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 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.
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.