PRODUCTS FOR SUSCEPTIBILITY to radiated fields above 1 GHz is on the
horizon. The new medical standard now stipulates testing to 2.5 GHz, and
this upper frequency may be adopted in the coming standard for fire detection
products. Though far from certain, it looks as if 6 GHz is a strong candidate
for the upper frequency for consumer products. When test facilities test
products for compliance to these new RF immunity standards, which antenna/amplifier
combination will prove the most effective at producing the necessary volts
per meter over the new band? What is the cost-benefit trade-off? This
article presents practical combinations using existing technology and
highlights the inevitable trade-offs faced during the selection process.
and power amplifiers of various frequency bands and power capabilities
are readily available within the EMC marketplace. Ideally, you would simply
select an antenna covering 1 GHz to 6 GHz, combine it with a power amplifier
covering 1 GHz to 6 GHz, and hey presto, you have the solution.
Life, as we all know, is never that simple. Antennas covering this frequency
band (and wider) that can handle the necessary input power are indeed
available from many excellent manufacturers. In fact, antennas covering
25 MHz to 7 GHz exist, possibly allowing coverage of the old and new frequency
in an uninterrupted sweep. Unfortunately, a power amplifier covering 1
GHz to 6 GHz and capable of delivering the necessary power level does
not exist. The band will need to be covered by at least two amplifiers
so that a switch is required to connect the antenna to the appropriate
amplifier during the immunity test. Or, if preferable, two amplifiers
and two antennas could be used. Clearly, the scene is set for the evaluation
of a selection of mix-and-match permutations.
RF immunity testing is done under the control of an automatic test system
button is pressed, and the product is subjected to a specific RF field
strength (say 10 volts per meter) for a fixed dwell time and over a series
of predetermined spot frequencies. The ATE software program steps the
system through the frequencies,
and the product is monitored
for susceptibility to the applied RF field. Figure 1 shows the antenna/amplifier
combination used to generate the necessary RF field. Also shown at a fixed
distance from the antenna is the imaginary measurement plane. The field
strength is measured at points across a section of this plane as part
of the system calibration.
selection of suitable antenna/amplifier combinations is part of the ATE
design process. The steps
in this process are:
- Establish what is required
of this part of the ATE system
- Identify possible solutions
- Assess and compare the
- Select the best solution
impact on the time taken to complete the testing of the product is important,
and the solution should represent good value in terms of performance,
reliability, and cost.
The fundamental requirement is that
the antenna/amplifier combination generates the necessary RF field strength
over the specified frequency range. A field strength of 10 V/m at a distance
of 1 meter will be used in this example.
(The exercise can be repeated for different field strengths and distances
as required). Sufficient power margin must be built in to cover:
These include dissipative (heat) loss in the cable feed to the antenna,
dissipative loss in the antenna itself, and any power reflected back
by the antenna. These losses affect the net power utilized by the antenna
to generate the RF field.
of the RF signal
The test signal is amplitude modulated at 1 kHz to a depth of 80%. This
modulation results in signal peaks that require 3.3 times more power
than the un-modulated signal. Also, waveform integrity must be maintained
waveform peaks caused by amplifier compression could cast doubt on the
validity of the test. In the frequency domain, flattened peaks will
show up as harmonic noise. Note: the upcoming standard is likely to
stipulate modulation on at the calibration stage.
- Field variation
An allowance must be made for field variation at different points on
the calibration measurement plane caused by the peculiarities of the
chamber, e.g., inconsistent damping of reflected signals and the effect
of locating the antenna in close proximity to the chamber walls. There
is a size constraint on the antenna since it must fit within an allotted
space in the chamber if the product is to be positioned at the prescribed
distance from the antenna. Fortunately, at frequencies above 1 GHz,
antenna dimensions are small compared to Sub-1-GHz broadband antennas
so this should be a non-issue.
For the 1-GHz to 6-GHz frequency range,
the main antenna options are microwave horn and log periodic since they
exhibit excellent performance over this band and are physically small.
The parameters of interest are input VSWR, radiation pattern, power handling
capability, and the input power required to generate 10 V/m at 1 meter.
The required input power provides the most useful information in terms
of assessing suitability so this data will be utilized most frequently,
and the other data will be used to confirm the selection.
2 shows samples of each type of antenna. For this exercise, the microwave
horn (SAS-571) will be compared with
log periodic antenna model (SAS-510-7).
The dimensions (L ´ W ´ H) of the microwave horn are 8.2 ´
5.6 ´ 9.5 inches and the dimensions (L ´ W) for the log periodic
are 24.9 ´ 20.1 inches.
1 and Table 2 show the power budget for each antenna. The figures in the
Watts Required column are actual measured data of the power
required at the antenna connector to generate 10 V/m at 1 meter. Parameters
such as power reflected back from the antenna and antenna dissipation
are already factored in. The cable loss is for nine feet cable length and
uses manufacturers data readily available on the Web.
Notice that cable loss increases
with frequency. The Peak Modulation Power is calculated by
multiplying by 3.3 (adding 5.2 dB). The Total Power Required
adds 3 dB (doubles the power) to allow for field variation. The figures
have been rounded where appropriate. The microwave horn can handle 300
watts input power, and the log periodic can handle 1000 watts so the power
levels shown are well within the capability of the antennas. The dimensions
of both antenna types are small so interaction with the chamber will be
POSSIBLE SOLUTIONS AMPLIFIERS
The amplifier options are solid-state
(GaAsFET) and traveling wave tube (TWT). Today, solid-state is the preferred
technology up to 4 GHz, but it still has a long way to go to beat the
price/performance capability of highpower octave band TWT amplifiers above
4 GHz. An octave represents a doubling of the frequency. The 4-GHz to
6-GHz power modules in the dualband solid-state amplifiers referred to
in this exercise are half an octave.
amplifier frequency band permutations that cover or exceed the 1- to 6-GHz
requirement are shown in Figure 3. Option A and Option C are all solid-state.
Option B uses solid-state and TWT technologies.
The power budget tables show that
the necessary linear power is 5.8 watts for the microwave horn and 12.8
watts for the log periodic antenna. Linear power is required to prevent
distortion of the modulated waveform. For the purposes of this exercise,
Option A (all solid-state) will be combined with the microwave horn and
Option B (solid-state/ TWT) will be combined with the log periodic antenna.
Rules of thumb for linear versus saturated power require backing off 1
dB from saturated for GaAsFET amplifiers and 3 dB for TWT amplifiers.
This adjustment equates to 7.3-W saturated power for Option A and 16.2-W/25.6-W
saturated power for the Option B solid-state/TWTA combination.
is needed irrespective of which option is decided upon. The next section
discusses how this is implemented.
are two basic approaches to switching the feed to the antenna. The first
is through an external band-switch box as shown in Figure 4. External
cables are used to connect the amplifiers to the band-switch box. With
both relays in the position shown (normal), Band 1 feeds the antenna.
With both relays operated, Band 2 feeds the antenna.
second approach is for the amplifier manufacturer to put both amplifiers
in one chassis and switch the bands internally. A schematic of this method
is shown in Figure 5. The principle of operation is the same, but there
are major space and cost savings since many of the key components are
shared. These include the power supply, the cooling system, control circuits,
and of course, the chassis itself.
Sharing components is possible since
only one amplifier is running at a time. Therefore only one power supply
is needed, and the cooling components need to dissipate the heat from
only one amplifier. Also, the internal RF cable runs can be shorter (compared
to externally run cables) resulting in reduced cable loss. As the power
budget tables indicate, cable loss can be significant, especially at 6
GHz. Once the main chassis design of a dual-band product is complete,
it is relatively easy to substitute RF modules with different frequency
bands and/or power levels. Unfortunately, this shared component approach
cannot be used with solidstate/TWT combinations since the power supplies
and cooling arrangements are radically different.
The ATE software provides the switching
signal at the appropriate place in the test run. The time for the relays
to switch is about one-tenth of a second so the impact on the overall
test time is negligible. In fact, for RF Immunity applications, a single-band
solution requiring no switching is a feature with little benefit and can
actually be detrimental to the harmonic noise performance of the system.
Cold-switching should be employed with both bandswitching methods.
That is, the switching sequence should be:
- Remove the RF input signal
- Switch over the band
- Apply the RF input signal.
cold-switching circuits that disable the power supply during relay switchover
can be implemented easily with internally switched amplifiers. This feature
is included in the dual-band amplifiers described in this article. RF
relays suitable for band-switching are available on the open market with
a loss of less than 0.1 dB at 6 GHz so the insertion loss of the relays
has not been factored into the calculations here.
COPING WITH REFLECTED POWER
The system losses included in the power calculations reduce the amount
of reflected power the amplifier has to handle. The factors reducing the
reflected power seen by the amplifier are:
- The power leaving the amplifier
is attenuated by the cable loss in the forward direction and is attenuated
again in the reverse direction. At 1 GHz, these losses represent at
least 2 dB total path loss in the cable alone. At 6 GHz, the return
path loss is 5 dB.
- The antenna does not transmit
a pencil beam (unlike a laser). Instead, the beam spreads and bathes
the measurement plane calibration area and its surroundings. Even with
the device under test in place, the absorptive tiles on the walls of
the chamber absorb much of the forward power; and because of the angle
of reflection, much of the reflected power from the device under test
as well. Even in the worst case of high reflection from the device under
test, only a small part of the forward power is returned via the antenna.
TWTA is operating in a backed-off condition and is delivering a fraction
of its forward power capability. This condition, together with the system
losses described above, means that the ratio of reflected power to forward
power capability is small. Also, GaAsFET amplifiers use an internal power
combining method that safely deflects reflected power away from the output
transistors. Collectively, these factors indicate that reflected power
via the antenna is not a crucial issue.
SUITABLE AMPLIFIER MODELS
The Option A power requirement can
be met by a dualband internally switched 0.8- to 6.0-GHz power amplifier
such as the BBS3Q9ACD. This contains a 0.8- to 4.2-GHz 15 watt amplifier
and a 4.0- to 6.0-GHz 10-watt amplifier providing 12 watts and 8 watts
of linear power, respectively. Model BBS3Q9ACD is shown in Figure 2(a).
The Option B power requirement can
be met with models BBS3Q7EEL a 0.8- to 4.2-GHz 25 W GaAsFET amplifier,
and model TWTA-7A8GFE, a 4.0- to 8.0-GHz 30 watt TWT amplifier. These
provide 20 watts and 15 watts (over 4.2 to 6.0 GHz) of linear power, respectively.
TWTA amplifiers produce significantly more power away from the band edges
so 15 watts of linear power is conservative at 6.0 GHz. External band
switching using a band-switch box is suitable for this option. Model TWTA-7A8GFE
is shown in Figure 2(b).
So far, both antenna/amplifier combinations
appear well suited for generating the necessary field strength for the
upcoming standard, with Option A seemingly providing the best value. However,
there is a major consideration that needs to be factored into the selection
criteriaintegration with the existing test setup.
INTEGRATION WITH THE EXISTING
If the sub-1-GHz test procedure and
the above-1-GHz test procedure are performed as separate events, then
it is merely a matter of manually replacing one test set-up with the other.
Under these circumstances, the microwave horn antenna/dual-band amplifier
combination is a good match. If the intention is to integrate the two
tests and, if possible, share test components, then other solutions need
to be considered.
- Using a single antenna for
the entire frequency sweep.
- Mounting antennas side by
- Manually substituting another
antenna part way through the
sweep, but capitalizing on the available antenna characteristics to
optimize the system performance.
Note: converting the test chamber
to make it suitable for above-1-GHz testing is beyond the scope of this
SINGLE ANTENNA COVERING
THE EXISTING AND
NEW FREQUENCY BANDS.
In 1994, the University of York and
Chase EMC collaborated on a hybrid biconical/log periodic antenna intended
for use in broadband emissions testing. The antenna exhibited poor performance
below 100 MHz, but this problem was corrected relatively inexpensively
by boosting the signal from the antenna with a low power amplifier. Also,
the poor match into 50 ohms at low frequencies was corrected by inserting
an in-line attenuator. Never intended for RF immunity testing, the antenna
would need a very expensive high-power amplifier to generate RF immunity
fields below 100 MHz. As regards immunity testing, a biconical antenna
is far superior below 100 MHz needing only around 70 watts of RF input
power to produce 10 V/m at one meter. The hybrid antenna would need around
900 watts to produce the same field.
More recent attempts to create a single
antenna to monitor RF emissions below 80 MHz and up to several GHz are
unwieldy in size (of the order of ten feet across and six feet long) and
have a poor match into 50 ohms at the lower band edge. The size means
there is a risk of interaction with the chamber and with the device under
test (except in the largest of chambers), and the poor match of up to
10:1 VSWR below 80 MHz means high power is required to generate the required
field strengths. Using one of these antennas for field generation makes
for an expensive antenna/amplifier combination as compared to using a
biconical antenna covering 20 MHz to 300 MHz (Model SAS-543)
followed by a log periodic antenna covering 290 MHz to 7 GHz (Model SAS-510-7).
Also, antennas designed for frequencies above 1 GHz are comparatively
small, and it would be a shame to lose this valuable feature. Very large
ultra-broadband antennas are intended for use in open area test sites,
and that is where they should stay.
MOUNTING ANTENNAS SIDE BY
covering different bands can be co-located without cross interference
it is feasible to place a biconical antenna next to a log periodic and
to switch amplifiers to the antennas at the appropriate time during the
test sweep. Mounting
the antennas and maneuvering them between vertical and horizontal polarization
could prove a challenge; but as long as each antenna adequately illuminates
the calibration plane, there is no fundamental reason this approach cannot
As with all the approaches
mentioned in this article, the feed to the high-frequency antenna should
be as short as possible. Most amplifiers are available with remote control
and monitoring facilities, and there is no written rule that the high-frequency
amplifier cannot be mounted up close to the chamber to minimize cable
length. A schematic of the switching arrangement is shown in Figure 6.
MANUAL SUBSTITUTION PART
WAY THROUGH THE SWEEP
With this approach, the biconical
antenna is used from 20 MHz to 300 MHz and is then manually substituted
for the log periodic to complete the rest of the test from 300 MHz to
6 GHz. The 20- to 1000-MHz amplifier feeds both antennas up to 1000 MHz,
and then the 1- to 6-GHz dualband amplifier feeds the log periodic antenna
up to 6 GHz. This method has the disadvantage of a break in the test run.
The break should be kept in context. Only one component is changed in
the test setup, and this adjustment takes a fraction of the time necessary
to tear down and to install a completely new test setup. The automatic
switching of the amplifiers to the antenna feed is retained.
A FURTHER RAMIFICATION OF
THE NEW STANDARD
There is a high probability that the
new standard will require modulation to be applied during calibration.
For the old test standard, many test houses were sold antenna/amplifier
combinations that included a 30-watt amplifier and a log periodic antenna.
A 30-watt amplifier will produce a guaranteed minimum linear power of
about 20 watts. At 80 MHz, log periodic antennas require about 5 watts
to produce 10 V/m at 1 meter. Multiply this power by the modulation factor
of 3.3, then add the allowances for system losses and chamber peculiarities,
and the performance of this antenna/amplifier combination may prove to
be marginal at best for this field strength and distance. As shown, it
does not take rocket science to determine field strength and
design within a sensible margin. It is never wise to accept and to pay
for goods blindly, and one size fits all with no guaranteed
margin could be construed as a prime example of this folly.
The anticipated RF immunity standard brings challenges and opportunities.
Although somewhat dependent upon the existing test setup, there are many
approaches to upgrading a test facility in readiness for the new standard.
This article lists pragmatic guidelines allowing independent determination
of how to meet the new requirement. This independence may help test houses
disregard marketing ploys that try to convince buyers that there is only
one viable solution.
Cohen founded A.H. Systems, Inc. in 1973 after working in several major
companies including Sandia Corporation, Packard Bell, Hughes Aircraft
and Litton Industries. He served in the Army during the Korean War as
Battalion Radar Section Chief. Art holds a BSEE from Brooklyn Polytechnic
Find out more about A.H. Systems at ww.1klic.com/184.
Thomas Mullineaux holds a B.Eng. degree from Portsmouth University, England,
and has led RF and Microwave Systems design teams for 15 years. tmullineaux@EmpowerRF.com.