+ All Categories
Home > Documents > Custom Interconnects

Custom Interconnects

Date post: 04-Feb-2022
Category:
Upload: others
View: 9 times
Download: 0 times
Share this document with a friend
30
Custom Interconnects Fuzz Button Coax interconnect Ø.020 RF Layout 50ohms Measurement and Model Results prepared by Gert Hohenwarter 7/8/14 GateWave Northern, Inc GateWave Northern, Inc . 1
Transcript
Page 1: Custom Interconnects

Custom InterconnectsFuzz Button Coax interconnect

Ø.020 RF Layout 50ohmsMeasurement and Model Results

prepared by

Gert Hohenwarter

7/8/14

GateWave Northern, IncGateWave Northern, Inc. 1

Page 2: Custom Interconnects

Table of Contents

TABLE OF CONTENTS.......................................................................................................................................................2OBJECTIVE..................................................................................................................................................................... 3METHODOLOGY...............................................................................................................................................................3

Test procedures....................................................................................................................................................... 4Setup....................................................................................................................................................................... 4

MEASUREMENTS .............................................................................................................................................................8Time domain........................................................................................................................................................... 8Frequency domain................................................................................................................................................ 12

SPICE MODELS.......................................................................................................................................................... 24Time domain......................................................................................................................................................... 25Frequency domain................................................................................................................................................ 27

..........................................................................................................................................................................................

GateWave Northern, IncGateWave Northern, Inc. 2

Page 3: Custom Interconnects

Objective

The objective of these measurements is to determine the RF performance of a

Custom Interconnects Fuzz Button Coax interconnect. A signal pin surrounded by

grounded pins is selected for the signal transmission. Measurements in both frequency

and time domain form the basis for the evaluation. Parameters to be determined are

pin capacitance and inductance of the signal pin, the propagation delay and the

attenuation to 40 GHz.

Methodology

Capacitance and inductance for the equivalent circuits were determined through a

combination of measurements in time and frequency domain. Frequency domain

measurements were acquired with a network analyzer (HP8722C). The instrument was

calibrated up to the end of coax probes that are part of the test fixturing. The device

under test (DUT) was then mounted to the fixture and the response measured from one

side of the contact array. When the DUT pins terminate in an open circuit, a

capacitance measurement results. When a short circuit compression plate is used,

inductance can be determined.

Time domain measurements are obtained via Fourier transform from VNA tests.

These measurements reveal the type of discontinuities at the interfaces plus contacts

and establish bounds for digital system risetime and clock speeds.

GateWave Northern, IncGateWave Northern, Inc. 3

Page 4: Custom Interconnects

Test procedures

To establish capacitance of the signal pin with respect to the rest of the array, a return

loss calibration is performed. Phase angle information for S11 is selected and

displayed. When the array is connected, a change of phase angle with frequency can

be observed. It is recorded and will be used for determining the pin capacitance.

Inductance of a pin is found in the same way, except the Fuzz Button Coax

interconnect array is compressed by a metal plate instead of an insulator. Thus a short

circuit at the far end of the pin array results. Again, the analyzer is calibrated and S11 is

recorded. The inductance of the connection can be derived from this measurement.

Setup

Testing was performed with a test setup that consists of a brass plate that contains

coaxial probes. The DUT is aligned and mounted to that plate. The opposite

termination is also a metal plate with embedded coaxial probes.

GateWave Northern, IncGateWave Northern, Inc. 4

Page 5: Custom Interconnects

Figs. 1 and 2 show a typical arrangement base plate and DUT probe:

Figure 1 Coax interconnect base plate example

Figure 2 DUT plate example

The Fuzz Button Coax interconnect and base plate as well as the DUT plate are then

mounted in a test fixture as shown below in Fig. 3:

GateWave Northern, IncGateWave Northern, Inc. 5

Page 6: Custom Interconnects

Figure 3 Test fixture example

This fixture provides for independent X,Y and Z control of the components relative to

each other. X, Y and angular alignment is established once at the beginning of a test

series and then kept constant. Z (depth) alignment is measured via micrometer and is

established according to specifications for the particular DUT.

Connections to the VNA are made with high quality coaxial cables with K connectors.

GateWave Northern, IncGateWave Northern, Inc. 6

Page 7: Custom Interconnects

Figure 4 Ø0.020 Fuzz Button Interconnect Configuration

The signal is routed through the center contact, all other contacts are grounded.

GateWave Northern, IncGateWave Northern, Inc. 7

Page 8: Custom Interconnects

Measurements

Time domain

The time domain measurements will be presented first. TDR reflection measurements

are shown below:

TDR open

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

-0.15 -0.05 0.05 0.15

t [ns]

rho System

DUT

Figure 5 TDR signal from an OPEN circuited Coax interconnect

The reflected signal from the Fuzz Button Coax interconnect (right trace) shows only a

small deviation in shape from the original waveform (left trace). The risetime is 28.5 ps

and is almost the same as that of the system with the open probe (27.0 ps). Electrical

open circuit pin length is 8.3 ps (one way).

GateWave Northern, IncGateWave Northern, Inc. 8

Page 9: Custom Interconnects

TDR SHORT

0.0

0.2

0.4

0.6

0.8

1.0

1.2

-0.15 -0.05 0.05 0.15

t [ns]

rho

System

DUT

G W N 502

Figure 6 TDR signal from a SHORT circuited Coax interconnect

For the short circuited Fuzz Button Coax interconnect the fall time is 28.5 ps. There is

no increase over the system risetime of 28.5 ps caused by the contact impedance

levels.

GateWave Northern, IncGateWave Northern, Inc. 9

Page 10: Custom Interconnects

TDR THRU

40

45

50

55

60

-0.15 -0.05 0.05 0.15

t [ns]

Ohm

s

G W N 1004

Figure 7 TDR measurement into a 50 Ohm probe

The thru TDR response shows primarily no perturbation to the signal. The peak

corresponds to an impedance of 50.8 Ohms. The dips below the 0 line go to 47.8

Ohms.

GateWave Northern, IncGateWave Northern, Inc. 10

Page 11: Custom Interconnects

The TDT performance for a step propagating through the contact arrangement was

also recorded:

TDT THRU

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

-0.05 0.00 0.05 0.10 0.15

t [ns]

rho

System

DUT

GW N 508

Figure 8 TDT measurement

The TDT measurements for transmission show an identical risetime from the pin array

(10-90% RT = 28.5 ps, the system risetime is 30.0 ps). The added delay at the 50%

point is 8.1 ps. There is a small plateau because of the low impedance level. If the

20%-80% values are extracted, the risetime is only 18.0 ps vs. 19.5 ps system risetime.

GateWave Northern, IncGateWave Northern, Inc. 11

Page 12: Custom Interconnects

Frequency domain

Network analyzer reflection measurements for a single sided drive of the signal pin

with all other pins open circuited at the opposite end were performed to determine the

pin capacitance. The analyzer was calibrated to the end of the probe and the phase of

S11 was measured. From the curve the capacitance of the signal contact to ground can

be determined (see Fig. 10).

S11 (f)

-250

-150

-50

50

150

250

0 10 20 30 40

f [GHz]

S11

[de

g]

GW N 502

Figure 9 S11 phase (f) for the open circuited signal pin

There are no aberrations in the response. The 360 degree jump is due to the network

analyzer data presentation which does not allow for values greater than +/- 180

degrees.

GateWave Northern, IncGateWave Northern, Inc. 12

Page 13: Custom Interconnects

S11 (f)

-3.0-2.5-2.0-1.5

-1.0-0.50.0

0 10 20 30 40

f [GHz]

S11

[dB

]

GW N 502

Figure 10 S11 magnitude (f) for the open circuited signal pin

While ideally the magnitude of S11 should be unity (0 dB), minimal loss and radiation

in the contact array are likely contributors to S11 (return loss) for the open circuited pins

at elevated frequencies.

GateWave Northern, IncGateWave Northern, Inc. 13

Page 14: Custom Interconnects

C (f)

0.0

0.5

1.0

1.5

2.0

0.0 10.0 20.0 30.0

f [GHz]

C [p

F]

GW N 502

Figure 11 C(f) for the open circuited signal pin

Capacitance is 0.16 pF at low frequencies. The rise in capacitance toward 27 GHz is

due to the fact that the pins form a transmission line with a length that has become a

noticeable fraction of the signal wavelength. The lumped element representation of the

transmission environment as a capacitor begins to become invalid at these frequencies

and so does the mathematical calculation of capacitance from the measured

parameters. This merely means the model is not valid anymore. As is evident from

time domain and insertion loss measurements this does not imply that the DUT does

not perform at these frequencies.

The Smith chart measurement for the open circuit shows no resonances. A small

amount of loss is present. The Smith chart covers frequencies to 40 GHz.

GateWave Northern, IncGateWave Northern, Inc. 14

Page 15: Custom Interconnects

GWN 903

Figure 12 Reflections from the open circuited Fuzz Button Coax interconnect

To extract pin inductance, the same types of measurements were performed with a

shorted pin array. Shown below is the change in reflections from the Fuzz Button Coax

interconnect. Calibration was established with a short placed at the end of the coax

probe.

GateWave Northern, IncGateWave Northern, Inc. 15

Page 16: Custom Interconnects

S11 (f)

-250

-150

-50

50

150

250

0 10 20 30 40

f [GHz]

S11

[de

g]

GW N 502

Figure 13 S11 phase (f) for the short circuited case

GateWave Northern, IncGateWave Northern, Inc. 16

Page 17: Custom Interconnects

S11 (f)

-3.0-2.5-2.0-1.5

-1.0-0.50.0

0 10 20 30 40

f [GHz]

S11

[dB

]

GW N 502

Figure 14 S11 magnitude (f) for the short circuited case

A small S11 return loss exists, likely the result of minimal loss and radiation.

GateWave Northern, IncGateWave Northern, Inc. 17

Page 18: Custom Interconnects

L (f)

0.00

0.50

1.00

1.50

2.00

2.50

3.00

0.0 5.0 10.0 15.0 20.0

f [GHz]

L [n

H]

GW N 502

Figure 15 L(f) for the Coax interconnect

The phase change corresponds to an inductance of 0.43nH at low frequencies.

Toward 12 GHz inductance increases. At these frequencies, the transmission line

nature of the arrangement must be taken into account.

GateWave Northern, IncGateWave Northern, Inc. 18

Page 19: Custom Interconnects

GWN 903

Figure 16 Short circuit response in the Smith chart

Only a small amount of loss is noticeable in the Smith chart for the short circuit

condition. The Smith chart covers frequencies to 40 GHz.

An insertion loss measurement is shown below for the frequency range of 50 MHz to

40 GHz.

GateWave Northern, IncGateWave Northern, Inc. 19

Page 20: Custom Interconnects

S21 (f)

-2.0

-1.5

-1.0

-0.5

0.0

0 10 20 30 40

f [GHz]

S21

[dB

]

GWN

Figure 17 Insertion loss S21 (f)

Insertion loss is less than -0.2 dB to 27.1 GHz. The 1 dB point is not reached before

40 GHz.

GateWave Northern, IncGateWave Northern, Inc. 20

Page 21: Custom Interconnects

Corner

GWN 903

Figure 18 Smith chart for the thru measurement into a 50 Ohm probe

The Smith chart for the thru measurements shows some reactive components toward

higher frequencies. The Smith chart covers frequencies to 40 GHz.

GateWave Northern, IncGateWave Northern, Inc. 21

Page 22: Custom Interconnects

S11 (f)

-45

-40

-35

-30

-25

-20

-15

-10

-5

0

0 10 20 30 40

f [GHz]

S1

1 [d

B]

GW N 502

Figure 19 S11 magnitude (f) for the thru measurement into a 50 Ohm probe

The value of the return loss for the thru measurement reaches a level of -20 dB at a

frequency of 25.5 GHz and -10 dB at >40.0 GHz.

GateWave Northern, IncGateWave Northern, Inc. 22

Page 23: Custom Interconnects

VSWR

1

2

3

4

5

0 10 20 30 40

f [GHz]

VS

WR

GW N 502

Figure 20 Standing wave ratio VSWR (f) [1 / div.]

The VSWR remains below 2 : 1 to a frequency of 40.0 GHz.

GateWave Northern, IncGateWave Northern, Inc. 23

Page 24: Custom Interconnects

SPICE Models

A lumped element SPICE model for the Custom Interconnects Fuzz Button Coax

interconnect is shown below:

Figure 21 Lumped element SPICE model

The resistance value (R4) approximates the loss term encountered.

The values for the elements are

C1+C2 L1 R40.226 pF 0.80 nH 1000 Ohms

Toward the cutoff frequency of the Pi section the lumped element model becomes

invalid. This happens above 29 GHz for the above model. Hence, the second model

developed is a transmission line model:

Figure 22 Transmission line model for the Fuzz Button Coax interconnect

The array configuration with signal pins surrounded by ground pins provides a

transmission line environment with the following parameters:

Zo L R447.8 Ω 8.1 ps 10000 Ω

GateWave Northern, IncGateWave Northern, Inc. 24

Page 25: Custom Interconnects

Time domain

The TDR simulation results indicate a capacitive response just as observed in the

measurement (see TDR THRU).

TDR thru model

-0.02

-0.01

0.00

0.01

0.02

0.03

0.04

0.05

-150 -100 -50 0 50 100 150

t [ps]

rho

DUT PI

DUT TL

GWN 0206

Figure 23 TDR model results

The transmission line models are better suited to the time domain simulation than the

lumped element models since the latter cause a dual downward response from the two

capacitors in the Pi section.

The risetime contributions of a signal transmitted through the pin are shown below:

GateWave Northern, IncGateWave Northern, Inc. 25

Page 26: Custom Interconnects

TDT thru model

-0.20

0.00

0.20

0.40

0.60

0.80

1.00

1.20

-150 -100 -50 0 50 100 150

t [ps]

rho Source

DUT PI

DUT TL

GWN 0206

Figure 24 TDT model

Risetimes and signal waveform for the transmission line case are comparable to those

measured.

GateWave Northern, IncGateWave Northern, Inc. 26

Page 27: Custom Interconnects

Frequency domain

The model’s phase responses are also divided into lumped element and transmission

line equivalent circuits.

S11 (f) model

-250

-150

-50

50

150

250

0 10 20 30 40

f [GHz]

S11

[de

g]

DUT TL

DUT PI

GW N 502

Figure 25 S11 phase (f) for open circuited case

The evolution of phase with frequency is comparable to that measured.

The response of the lumped element model illustrates that it is limited to a maximum

frequency of 29 GHz.

GateWave Northern, IncGateWave Northern, Inc. 27

Page 28: Custom Interconnects

S11 (f) model

-250

-150

-50

50

150

250

0 10 20 30 40

f [GHz]

S11

[de

g]DUT TL

DUT PI

GW N 502

Figure 26 S11 phase response (short circuit)

The short circuit phase evolution with frequency is also comparable to that actually

measured.

The insertion loss results below also clearly demonstrate the limits of the lumped

element model. As the frequency approaches the cutoff frequency for the Pi section,

the insertion loss increases significantly. This can be avoided by splitting the lumped

element model into more than one section while keeping the sum of capacitances and

inductances the same as for the single element. The transmission line model does not

suffer from this shortcoming.

GateWave Northern, IncGateWave Northern, Inc. 28

Page 29: Custom Interconnects

S21 (f) model

-5

-4

-3

-2

-1

0

0 10 20 30 40

f [GHz]

S2

1 [d

B]

DUT TL

DUT PI

GW N 502

Figure 27 Insertion loss as a function of frequency

GateWave Northern, IncGateWave Northern, Inc. 29

Page 30: Custom Interconnects

Custom InterconnectsFuzz Button Coax interconnect

Ø.020 RF Layout 50ohms

7/8/14

Measurement results:

PI equivalent circuit component values:

It should be noted that there are 2 capacitors in the PI equivalent circuit. Each of them has half the value listed here.

Transmission line equivalent circuit values:

The impedance listed is that measured. It is different from that calculated from the measured L,C parameters because of the time domain signal risetime.

GateWave Northern, IncGateWave Northern, Inc. 30


Recommended