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` Balun Transformers
` A Balun is a device which converts balanced impedance to unbalanced and vice versa. In addition, baluns can also provide impedance transformation,
`hence the name Balun Transformers.
`The following sections describe the properties of various commercially available baluns.
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` Types of Transformers
`Following are the most commonly available balun transformers:
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` I. Ruthroff1 Balun Transformers.
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`Figure 1a
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`Figure 1b
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`Figure 1c
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` In the most common form, these use a pair of twisted magnet wire wound around a ferrite or powdered iron core.Figure 1(a) shows an equivalent
`circuit of the balun, and Figure 1b shows its actual implementation. Baluns of this type provide multi decade bandwidth and are generally limited to
`frequencies below 1.5 GHz. They also provide isolation from primary to secondary, and can provide a variety of impedance ratios. The higher the
`impedance ratio, lower the bandwidth. Variations are constructed with secondary center tap, Figure 1(c).
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` Figure 2, shows performance of such a Balun, having 1:4 impedance ratio and centertapped secondary. (Model TC414+)
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` II. Guanella2 or Transmission line transformers
` As frequency of operation increases, insertion loss of Ruthroff transformers increases; so also unbalance and VSWR. Transmission line transformers
`overcome these limitations.
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`Figure 3(a) shows the equivalent circuit of a 1:1 balun. Figure 3(b) its implementation in simplest form. Figure 3(c) is its alternate implementation. Figure
`3(d) shows a 1:4 balun.
`Transmission line transformers provide very wide bandwidth and operate up to 3 GHz and higher.
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`Figure 3(b)
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`Figure 3(c)
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`Figure 3(d)
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`Figure 3(a)
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`Figure 4 shows the performance characteristics of a transmission line balun implemented in LTCC. (Model TC1113MG2+)
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` III. Marchand Balun3 Transformers
` Transmission line transformers do not provide isolation from primary to secondary. When such isolation is essential for the performance of the circuit,
`external DC blocks need to be used. Marchand Balun overcomes this problem. Fig 5 shows its schematic.
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`Balun
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`Figure 5
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` In its original form it used coax/cavities and was very bulky. Over years of research, it was implemented in microstrip and in recent years in LTCC
`(Example: Some MiniCircuits models with prefix TCN and NCS). LTCC baluns are very compact (such as 1206 or 0805 size). Commercial Marchand
`baluns operate above 600 MHz. Theoretically, they can provide any impedance ratio, but commercially available baluns are generally limited to 1:1, 1:2,
`1:3 and 1:4 ratios. Figure 6 shows the performance of a Balun implemented in LTCC, (Model TCN422+) In addition to being compact, LTCC baluns
`also provide stable performance over a wide temperature range such as 55° to 100°C.
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` Characterization at arbitrary impedances
` Balun transformers are generally characterized in 50 or 75 ohm systems until now due to the limitation of the test instrumentation. Thanks to the
`availability of impedance transforming capabilities of the new network analyzers (such as Agilent’s ENA/PNA series), it is possible to characterize them
`at any other impedances.
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` Explanation of terms used
`Insertion Loss
` Prior to the availability of modern network analyzers, the baluns were connected back to back and the insertion losses of two baluns were measured together.
`Insertion loss of a single balun was calculated by dividing the measured loss by two.
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` In recent years, baluns are characterized as 3 port networks, like a twoway 180° splitter. As the impedance at the secondary ports is generally not 50 ohms,
`impedance transformation is essential to do an accurate measurement. One method is to use resistive matching pads at the secondary4 for that purpose. In this
`method insertion losses from primary dot to secondary dot and primary dot to secondary (after subtracting loss of matching pad and 3 dB for loss due to theoretical
`split) are measured. The average of these two losses is specified as insertion loss.
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`Balun
` New network analyzers such as Agilent’s PNA series provide impedance transformation and port extension capabilities and hence there is no need to add
`resistive matching pads. This also enables measurement for any userspecified input and output impedances.
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`Unbalance Amplitude and Phase
` In an ideal Balun, with input at primary (unbalanced port), the output voltage at the two secondary ports should be identical in amplitude but differ in phase by
`180°. In practical Baluns there is always a difference, amplitude unbalance (expressed in dB) and phase (deviation from 180°) expressed in degrees. The set up
`used for charactering a balun as a 3port network, provides two insertion losses (primary dot to secondary dot and primary dot to secondary). The difference of
`these two powers in dB is called amplitude unbalance. The phase angle deviation from 180° between the secondary ports is phase unbalance.
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`Input Return Loss
` When the secondary is terminated in its ideal impedance, the return loss measured at the primary is the input return loss. It is a measure of the effectiveness of
`the balun in transforming impedance.
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`References:
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`1) Ruthroff, C.L., “Some Broadband Transformers,” Proc IRE, vol 47, August 1959, pp 13371342
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`2) Guanella, G., “ New Method of Impedance Matching in RadioFrequency Circuits”, Brown Boveri Review, September 1944, pp. 327329
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`3) Marchand, N., “TransmissionLine Conversion transformers”, Electronics, Vol 17, December 1944, pp 142145
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`4) MiniCircuits Application Note, “How RF Transformers work and How they are measured,” Click here to review this article
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`https://www.minicircuits.com/pages/BalunApplicationNote.htm
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