Five Best Practices for Quality RF ATE Measurements:
Achieving Quality RF Results through Variation and Repetition
INTRODUCTION
In order to make quality measurements, repeatability is nonnegotiable. Studying variation in
reproduced measurements determines the character of a desired RF ATE test measurement.
Repeatability
If you cannot make repeatable measurements, you cannot make good
measurements. How do you know that you can make repeatable
measurements? Duh. Make the same measurement over and over
again and examine the variation.
Connector torque is an important consideration for repeatable RF
measurements. Know your connectors and know their torque
requirements, and, use the proper tools. They are not hydraulic fittings
that must withstand 3000 psi! And, use the proper tools properly. Brass
connectors often have lower torque specs than their stainless steel
counterparts. And, use proper tools properly with the proper torque.
Test your measurements with varying amount of settling time or
averaging. You should more or less know the expected level you are
about to measure. Higher signals do not suffer so much from signal to
noise issues as do lower amplitude signals. With lower level signals,
you can expect a lower signal to noise ratio. Because noise is
generally randomly distributed, a typical solution to battling low level
signals is to average your readings over a longer measurement period.
Or, take many readings and average them together. Someone says “I
can measure the RF power in one millisecond with repeatability!” Yes,
but how accurate is that measurement? It may be possible to
consistently get the same measurement over and over again and still
not get an accurate measurement with time varying signals, or if your
instrument is not settled from a previous configuration change. You
need to know your signal characteristics before you can measure it
accurately. Measure it several different ways if possible. For example,
use a spectrum analyzer in addition to the power meter.
"It may be
possible to
consistently get
the same
measurement
over and over
again and still not
get an accurate
measurement with
time varying
signals."
Let’s go with a typical measurement scenario: You want to measure
the signal strength output from a cable attached to an RF signal
generator. You might start by averaging a large number of readings
(maybe something ridiculous like 100) and recording the results. Try
this several times to give you confidence in your measurement value.
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Then, try cutting the number of samples in half. Record and cut in half
again until you see a significant change. When you see a significant
change, you can revert to the previous higher number of samples in
order to average out the noise. Higher amplitude signals may only
need one (1) to ten (10) samples to yield satisfactory stability in
measurements. You can keep cutting the number of samples in half
and converge on an appropriate number of samples that gives you
repeatability that you desire. If you are using triggered or windowed
measurements, you may want to vary the trigger delay around to
make sure you are measuring when you think you are. A familiarity
with the nature of the signal in question will help you with this
exercise.
When measuring time-varying signals, using a spectrum analyzer in
zero span mode can render a nice picture of how that signal is varying
with time. Using a spectrum analyzer in zero span mode is like using
an oscilloscope, but to measure RF waveform power. That can be
tuned. High slew rate RF signal measurements are limited by the
resolution bandwidth (RBW) of the analyzer. For example, if your
analyzer has a maximum RBW of 10 MHz, you are not going to be
able to measure RF signals that change with nanosecond rise times.
The effect is as if a low pass filter is applied to the rise time response.
Frequently, RBW adjustability comes in handy when measuring an
RF signal that is concentrated around a single RF frequency or small
range of frequencies, because you can adjust the RBW to help filter
out noise that you don’t care about.
If given the chance,
make relative RF
measurements
instead of absolute
RF measurements.
Accuracy vs. Relative Measurements
If given the chance, make relative RF measurements instead of
absolute RF measurements. Sometimes you can avoid characterizing
your test setup if you make relative measurements. A common
example is making gain measurements of an RF amplifier. Since
there is no frequency translation in an amplifier, you can first connect
the measurement system without the amplifier in place. Carefully
record the frequency response of your system sans amplifier. This will
be your reference response. Then, insert the amplifier into the test
system and re-measure the frequency response. Subtract the
reference response from the response with the amplifier to obtain the
frequency response of the amplifier alone. You have just made a
relative measurement.
This relative measurement strategy is sometimes complicated by nonmatching UUT connectors and might require a new RF adapter to be
able to connect the UUT input and output cables together. If you are
extremely lucky, you can connect the cables directly together without
an extra RF adapter. If not, seek to balance out the number of
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adapters used between the reference measurement and the final
measurement. If you can use the same number of adapters in both
measurements, and they have about the same insertion loss and
response vs. frequency, then their contributions to each
measurement will mostly cancel out.
Pathloss
Sometimes you need to make absolute measurements of an RF signal
instead of relative measurements. In this case, it is important to know
the loss of any RF components between the signal of interest and your
measuring instrument. These might include any number of RF relays,
interconnecting cables, attenuators, and adapters. These pathloss
contributions must be carefully characterized because they become
part of every subsequent measurement in a one-to-one proportion. It is
important to get these right!
As an example, suppose I wanted to apply exactly -10 dBm to the input
of my RF widget during testing. If only a single frequency was required,
I could simply replace the widget with a power meter and adjust the
power output of my signal generator to as close to -10 dBm as
needed. Then I would disconnect the power meter and connect my
widget for testing. I’ll arbitrarily call this Pathloss Technique #1.
What if I needed to apply -10 dBm over a range of frequencies? Too
many to setup and calibrate manually. The preferred method would be
to setup the signal generator and power meter under computer control
and perform the power adjustments at each frequency, recording the
value needed on the signal generator at each frequency. For each
frequency, I would tweak the output power of the signal generator until
my target power value was met. I could then apply these amplitude
values each time I changed frequency and I would have -10 dBm for
every frequency. Assuming your RF signal generator is very
repeatable, you can recall these settings each time you need to
change frequency. You could even expand this technique for different
power levels if you know beforehand what the required power levels
will be. I’ll call this Pathloss Technique #1 Extended. You might end up
with something like Table 1.
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From Table 1, you can see that at 1 GHz, there is about 1.7 dB loss (8.31 – (-10)) thru the test system to the UUT. The loss increases to
around 5.01 dB (-4.99 – (-10)) at 4 GHz. There is probably a six foot
cable between the RF signal generator and the UUT judging by the
losses.
An alternate way of slaying this problem is to setup your signal
generator at each frequency to a known displayed power level. Then,
measure the resulting actual power with your power meter. For
example, you would set the signal generator to 0 dBm as display on its
front panel at the first frequency. Then you would read the power
meter, proceeding for each frequency in turn. You might expand this
technique for different power levels. This is a job for computers. Under
computer control you could build up a matrix of power vs. frequency
similar to that below in Table 2 shown graphically in Figure 1. I’ll call
this Pathloss Technique #2 Extended.
Pathloss Technique #2 appears similar to Pathloss Technique #1, but
is a little different. In the first technique, the signal generator output is
adjusted incrementally (either manually or under computer control)
until your desired target power is achieved. When the target is spot
on, you record the displayed amplitude from the signal generator
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and use this value any time you want the same target power. The
results are a simple lookup table.
As an aside, Pathloss Technique #1 may require a bit of time
depending on how close you wish to get to your target power. If
your tolerance is small, sometimes it takes several iterations to find the
target. Sometimes, you just cannot quite reach the target power,
oscillating back and forth around the intended target. Your algorithm
must be smart enough to know when to give up and continue when
building the lookup table.
In the second technique (example results in Table 2), you set the signal
generator display to fixed levels and measure the resultant power at
your UUT. To make this really accurate, you would want to premeasure at as many frequency points and at as many power points as
possible. However, this takes a lot of time. You can compromise by
taking fewer frequency and power points. Then, if you ever want a
power in between any two frequency points or power points, you must
interpolate between them to get the power level to send to your RF
signal generator at that frequency and target power output. Typically,
when interpolating, you can assume a linear curve for frequency.
However, you must assume a logarithmic curve for power expressed in
dBm. Then you record the difference between the displayed value and
the calculated value, add that to the displayed value, and write the new
displayed value to the signal generator. For example, in Table 2, if you
wanted -13 dBm at 1.0 GHz, you notice that at 0.00 displayed value,
the level is actually -7.50. At -10.00 displayed value, the level is actually
-17.59 dBm. Solving, first to milliwatts, use straight line interpolation,
and convert back to logarithmic power. For -13 dBm actual power
output, you should program the signal generator to a level of -5.47
dBm.
Now in this second technique, there are some assumptions you must
work with. You must trust the output leveling attenuator on your signal
generator to be fairly linear, accurate, and having sufficient resolution.
This is often the case with modern signal generators. Attenuation in
modern RF signal generators can be achieved either by a variable
attenuator or by varying the signal level before the output attenuator,
such as by adjusting DAQ gain. It turns out DAQ gain can be much
more accurate than a variable attenuator. But if the accuracy of the
variable attenuator built into the output of your RF signal generator is
adequate, this simplifies things.
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The advantage of Pathloss Technique #2 is the time required to build
up the matrix. You don’t need to iterate around to find your target
value. You simply configure the displayed value, sample the power
with adequate averaging, and go to the next point. The disadvantage
of Technique #2 is you must build a proper interpolation algorithm to
find those points in between.
Some RF signal generators have the ability to store the delta between
the displayed value and the actual value. Then, they can be configured
to display an offset value. Looking at the first entry in Table 2, we see
that the displayed value is 0 dBm while the actual value at the UUT is 7.50 dBm. Writing -7.50 to the appropriate instrument register and
turning on the offset results in the displayed value of -7.50 dBm (with
an actual value at the UUT of -7.50 dBm)! This is pretty useful to keep
power levels straight when testing. Warning! This might confound
bystanders that are not familiar with this feature of some signal
generators.
So far, I have only addressed the pathloss for devices that generate
RF signals. The preceding examples are applicable whether using an
RF signal generator or measuring the signal generated by your UUT.
What about measuring pathloss for measurement instruments such
as a power meter or vector signal analyzer? The short answer is that
you need to establish an accurate reference level at the point of
measurement and take the measurement with your instrument.
Subtratcting the reference will give you the pathloss between your
reference point and the measuring instrument.
Let me describe the process in more detail. Suppose you want to
characterize the loss from the UUT output thru a flexible RF cable,
back thru your interface drawer, thru a set of RF relays, then to your
RF signal analyzer. After all, you want to know the power at the UUT
output, not the power at your signal analyzer. Set up a reference power
by, for example, connecting a cable to an RF signal source with a high
quality cable and a high quality attenuator (6 to 10 dB) at the end.
Connect this directly to your power meter. Set the signal source to all
the various frequencies you might need and take careful readings on
the power meter. These numbers become your references.
Then, disconnect the power meter and connect the reference point to
the cable that will be connected to your UUT output. Replay all the
frequencies you previously recorded while making level measurements
with you signal analyzer. Subtract the measurements on your signal
analyzer from the reference powers. The delta values are the pathloss
between the UUT output cable and the signal analyzer.
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The process of calibrating measurement pathlosses takes twice as
long as stimulus pathlosses because you must make twice as many
measurements. There are also more chances for mistakes. Keep any
cabling as stationary as practical to prevent changes in reference
power during the two runs. Try to minimize any additions or
subtractions of adapters between the reference run and the final run.
Make certain the readings are realistic and repeatable.
Isolation or Crosstalk
Good isolation inside your RF ATE is important for quality
measurements. When dealing with sensitive measurements like
phase noise or EVM, it doesn’t take much crosstalk to ruin your
reading. One of the first overlooked items when dealing with crosstalk
is your RF cables. Quality RF cables have their shielding
effectiveness specified on their datasheets. If it is not specified on the
data sheet, and you are dealing with sensitive measurements, you
may want to pass on that brand. If you want good shielding to improve
your isolation, you may want to go with a foil wrapped and braided or
double-shielded outer braid on your coax. The ultimate in shielding is
obtained with semi-rigid coaxial cable. At the expense of flexibility.
Semi-rigid is a good choice inside the ATE where cables should be
fastened down anyways. Use flexible cables only where necessary if
you are looking for the best isolation with the least amount of potential
crosstalk.
"Good isolation
inside your RF
ATE is
important for
quality
measurements."
Another widely used component that influences isolation is the RF
relay (or RF switch). You should choose your relays appropriately
based on their isolation between open terminals at your highest
frequency of interest. Shown in Figure 2 below is a simple RF
multiplexer built by cabling together three (3) single-throw double-pole
(SPDT) RF relays. These are very useful and sometimes can be built
more cheaply than buying a dedicated 4:1 or 6:1 relay. Especially if
you have orphaned relays leftover in your ATE PXI chassis. Keep the
cables identical with identical lengths and since the relays have very
symmetrical loss from each pole to the common, you will end up with
very consistent pathloss thru this 4:1 MUX. One thing to watch out for
is crosstalk. You will notice that in Figure 2, channel 2 is connected
thru the first level and goes to the last common relay before the
measurement instrument. The implication is that the last relay sets the
isolation specs for this MUX architecture. If you have the opportunity,
you can connect a signal with a low level to channel 2. Thus when you
want to look at channel 0, any crosstalk is that much lower. But
flexibility in picking channels isn’t always available.
Improvements can be made if the first level relays are preceded by
single-pole single-throw RF relays (SPST) as shown if Figure 3. This
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effectively doubles the isolation of the RF switches with all else
being equal. Any signal present at channel 2 input is reduced by the
isolation of two (2) relays before reaching the measurement
instrument.
Another consideration when using RF switches is lifetime. There is a
mechanical lifetime specified for the switches that should exceed your
requirements. There are only so many times the little metallic contacts
can bang together and keep their sanity. There is also an electrical
lifetime associated with the switches if you switch live signals thru the
relays. The electrical lifetime is always less than the mechanical
lifetime. Most of the wear due to switching live signals occurs during
the make or break process. You can extend the lifetime of the relays
by using cold switching. That is, turning off the signal, switching the
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signal, and turning the signal back on. This strategy only works if the
mechanism turning the RF signal on and off and back on is an
electronic switch and not another overworked mechanical relay.
RF relays exhibit a slight improvement in insertion loss once they are
broken in. You might measure 0.76 dB loss when the switch is brand
new and later measure 0.61 dB insertion loss after break-in. It’s good
to know in case small amount affects your measurement accuracy.
Break-in usually takes several hundred switch cycles.
Everywhere there is an RF adapter, there is a chance for signal
leakage. Minimize the use of adapters. Use high quality adapters. Stick
with the same connectors throughout your ATE to reduce the need for
adapters.
Return Loss / Reflections
High Standing Wave Ratio (SWR), caused by high Reflectivity (Γ) is
high up on the RF ATE designer’s list of enemies. Reflections are
caused by mismatches from the primary impedance (Z0=50 ohms) of
various system components such as adapters, attenuators, splitters,
cables, and connectors. One of the more popular impedances for RF
components is 50 ohms with 75 ohms a distant second. Sometimes,
the mismatch comes when you connect your ATE to the UUT.
Sometimes you don’t see this mismatch when calibrating your ATE. It
only becomes visible when the UUT data is examined. Frequently, it is
worth it to invest in high quality components in order to minimize the
effects of reflections.
There is a common uncertainty in RF measurements called Mismatch
Uncertainty. Because many measurements are made without knowing
phase information represented in your signal, there will be uncertainty
when the signals are combined together. Do they add constructively or
do they cancel or is the signal somewhere in between? Mismatch
Uncertainty is given by the following equation.
Let me illustrate the problem of reflections using a recent project I was
working on. In the process of measuring the pathloss of a test setup so
that I could generate accurate power levels, I came across the
following results seen in Figure 1. I was measuring the insertion loss
between an RF signal generator and the connector that would connect
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to the UUT when testing. I first measured the power at the output of the
RF signal generator using a GPIB controlled RF power meter at over
one hundred (138) frequencies. This became my reference power vs.
frequency.
Then, I connected the cable to the RF signal generator and connected
the power meter to the UUT end of the cable. Again, I measured at the
same frequencies, subtracted the reference power, and plotted the
pathloss vs. frequency in Figure 4.
All these operations were controlled by a software program written in
LabVIEW 2014™. Performing these types of measurements manually
would take too much time and be error prone from hand operations.
The data taken for these graphs took about 5 minutes. I intentionally
had the software average the power meter readings heavily in order to
measure very accurately for both the reference and last pass. These
pathloss calculations would be used for all subsequent UUT testing for
the duration of this project. I wanted them to be accurate, not fast.
Experiments showed a very repeatable set of numbers from the
LabVIEW application.
Examining Figure 4 shows that there is significant ripple in the
frequency response between the RF Generator output and the UUT
connector. There is an overall downward slope which we would expect
to see (Figure 5). But, this downward slope is dwarfed by the ripple
which eyeballs out to roughly 0.6 dB peak-peak. This ripple is caused
by reflections in the cabling between the RF generator and the UUT
end of the cable. I was using a couple of cables connected by an
adapter. I think the cables were of the RG-142 variety with standard
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male SMA connectors. Overall, their combined length was about 5 feet
plus or minus a couple of inches. (Actually, the customer I was working
with found these cables laying around and decided to use them in the
test setup. I was stuck with them.)
If you take the frequency span by looking at the first peak and last
peak (or first and last valley) you can see the ripples span about 1.325
GHz. If you divide that span by the number of peaks (or valleys) which
number 20, you will notice that the average distance between peaks is
about 66.25 MHz.
If you look up the specs for the RG-142 cable that we were using, you
find out the velocity factor is 70% of the speed of light. If you take the
speed of light (~ 3x108 meters per second), multiply by the velocity
factor (0.70) and divide by 66.25 MHz, this works out to a total length
of 3.16 meters. Now this is the round trip length for the reflections, so
we divide 3.16 meters by 2 and convert meters to feet and we get 5.2
feet. Eureka! That is the combined length of my RF cable between the
RF signal generator and the UUT connector. Rats! There are
significant reflections at the UUT connector that bounces back toward
the RF signal generator, hits it and then combines with the forward RF
wave to produce these terrible ripples.
One well known technique to reduce these ripples comes at the
expense of increased insertion loss. Since the input power of my UUT
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was quite a bit lower than the output of the RF signal generator, I knew
that increased insertion loss would not be a problem. I added a 6dB
attenuator to the end of the cable where the UUT would connect. Now,
when the signal traveling from the RF signal generator goes thru the
attenuator, hits the UUT, and reflects back, it would be reduced by
twice the attenuator value or 12 dB. When using this technique, you
should use high quality attenuators to minimize their contribution to
reflections. See Figure 6.
Another aside: The problem really manifests itself because the RF
signal generators are not known for having a really good output match
to 50 ohms. Consider if the forward wave that bounces off of the UUT
and returns to the signal generator is completely absorbed by a good
match. There is no secondary forward wave and consequently no
ripple because of reflections!
Don’t be deceived by the scale of Figure 6. The straight line fit went
from about 0.55 dB per GHz to about 0.6 dB per GHz of slope. An
insignificant change is not entirely unexpected. Pathloss has increased
by about 6 dB from the attenuator. However, the ripple has decreased
dramatically to around 0.25 dB peak-peak average across the band.
Since the accuracy of the power delivered to the UUT was specified to
be ±0.25 dBm, this test setup was good to go.
Greater improvements in ripple could have been had if larger
attenuators had been available. Also, eliminating the extension cable,
only using a single cable is always an improvement in impedance
mismatches. Using high quality connectors and high quality cables is
important in RF testing. Minimizing reflections results in quality RF
measurements.
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