The N2PK VNA Brian Flynn GM8BJF · What is a VNA? • Measures the transmission and reflection...
Transcript of The N2PK VNA Brian Flynn GM8BJF · What is a VNA? • Measures the transmission and reflection...
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The N2PK VNA
Brian Flynn GM8BJF
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What is a VNA?
• Measures the transmission and reflection properties of a network.
• Technique has been around since the 1970s
• Instrumentation is expensive• Normally beyond the scope of amateurs
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The H-P 8410 (History!)
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The H-P 8510 (Current)
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The H-P 8573C (Current)
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Why reflection and transmission?
• The reflection and transmission properties can completely describe a network.
• A “network” can be any device• A load is a one port network• Filters or amplifiers are two-port networks• Directional couplers are 3 or 4 port
devices
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A one –port networkjX
Zo
incident
reflected
RR+jX
-0.2
-0.5
-1.0
-2.0
0.5
0.2
0.2 0.5 1.0
1.0
2.0
0 oo
Smith chartReflecttion coefficient
O
z=r=jx
| |
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The Smith Chart
-0.2
-0.5
-1.0
-2.0
0.5
0.2
0.2 0.5 1.0
1.0
2.0
0 oo
ρ or Γshort
circuitopencircuit
"pure"resistance
capacitivereactance
inductivereactance
-90 degrees
90 degrees
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A two-port network
• S11 is complex input reflection coefficient• S22 is complex output reflection coefficient• S21 is forward transmission coefficient (gain)• S12 is reverse transmission coefficient (isolation)
X
X
Y
Y
ZL Z
L
Zo Zo
a
b
b
a
21 S
S S
S11
22
21
121 2
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Black box representation of circuits
• Approach is very versatile• A knowledge of 4 S-parameters
completely characterises the network.• The VNA is the means of making these
measurements.
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The N2PK approach to VNA design
• Commercial instruments work with a superhet based architecture
• N2PK uses a direct conversion approach• Depends on direct digital synthesis of test
signals and accurate phase detection
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N2PK VNA Block diagram
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PCB layout
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Back view
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The Reflection bridge
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Calibration
• The accuracy of measurements depend on comparison with known standards.
• Open circuit• Short circuit• 50 ohm load• Through piece
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Software
• A large part of the instrument’s functionality is dependent on the software (which is freely downloadable).
• N2PK• W3WWV – EXETER• GM3SEK – VNA4WIN
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N2PK
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N2PK
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Exeter
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Exeter
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VNA4WIN
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VNA4WIN
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N2PK a la GM8BJF
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The Reflection Bridge
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The Calibration Standards
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Plot of My 7MHz Dipole
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A Live Demonstration.
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Further Plans
. Convert to USB (Plug n’ Pray)Extend frequency rangeFast ADC conversion (allows real-time measurement)
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Acknowledgements
• Paul Kiciak, N2PK• Greg Ordy, W8WWV• Ian White, GM3SEK• Giancarlo Moda, I7SWX• Lou Macknik, W8LI
and Suja Raviraj