Design Dengan Microstrip

Sukiswo

[email protected]

Elektronika Telekomunikasi, Sukiswo ST, MT 1

Outline

 Pengertian  Design Dengan Microstrip  Microstrip sebangai saluran transmisi (line transmission)  Microstrip as Equivalent Componen  Distributed Equivalent Component Design Elektronika Telekomunikasi, Sukiswo ST, MT 2

Pengertian



Microstrip

is a type of electrical transmission line which can be fabricated using printed circuit board [PCB] technology, and is used to convey microwave -frequency signals.

 It consists of a conducting strip separated from a ground plane by a dielectric layer known as the

substrate

. Elektronika Telekomunikasi, Sukiswo ST, MT 3

Pengertian

Cross-section of microstrip geometry. Conductor (A) is separated from ground plane (D) by dielectric substrate (C). Upper dielectric (B) is typically air.

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Pengertian

Band Stop Filter 1 GHz Elektronika Telekomunikasi, Sukiswo ST, MT 5

Pengertian

Power Divider 1 GHz Elektronika Telekomunikasi, Sukiswo ST, MT 6

Pengertian

 Microwave components such as antennas , couplers , filters , power dividers etc. can be formed from microstrip, the entire device existing as the pattern of metallization on the substrate.  Microstrip is thus much less expensive than traditional waveguide technology, as well as being far lighter and more compact. Elektronika Telekomunikasi, Sukiswo ST, MT 7

Pengertian

 The disadvantages of microstrip compared with waveguide are the generally lower power handling capacity, and higher losses.  Also, unlike waveguide, microstrip is not enclosed, and is therefore susceptible to cross-talk and unintentional radiation.

 For lowest cost, microstrip devices may be built on an ordinary FR4 (standard PCB) substrate.  However it is often found that the dielectric losses in FR4 are too high at microwave frequencies, and that the dielectric constant is not sufficiently tightly controlled.  For these reasons, an alumina substrate is commonly used.

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Pengertian

 On a smaller scale, microstrip transmission lines are also built into monolithic microwave integrated circuits [MMIC]s.

 Microstrip lines are also used in high-speed digital PCB designs, where signals need to be routed from one part of the assembly to another with minimal distortion, and avoiding high cross-talk and radiation.

 Microstrip is very similar to stripline and coplanar waveguide [CPW], and it is possible to integrate all three on the same substrate.

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Design With Microstrip

 At microwave frequencies, microstrip is employed not only as a transmission line on PCBs, but also as equivalent passive components, tuned circuits, and high-

Q

microwave filters and resonators.

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Design With Microstrip

 Microstrip transmission line is used in microwave applications due to its low loss, low cost, small size, ease of implementation, and the ability to mount components, such as surface-mount capacitors, resistors, and transistors, directly onto the microstrip itself.

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Design With Microstrip

   Most microstrips are unbalanced transmission lines and, because of their unshielded nature, can radiate some RF. However, radiations from properly terminated microstrips are fairly small.

Stripline

(Fig. 1.42) is similar to microstrip, but is placed between the metallization layers of a PCB and, due to these balanced twin groundplanes, does not radiate.

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Design With Microstrip

 The characteristic impedance of microstrip transmission line is governed by the width of the conductor, the thickness of the dielectric, and the dielectric constant; with low impedance microstrip lines being wide, and high impedance microstrip lines being narrow  But the most important attribute of terminated microstrip as transmission line is that its impedance does not change with frequency, or with length.  The normal characteristic impedances of microstrip and stripline are designed to be anywhere between 10 to 110 Ω, with 50 Ω being the universal norm for RF transmission line use.

 Microstrip is very common in frequencies of operation at 250 MHz and above Elektronika Telekomunikasi, Sukiswo ST, MT 13

Microstrip as Transmission Line

  A 50-Ω microstrip is utilized in microwave circuits to prevent reflections and mismatch losses between physically separated components, with a calculated nominal width that will prevent the line from being either inductive or capacitive at any point along its length. In fact, with a source’s output impedance matched to the microstrip, and the microstrip matched to the input impedance of the load, no standing or reflected waves will result.  Consequently, there will be no power dissipated as heat, except in the actual resistance of the copper and dielectric as

I

2

R

losses.

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Microstrip as Transmission Line

 In microstrip, the dielectric constant (

Er

) of the PCB’s substrate material will not be the sole

Er

that the microstrip transmission line itself “sees.”  This is due to the flux leakage into the air above the PC board, combined with the flux penetrating into the dielectric.  So the actual

effective dielectric constant

(

E

EFF), which is the true dielectric constant that the microstrip will now see, will be at some value between the surrounding air and the true dielectric constant of the PCB.

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Microstrip as Transmission Line

 Due to the small RF field leakage that emanates from all microstrips, these types of transmission lines should be isolated by at least two or more line widths away from other traces and circuits in order to decrease any mutual coupling effects.  To lower the chances of crosstalk even further an isolation ground trace may possibly be necessary between two such lines.

 To decrease any impedance bumps at high microwave frequencies, microstrip should always be run as short and as straight as possible, with any angle using a mitered or slow round bend (Fig. 1.43) Elektronika Telekomunikasi, Sukiswo ST, MT 16

Microstrip as Transmission Line

  Another issue to watch for when designing microwave circuits with microstrip transmission lines is the

waveguide effect

(see

RF Shielding Resonances

): Any metal enclosure that is used to shield the microstrip, or its source or load circuit, may act as a waveguide, and drastically alter the circuit’s behavior. This effect can be eliminated by changing the width of the shield to cover a smaller area, or by inserting a special microwave foam attenuator material within the top of the enclosure.

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Design with Microstrip

 Use the following equation to plug in different microstrip widths to obtain the desired impedance: Where

Z

O = characteristic impedance of the microstrip, Ω

W

= width of the microstrip conductor (use same units as

h

below)

h

= thickness of the substrate between the groundplane and the microstrip conductor (use same units as

W

)

E r

= dielectric constant of the board material Elektronika Telekomunikasi, Sukiswo ST, MT 18

Microstrip as Equivalent Componen

 Distributed components, such as inductors and capacitors, can be formed from microstrip transmission-line sections on PCBs at microwave frequencies.  A series or shunt inductor can be formed from a thin trace (Fig. 1.44),  A shunt capacitor can be formed from a wide trace (Fig. 1.45),  And even a transformer can be formed by varying the width of the microstrip (Fig. 1.46).

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Microstrip as Equivalent Componen

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Microstrip as Equivalent Componen

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Microstrip as Equivalent Componen

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Microstrip as Equivalent Componen

 Distributed equivalent microstrip works in replicating a lumped component because high impedance (thin) microstrip traces possess very little capacitance (due to the small surface area of the structure), but possessing a useful amount of inductance.  This inductance is caused by the current flowing through the constricted microstrip element, which is both thin and long.  In other words, we are taking a normal 50-Ω microstrip, with its built-in distributed inductance and capacitance that

makes

the microstrip line 50 Ω throughout its entire length, and simply minimizing the capacitance, while maximizing its inductance. Elektronika Telekomunikasi, Sukiswo ST, MT 23

Microstrip as Equivalent Componen

 The reverse is also true if we want to create a distributed capacitor, since the 50-Ω microstrip can now be formed as a wide strip, with the PCB’s copper groundplane and its dielectric being located directly beneath it.  Thus, the much larger surface area of this fatter microstrip (over its 50-Ω configuration) appears as a capacitor at RF frequencies.  However, we cannot accurately, nor with any value above 1 pF or so, replicate

series

capacitors in distributed form, so we should select an appropriate lumped matching network that is devoid of any such series capacitors, and then convert it to a distributed structure.

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Microstrip as Equivalent Componen

 The accuracy of the equivalency itself will only be exact for frequencies that are less than 30° long across the distributed equivalent shunt component.  Equivalency is still possible with longer lengths, but keeping all equivalent component elements at less than 30° will supply the best performance.  Maintaining this shorter length in the distributed component design will sometimes demand compensating, which we can do by narrowing the distributed inductor’s trace width (increasing impedance), or widening the distributed capacitor trace width (decreasing impedance), to keep the total length under 30°.

 Due to the increased losses and the lack of width repeatability during board fabrication, we cannot normally use equivalent inductors that are narrower than 6 mils.

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Distributed Equivalent Component Design

 As stated above, it is important to strive to make a distributed component shorter than 30° out of the 360° of an entire wavelength, or the lumped component.

equivalent component effect

will begin to depart more and more from that of an ideal  To calculate how long 30° is out of 360°, simply divide 30 by 360, then multiply this value by the actual wavelength of the signal on the PCB, keeping in mind that the signal’s wavelength in the substrate will not be the same as if it were traveling through a vacuum.

 To find the actual wavelength of the signal, which is being slowed down by the PCB’s substrate material, calculate the microstrip’s

velocity of propagation

(

VP

).  First, find the

effective dielectric constant

dielectric mediums.

(

E

EFF) of the microstrip, since the signal will be partly in the dielectric and partly in the air above the microstrip, which affects the propagation velocity through this combination of the two Elektronika Telekomunikasi, Sukiswo ST, MT 26

Distributed Equivalent Component Design

 First, find the

effective dielectric constant

two dielectric mediums.

(

E

EFF) of the microstrip, since the signal will be partly in the dielectric and partly in the air above the microstrip, which affects the propagation velocity through this combination of the Elektronika Telekomunikasi, Sukiswo ST, MT 27

Distributed Equivalent Component Design

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Distributed Equivalent Component Design

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Distributed Equivalent Component Design

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Perhitungan Microstrip

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Perhitungan Microstrip

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Perhitungan Microstrip

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Perhitungan Microstrip

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Perhitungan Microstrip

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Perhitungan Microstrip

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Perhitungan Microstrip

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Perhitungan Microstrip

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