Category Archives: Baluns
What is the difference between the BAL line of products (BAL-0003/6/9SMG and BAL-003/6/10) and the equivalent BALH products (BALH-0003/6/9SMG and BALH-0003/6/10)? Which one is a 1:1 (50 Ω single ended to 50 Ω differential/25 Ω single ended) transformer and which one is a 1:2 (50 Ω single ended to 100 Ω differential/50 Ω single ended) transformer?
A while ago we wrote a product feature for Microwave Journal for our isolation baluns. Basically we showed why they were better than 4 port VNAs for high speed differential testing.
These baluns are now available up to 67 GHz:
in the literature on the internet about baluns, a distinction that will often come up is ‘current’ baluns vs. ‘voltage’ baluns. I’ve always found this distinction confusing, because an electromagnetic wave converted by a balun from differential to unbalanced modes consists of both current AND voltage waves. They interplay to make a single electromagnetic wave. I can understand the terms applied to transistors, i.e. the current driven BJT vs. the voltage driven FET, since in this case the charges are either transmitted across the junction (current) or they only create a field (voltage).
If you really want to learn a lot about baluns, you need to talk to amateur radio operators. Jerry Sevick was probably more obsessed with baluns than anyone else in history. This means that the modern understanding of baluns is as viewed through the lens of antenna operation. The roots of the terms voltage balun and current balun trace to an article by Roy W. Lewallen, call sign IVTEL, in the ARRL Antenna Compendium, Baluns: What They Do And How They Do lt. In it, Lewallen describes the following two baluns, and differentiates one as the ‘current’ balun, and one as the ‘voltage’ balun:
This paper described the 1:1 transmission line transformer on top, the basis of our surface mount and 10 GHz and below connectorized baluns, as a current balun. This is because they force the currents to be equal and opposite, regardless of the differential impedances. This is easy to understand in an ideal flux coupled balun, as the signal transfers through the magnetic flux from one wire to the other wire only due to the current induction.
The second balun is Fig. 2 from Ruthroff’s seminal and diminutive named paper, “Some Broad-Band Transformers”. The connoisseur will recognize this as a compensated version of the above 1:1 balun. Lewallen describes this as a voltage balun, since the extra line forces the voltages at the output to be equal and opposite, regardless of what the impedances are.
I believe there are two problems with Lewallen’s analysis, one that is sort of his fault and one that is not. First, Lewallen points out in his paper that the issues arise with the different types only when the impedances are unmatched between the differential ports, as one would see with an antenna but not in many of the modern high speed differential setups that Marki baluns are suited for.
Second, it is based on a DC analysis, assuming that the voltages are not time varying. This is true at the low frequencies Lewallen was working at, but it breaks down for transmission line transformers generally. Also the impedance mismatches can cause reflections at higher frequencies that break down the ‘current’ balun argument.
In conclusion, I think that these distinctions may be valid and meaningful for low frequency baluns used in amateur radio setups, but for high speed test baluns the most useful metric is not voltage vs. current, but how much isolation is there?
As detailed in this blog post, there was a previously unaddressed problem with using baluns back to back in a test setup, with a VNA for example. The problem was that the baluns did not have isolation, which would cause a signal input to one differential port to show up at the other differential port. This would cause a resonance in the S parameters, and an ‘echo’ in the time domain behavior of the balun.
Starting now, this problem is a thing of the past. Marki Microwave just released a new line of ‘Isolation Baluns’. These are baluns built using a through line and an inverter, as has been sold before, but with a Wilkinson power divider between them. As I showed in this blog post, a Wilkinson power divider is capable of splitting data. It is also capable of combining data, so long as the data is common mode (identical on each arm). This is the circumstance with the new baluns, when placed back to back.
Previously this would cause an insertion loss ripple, like this:
As you can see in this second plot taken with back to back baluns there is very little ripple in the insertion loss, vs. the large ripple in the insertion loss of the back to back baluns in the case with no isolation. Additionally, the return loss does not exhibit the same high values, for the same reason.
This effect can be seen in the time domain as well. When two non-isolation baluns are placed back to back, a step echo can be seen in the oscilloscope trace of a square wave input:
The top trace here is the input, and the bottom trace is the output of two back to back baluns without isolation (not to scale). Compare this with the effect when two isolation baluns are placed back to back:
This implies that these isolation baluns can be used to extend a 2 port VNA to differential testing simply by de-embedding the S-parameters of the back to back baluns, something that was not possible before. This is something that we are still working on, but I will write an update when we have an exact procedure.
I got an email asking if our baluns would work to 16 Gb/s data signals, so I tested them with the fastest data signal we can generate here before our BERT starts to go loopy. Here are the results:
As I’m detailing in an upcoming app note, baluns are extremely useful devices. They can be used to interface with differential chips, build balanced amps and mixers, and drive antennas. They can also be used to create differential signaling lines that are immune to common mode noise.
There is a problem, however, if the signaling lines are not sufficiently lossy enough. If two baluns are placed back to back, the insertion loss is about what is expected, but it has tremendous ripple in both insertion and return loss. The frequency of this ripple is determined by the length and amount of loss of the lines connecting the two baluns. Here is what the output of two BALH-0006 baluns back to back looks like:
As you can see the insertion loss ripples along the expected insertion loss curve, along with the return loss increasing dramatically at each insertion loss suckout. So there is obviously some resonant behavior going on, the question is where does it come from exactly?
The frequency of the insertion loss ripple depends on the length of the cables between the baluns, therefore there must be a stray signal rattling around on the cables. There are two potential sources for this signal: bad return loss on the output, and the lack of isolation. EIther the signal is reflecting from the differential ports of the second balun, or it is being created by the inputs traveling from one differential port to the other. Previously this signal has been blamed on the poor return loss, but this seems unlikely.
.The BALH-0006 has good output return loss, which is odd for a balun with no isolation. In general you would expect to see common mode signals (which is half of any input signal when the other port is grounded) reflected entirely. In the case of the BALH-0006, the return loss is actually better than 20 dB at low frequencies, and better than 15 dB across the band. Therefore it seems more likely that the cause is lack of isolation. The BALH has only 1 dB better isolation than it’s insertion loss, which means that almost as much power goes from one balanced arm to the other as from the balanced side to the unbalanced side. How do we prove that it is one or the other?
This is where time domain techniques become valuable. First define the delay time for each step in the balun connection. Through the balun is t1, through the cables is t2, and from one balanced input out to the other (through lack of isolation) is t3.
The through path, is delayed by 1 balun t1, then the cables t2, then the second balun t1, for a total delay time of 2*t1+t2.
A pulse generated by a bad return loss will see the first balun t1, then the cable delay t2, then it will reflect from the other balun’s balanced outputs and go back through the cables for another t2, then again reflect for another t2, and finally pass through the last balun t1. This gives a total delay of 2*t1 + 3*t2. We will see the step arrive at a time 2*t2 after the first step output.
If the ripple is dominated by the lack of isolation, the step will travel through the first balun t1, then the cable t2, then from one balanced output to the other t3, then through the cable t2, then from one balanced output to the other t3, then through the cable again t2, and finally out the last balun t1. The total time is 2*t1 + 3*t2 + 2*t3. The step will arrive 2*t2 + 2*t3 after the first step, and 2*t3 after the return loss step.
First we build a square wave generator with a low rep rate (200 MHz with a 20 ps risetime) by cascading several square wave amps. Next we measure each of the time delays. First the transit time of the balun is measured to be t1 = 330 ps (not needed for the experiment).
Then we look at the delay of the cable, measured to be t2 = 300 ps.
Finally the transit time from one balanced input to the other is found to be t3 = 100 ps. You can also see in the screenshot below that the output pulse from the second unbalanced input is still quite large.
Here is what the output of the two back to back baluns looks like, overlaid with the input.
You can clearly see that despite a clean input, there is a delay in achieving the full output power until some time after the initial step arrives, and further that there is some ripple on the step function in between. If we zoom in on the output pulse we can see the details.
Here we can clearly see that the initial step, and after a delay of about 600 ps (=2*t2) there is another very small step, and finally the big step 200 ps (=2*t3) after that.
So now we have the full story of what happens when you put two baluns back to back and try to send data through them. Even with a perfect return loss, the non-isolated path creates a delayed second version of the input step, distorting the output pattern. Eventually all of the power arrives at the end, but only after a long delay equal to the twice the cable delay (plus some small amount). This contradicts what is currently found in the applications literature, which blames this phenomenon on return loss problems.
Since we released our BAL-0006SMG (and now BALH-0006SMG) broadband surface mount baluns, we have received a lot of interest in them from people using high speed analog to digital converters, and have wondered why they were so driven to find the best balun/transformer available. Today I figured out why.
Rob Reeder, of Analog Devices has a paper on why phase balance in particular matters for A/D converters. The basic idea is that while an ideal ADC is perfectly linear, an actual ADC has a slightly non-linear transfer function, with some remnants of the second and third harmonics appearing in the output of the ADC. This is the limiting factor in the dynamic range of the A/D, and that is the reason that spur-free dynamic range (SFDR) is one of the most important specs for an ADC. The most significant source of signal distortion is frequently the second or third harmonic, and this is specified on the datasheet.
A differential ADC using a balun/transformer at the front end can cancel out the second harmonic significantly, but only if it is well balanced. Any imbalance, and in particular the phase imbalance, will lead to a significant increase in the second harmonic spur in the ADC output. The phase balance is more important than amplitude, because the distortion due to phase imbalance is proportional to the amplitude squared of the input signal, while the distortion due to amplitude imbalance is proportional to the difference in the square of the amplitudes, which is much lower.
So I took Reeder’s MATLAB code to compare the BAL-0006SMG to a competing 6 GHz balun. Here are the results. First the signal output with the competitor balun (amplitude balance .5 dB, phase balance 12 degrees at around 3 GHz), assuming an ADC that would be second harmonic limited:
In this case the dynamic range is limited by the second harmonic to 73 dB. Now if we replace it with a Marki BAL-0006SMG at the same frequency, the amplitude balance improves from .5 dB to .2 dB typically, and the phase improves from 12 degrees to 3 degrees typically. This is the result:
As you can see the spur is reduced to 85 dBc, near the third harmonic. In this case the change of the balun improved the dynamic range of the ADC by more than 10 dB. When high signal sensitivity in broadband applications is critical, the BAL-0006SMG is by far the best choice available on the market today.
Baluns, transformers, and pulse inverters are related products in that each will take a single ended input and provide an inverted signal, in the sense that the negative voltages become positive and the positive voltages become negative. A pulse inverter performs this function in the most straightforward way, simply reversing the voltage of an input signal. A ‘transformer’ in the microwave sense generally means a device that will transform one voltage into another at a given current, and in doing so will transform the impedance. The word ‘balun’ is a portmanteau of ‘balanced’ and ‘unbalanced’; a single ended input to a balun will create an image version of itself with half of the power (on the positive output) and an inverted version with half the power (on the negative output). In this sense it is similar to an out of phase or 180° power divider, except that a balun is inherently a 3 port device, while a 180° hybrid is a 4 port device.
There are two main types of baluns: those that transform the input impedance and those that do not. If the balun divides the input voltage into two parts, while keeping the input current, then the output impedance on each line with be half of the impedance of the input line impedance. This is a 1:1 transformer, as no impedance transformation takes place (although generally the input lines are 50 ohms and the output lines are 25 ohms, because two 25 ohm lines makes 50 ohms differential). If the balun cuts the current in half to match the voltage drop then it is an impedance transforming balun, called a 1:2 transformer (because the 100 ohm differential output impedance is twice that of the 50 ohm input).
Here are some relevant performance criteria for Marki Microwave balun/transformers and pulse inverters:
While not a figure of merit in it’s own right, each performance metric of a balun is limited to a certain band. On the low end it tends to be limited because the device is only a fraction of a wavelength long. On the high end it is limited because the symmetry required to maintain balance is difficult to maintain at shorter wavelengths, just as with an in-phase or quadrature power divider.
Related to the bandwidth, the rise and fall time determine how well the balun can handle data. The risetime needs to be less than around 1/3 of the bit period to ensure good data transmission with minimal amplitude eye closure. For example, the BAL-0010 has a rise/falltime of 20 ps, allowing it to transmit with a period up to 60 ps or around 17 Gb/s.
Amplitude and Phase Balance
In an ideal balun the two outputs will be identical inverses of each other at every frequency. In reality the two outputs will have slightly different amplitudes and slightly different time delays, resulting in the phase of the output of the two signals being slightly off from each other. A typical balun application will require an amplitude balance of better than .5 dB and phase balance of 5° or better. Most applications are more sensitive to phase imbalance than they are to amplitude imbalance.
Common Mode Rejection
A balun can be used to reject common mode noise (noise that is present on both the positive and negative polarity signals); this is one of the major benefits of differential signaling. How well a balun can cancel out the common mode noise from two differential inputs is called the common mode rejection of the balun. How much is necessary depends on the application.
Unlike a magic tee or 180° hybrid power divider a balun does NOT have any intrinsic isolation. The isolation indicates how much power from a signal going into one arm will show up in the other arm, assuming all ports are terminated with matched loads. Hybrids resistively terminate in-phase signals, leading to isolation. Baluns reactively terminate signals, and so the isolation is simply equal to the insertion loss of the device.
Insertion Loss, Return Loss, and VSWR
As with all passive RF and microwave components, the insertion loss refers to how much power is lost in transmission. In 2:1 baluns this is 6 dB (3 dB for the power splitting and another 3 dB for the matching), but in 1:1 baluns the nominal insertion loss is only 3 dB.
The return loss and VSWR for baluns tends to be better at the common port than at the differential ports.