded. The ‘open’ circuits become capaci-
tive and the ‘short’ circuits become inductive, hence measurement
accuracy is destroyed. However, other measurement techniques
can be used to solve these problems. These techniques use the
behavior of electric ‘waves’ travelling along a transmission line, e.g.
a Co-Axial Cable or a Micro-Strip Line.
If the transmission line is terminated by an unknown impedance,
e.g. the capacitor under test, then a reflected wave is created
which is sent back towards the Test Signal Generator and has a
magnitude and phase angle dependent on the unknown imped-
ance. We now have two waves, travelling in opposite directions,
giving, in effect, the required four terminal connections to the
capacitor, provided only that these waves can be separated out
and independently measured.
This separation is easily possible using variations on standard
Wheatstone Bridge principles. Hence by the measurement of the
magnitudes and phases of three travelling waves, which are
called Scattering of ‘S’ waves, the capacitor parameters can be
calculated.
It should be noted that since these measurements rely on
reflected waves, any changes in physical size, or changes in char-
acteristic impedance between the measurement system and the
points to which the capacitor is connected, will create additional
and unwanted reflected waves, which will degrade the measure-
ment accuracy.
Notes
1) The swept frequency range over which all measurements
were taken was 1MHz to 3GHz with measurements at 10MHz
increments below 1GHz, increments of 50MHz above 1 GHz.
2) For the very low capacitance values, the lowest frequencies
at which sensible data was obtained appeared to be greater than
50 MHz, the data is thus presented.
3) The curves showing the resonant points for the capacitors
have been left in as a guide to these points of resonance.
However, due to the rapid changes in all aspects of the capacitors’
parameters near to the resonant point, such measurements
should be treated with caution. Above resonance the capacitance
curves are dominated by the self-inductance of the capacitor.
H
L
2
L
3
W
L
1
Features:
•
•
•
•
•
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High ‘Q’ Factor at high frequencies
High RF power capabilities
Low ESR
High self resonant frequencies
Excellent stability across temperature range
Small size
26
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