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Application Note on Transformers

Application Note on Transformers

Application Note on Transformers

(AN-20-002)

1 Introduction

The purpose of this application note is to describe the fundamentals of RF and microwave transformers and to provide guidelines to users in selecting proper transformer to suit their applications. It is limited to core-and-wire and LTCC transformers.

2 What is a Transformer

A Transformer is a passive device that “transforms” or converts a given impedance, voltage

or current to another desired value. In addition, it can also provide DC isolation, common mode rejection, and conversion of balanced impedance to unbalanced or vice versa, as explained later. Transformers come in a variety of types; our focus is on transformers used in RF and Microwave signal applications. Essentially, an RF transformer consists of two or more windings linked by a mutual magnetic field. When one winding, the primary has an ac voltage applied to it, a varying flux is developed; the amplitude of the flux is dependent on the applied current and number of turns in the winding. Mutual flux linked to the secondary winding induces a voltage whose amplitude depends on the number of turns in the

secondary winding. By designer’s choice of the number of turns in the primary and

secondary windings, a desired step-up or step-down voltage/current/impedance ratio can be realized.

3 Why are Transformers needed

Transformers are used for1:

?Impedance matching to achieve maximum power transfer between two

devices.

?Voltage/current step-up or step-down.

?DC isolation between circuits while affording efficient AC transmission.

?Interfacing between balanced and unbalanced circuits; example: push-pull amplifiers, ICs with balanced input such as A to D converters.

?Common mode rejection in balanced architectures

4 How are they made

An RF transformer usually contains two or more insulated copper wires twisted together and wound around or inside a core, magnetic or non-magnetic. Depending on design and performance requirements, the core can be binocular as in Figure 1, toroid (doughnut shaped) as in Figure 2 etc. Wires are welded or soldered to the metal termination pads or pins on the base. The core and wire ensemble is housed in a plastic, ceramic or metal case.

Application Note on Transformers

Wire welded to termination pads

Application Note on Transformers

Wrap around Termination pad

Core

Twisted wire

Ceramic/Plastic base

Application Note on Transformers

Figure 1 Open Case Transformer (Binocular Core)

Application Note on Transformers

Figure 2 Toroidal Core

5 Ideal transformer

At low frequencies, an alternating current applied to one winding (primary) creates a time-varying magnetic flux, which induces a voltage in another (secondary). At high frequencies, the inter-winding capacitance and magnet wire inductance form a transmission line which helps propagate the electromagnetic wave from primary to secondary. The combination of magnetic coupling and transmission line propagation helps the transformer to achieve outstanding operating bandwidths (1:10000 or more). Figure 3 shows ideal circuit of a simplified two-winding transformer.

Application Note on Transformers

Primary Secondary

Sec N2N1

Application Note on Transformers

Balun

Sec

Sec

Application Note on Transformers

+Sec

-DI

Port #2

T1

+Sec

+

-

CI

Port #2

CO

T1

Application Note on Transformers

+Sec

+

-DI

V DI

Port #2

T1

Application Note on Transformers

9.2 Push-Pull amplifiers3

Benefits:

Even-order harmonic suppression, which is a big deal in wideband Cable TV application ~3 dB higher Pout & IP3 than a single device.

Wideband communication systems have signals occupying multi-octave frequency range.

For example, CATV signals occupy 50-1000 MHz range, which is more than four octaves.

Such signals when amplified in conventional amplifiers can be distorted due to the second order products generated inside the amplifier. For example, second harmonic of 50 MHz signal is 100 MHz, so also second harmonic of 400 MHz which is 800 MHz and both are within the band.

An ideal push-pull amplifier can cancel the internally generated products and preserve the signal quality. Figure 10 shows a simplified schematic of such an amplifier. It consists of two baluns and two identical amplifiers. When a signal is applied to the input of the first balun (Balun #1), the output signal from the same balun consists of two signals of equal amplitude and out of phase. These signals are amplified combined in output balun (Balun #2).

Application Note on Transformers

Figure 10 Simplified Schematic of Push-Pull Amplifier

The gain of a push-pull amplifier is same as that of an individual amplifier, whereas the output power is twice that of an individual amplifier. Push-pull connection is frequently used for combining power of individual amplifiers.

An additional benefit, push-pull amplifiers cancel even-order harmonics, as even-order harmonics are in-phase. An example is shown in Figure 11 for second harmonic. Same is true for other even order products falling within the operating bandwidth of the transformer.

Application Note on Transformers

Sec

Figure 12 Push-Pull amplifiers using Transistors & Baluns

Application Note on Transformers

Sec

Diode Quad

Application Note on Transformers

Application Note on Transformers

Mini-Circuits Transformers

Figure 15 Baluns used at input & output to convert from and to single ended

10 Transformer Configurations6

Transformer configurations can be broadly classified as:

Conventional; core-and-wire based(Configurations A,B,C,D,F)

Transmission line; core-and-wire and LTCC (Configuration G,H,K)

Marchand; LTCC (Configuration J)

See Table I for the schematics, frequency of operation, impedance ratio, important

electrical parameters and applications

Conventional transformers made of core-and-wire optionally have center tap on primary or secondary or on both sides and are limited to an upper frequency of 2 GHz. Most

configurations have DC isolation from primary to secondary.

Transmission line type transformers using core-and-wire operate to 3 GHz and using LTCC to 5 GHz or higher and do not have DC isolation from primary to secondary. External

blocking capacitors are needed to realize isolation.

Marchand (named after the inventor) transformers operate to 6 GHz and higher and are realized in LTCC form and have DC isolation from primary to secondary.

Selection of a transformer for an application can often be confusing and sometimes results in the wrong choice. The following guidelines attempt to clarify the options and state the benefits of various configurations.

For impedance matching (unbalanced to unbalanced) applications; choose auto

transformer (Configuration –D), in general it provides lowest insertion loss.

For Balun applications, choose a balun with center tap on balanced side as it provides excellent amplitude and phase balance (Configurations A, B, H, J).

Application Note on Transformers

For Balanced to balanced transformation, choose a transformer with center tap on both primary and secondary (Configuration B, L) as it provides excellent amplitude and phase balance on both sides.

For DC isolation between primary and secondary, do not choose transmission line configurations (G, H, K,). If needed; add DC blocking caps to isolate primary and secondary.

Applications C o n f i g u r a t i o n

Schematic

Description

Frequency (MHz) Typical maximum range

Impedance ratio Typical maximum ratio

U n b a l a n c e

D C

I s o l a t i o n

Power handling Typical use I m p e d a n c e C o

n v e r s i o

n

B a l u n B a

l a n c e d t o

Application Note on Transformers

B a l a n c e d A

DC isolated

primary and secondary, center-tap secondary 0.01 to 1400

1 to 16 Excellent Yes

Up to 1W

YES YES YES

Application Note on Transformers

B

DC isolated primary and secondary, center-tap primary and secondary 0.004 to 500

1 to 25 Excellent Yes

Up to 0.25 W

YES YES YES

Application Note on Transformers

C DC isolated primary and secondary 0.01 to 1200 1 to 36 Average Yes Up to 0.25 W YES YES YES D

Auto

transformer 0.05 to 2200 0.1 to 14

N/A

No

Up to 0.25 W YES

--

--

Application Note on Transformers

Application Note on Transformers

F

DC isolated, three open windings, Tri-filar transformer 0.01 to 200

1 to

2 Good No Up to 0.25 W YES YES YES

G

Transmission line

transformer 0.5 to 3000 1 to 4 Good No Up to 2 W YES YES YES

Application Note on Transformers

Application Note on Transformers

H

Transmission line

transformer-four

windings 10 to 4500 2 & 4 Good No Up to 5 W

YES YES YES

Application Note on Transformers

J

Marchand Balun

600 to 6200 1 to 4

Excellent Yes 3 W YES YES --

K

Transmission line

Application Note on Transformers

transformer: Tri-Filar

5 to 3000 1

Excellent No

Up to 0.5W

--

YES YES

Application Note on Transformers

Application Note on Transformers

Application Note on Transformers

Sec

Application Note on Transformers

loss of two units was measured. Insertion loss of a single device was calculated by dividing the measured loss by 2. This overcame the need to match impedance of devices having output impedance other than 50 ohms.

In recent years, baluns have been characterized as 3-port networks, like a two-way 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 secondary1 for that purpose. In this method insertion losses from primary dot to secondary dot and primary dot to secondary are measured. The average of these two losses after subtracting the loss of the matching pad and 3 dB for loss due to theoretical split, is specified as insertion loss.

New network analyzers such as Agilent’s PNA series provide impedance transformation and port extension capabilities so that there is no need to add resistive matching pads. A PNA analyzer enables 3-port measurement for any user-defined input and output

impedances.

12.2 Unbalance: Amplitude and Phase

The set up used for charactering a transformer as a 3-port network provides two insertion losses (primary dot to secondary dot and primary dot to secondary) in vector form. The difference of these two magnitudes in dB is called amplitude unbalance. The phase angle deviation from 180° is phase unbalance.

12.3 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.

12.4 S-Parameters

By using a multi port network analyzer, s-parameters can be measured. The resulting “.snp” file is in Touchstone format and can be used in simulators such as Agilent ADS.

When an application needs impedance other than the one specified in the data sheet, “.snp” can be used in simulation software such as Agilent’s ADS (or equivalent ) to analyze its performance.

13 Summary

This application note is to describe the fundamentals of RF and microwave transformers, most common applications, guidelines to users in selecting proper transformer to suit their applications and measurement methods.

14 References

Application Note on Transformers

1) Mini-Circuits Application Note, “How RF Transformers Work”, http://www.wendangku.net/doc/daf5d8b2cf84b9d529ea7ae3.html/pages/pdfs/howxfmerwork.pdf

2) Nathan R.Grossner,”Transformer for Electronic Circuits”, McGraw-Hill Book Company, Second edition, 1983

3) R.Setty, “Push-pull amplifiers improve second-order intercept point”, RF Design, P76, Nov 2005

4)Mini-Circuits website, http://www.wendangku.net/doc/daf5d8b2cf84b9d529ea7ae3.html/cgi-bin/modelsearch?model=hela-10 , click link ”Data Sheet”

5) Dorin Seremeta, “Accurate Measurement of LT5514 Third Order Intermodulation Products”, Linear AP note 97-3

6) “Transformers RF/IF”, Mini-Circuits web page

http://www.wendangku.net/doc/daf5d8b2cf84b9d529ea7ae3.html/products/transformers.html