Basic Investigations for Switching of RD Signals

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Basic Investigations for Switching of RF Signals
Werner Johler, Senior Member IEEE
TYCO ELECTRONICS Logistics AG, Werk AXICOM Au
Seestr. 295, 8804 Au-Waedenswil, Switzerland
werner.johler@tycoelectronics.com
Abstract – A major advantage of electromechanical RF
relays is their capability to carry and to switch signals
from DC up to the GHz range. Up to now the switching
capability of miniature RF relays was limited to a few
Watts, which was not really satisfactory.
Basic investigations were performed to understand the
mechanism when High Frequency signals are switched,
as no data was previously available.
When High Frequency signals are interrupted, the arc
performs like a DC arc due to its thermal characteristics and burns for several periods of the signal. The arc
is an adjustable resistor and just changes the amplitude
of the output signal. When the contact gap increases,
the arc finally extinguishes at current zero in a similar
way as an AC arc. At the time the arc extinguishes the
signal makes a phase shift as the open contact can be
considered to be a capacitance.
Practical tests were performed on a new type of high
performance low-cost ultra-miniature RF relay with
DC loads up to 60 W, low frequency AC loads up to
62.5 VA and RF loads with a maximum power of 37 W.
The results obtained confirm the theoretical approach.
Major advantages of bridge contacts providing two contact gaps in series were found, yielding excellent RF
characteristics as well as outstanding switching performance in all frequency ranges from DC to RF. Even
after 1 million operations no relevant changes of the RF
characteristics were found, although a load of 37 W was
hot switched.
Keywords: RF relay, RF switch, RF switching, hot
switching
I.
INTRODUCTION
In modern telecommunications more and more information
has to be transmitted faster and faster. In order to achieve
higher transmission rates, more bandwidth is required. Although most information is digitalized, the transmission
frequency increases proportionally. For modern radio frequency (RF) and microwave applications as well as for the
related test equipment, suitable switching elements are required for cold as well as hot switching.
Several competitive technologies are used to conduct and
to interrupt RF signals.
A.
Comparison of Switching Technologies
The most commonly used devices are traditional electromechanical RF relays, pin diodes and GaSFET semiconductor switches. Recently RF MEMS have also been discussed but are not commercially available yet for commercial applications.
The advantages of solid-state solutions (GaAs FET and Pin
diodes) are the high lifetime (in a laboratory environment),
small size, high switching speed and relatively low costs.
Electromechanical solutions generally provide better RF
characteristics with higher isolation level and lower insertion loss within the whole frequency range. In addition,
contacts are able to carry a DC signal superimposed on the
RF signal and to transmit higher RF power. They yield true
broadband RF characteristics from DC up to the specified
frequency.
Excellent isolation properties can be achieved regardless of
the ambient temperature since the isolation is made of air
or plastics. Metal contacts provide the lowest possible insertion loss. Furthermore electromechanical RF switches
are able to transmit RF signals without any signal distortion, neither amplitude distortion nor phase distortion and
without any signal modulation, neither cross modulation
nor mutual modulation, regardless of the temperature.
While electromechanical RF relays have traditionally been
large, recently introduced designs, offering the combination
of small physical size and superb RF characteristics, with
isolation values up to more than 80 dB @ 1 GHz, insertion
loss 0.3 dB, and VSWR values less than 1.3, are impressive. In addition to the excellent RF characteristics, RF
relays yield about the same performance as standard telecom relays. Both latching and non-latching switching characteristics are available, requiring a power consumption of
only 140 mW (non-latching) and 70 mW (1 coil latching).
Good mechanical and electrical endurance values and high
dielectric characteristics result in an extremely robust and
reliable design. Standard assembly technologies can be
used such as lead-free SMD soldering.
B.
Limiting Factors for carrying and switching RF
power
Miniature electromechanical RF relays with a size between
0.5 and 4 cm3 typically are able to carry 10 W and to
switch a few Watt of RF power.
The RF power carrying capability of relay contacts is defined mostly by the heat generated when the power passes
the contacts. The heat is mainly generated by intrinsic ohmic loss. As electromechanical relays inherently have low
contact resistance values, the ohmic loss is relatively small.
As shown in [1], the constriction resistance of contacts
depends on the frequency of the signal as skin effects result
in a reduction of the spot diameters. In order to minimize
this resistance, high conductive materials are used, and
even coated with silver. Contacts are gold plated to keep
the resistance low and stable during their entire lifetime.
Depending on the ambient temperature, therefore, only a
certain RF power can be carried, as the permitted tempera-
Proceedings of the 53rd IEEE HOLM 2007 Conference on Electrical Contacts, September 2007
(Winner of Best Paper Award)
ture level of the relay must not be exceeded. Typically this
temperature is 85 °C, but for relays which are suitable for
SMD soldering, a temperatures of 125 °C is possible.
The switching capability of RF relays is determined by the
design, the contact material used and the switching characteristics. Welded contacts or contact rivets – as used for
signal or power relays – cannot be used in RF relays, as the
contacts would generate discontinuities in the signal path
and cause signal reflections. Whenever perfect RF performance is required welded contacts are not a viable
choice. Most designs achieving low signal reflections use
galvanized contacts. As there are only a few alloys suitable
for switching contacts which can be deposited and the
thickness of the precious metal layer (maximum 2…3 µm)
which can be deposited economically is limited, the switching performance especially under load is modest.
Especially when DC loads were switched, only a very limited number of switching cycles were achieved. Until now
no low cost and at the same time high performing products
were available.
II.
SWITCHING LOW LEVEL SIGNALS
A.
Basics
If a relay is to interrupt a few amperes in the total range of
frequencies from zero (DC) to 3 GHz, the mechanism for
current interruption under these conditions has to be the
primary consideration. At any rate, a switching arc is
needed, which is not just an unwanted troublemaker as
some people still believe, it is actually the ideal and unique
switching element. If an arc did not appear spontaneously
when a circuit is interrupted by opening the contacts, the
current would be chopped discontinuously. As a consequence, the magnetic energy LI2/2 stored in the inductance
of the circuit could not be dissipated in the arc but would
have to be converted into capacitive energy CU2/2 in order
to be stored in the stray capacitance C of the circuit. Because of the low value of C, a detrimental switching over
voltage would appear even at quite low current values. Fortunately, the arc appears reliably and solves three problems
at once: first, it limits the overvoltage LdI/dT, necessarily
caused by any current change, to the value of its welldefined arc voltage; second, it converts the magnetic energy of the circuit LI2/2 into heat, while, third, its increasing resistance reduces the current continuously to zero until
eventually the arc disappears when no longer needed - provided it is long enough not to be continuously sustained by
the power source.
B.
DC Interruption
The arc length required to interrupt current reliably depends on both the e.m.f. of the power source U0 and the
current to be interrupted I0, i.e. the amperage when the contacts were still closed.
By using known voltage/current arc characteristics [5], the
critical arc length needed for interruption can be predicted
easily. In the diagram of arc voltage vs. arc current, Fig. 1,
only the points (0, U0) and (I0, 0) have to be connected by a
straight line; the arc length belonging to the characteristic
curve just touched by this "resistance line" gives the critical
arc length required to interrupt the current in this circuit.
U0
I0
Fig. 1: Voltage vs. current characteristics of arc burning on gold contacts in air [6].
C.
AC interruption
While DC interruption is an energy problem necessarily
producing some overvoltage due to the circuit inductance
when the current decreases but needed to convert the magnetic energy stored there, AC interruption is just a problem
of timing because there is no energy stored in the inductance at the exact moment of current zero. Unfortunately, it
is practically impossible to open the contacts exactly at
current zero and to do it fast enough to prevent a breakdown caused by the recovery voltage. Fortunately, however, the arc which appears spontaneously carries the current during the half cycle of contact opening but ceases,
also spontaneously, at the next current zero, thereby avoiding any overvoltage except the recovery voltage of the circuit - provided the recovery voltage is lower than a certain
reignition value, which is not of relevance for low power
signals.
Certainly, the arc column does not disappear fast enough at
current zero because the thermal time constant of a low
current arc, e.g., 2 Arms, is of the order of 1 ms [7]. Therefore, the arc plasma does not lose its conductivity within
the first millisecond after current zero until it has cooled
down to about 3000 °C. But the problem is the supply of
electrons from the new cathode, which was the anode just
before and where no mechanism has been established, requiring very high positive ion density for field electron
emission. Thus, in the very first moment after current zero,
the recovery voltage appears across the arc gap, now in
"reverse" direction. The very mobile free electrons of the
hot arc plasma immediately move toward the new anode;
but they are not replaced by other electrons leaving the
cathode. Thus, "immediately" a zone of 1000 times less
Proceedings of the 53rd IEEE HOLM 2007 Conference on Electrical Contacts, September 2007
(Winner of Best Paper Award)
mobile and therefore practically immobile positive ions
remains in front of the new cathode and the total amount of
the recovery voltage drops across that positive ion zone, as
shown in [8]. The density of these remaining ions, however, is far too low to cause any field electron emission
from the cathode. Thus, the recovery voltage of the circuit
is held by the residual ion sheet which, fortunately from
our point of view, is able to withstand a certain voltage
value "instantaneously". This value depends on very many
parameters, but mainly on the temperature of this ion sheet,
which is about the temperature of the adjacent cathode surface and therefore limited by the boiling point of the contact material. But it also depends on the way the contact
surface works and the characteristics of the gas, of course.
Materials such as Cu, Fe, and Ni yield various values of
instantaneous reignition voltage depending mainly on their
boiling point and the way they work. It is well known that
this "instantaneous" reignition voltage is established within
much less than 1 µs, i.e. faster than any usual recovery
voltage. And, we also know that even much higher power
frequencies than 60 Hz or 400 Hz can be interrupted in that
way.
D.
RF interruption
Little is known, however, about the mechanism of RF current interruption in the range of MHz and even GHz. Not
even a single paper has been published on this topic so far.
It has to be emphasized that there is not a quasi-steady long
arc in air when such high frequencies have to be interrupted, since a current loop lasts less than a microsecond in
the MHz range and less than a nanosecond in the GHz
range.
What is the state of matter 1 ns after contact separation?
During the last millisecond before contact separation, the
contact constriction area decreases, the contact resistance
increases and so does the temperature of the contact site, up
to melting point and beyond it. A molten metal bridge is
formed by the surface tension of the melt, preventing contact separation at the very moment a gap would appear if
there were no current. Instead, the bridge increases in both
length and diameter while its temperature approaches boiling point. During that stage, the surface tension is decreasing while both the kinetic energy and the mean distance of
the molecules are increasing until, at boiling point, the molten bridge disintegrates explosion-like. During these nanoseconds, a more or less continuous transition from liquid to
gaseous (plasma) state is to be expected, followed by a
rapid increase in temperature and a decrease in the density
of the metal vapor. "Suddenly", the distance between the
remaining solid surfaces increases while the liquid becomes vaporized or, mainly, is splashed away and the voltage across the contact gap jumps from the boiling voltage
(0.75 V for Ag) to the anode arc voltage (about 8 V).
"Suddenly", a so-called anode arc is established which is a
rather unstable phenomenon existing inside the expanding
metal vapor cloud. It is hardly known how many fractions
of a nanosecond or even how many nanoseconds "suddenly" means. The anode arc, however, may last some time
in the range between 1 ns to 10 µs depending on contact
material, current and opening velocity, until at a contact
distance of about 0.5...5.0 µm, it changes discontinuously
into a so-called cathode arc, which is still a discharge existing in the expanding metal vapor cloud which remains
from the disintegrated contact bridge but requiring some
more arc voltage.
Only 1...100 µs after contact separation or when about
10...1000 µm gap length is reached, the surrounding air
diffusing into the arc column has replaced the expanding
metal vapor and the discharge changes from the state of a
metal vapor arc to a gas arc.
Comparing this break arc history with the duration of RF
current loops, it becomes evident that arcs at frequencies
even much lower than 1 GHz attain their first current zero
during the anode arc mode and the ambient of a MHz arc
loop might still be metal vapor rather than air at current
zero. Our limited knowledge about the short arc modes
existing in an expanding high pressure metal vapor cloud is
not sufficient to predict their current zero behavior.
III.
SWITCHING RF AND MICROWAVE SIGNALS
No published information is available about make and
break of RF and Microwave signals. As already described
before, the most interesting question is whether the arc
behaves like a DC arc due to the time constant of the arc
which is longer than the period of the signal. Furthermore it
is of vital interest, whether the arc is extinguished by the
increase in the contact gap and/or by crossing current zero.
A.
Test setup
The traditional way of monitoring the contact voltage and
contact current is not applicable for signals in the GHz
range, as the sampling rate of the fastest commercially
available oscilloscope is currently 20 Gsamples/s. The test
setup for measuring the break and make operation is given
in Fig. 2. The signal generator and the amplifier available
for these tests allowed a frequency range between 0.8 and
3 GHz to be covered, and a power range between 1 and
37 W. The following procedure was applied to investigate
the make and break process. For breaking the contacts,
power was applied to the closed contacts. The relay was
activated and the input and output signals were compared.
For making the contacts, power was applied to the closed
contact, the relay activated and the signals on the closing
contacts were monitored.
B.
Make contact
Make operation is given in Fig. 3 for a RF load of
33 W @ 75 Ohm, frequency 1.4 GHz. In order to get an
overview of all relevant phenomena, the overview was
taken at a reduced resolution. In Fig. 3a, the input signal is
given in pink, the output signal in blue. During the make
operation, 3 steps can be observed in the output signal.
Additionally after the first closing a bounce can be detected.
The first step in the output signal appears when the contact
bridge leaves the grounded position (Fig.3b). As the contact bridge approaches, a pre-strike arc is ignited (Fig.3c).
Proceedings of the 53rd IEEE HOLM 2007 Conference on Electrical Contacts, September 2007
(Winner of Best Paper Award)
When both contacts are closed the output signal is similar
to the input signal reduced by the intrinsic loss.
grounded position. Grounding the contact bridge results in
a further increase of the RF isolation by about 15 dB.
Fig. 3a: Overview make contact 33 W @ 50 Ohm 1.4 GHz,
Amplitude 20 V/div - Time base 20 µs/Div
Fig. 3b: 1st step – contact leaves grounded position.
Amplitude 20 V/div - Time base 2 ns/Div
Fig. 2: Test setup for the investigation of the make and
break process on RF and microwave signals.
C.
Break contact
Break operation is given in Fig. 4 for a 25 W @ 50 Ohm
load, frequency 1.4 GHz. In the overview picture Fig. 4a
was taken at reduced resolution, in order to get an overview
of all relevant phenomena during the make operation.
While the dark signal shows the input power, the light signal shows the output power. The three interesting parts for
breaking the contact are shown in Fig.4a to Fig.4d.
Fig. 4b represents the first step in the output signal, in
which amplitude is suddenly reduced. This step is caused
by the opening of the first contact of the contact bridge as
an arc appears in the contact gap. Both input and output
signal are still in phase, as the arc can be considered as
resistor. The arc burns continuously and the signal has several current zero crossings.
In Fig. 4a further reduction of the output signal can be detected, which is caused by the opening of the second contact of the contact bridge. As the arc does not have enough
e.m.f. available it extinguishes at the current zero crossing.
At the same time a phase shift can be detected, at the moment when the resistive arc extinguishes and the signal
transfer is based on capacitive conduction.
Fig. 4c shows another reduction of the output signal. This
last step appears when the open contact bridge reaches the
Fig.3c:2nd step – pre-strike when the contact approaches.
Amplitude 20 V/div - Time base 1 ns/Div
Fig.3d: 3rd step – contacts are closed.
Amplitude 20 V/div - Time base 1 ns/Div
Fig. 3: Make operation of a high frequency signal
33 W @ 75 Ohm – 1,4 GHz
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IV.
Fig.4a: Overview-Break contact 25 W @ 50 Ohm load,
1.4GHz. Amplitude 20V/div-Time base 200µs/Div
Fig.4b: 1st step – Contacts open and arc appears.
Amplitude 20 V/div - Time base 2 ns/Div
EXPERIMENTAL
A.
Test device
All hot switching tests were performed on HF3 relays [10,
11]. The HF3 relay is given in Fig. 5. The HF3 relay is a
SPDT (single pole double throw) relay designed for high
frequency applications. The principle design of the carrier
shown in Fig. 5. is based on a Y structure. The contact carrier is designed on the coplanar principle, the switching
contact bridge is based on microstrip design. Besides the
terminals for the coil connections and the three terminals
for the SPDT configuration, there are eight additional terminals to provide optimum shielding.
Fig. 5: Design of a low cost ultra-miniature RF relay
(HF3) – dimensions 14.6 x 7.2 x 10 mm. Contacts
are formed as bridge contacts, grounded in open
condition (75 Ohm).
HF3 Relay
Coil characteristic
Coil voltage
Coil power consumption
Contacts:
Type
Fig. 4c: 2nd step – arc extinguishes – phase shift.
Amplitude 20 V/div - Time base 1 ns/Div
Fig.4d: 3rd step – contact bridge reaches ground position.
Amplitude 20 V/div - Time base 2 ns/Div
Fig. 4: Make operation of a high frequency signal
25W @ 50 Ohm – 1.4 GHz
Monostable and
Bistable 1 or 2 coils
1.5 ... 24V
70 ... 140 mW
1 change over
1 form C / SPDT
Contact material
Gold over silver
Maximum carrying power
50 W dc, ac and RF
Maximum switching power
50 W dc, ac and RF
Mechanical endurance
107 operations
Electrical endurance
5 x 105 operations
RF Characteristics
50 Ohm
75 Ohm
Isolation
- 72 dB
- 72 dB
@900 MHz min.
- 56 dB
- 56 dB
@3 GHz min.
Insertion loss
- 0,15 dB
- 0,09 dB
@900 MHz max.
- 0,30 dB
- 0,20 dB
@3 GHz max.
V.S.W.R.
1,16
1,11
@900 MHz max.
1,25
1,15
@3 GHz max.
Table 1: Characteristics of the newest generation of
electromechanical RF relays
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(Winner of Best Paper Award)
C.
Test evaluation
The contact resistance, the number of occurrences of bridging, and the number of occurrences of sticking during the
electrical endurance tests were evaluated for all AC load
tests. For the evaluation of the contact resistance values,
the endurance test is divided into 50 intervals. For every
interval the minimum, average and maximum contact resistance values are plotted. Contact resistance limit RF load
tests were performed without monitoring. After the endurance tests, the relays were opened and the contacts were
analyzed using an SEM (scanning electron microscope).
V.
Contact circuit resistance [ohm]
10
1
0,1
0,01
20
10
1
0,1
0,01
0
20
40
60
80
Number of operations [thousands]
100
RESULTS
A.
AC Loads
1)
30 V and 1 A -30 VA @ 50 Hz
100,000 operations lifetime were expected at 30 V, 1 A @
50Hz. Fig. 6 shows indeed that the contact resistance remained low and only a little erosion occurred.
0
2)
30V and 1 A – 30 VA @ 1000 Hz
The same load was tested at 50 Hz and 1000 Hz, in order to
investigate the impact of the load frequency. Fig. 6 and
(Fig. 7) however do not show any relevant differences of
contact resistance and erosion. The load was interrupted at
any point on the sine wave. A short arc (Fig. 8) – duration
about 1 µs – appeared and extinguished as soon as the second contact of the contact bridge opened. The duration of
the arc, as already seen before, only depends on the asynchronous opening of the bridge contacts.
Contact circuit resistance [ohm]
B.
Tests performed
The aim was to compare the switching performance of the
HF3 relay at:
- AC loads: - 30 V and 1 A – 30 VA @ 50 Hz
- 30V and 1 A – 30 VA @ 1000 Hz
- RF loads: - 25 W @ 50 Ohm - 0.8, 1.5 and 3 GHz- 37 W @ 50 Ohm – 1.5 GHz
- 37 W @ 75 Ohm – 1.5 GHz
Number of operations:
1 x 106
Contact material
• Fixed contacts:
3 µm Ag + 0.5 µm AuCo 0.5
• Moveable contacts: 3 µm Ag + 0.5 µm Au
Ambient temperature: 85°C for AC tests, 23°C for RF tests
40
60
80
100
Number of operations [thousands]
Fig. 7: Resistive load 30 V and 1 A @1000 Hz. Minimum, average and maximum contact resistance
values (top figure) and SEM photos of the bridge
contacts.
Power [W] Impedance=50 Ohm Impedance=75 Ohm
[W]
U [Vp]
I [mA]
U [Vp]
I [mA]
1
10
141
12
115
5
22
316
27
258
6
24
346
30
283
9
30
424
37
346
10
32
447
39
365
20
45
632
55
516
25
50
707
61
577
37
61
860
74
702
50
71
1000
87
816
Table 2: Correlation between RF power and impedance
on the one hand and voltage and current on the
other hand.
Fig. 6: Resistive load 30 V and 1 A @50 Hz. Minimum,
average and maximum contact resistance values
(top figure) and SEM photos of the bridge contacts.
B.
RF Loads
While all AC electrical endurance tests were fully monitored (measurement of contact and isolation resistance after
every single operation) this was not possible for hot switching RF loads. On these relays contact resistance as well as
insertion loss was measured before and after the switching
tests. In addition the contacts were analyzed using a SEM
Proceedings of the 53rd IEEE HOLM 2007 Conference on Electrical Contacts, September 2007
(Winner of Best Paper Award)
(Scanning Electron Microscope). In RF engineering, the
load is not given in voltage and current values, but it is
usually defined by the power and by the system impedance.
A correlation between power and voltage and current respectively is given in .
Voltage
20V/DIV
2)
RF Load 25 W @ 50 Ohm – 3GHz
In order to judge the impact of the load frequency the same
test was performed also at a frequency of 3GHz. As the
pictures in Fig. 10 indicate, the erosion is approximately
the same. The precious metal layer is still present in the
contact area. Arcing during the hot switching tests showed
some impact on the contacts but still not critical.
Detail
Current
0.5A/DIV
500µs/DIV
10µs/DIV
Fig. 8: Voltage and current during break contact.
Fig. 10: RF Load 25 W @ 50 Ohm – 3 GHz - SEM photos of the contacts after 1 million operations.
Fig. 9: RF Load 25 W @ 50 Ohm - 0.8 GHz - SEM photos of the contacts after 1 million operations.
1)
RF Load 25 W @ 50 Ohm - 0.8 GHz
The HF3 relay was switched for 1 million operations (make
and break). The power was monitored during this time.
After the tests the insertion loss was measured once more
and than opened for SEM analysis. The analysis of the contacts showed some minor erosion of the contacts after the
hot switching tests (Fig. 9). Considering the load of 25 W
and the 1 million operations the results are remarkable.
Only the gold layer is removed in the contact area, but the
thin silver layer was able to withstand the arcing.
3)
RF Load 37 W @ 50 Ohm – 1.5 GHz
Although the power which has to be carried and switched
in systems with a 50 and 75 Ohm impedance is the same,
there are significant differences in the voltage and current.
While the 37 W in 50Ohm systems result in a contact load
of 61 Vp / 860 mA, in the 75 Ohm system the contact load
is 74 Vp / 702 mA.
Apart from the different contact load the design of the 50
and 75 Ohm type is only slightly different, but might have
a significant impact for power switching. While the relays
suitable for 50 Ohm impedance have a bifurcated contact
bridge, the 75 Ohm version has only a single contact
bridge.
In order to compare the hot switching performance of both
versions, tests at 37 Watt were performed.
The results achieved for hot switching of 37 W @ 50 Ohm
- 1.5 GHz (Fig. 11) show remarkably low contact erosion.
After 1 million operations no significant contact erosion
can be detected. Measurements of the contact resistance
values before and after the hot switching tests also did not
show any differences.
4)
RF Load 37 W @ 75 Ohm – 1.5 GHz
The contact erosion for the 37 W load @ 75 Ohm impedance shows very similar results to those achieved at 50
Ohm. The erosion is visible but still in the precious metal
layer (Fig. 12), and therefore absolutely not critical. Just
the opposite in fact, as the erosion is remarkably low for
Proceedings of the 53rd IEEE HOLM 2007 Conference on Electrical Contacts, September 2007
(Winner of Best Paper Award)
this very high load of 37W which was hot switched for 1
million operations.
0.0
-0.1
[dB]
-0.2
-0.3
-0.4
-0.5
0
500
1000
1500
2000
2500
3000
Frequency [Mhz]
NO-Init
NC-Init
NO-Finale
NC-Finale
Fig. 13: Change of insertion loss caused by hot switching - Load 37 W @ 50 Ohm – 1.5 GHz
VI.
Fig. 11: RF Load 37 W @ 50 Ohm – 1.5 GHz - SEM
photos of the contacts after 1 million operations.
5)
RF Load 37 W @ 75 Ohm – 1.5 GHz
The contact erosion for the 37 W load @ 75 Ohm impedance shows very similar results to those achieved at 50
Ohm. The erosion is visible but still in the precious metal
layer (Fig. 12), and therefore absolutely not critical. Just
the opposite in fact, as the erosion is remarkably low for
this very high load of 37W which was hot switched for 1
million operations.
DISCUSSION AND PROSPECTS
The HF3 relay has been designed to optimize RF characteristics. For good RF isolation in the open position, bridge
contacts were used and grounded in the open state. The
electrical endurance tests performed with AC and RF loads
showed that bridge contacts improve the switching performance even in load areas where bridge contacts are not
necessarily required in order to interrupt the load circuit,
e.g., at higher DC voltages.
A.
AC loads
According to published data, current interruption of the
loads was expected at power frequency current zero. For
the tested loads, this breaking mechanism was not confirmed. This can be explained for the 30 V / 1 A load.
When the second contact of the contact bridge opened, the
minimum voltage to maintain an arc was determined to be
about 35 V. As this voltage was not available, the arc extinguished immediately.
For the 30 V / 1 A load, the load was interrupted at any
point of the sine wave. The typical arcing time was always
less than a few µs.
Current
0.5A/DIV
Fig. 12: RF Load 37 W @ 75 Ohm – 1.5 GHz - SEM
photos of the contacts after 1 million operations.
Contact resistance values and contact erosion are good indicators for the RF characteristics. Hot switching does not
have an impact on isolation but on the insertion loss values,
due to potential changes of the contact surface. The insertion loss before and after a hot switching test of
37 W @ 50 Ohm for 1 million operations was measured.
The results presented in Fig. 13 show no change in the insertion loss, caused by the switching operations.
Voltage
20V/DIV
5µs/DIV
Fig. 14: Voltage and current during break contact.
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(Winner of Best Paper Award)
A.
RF loads
For the first time ever, the switching of RF signals was
investigated in detail.
The analysis of the break and make operation showed, that
arcing appears in the same voltage/current range as for DC
loads. As long as the voltage is less than the minimum arc
voltage no arcing occurs. When the load is increased at
about the same load levels as for DC loads an arc appears
during make or break contacts.
During break operation of the contacts, an arc is ignited
when the contact opens and burns for the duration of several periods of the signal – similar to a dc arc. This can be
explained with the thermal time constant of the arc, which
is longer than the duration of a period. As the contact gap
increases, the minimum arc voltage increases and the voltage required for the arc cannot be maintained anymore.
Finally the arc extinguishes in a similar way to an AC arc,
during system frequency current zero. At the same time
when the circuit is interrupted the signal makes a phase
shift from a resistive to a capacitive circuit.
Although loads up to 37 W at both 50 and 75 Ohm impedance were tested, the arcing during make and break did not
cause significant damage to the contacts. This is a remarkable result, as 1 million operations were made under load.
The excellent performance was also confirmed by the unchanged contact resistance and insertion loss values of contacts which were subjected to the hot switching tests.
VII.
CONCLUSIONS
Electromechanical RF switches provide significant advantages compared to solid-state RF switches. As the isolation
is made from air or plastics, excellent isolation can be
achieved regardless of the ambient temperature. Metal contacts provide the lowest possible insertion loss. Furthermore, electromechanical RF switches are able to transmit
RF signals without signal distortion and signal modulation.
Electromechanical RF relays provide real broadband characteristics over the whole frequency range from DC to
GHz.
For the first time ever, detailed investigations into the
switching phenomena of RF signals were performed. It was
found that making or breaking RF loads causes arcs which
burn over several periods of the signal, as the time constant
of the arc is longer than the cycle time of the signal. The
arc extinguishes as the contact gap increases by frequency
current zero.
Excellent switching characteristics were found for
• DC loads up to 60 W
• AC loads up to 30 VA at 50 Hz and 1kHz
• RF loads up to 37 W @ 50 and 75Ohm impedance and
frequencies up to 3 GHz
Due to the bridge contacts, which were initially designed in
order to optimize RF isolation values, excellent switching
performance was also achieved. The two contact gaps in
series resulted in a minimum arcing voltage of a total of
more than 30 V. All loads with an open circuit voltage less
than approximately 35 V can be interrupted with no or only
a very short arc. The arcing time is defined by the synchro-
nism of the contact bridge. Due to manufacturing tolerances, the two contact spots of the bridge contacts do not
open at exactly the same time. Then the arc burns on the
contact pair which opens first and until the second contact
pair open too.
The switching mechanisms can be characterized as follows:
• On AC loads, breaking at any point on the sine wave
was detected, even at 360 V peak voltages. On all AC
loads tested, the arc extinguished before the power frequency current zero.
• On RF loads basically a mixture between DC and AC
was found. The arc burns over several periods and finally extinguishes at power frequency current zero.
Regardless of the mechanism involved, the switching characteristic is remarkable for the present design. Before the
tests were performed, this level of switching capability for
DC, AC and RF loads, with such a thin precious metal
layer, would not have been expected.
VIII.
ACKNOWLEDGMENT
The author thanks Dr. Klaus Poefel, Mike Ludwig and Bert
Krapfenbauer for conducting the tests and preparing the
analyses.
WERNER JOHLER Werner Johler received his Ph.D.
degree in electrical engineering from the Technical University of Vienna, Austria in 1988 and his MBA in 2003.
From 1984 to 1988 he was a
scientific staff member at the
Institute of Switchgear at the
Technical University of Vienna. Since 1988 he has been
with Tyco Electronics in Au,
Switzerland. He is Director
for AXICOM relays. He has
been Chairman of the
Technical Committee TC94
“All or nothing relays”
within CENELEC since 1999
and member of the Board of Directors of IRSTC (International Relay and Switch Technology Conference) since
2002 (former NARM). He received the Scientific Award
from the state of Vorarlberg, Austria in 2004, the Albert
Keil Award from VDE in 2005 and the IEC 1906 award in
2007.
He has published more than 60 papers on relay technology,
miniaturization of electromechanical devices, contact physics and reliability of electromechanical relays
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