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Keysight Technologies
Connector Pin Recession and its Effect
on Network Analyzer Accuracy
White Paper
Abstract
The apparent effect of a recessed connector upon network
analyzer measurements is often talked about, but without
any quantitative measurements it is very hard to either
predict or correct for the resulting errors. This article outlines
an experiment undertaken to assess the impact on the
measurement of reflection coefficient when using 3.5 mm
connectors.
Introduction
The ideal connector would not produce any discontinuities in the transmission line. In reality,
as a result of their dimensional tolerances, there will normally be some small gap between
mated connectors. This gap is often referred to as recession and it may be both or just one of
the mated connectors which are recessed. The opposite of recession, protrusion, is likely to
cause mechanical damage to 3.5 mm devices unless the mating connector is at least equally
recessed and is undesirable. The electrical effect of recession is to produce a very short section
of line having different impedance characteristics due to the difference in diameter of the center
conductor and the exposed part of the connecting male pin. The theoretical effect of this change
in diameter could be calculated but because there are many more effects in operation with such
a short length of line, it is simpler to assess practically.
Pin recession
Mating end of connector
Pin protrusion
Figure 1. Connector recession defined
This white paper may contain references to HP or Hewlett-Packard. Please note that Hewlett-Packard's former test and
measurement and chemical analysis businesses are now part of Keysight Technologies. Some of the products mentioned
may be obsolete or have been replaced by Keysight products.
Procedure
The measurements were performed using an HP8722D Vector Network Analyzer
over the range 3 GHz to 27 GHz at 1601 trace points and with the bandwidth
set to 300 Hz. Calibration (network analyzer error correction) was performed
using an HP85052B Precision-grade Calibration Kit. Three separate calibrations
were performed; first with the sliding load set to zero recession, another with it
locked at 2.5 thousandths of an inch (0.0025") recession, and finally with 0.005"
recession.
It was important that the characteristics of the item to be measured remained
stable throughout the test, even though the connector's pin recession was
to be varied. The item selected for this task was another 3.5 mm sliding load
(HP911D); the type where the connector recession can be adjusted using
a small screwdriver. This enabled the connector recession to be precisely
incremented from zero to 0.005" in steps of one thousandth of an inch without
damaging any of the components involved in the tests.
Zero recession for both calibration and measurement
0.050
Voltage reflection coefficient
0.040
0.030
0.020
0.010
0.000
5 10 15 20 25
Figure 2. Frequency GHz
Figure 2 forms the reference for the following measurements and is shown
on all plots as the red line. It was performed with both the calibrating and
measured sliding load set to zero recession. This is considered to be the correct
representation of the voltage reflection coefficient (VRC) of the tested sliding
load.
3
Calibration and measurement with equal recession of:
Zero 0.0025" 0.005"
0.035
Voltage reflection coefficient
0.030
0.025
0.020
0.015
5 10 15 20 25
Figure 3. Frequency GHz
When the significance of recessed connectors was more generally recognized, it
was suggested that if the test port of the network analyzer were also recessed
and the calibrating and measured devices had equal recession, then a "good"
measurement could still be performed. Whilst it is no easy task to recess the
network analyzer's test port connector, it is possible to recess the calibration
and measured devices. Figure 3 seems to prove that as long as the recession is
the same during calibration and measurement, the results are very similar. The
small deviations seen (in the order of 0.0025 VRC) could be accounted for by the
unrepeatability of the connectors and possibly because the reference short and
open circuits were not recessed by the same amount as the sliding load.
Zero recession calibration and then
measurement recession of 0, 1, 2, 3, 4, 5
thousandths of an inch
0.050
Voltage reflection coefficient
0.040
0.030
0.020
0.010
0.000
5 10 15 20 25
Figure 4. Frequency GHz
Figure 4 shows an ideal calibration performed with no recession and the mea-
sured load adjusted in 0.001" increments from zero to 0.005". At approximately
22 GHz it is apparent that the predictability of the results changes. As the reces-
sion continues to increase, the VRC reaches a minimum and starts to return
towards its original value.
4
Calibration recession of 0.0025" and then
measurement recession of 0, 1, 2, 3, 4, 5
thousandths of an inch
0.050
Voltage reflection coefficient
0.040
0.030
0.020
0.010
0.000
5 10 15 20 25
Figure 5. Frequency GHz
Figure 5 is similar to Figure 4 but with the errors "reversed". With 0.005"
recession during calibration then zero recession during measurement, the effect
is similar to that for a connector with protrusion (providing the test port has suf-
ficient recession to avoid damage). However, the peaks occur where the troughs
were positioned in the previous chart.
Calibration recession of 0.0025" and then
measurement recession of 0, 1, 2, 3, 4, 5
thousandths of an inch
0.050
Voltage reflection coefficient
0.040
0.030
0.020
0.010
0.000
5 10 15 20 25
Figure 6. Frequency GHz
In Figure 6, recession during calibration was set midway between the extremes
of recession for measurement. This produced a plot that resembles an
amplitude-modulated signal and clearly shows the combination of the plots from
Figures 4/5.
5
Conclusions
The results are modified by approximately the same degree but in the opposite
direction depending whether the calibrating, or measured, sliding load connector
was recessed. The small differences are possibly due to the open and short not
being recessed. In addition, the uncertainty due to use of the connector dial
indicator gauge to set the recession has an effect on measured values.
A Rule-of-Thumb:
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