Attenuators and Circulators in RF Spectrum Analyzer Measurements
Getting the most out of your Spectrum Analyzer requires
understanding the concepts of Input Impedance, Return Loss and Isolation.
A firm grasp of these phenomenon will allow you to minimize interactions
in your measurement process. By using attenuators and circulators
to control Return Loss and Isolation in your test setup accuracy and
repeatability can be greatly improved. Without proper attention to
Return loss and Isolation, interactions may take place that can corrupt
the measurement process. It is hoped that this article will clear
some of the mystery and confusion in using attenuators and circulators
in RF Spectrum Analysis.
Understanding Return Loss or VSWR
As an example of return loss lets take a look at a typical
Spec Ana. input circuit as shown if figure 1. From the figure we see
that the typical input circuit consists of a programmable attenuator
followed by a mixer. Mixers are very nonlinear devices and have complex
input impedances that vary with frequency and some extent the signal
level or power. Lets look at the case where the attenuator is set
to 0dB, i.e. a direct short to the input of the Mixer. The stated
nominal impedance of the instrument is 50 Ohms and the Impedance of
the signal source driving the instrument is a perfect 50 Ohms. When
the signal from the generator reaches the mixer of our instrument
it encounters a non perfect 50 ohm impedance. This causes a portion
of the incident wave to reflect and travel back toward the Signal
generator which is perfectly terminated in 50 ohms. If the mixer had
a perfect impedance of 50 Ohms no reflection would occur and all the
incident wave would be absorbed. Instead the wave reflects and is
completely absorbed or dissipated in the 50 Ohm impedance of the generator.
Of course a real world generator does not have a perfect 50 ohm impedance
and a portion of the reflected wave would again reflect back towards
the Analyzer. This process of partial reflection and partial absorption
continues until the energy of the wave dissipates. The ratio of the
incident wave power to the reflected wave power is called the Return
Loss. An equivalent measure is called the Voltage Standing Wave Ratio
or VSWR. VSWR is the voltage ratio of the incident wave to the reflected
wave. The conversion between these two equivalent measures is shown
in equation 1. Return loss is normally expressed in dB. The higher
the return loss the less reflected wave and the closer to the ideal
we get. For example a return loss of 15 dB indicates that the reflected
wave power is 15dB lower in power than the incident wave power. In
the real world Return Loss is always a function of frequency .
Almost all Spectrum Analyzers on the market today claim
to have a 50 Ohm or 75 Ohm input impedance. In practice no Analyzer
has 50 or 75 ohms input impedance. A more informative and meaningful
measure is the return loss of the input port. Instrument makers typically
give a worst case return loss over the frequency band of the instrument.
A typical value might be -15dB(1.4:1 VSWR). The question arises, What
is a good enough Return Loss for my measurement? In table 1 a range
of Return Losses are rated. These are somewhat subjective assessments
but the idea is to impart a feel for the units involved. Generally
speaking one would like the highest return loss possible. Some measurements
are more sensitive than others and require higher return losses. Generally
speaking if all the return losses in your system under test are 20dB
or greater very little interaction between devices/instruments will
take place and you can believe what you are seeing.
Isolation
Isolation is best understood by example. Figure 2 shows
a typical three port mixer. RF and LO signals are injected at there
respective ports. The mixer is not perfect so some RF appears at the
IF and LO ports, conversely some of the LO leaks out through the RF/IF
ports. This not desirable undermost circumstances and a perfect Mixer
would have no leakage or equivalently infinite isolation. The higher
the isolation the lower the leakage. Typically the isolation is expressed
in dB as the difference in applied power at one port to that leaking
out at another.
Effects of Poor Return Loss and Low isolation
The effects of not having high return loss and isolation
both in your instrumentation and your DUT's is that the devices you
are trying to measure could interact with each other and your test
equipment. These interactions can range from the subtle to the extreme
depending on what exactly you are trying to do. Subtle effects might
only effect your measurements by a couple of dB. An example of extreme
effects is an oscillator that will not operate or turns into a broad
band noise source. The goal is to shoot for the highest return losses
feasible and the highest isolation possible in your test setup to
minimize these effects.
Improving Return Loss and Isolation Using Attenuation
Pads and Circulators
Now lets look at the case where the programmable attenuator
is set to 6dB in fig 1. We will assume for now that the attenuator
is perfect and its own return loss is infinite. The incident wave
power now must pass through 6 db of attenuation as well as the reflected
wave power. The net effect is increase the return loss by 12 dB, or
double the attenuator rating. If our instrument has a poor return
loss, say 6dB we can transform it very good rating of 18dB by just
adding 6 dB of attenuation. The down side is we lose some signal strength.
Signal strength allowing it always best to use 10dB or more of your
Analyzers programmable attenuation if available.
The above improvement will happen only if we use a high
return loss attenuator. High end Spectrum Analyzers have such programmable
attenuators built in which can have return losses above 25 dB well
into the GHZ frequency range. Lower end instruments may not have programmable
attenuators or if they do they may have low return losses(high VSWR's).
The most economical method to improve return loss is
to use fixed attenuators. These are available with a variety of connectors
and values typically from 1 to 60 dB. Fixed attenuators consistently
deliver higher return losses for the dollar than any programmable
attenuator. Even low cost 75 ohm "F" type attenuators can
deliver return losses of 20 db up to 1 GHz. Generally speaking the
return loss of fixed attenuators decreases with frequency. Before
using an attenuation "pad" be sure the R.L. vs. frequency
is adequate for the frequency range you are working in
. A circulator is a device that allows a forward wave
to pass with very low loss but reflected or backward traveling waves
are redirected to a termination, typically 50 or 75 Ohms. The reflective
wave is absorbed in the termination resulting in very good return
losses. Essentially circulators have all of the advantages of an attenuation
pad w.r.t. improving Return Loss with (almost) no losses in the forward
direction as is the case with attenuators. The down side of circulators
is that they are not nearly as broad band as a high quality attenuator
pad and they cost five to ten times more. Circulators can typically
have Return Losses of 15dB or more with less than 1 dB of forward
wave loss.
Another benefit of adding attenuator pads and circulators
to the test setup is increased isolation. By adding these components
in the right places of the test setup isolation is provided between
the various elements which inhibits interactions. Attenuators improve
isolation by the amount of attenuation while circulators typically
provide 15 to 20 db isolation. A case in point is the RF leakage out
the LO port in figure 2. A circulator on the LO port will not only
improve the LO port return loss it will also attenuate any signal
from RF port leaking out the LO port.
A Preferred Measurement Technique using fixed Attenuators
Figure 3 shows a typical test setup where we wish to
characterize some aspect of the DUT using a signal source and a spectrum
analyzer. Notice that 6 to 10 db pads are used on both the input and
output of the device under test. With the input and output attenuators
we ensure that reflections will be a minimum and that no reflection
caused interactions will occur both in the generator and in the spectrum
analyzer. If our DUT has moderate return losses of 8 to 10dB the attenuators
transform this to 20dB or more of Return Loss. With 20dB of return
loss we can fairly confident that interactions will be negligible
and we can trust our measurements.
As a general rule attenuators almost always have better
return loss than any amplifier, mixer or filter network. For this
reason a liberal spreading of attenuation pads throughout your test
setup is highly desirable.
Converting 75 ohms to 50 ohms
A lot of the world is 75 ohms not 50ohms, most notably
our cable TV system. The question is can I use my "50 Ohm"
Spec An. to make accurate amplitude measurements on 75 Ohm systems?
This question is a little misleading. The fact is that a 75 ohm system
is a good approximation to 50 ohms and represents a return loss of
about 14 dB. This is descent and may actually be better than the return
loss of your "50 Ohm" test equipment. To improve the accuracy
and return loss you can get a minimum loss 50/75 ohm attenuator pad.
Low cost units typically have a loss of about 6 db and provide return
losses from either direction of 20 db or more up to 1 GHz. In a pinch
use a 6 db 75 Ohm attenuator between your instrument and your 75 Ohm
system. With even moderate cost pads you should see Return Losses
of around 20 dB. With either technique your "50 Ohm" instrument
now looks pretty close to an ideal 75 Ohm system. Corresponding power
measurements will be lowered by approx. 6 dB due to pad losses. With
our min loss pad we now read power on spectrum analyzer(or power meter)
as if it were calibrated for a 75 Ohm system simply by subtracting
6 dB. Even without the matching pads the indicated power will typically
be within 1 to 2 db of actual.
Summary
Return Loss is a measure of reflected wave power. Return
loss also can thought of as measure of how close the input impedance
of our instrument or DUT "matches " an ideal termination
, say 50 ohms. It can also be expressed as VSWR. The proper use of
resistive attenuators and circulators if properly used can improve
the Return Loss and isolation present in the test setup which will
improve accuracy and repeatability. We can use our 50 Ohm equipment
to make measurements in a 75 Ohm environment with some degradation.
Using a low cost 75/50 Ohm matching pad will improve our accuracy.
Back
to top