Millimeter-Wave Concepts can be used to Optimize the

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John Coonrod

Technical Marketing Manager

Millimeter-Wave Concepts can be used to Optimize the Performance of High Speed Digital Circuits

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Millimeter-Wave Concepts can be used to Optimizethe Performance of High Speed Digital Circuits

Agenda:

• Basic millimeter-wave concepts

• Basic High Speed Digital concepts

• High Speed Digital measurements

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Basic millimeter-wave concepts, terminology


• Print Circuit Board (PCB) also known as Printed Wiring Board (PWB)

• RF (Radio Frequency) a generic high frequency term

• Microwave is a range of frequency from 300 MHz to 30 GHz

• Millimeter-wave (mmWave) is a range of frequency from 30 GHz to 300 GHz

• Dielectric constant also known as• Dk, relative permittivity, εr , Er , K’, ε’• It is related to the real component of complex permittivity (ε = ε’ + jε”)

• Dissipation factor also known as• DF, Tanδ (“Tan Delta”), Loss tangent, K”, ε”• It is related to the imaginary component of complex permittivity (ε = ε’ + jε”)

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Basic millimeter-wave concepts, waves


Wavelength (λ) is the physical length from one point of a wave to the same point on the next wave; as reference, the point shown here is a phase angle of 90°

Long wavelength = low frequency

Short wavelength = high frequency and more waves in a given same time frame

Amplitude is the height of the wave and often related to power

High electric field = High magnetic field = High amplitude = High power

90°

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Basic millimeter-wave concepts, waves


90° or

Phase velocity is how fast the wave propagates in reference to one point on the wave

The point is a phase angle and in this case it is 90°but it could be any phase angle on the wave and one wave has the potential for 360°phase angle

Fractions of wavelength are also very important and 90°is ¼ wavelength (λ) or

Important fractions of wavelength are ½, ¼ and sometimes 1/8

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Basic millimeter-wave concepts, Dk


Circuit with low Dk Circuit with high Dk

• A higher Dk material will

• cause the wave to have a shorter wavelength

• slow an electromagnetic wave

• can have lower amplitude

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Basic millimeter-wave concepts, transmission lines


• Impedance is the property of the circuit which impedes the propagation of the wave

• A matched system will have the same impedances for all components of the system

• Matched impedance systems allow efficient transfer of energy from generator to load

• However there will always be some losses due to the transmission line, called insertion loss

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• Transmission lines are often used to connect modules together

• The above illustration is an example of a transmission line with 3 dB insertion loss

• 3 dB is a loss of half of the power (dB is a logarithmic unit)

• The load receives half of the power that the generator sent


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• A mis-matched system will have losses due to reflections

• Differences in impedance, along the wave propagation path, can cause reflections

• There are two reflection points shown for this example:

• One reflection occurs at the generator going from 50 ohms to 30 ohms

• The second reflection occurs at the load going from 30 ohms to 50 ohms

• The losses due to these reflections are called return loss or reflected loss

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• When a transmission line is evaluated, the accuracy of the insertion loss is strongly dependent on the return loss

• The evaluation system knows how much energy left the generator and how much energy was received at the load and assumes all lost energy is due to insertion loss of the circuit

• However, if return loss is significant, some energy is reflected before getting to the load and that is not due to the insertion loss of the transmission line

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Basic millimeter-wave concepts, S parameters

S parameters are wave Scattering parameters, related to transmission and reflection

S21 is commonly referred to as insertion loss

S21 is the amount of energy that arrived at port 2, which originated from port 1

S11 is return loss (reflected energy at port 1)

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Basic millimeter-wave concepts, S parameters

Knowing how much energy left port 1 and how much energy arrived at port 2 is a measure of insertion loss (S21), but that measurement is influenced by S11

If S11 is large, then the value of S21 is not just insertion loss and the assumed value of S21 is incorrect because some energy was reflected back into Port 1

S11 is an indicator of the quality of the measurement for S21 and S11 should be small

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Basic millimeter-wave concepts, Insertion loss and return loss

x-axis is frequency

S21, insertion loss

S11 and S22 return loss

Microstrip transmission line circuitExample of insertion loss for a microstrip transmission line

Return loss is a logarithmic value

A return loss value of 0 dB means all energy was reflected and no energy went down the transmission line

A return loss of 15 dB means very little energy was reflected and most went down the transmission line

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S21, insertion loss

The circuit was 6 inches long using 10mil RO4350B™ laminate with a 21mil wide conductor

At 29.5 GHz the circuit had 6.8151 dB insertion loss which is a little more than 1.1 dB/inch



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S11 and S22 are return losses(S11 – yellow, S22 – magenta)

Rule of thumb at less than 50 GHz, return loss should be 15 dB or better (better is more negative on this scale)

S11 is good in this entire frequency range

S22 is poor at 35.5 GHz and can interfere with the wave, causing more insertion loss



Notice the dip in insertion loss at 35.5 GHz due to the effect of return loss

Dip in insertion loss due to poor return loss

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Basic millimeter-wave concepts, phase response

Microstrip transmission line circuit

• The previous slides were related to loss, however phase angle can be extremely important

• For simplicity, assume the circuit has sine waves propagating on it

• At a very specific frequency, it is possible for a circuit to have one sine wave (or 1 wavelength) on the circuit and that means the circuit would have a 360 degree phase angle

Input = 0° Output = 360°

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Basic millimeter-wave concepts, phase responseInput = 0° Output = 360°

Reference circuitDk = 3.0Length = 1 inch (25mm)Frequency = 7.43 GHzCircuit phase angle = 360°

Input = 0° Output = 432°

Dk = 3.0Length = 1.2 inch (31mm)Frequency = 7.43 GHzCircuit phase angle = 432°

Dk = 4.0Length = 1 inch (25mm)Frequency = 7.43 GHzCircuit phase angle = 410°

Output = 410°

Dk = 3.0Length = 1 inch (25mm)Frequency = 14.86 GHzCircuit phase angle = 720°

Output = 720°

Input = 0°

Input = 0°

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Basic millimeter-wave concepts

Specific mmWave Concerns

• Spurious Mode Propagation

• Radiation loss

• Thin circuit issues

• Signal launch

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Basic millimeter-wave concepts, spurious mode propagation

The desired wave propagation mode for microstrip is typically quasi-TEM

Other wave modes can propagate on the transmission line

These other modes are typically not desired and termed spurious parasitic wave mode propagation

These spurious modes can interfere with the quasi-TEM mode

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Resonances can set up between the signal conductor and the ground plane

Resonances can set up between opposite edges of the signal conductor

When W is ½ or ¼ wavelength, resonances will occur

A resonance can generate its own EM wave

W

W

The resonance generated EM wave will be a spurious wave that can interfere with the intended quasi-TEM wave; a good design limit is no feature > 1/8 wavelength


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Basic millimeter-wave concepts

Thickness comparisons, microstrip insertion loss; thicker laminate has more issues at higher frequencies

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Thickness can be a concern, but typically conductor width is more significant

This example: conductor width is 0.036”

1/4 wavelength (λ) is 0.036” at 46.5 GHz

1/8λ is 0.036” at 23.8 GHz

The insertion loss curve after 1/8λ has increasing amounts of noise

1/8λ

1/4λ


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Basic millimeter-wave concepts, signal launch

50 ohm microstrip transmission line circuit

The point where the pin of the connector touches the circuit, there will be an impedance mismatch

Recall

impedance mismatch can cause reflected energy and poor return loss

Poor return loss causes poor wave propagation properties and increased losses

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Basic millimeter-wave concepts, signal launch• It can be seen that the large inductive spike of the circuit with poor signal launch is

eliminated with the circuit having a good signal launch

• It is also very important to notice the difference in return loss curves (S11, S22)

Poor signal launch Good signal launch

Pin of connector contacting circuit Pin of connector

contacting circuit

Green curve is the impedance curve Good return loss is -15 dB or better

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Basic millimeter-wave concepts, signal launch

The insertion loss charts shown are using the exact same set of circuits, however after the good signal launch measurements were made, the circuits were modified to have poor signal launch

Poor signal launch Good signal launch

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Basic High Speed Digital concepts

• RF, microwave and millimeter-wave technology mostly uses frequency domain

• High Speed Digital (HSD) technology mostly uses time domain

• There are relationships between frequency and time domain, however

• that which is important to frequency domain may not be important to time domain and vise versa

• A significant relationship between frequency and time domain is how a digital signal is produced

• The HSD signal is basically a square wave and there are no naturally occurring square waves

• The square wave must be generated from naturally occurring RF waves

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( ) ( ) ( ) ( )

⋅+⋅+⋅+⋅= tttty ωωωω

π7sin

715sin

513sin

31sin4

( )ty ωπ

sin4⋅=

( ) ( )

⋅+⋅= tty ωω

π3sin

31sin4

( ) ( ) ( )

⋅+⋅+⋅= ttty ωωω

π5sin

513sin

31sin4

fundamental

Fundamental + 3rd harmonic

Fundamental + 3rd and 5th harmonic

Fundamental + 3rd , 5th and 7th harmonic

A digital signal is made up of several RF waves

A summation of a RF wave and many of its odd harmonics can generate a square wave

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Basic High Speed Digital concepts( ) ( ) ( ) ( )

⋅+⋅+⋅+⋅= tttty ωωωω

π7sin

715sin

513sin

31sin4

Fundamental + 3rd , 5th and 7th harmonicExample for a 28 Gbps signal

Fundamental = 14 GHz3rd harmonic = 42 GHz5th harmonic = 70 GHz7th harmonic = 98 GHz

Example for a 56 Gbps signalFundamental = 28 GHz3rd harmonic = 84 GHz5th harmonic = 140 GHz7th harmonic = 196 GHz

The relationships shown above, between fundamental / harmonics and digital rate, are approximate and assuming a NRZ digital format

A PAM4 digital format will have approximately half the frequency values shown

Regardless of digital format, the examples suggests that millimeter-wave properties may have an influence for these very high speed digital applications

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• Transmission lines are also used in HSD applications, similar to RF applications

• The RF transmission line is very concerned with moving energy from module-to-module

• The HSD transmission line is very concerned with moving data from module-to-module

• The emphasis for HSD transmission lines is to have good signal integrity

• In other words, the 0’s and 1’s are not distorted by the transmission line

0

1

0

1

0 …

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• Electrical concerns which can cause signal integrity problems (distorting the digital signal)

• Increased insertion loss, can lower the amplitude of the 1’s

• Impedance differences and reflections, can cause noise in the bit pattern

• Phase angle differences can cause timing problems

• When the 0’s and 1’s are not properly defined for the receiver there can be increased BER (Bit Error Rate)

0

1

0

1

0 …

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• As HSD technology developed, it became obvious that moving large amounts of data quicker was mandatory

• The parallel data systems could be fast, but they required a lot of real estate on the PCB

• A better system (serial) is to code the parallel data at the receiver, send it down one path very fast and then decode the data at the receiver

• SerDes is an acronym for Serialize / Deserializer and it codes and decodes the digital signals

Parallel data system Serial data system

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• Nowadays, there is a lot of attention that must be put on the transmission line which connects these very high speed SerDes modules

• If there is a single transmission line which carries the data and an outside source causes electrical noise to be resident on the circuit, the digital data (0’s and 1’s) can be corrupted

• In order to minimize the possibility of poor signal integrity (corrupted data), differential pair transmission lines have been employed for HSD

Single ended transmission line connection Differential pair transmission line connection

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• Simple overview of differential pair operation

• The original digital data stream is copied and inverted

• One data stream has 0’s and +1’s and the other data stream has 0’s and -1’s

• Example of a positive corrupting pulse being coupled to the data stream is shown

• At the end of the transmission line, the differential receiver will compare differences of the two data streams

• For a data bit of 1, there will be 2 times the amplitude of the original signal

• The corrupting signal will be the same or “common” for both data streams and since there is no difference it will be ignored

Differential pair transmission line connection

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Simple and ideal impact of increased insertion loss

Simple and ideal impact of impedance mismatches

Simple and ideal impact of phase angle differences

• The issue related to phase angle differences are multiple

• Different physical lengths of the two conductors making up the diff pair

• One conductor having a different Dk medium than neighboring conductor

• Any difference between the two conductors for pulse propagation

• These differences relate to timing and they are called “skew”

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• In reality, there are a multitude of issues which can impact signal integrity

• If there is a slight difference in the symmetry of the differential pair, there will be an unwanted and problematic common mode generated, which is called mode conversion

• The HSD industry adapted very smart evaluation techniques for signal integrity and some items they consider:

• Impedance measurements (time domain); the reflected impedance of S11 is evaluated and differential pair is TDD11 and mode conversion TCD11 (due to common mode distortion issues)

• Insertion loss (S21) is frequency domain and for differential pair is SDD21 and the impact mode conversion has is evaluated as SCD21

• Eye-diagram is a time domain measurement and it is an overlay of multiple 0’s and 1’s from a data stream; typically evaluated is TDD21 but there are other formats considered

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Basic High Speed Digital concepts, impedance measurements

1 2

3 4

Top view of differential pair transmission line circuit

Impedance measurement of 1-2 single conductor

Differential pair impedance measurement TDD11 The impedance target for most

HSD differential pair is 100 ohms

1 2

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Basic High Speed Digital concepts, insertion loss measurements

1 2

3


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Insertion loss (S21) measurement for 1-2 single conductor

Differential pair insertion loss (SDD21) measurement out to 110 GHz

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Basic High Speed Digital concepts, eye-diagram measurements

1 2

3


4

• The eye-diagram to the left can be used to compare to other similar circuits

• This particular circuit was a microstrip differential pair and when using the same material, a stripline differential pair could have different eye-diagrams

• Also the format of the eye-diagram will matter significantly when comparing to other circuits

Eye-diagram is an overlay of many 0’s and 1’s from a large data stream

Ideal eye-diagram

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Basic High Speed Digital concepts, evaluations

• When doing evaluations for HSD applications, they are typically conditional

• Generally the evaluations are done as comparisons to specific items of interest

• A SerDes operating at 10 Gbps will have different needs than a SerDes operating at 28 Gbps; additionally there could be differences in rise time and eye-opening requirements

• The type of circuit being evaluated should be the same for a valid comparison

• Microstrip will have very different responses than will stripline

• Single ended transmission lines have very different responses than differential pair

• If comparing test vehicles for HSD and evaluating differences in materials

• The circuits should use the same thickness of substrate

• The Dk values of the materials should be very similar so the same circuit patterns for the test vehicles can be used

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Basic High Speed Digital concepts, material evaluation example

Microstrip differential pair insertion lossUsing 5mil thick PPE based material, smooth copper

Microstrip differential pair insertion lossUsing 5mil thick XtremeSpeed™ RO1200™ material with rolled copper

This material is a non-Rogers materials and has been used extensively in HSD applications

Dk = 3.70, Df = 0.002 at 1 GHz

This material is a Rogers material that is specially formulated for extremely high speed digital applications

Dk = 3.05, Df = 0.0013 at 1 GHz

The insertion loss curves are for a 5 inch long circuit and has the loss of the connectors included

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Basic High Speed Digital concepts, material evaluation example

Microstrip differential pair eye-diagram at 28 GbpsUsing 5mil thick PPE based material, smooth copper

Microstrip differential pair eye-diagram at 28 GbpsUsing 5mil thick XtremeSpeed™ RO1200™ material with rolled copper

This material is a non-Rogers materials and has been used extensively in HSD applications

Dk = 3.70, Df = 0.002 at 1 GHz

This material is a Rogers materials that is specially formulated for extremely high speed digital applications

Dk = 3.05, Df = 0.0013 at 1 GHz

511 mV378 mV

Circuits evaluated in this test were 5 inches in length

As an important side note, the glass reinforcement layer found in a laminate can be problematic for skew, due to glass-weave effect. The XtremeSpeed RO1200 materials use a special layer of glass reinforcement, which is spread glass and does not influence skew

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Circuits are 5 inch long microstrip differential pair RO1200 is reference to Rogers Corporation XtremeSpeed™ RO1200™ material

Circuits using PPE material at 28 Gbps Circuits using PPE material at 56 Gbps

Circuits using RO1200 material at 28 Gbps Circuits using RO1200 material at 56 Gbps

511 mV

378 mV 264 mV

456 mV

30% reduction

11% reduction

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Summary

• Insertion loss is a RF topic and is frequency dependent (low frequency will have lower insertion loss, higher frequency has higher insertion loss)

• Making accurate insertion loss measurements at millimeter-wave frequencies involves understanding signal launch, return loss optimization and other RF issues

• Insertion loss does impact HSD signal integrity and specifically eye-diagrams

• Older technology used slower digital speeds, which the data signals were made up of lower frequency components and insertion loss was not a significant factor

• New technology using very high speed digital is using higher frequency components and insertion loss is a mandatory issue to address for good signal integrity

• As shown on the comparison of circuits using the PPE based and circuits built on very low loss XtremeSpeed™ RO1200™ materials, a change in digital rate from 28 Gbps to 56 Gbps caused a significant decrease for the eye-diagram of the PPE based circuit and a minimal decrease for the eye opening for the circuit using the XtremeSpeed RO1200 material

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Millimeter-Wave Concepts can be used to Optimize the