Download Return Loss

Amphenol RF Connector Training
Course
Sept. 9, 2002
There are basic RF questions that should be asked before helping a customer
choose a connector
However, first we should discuss
• Why is it necessary to ask these questions?
• The Sales Engineer must be able to speak to the customer and understand
his needs
• A basic understanding of RF as it relates to helping the customer choose a
connector is essential to accomplish this
• The Sales Engineer is the means by which this information is transmitted
from the customer to the design engineer
• Clear, specific information leads to quick, correct designs
Understanding RF and how it relates to cable and connectors
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
Transmission Lines
Impedance
Frequency Range
Return Loss-VSWR
Insertion Loss
Passive Intermodulation Distortion
Power Handling-Voltage
Isolation-Crosstalk
RF Leakage
Cable Assembly
Amphenol RF Design Engineering Support and Capabilities
Connector Anatomy
•
What is a Connector?
•
•
A connector is a device used to connect to cables or other devices through
which electromagnetic energy is transferred from one place to another
Body-Contact-Insulator
Body
Contact
Insulator
Connector Anatomy
 Lots of changes in a very short length
 Mechanical rigidity
 Hold Contacts in place
 Prevent Insulator rotation or lateral movement
 Adapt to different cable sizes
Transform between connector series
Creates many impedance variations or discontinuities in a
very short distance
Reflections are important, Attenuation not as
important
Connector Anatomy
 Various Captivation Methods
Barbs
Grooves
Shoulders
Knurls
Staking
Connector Anatomy
Slotted Contact
Contact
Barb
Dielectric
Support Bead
Discontinuity
Compensation
Steps
Cable Anatomy
•
What is Cable?
•
•
Cable is a transmission line through which electromagnetic energy is
propagated and transferred from one place to another
Jacket-Braid-Shield-Dielectric-Center Conductor
Conductor
Jacket
Dielectric
Shield
Braid
Cable Anatomy
 No changes in a very long length
 No impedance changes or discontinuities
Usually very few reflections, but Attenuation is
important
Assistance on the Web
 Amphenol RF Newsletter
 Technical Questions:
http://www.amphenolrf.com/rf_made_simple/techquestions.asp
VSWR Conversion Charts:
http://www.amphenolrf.com/rf_made_simple/vswr.asp
Glossary: http://www.amphenolrf.com/rf_made_simple/glossary.asp
# 1. Transmission Line
•
What is a transmission line?
•
A transmission line is a conduit by which electromagnetic energy is transferred
from one place to another
• Coaxial Cable- Unbalanced Line: Center conductor surrounded by a
concentric dielectric and outer conductor-Most popular type of
transmission line
# 1. Transmission Line
•What
is a transmission line?
•A transmission
line is a conduit by which electromagnetic
energy is transferred from one place to another
•Waveguide: Rectangular, Circular
# 1. Transmission Line
•What
is a transmission line?
•A transmission
line is a conduit by which electromagnetic
energy is transferred from one place to another
•Planar Transmission Line:
Microstrip, Stripline,
Coplanar waveguide are most common
# 1. Transmission Line
•What
is a transmission line?
•A transmission
line is a conduit by which electromagnetic
energy is transferred from one place to another
•Twin
Line- balanced line: two parallel conductors
separated by a dielectric
..
# 1. Transmission Line
•
The type of transmission line will determine the connector
style
•
•
•
•
•
Cable Connector-Coaxial Cable, Twin Line
Surface Mount Connector-Microstrip
Tab Launch Connector-Microstrip, Stripline, Coplanar
End Launch Connector-Microstrip, Coplanar
Pin Launch Connector-Microstrip, Stripline, Coplanar
# 2. Impedance
Ohm’s Law
# 2. Impedance
•
What is the Characteristic Impedance?
•
The ratio of Voltage to Current at any point in a transmission line
•
A transmission line can be represented as shown, with the values of C. L,
R, and G determining the impedance
Coaxial Line Equivalent Circuit
# 2. Impedance
•50
and 75 Ohms are the most common impedances
•Do not confuse impedance with LOSS:
•A 50 ohm impedance does not have less loss than a 75 ohm
impedance. It is not like resistance
•Impedance is independent of the length of the cable or
connector
•Impedance is independent of frequency
•The
Impedance will help determine the connector series
•Some series are only one impedance: C, SC, HN, 7-16
•Some series can be both 50 or 75 ohms: BNC, TNC, N
# 2. Impedance
•Characteristic
Impedance is determined by the geometry and dielectric constant
of the transmission line
of Coaxial Cable: Zo=(138/(E))*log(D/d))= (L/C)
•L=.0117*Log (D/d) uh/inch C=.614*E/Log (D/d) pf/inch
•Impedance
# 2. Impedance
# 2. Impedance
Impedance: The impedance of the connector generally must match that of the transmission line
Non-Constant
50, 75 ohm
50 ohm
75 ohm
BNC Twinaxial
UHF
Twinaxial
BNC
SMB
MCX
1.0/2.3
TNC
N
7/16
C, SC, HN
Mini-UHF
MMCX
SMA
1.6/5.6
Type F
Type G
Outer Diameter Inner Diameter
.063
.063
.276
.020
.012
.120
Dielectric Constant
2.0
2.0
1.0
Impedance
50 Ohms
75 Ohms
50 Ohms
# 3. Frequency Range
•
•
•
Frequency is the number of electromagnetic waves that pass a given
point in 1 second
Hertz is the unit of frequency measurement
Generally, the RF performance of a connector degrades as the
frequency is increased (c=f)
•
•
•
Wavelength decreases, therefore smaller disruptions cause more problems
Specifying the frequency will make it easier for the design engineer to
optimize the performance
Whenever possible, don’t specify a high frequency connector when a
low frequency connector will work do the job
# 3. Frequency Range
•
If a frequency range is not specified, then the connector will
be designed to catalog specs and this could cause the design
process to take a lot longer
•
•
For example-A customer needs a new SMA to operate up to 12
Ghz. The catalog specifies 18 Ghz for some SMA connectors. If
the connector is optimized for 18 Ghz, it will likely take a lot
longer than necessary to design
Give as much information about the application of the
connector to the design engineer as possible
•
•
Is it used in a high power, narrow frequency band amplifier?
Is it used in a band pass filter?
# 3. Frequency Range
Giga
Mega
Kilo
Milli
Micro
Nano
Pico
=
=
=
=
=
=
=
1,000,000,000
1,000,000
1,000
1/1000
1/1,000,000
1/1,000,000,000
1/1,000,000,000,000
Billion
Million
Thousand
One thousandth
One millionth
One billionth
One trillionth
Some Typical Frequencies:
House current
60 Hz in the US (50 Hz in many other countries)
AM Radio
500 - 1500 kHz
Shortwave Radio
10 MHz
TV (channels 2-13)
60 - 250 MHz
Cellular Phone
824 - 894 MHz
Digital (PCS) Phone
1850 - 1990 MHz
Radar
6 - 26 GHz
Direct Broadcast Satellite (DBS) 12 GHz
Frequency Chart (GHz)
26.5
SM A (Hi P erf.)
18
SM A (SR)
12.4
SM A (Flex Co ax)
11
11
N
TNC
10
10
1.0/2.3 (Threaded)
SM C
7
7/16 (Co ax, SR)
6
6
5.2
M CX
M M CX
7/16 (Co rrugated)
4
4
4
1.0/2.3 (P ush P ull)
B NC
SM B
Type F
3
Type G
3
2.5
M ini-UHF
1
0.3
0.2
0.1
1.6/5.6
UHF
Twinax
B NC Twin
0
5
10
15
20
25
30
# 4. Return Loss or VSWR
•
A measure of how much power is reflected
 Return Loss: The portion of a signal that is lost due to a reflection of
power at a line discontinuity. Return Loss is similar to VSWR and is
generally preferred in the CATV industry to a VSWR specification
• VSWR: Acronym for Voltage Standing Wave Ratio. VSWR is the ratio of
voltage applied to voltage reflected. It is the major factor contributing to
the total signal efficiency of the connector.
•
Best performance is achieved when the impedance of the cable and the
connector are the same (matched)
# 4. Return Loss or VSWR
Reflections are created by deviations from the
characteristic impedance caused by:
•variations in machining tolerances
•Variations in the dielectric constants of insulators
•transitions within the connector:
•transitioning from the cable size or stepping the
connector from one line size to another line size
•
# 4. Return Loss or VSWR
•
•
•
•
•
•
Reflection Coefficient is the basic measure of reflection:
r=abs(Zo-Zl/Zo+Zl) where Zo is the characteristic impedance
and Zl is the actual impedance
Generally, this is the most important RF figure of merit for a
connector
VSWR=(1+r)/(1-r)
Return Loss=-20*log (r), in dB (decibels)
These are all the same thing, just expressed in different ways
Return Loss and VSWR are most commonly used
Incident Power
Power transmitted
into component
Reflected Power
Cable
Component
Return Loss = Ratio of reflected to incident power
in dB
VSWR = Ratio of maximum to minimum electric
field (Voltage)
Relative Magnitudes:
Power
Reflected
1%
Power Transmitted
into Component
99%
Return
Loss
20 dB (1/100=10-2)
VSWR
1.25
5%
95%
13 dB
1.58
10%
90%
10 dB (10/100=10-1)
1.95
50%
50%
3 dB
5.80
Try to get a realistic idea of the Return Loss really required for a specific
application
•
Trying to design very low VSWR connectors, when not really needed, can
take a long time and can add to the cost
•
dB Notation
Increase
of Signal
Decibel (dB)
Equivalent
1
=
100
=
0dB
2
=
100.3
=
3dB
10
=
101
=
10dB
20
=
101.3
=
13dB
100
=
102
=
20dB
1000
=
103
=
30dB
1/10
=
10-1
=
-10dB
1/100
=
10-2
=
-20dB
1/1000
=
10-3
=
-30dB
Rather than say “The gain of the amplifier is 100 times”,
we say, “The gain is 20 decibels.”
# 5. Insertion Loss
•
Insertion Loss is expressed in dB, and is a measure of the total loss of power
going through a device
•
IL = -20*log (Pout/Pin)
•
Includes losses due to reflection (usually the dominant factor unless the Return
Loss is very low <-26 dB), plus losses due to the dielectric and metal
conductors (Attenuation)
Long Cable assembly-Connector insertion loss not usually significant
Short cable assembly- Connector insertion loss can be significant
•
Typically, connector insertion loss is very small (.1-.25 dB)
•
•
# 5. Insertion Loss
•As
frequencies increase, the insertion loss increases (as a
square law function (P=E^2/Z)
•Most
of the electromagnetic energy (current) travels
through the conductors in a circumferential ring
•Most
of it in center conductor, but there is some impact
from outer conductor
•Current
flow is restricted to the surface layer or “skin” of
the conductor
• Approximately
98% of the current density travels
within 4.6 skin depths
# 5. Insertion Loss
•
The length of the connector and the materials chosen will impact
the insertion loss
•
•
shorter is better
Plate the conductors with a high conductivity material
•
•
•
•
Nickel-Inexpensive, hard material with good conductivity, but high
relative permeability resulting in higher insertion loss
Gold-Hard material and an excellent conductor, but expensive
Silver-Excellent conductor, less expensive than gold, better
permeability than nickel, but softer, and tarnishes
Stainless Steel-Rugged material for small connectors such as SMA,
but steel has high relative permeability
# 5. Insertion Loss
# 6. Passive Intermodulation Distortion
•
Not well known until mid 1990’s
•
Primarily concern to satellite, microwave relay industries
•
Modern Frequency plans
•
High Power levels
•
Sensitive Receivers
•
Spurious Signals created by non-linear mixing of 2 or more
frequencies in a passive device
•
Active PIM-generated by amplifiers-is reduced by filtering
•
Passive PIM-filtering not possible
• Common to many channels
• Must be low PIM designs
# 6. Passive Intermodulation Distortion
• Spurious Signals created by non-linear mixing of 2 or more
frequencies in a passive device
• PIM products fall in receive (uplink) band
• Block Channels
• 3rd order generally greatest amplitude
• 5th and 7th may be of concern
• fIM = mf1 +/- nf2
• (2f1-f2), where m = 2 and n = 1 is a 3rd order product
# 6. Passive Intermodulation Distortion
F1 = 930 Mhz and F2 = 955 Mhz, then Fim = 905 Mhz
# 6. Passive Intermodulation Distortion
Base Station Antenna Systems
•Simplex•Most prone to PIM effects
•Most economical
•Duplex
•Less Prone to PIM
•More expensive
•Cross Polarization
•Least PIM susceptible
•May require more space
# 6. Passive Intermodulation Distortion
•dBm-measure of power relative to 1 milliwatt
•dBc-measure of dB below a specified carrier level
•+43 dBm input
•PIM: -120 dBm
•Spec: -163 dBc
•Common Spec is -143 to -163 dBc (-100 to -120 dBm)
# 6. Passive Intermodulation Distortion
•
Causes of PIM
• Poor Contact Junctions-Non linear rectifying
• Solder outer-Solder inner- over molded design are best
• Most stable
• Ferromagnetic materials-Non-linear hysteresis
• No Nickel, Stainless Steel
• Contamination
•
Types of Connectors
• 7-16 DIN
• Type N
• TNC-Occasionally
• Never use Bayonet (BNC) or Push on styles
# 7. Power Handling Capability
•
There are 2 types of power handling (expressed in watts)
that must be considered
•
•
•
Average Power
Peak Power
Average Power-the input power to a cable/connector which
will produce a maximum safe center conductor temperature
under steady state conditions when terminated with a
matched load. A safe center conductor temperature is one
that will not melt the dielectric
# 7. Power Handling Capability
•
Average Power is inversely proportional to frequency and
must be derated accordingly
•
•
Average Power=Power Rating @ 1 Mhz/ (Frequency in Mhz)
Connectors generally have higher power ratings than the
cable to which they are attached
•
•
•
They have metal shell-cables have braids covered by plastic jackets
They can be attached to bulkheads which help dissipate heat
They usually have lower attenuation per unit length due to air
sections within the connector
# 7. Power Handling Capability
•
•
•
•
•
Peak Power-is limited by the voltage rating of the connector. The peak
power is determined by the equation V^2/Z where V=the peak voltage
rating and Z is the characteristic impedance
Peak Power is not a function of frequency
Peak Power is an inverse function of VSWR and modulation schemes
and must be derated
Peak and Average Power are functions of altitude and must be derated
accordingly
Maximum power ratings will always be the lesser of the
cable/connector combination
Max. Operating Voltage (volts)
Used to determine Peak Power Ratings
1000
N (Typical)
750
UHF (Typical)
500
500
500
TNC (Typical)
BNC (Typical)
SMA (RG-402)
335
335
335
335
250
250
250
170
SMA (RG-58, 142, 405)
MCX (RG-316)
SMB RG-179)
SMB (RG-316, 405)
SMA (RG-316)
MCX (RG-178)
SMB (RG-178)
SMA (RG-178)
0
200
400
600
800
1000
1200
# 8. Isolation-Crosstalk
•
•
Isolation and Crosstalk are used interchangeably
They are a measure of how much signal is picked up by an adjacent
line
•
•
•
•
•
Ganged style connectors on PC boards
Harnessed or “parallel run” cable assemblies
They are measured in dB and usually range from –60 to –100 dB
If frequency increases or the length of the lines increase, crosstalk gets
worse
If the distance between the lines increases, crosstalk gets better
# 8. Isolation-Crosstalk
There will be significant
crosstalk between the
lines on this ganged
connector unless some
precautions (such as
shielding) are taken
Question # 9. RF-Leakage
•
•
•
•
RF Leakage is a measure of how much signal leaks out from a
connector in dB at both the interface and at the cable entry
As frequency increases, the leakage gets worse
Typical RF Leakage values range from –40 dB for Push-On types to
-90 dB for threaded styles on Semi Rigid cables
Generally not a big concern except if epoxy captivation is used
#10: Is the connector used on a cable assembly
•
•
•
2 connectors separated by a distance on a cable
At specific frequencies, all of the reflections can add up
(both connectors and cable)
When specifying a connector for a cable assembly, the cable
assembly requirements must be known
•
Catalog connectors, even if performance levels meet MIL Spec
requirements, may not be able to perform to the cable assembly
specifications
#10: Is the connector used on a cable assembly
•
Calculate the total worst case VSWR by multiplying all of
the VSWR’s: For example- The cable assembly
specification is 1.45 maximum
•
•
•
•
•
1st connector VSWR=1.25
2nd connector VSWR=1.15
Cable VSWR=1.05
Total worst case VSWR=1.25*1.15*1.05=1.51
Choosing a catalog BNC connector with a VSWR=1.25 and
a catalog SMA connector with a VSWR of 1.15 obviously
won’t work. Special connectors are needed.
#10: Is the connector used on a cable assembly
#11: How can Amphenol RF adequately support the design
and development of high performance, RF connectors?
•
RF Design Capabilities
•
•
ANSOFT High Frequency Structure Simulator
Test Capabilities-Design Verification
•
•
Network Analyzers
PIM Test Capabilities
RF Simulation Capability
• ANSOFT 3D High Frequency Structure Simulator
• Model any Geometry
• No Frequency Limitation
• S Parameter Analysis
• Return Loss, VSWR, Insertion Loss etc.
• Radiated Power
• E Field Plots
• Time Domain Analysis
• Optimization Capability
RF Simulation Capability
• The connector is designed using standard RF practices and 2D linear analysis
programs for “ballpark” performance
• Calculate impedances within the connector
• Calculate nominal compensation steps within the connector
• Draw the problem in HFSS-import from PRO-E: IGES (3D), or DXF (2D)File
• Assign the materials
• Set the ports and boundary conditions (symmetry)
• Solve
• Analyze frequency and time domain plots
RF Simulation Capability
Draw the RF Model
from the Mechanical
drawing
RF Simulation Capability
Plot the desired S
Parameters
RF Simulation Capability
View Time Domain
response to determine the
location of impedance
mismatch
RF Simulation Capability
•All design changes are made on the computer (No
samples made until the design is optimized)
•Simulations in a matter of minutes, or hours at most
•Numerous iterations in a matter of hours or days
•Final modifications (if needed) made after testing
Surface Mount Connector on Microstrip
Customer must supply board
characteristics:
1.
Thickness
2.
Trace width
3.
Material (dielectric constant)
4.
Transmission line type (i.e..
Microstrip, stripline)
Connector has excellent Return Loss (-35 to –40 dB)
When mounted on board, performance deteriorates (-20 dB) due to
the mismatch at the launch
Surface Mount Connector on Microstrip
Initial simulation results
Return Loss
Insertion Loss
Surface Mount Connector on Microstrip
.010 wide
.015 wide .022 wide
Modify launch area to reduce
the negative (capacitive)
discontinuity at the launch
area
Capacitance
due to launch
Time Domain
Surface Mount Connector on Microstrip
Final Insertion Loss
Final Return Loss
Return Loss
Insertion Loss
Able to achieve a significant improvement in Return Loss and
Insertion Loss by modifying the launch area
Antenna Isolation Board
•
•
•
Design Capabilities are not limited to connectors
Can model and simulate entire assemblies
Example:
•
•
•
MCX angle PC connector on a capacitively coupled microstrip
board
4” of RG-316 cable
Straight MCX connector and angle MCX PC connector on ends of
cable
Antenna Isolation Board
919-101P-51SX
4” RG-316/U
Board, Top View
.063 thick, FR4
47pf, 4000 V,
capacitor
919-119J-51AX
919-134C-51P1X
.115 wide trace
Antenna Isolation Board
Board, Bottom View
Ground Plane
47 pf Cap, 4000 V,
capacitor
Antenna Isolation Board (Simulated vs. actual test results)
Return Loss
Insertion Loss
Spec: -15 dB Return Loss and -1.5 dB Insertion Loss at 900 Mhz
Example:
Angle Plug for LMR400 Cable VSWR Improvement
Contact too close to body
Initial VSWR
5 mm Diameter too small (35 ohm
impedance)
ANSOFT HFSS
Model
Angle Plug for LMR400 Cable VSWR Improvement
Recommended Design Changes
1. Remove Chamfer at solder post on contact
2. Increase 5mm diameter to 6.3 mm diameter on Body
Remove chamfer and shorten contact by
1.25 mm
Increase diameter to 6.3 mm
ANSOFT
Model
Final improved VSWR
Test Capability-S Parameters
•
•
•
•
State of the art Network Analyzers
HP 8510: 26.5 Ghz Vector Network Analyzer
HP 8753D: 50 Ohm 6 Ghz Vector Network Analyzer
HP 8753D: 75 Ohm 3 Ghz Vector Network Analyzer





Return Loss
Insertion Loss
VSWR
Crosstalk
RF Leakage
Vector Network Analyzer (S Parameter Measurements)
RF Leakage Test chamber
Passive Intermodulation Distortion Testing
•
•
•
•
•
There are no “high tech” computer programs to predict IMD
performance
Devices must be built and tested
State of the art measurement test set using 20 watt (+43 dbm)
signals with a system noise floor of -130 to -135 dbm
Computer Automated-in house programming capabilities to
customize test measurements
Typical specifications of –116 to –120 dBm for 7-16 and Type N
connectors on helical and annular cables
PIM Testing – Cont’d
PIM Testing – Cont’d
Computer Control
(HP VEE Interface)
How to Select an RF Connector
Select a connector based on the information learned
from asking questions about the 10 RF parameters:
1. Impedance Typical impedance of a system is 50 or 75 ohm. See
Overview in catalog for impedance by series.
2. Frequency Range Connector series range from 100 MHz to 26.5 GHz.
See Overview in catalog for frequency range by series.
3. Cable Type Connector series are designed to terminate to a limited
number of cable types. Is it a new cable required by the customer? Is it a PC
style? See the “Cable Selection Chart” in the catalog.
4. Electrical/Mechanical requirements VSWR, Voltage Rating,
Temperature Range, and other environmental requirements are all key
specifications.
5. Coupling Type Choose between Threaded, Bayonet, Snap-on, and Pushpull based on all of the above.
Use all of the information gathered to make a final decision
•
Coupling style
•
Frequency range: 6 Ghz
• Power Handling: 5 Watts Average
• RF Leakage: -70 dB (Eliminates Push on or
Bayonet styles)
• PIM requirements: -None
•
N, TNC, SMA, 7-16
Connector style
•
Impedance: 50 Ohms
• Return Loss: -20 dB
• Insertion Loss: -.1 dB
• Mechanical Restrictions
•
•
•
SMA, TNC
Available Real Estate: .5 “ long
Cable: RG-142
Cost, other mechanical requirements, etc.
Final Connector Choice