Transcript VHF Propagation Modes - Rochester VHF Group

Preamps & Noise Figure
Rochester VHF Group
12 December 2008
Preamps
• Most VHF/UHF receiving systems with nominal sensitivity can use
some additional gain and improved noise figure.
• A Preamplifier if properly selected allows you to hear weak
signals with improved Signal to Noise Ratio.
• A reasonable investment yields significant more DX at VHF/UHF
• Proper selection requires a “System Approach”… understanding total
receiving system impact.
• Good Preamps are reasonably easy to homebrew.
• There are cost effective commercial solutions.
• Measurements are a little sophisticated, but expensive test
equipment is not a necessity.
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Modern Preamps
• Bipolar Transistor
• Field Effect Transistor
• Dual Gate FET
• MOSFET – Metal Oxide Semiconductor FET
• MESFET – Metal Semiconductor FET
• HEMT High Electron Mobility Transistor (low resistivity, doped to
achieve advanced results)
• PHEMT Pseudomorphic HEMT (small gate length FETs)
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Preamps
• 40’s Open Mixers, tubes, 15-25 dB NF
• 50’s Low noise tubes, nuvistors, first xstrs 3-6 dB NF
• 60’s Transistors, JFETs, Parametric Amps 2-3 dB NF
• 70’s Hot JFETs, Dual-Gate FETs, Bipolar Xstrs, MASERs
• 80’s GaAs FETs in TVRO industry
• 90’s Small gate length FETs, MMICs
• Today PHEMT, BJT, GaAs MMICs
• Future?
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Preamps
• Filtering/Z-Matching – Exclude unwanted signals to
inconsequential levels (pre-selection filtering) and
provide necessary impedance transformation
• Gain – Enough to determine system RX sensitivity (p/o
overall amplification needed to detect signals and mask
the effects of subsequent stages)
• Noise Figure – As low as can be obtained at good economy
• Linearity – Amplify without creating new spurious signals
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Effects of Filtering / Matching
• Improved rejection of unwanted signals – off channel
• Provide impedance transformation – matching input to output
for best Gain or Noise performance (sometimes a controlled
mis-match or “noise match”)
• Off-frequency complex impedances – maybe not unilaterally
stable amplifier
• Filter elements don’t usually introduce additional IM but the
net effect on the device is important to the amplifier
performance
• Sharp filters may exclude wide band noise that the measuring
instrument is including in calibration – adds uncertainty
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Typical Receive Chain
25 dB Gain
0.3 dB Noise Figure
20 dB Gain
3 dB Noise Figure
-10 dB
Conversion Loss
20 dB Gain
3 dB Noise Figure
Mixer
X
Filter
IF
Amp
Filter
RF
Amp
RF
Amp
Filter
Filter
Multiplier
Oscillator
Xtal
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Down East Preamp – 144 MHz
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Noise and Gain
• Noise is amplified along with the signal
• Both S + N eventually presented to the detector
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Noise Figure
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Thermal Noise Power
Noise Power of a 3 dB NF preamp = 1.2E-17 Watts = -139.2 dBm
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Noise Figure vs Sensitivity
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Y-Factor NF Measurement
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Excess Noise Ratio
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Using a Noise Source
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Y-Factor
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Noise Figure Measurement
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Noise Figure Measurement
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Linearity
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Effects of Non-Linearity
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Intercept Point
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Intercept Point
• High IP3 and Low NF designs don’t usually coincide, but may overlap
• Best performance is lowest NF with acceptable IP3
• 1 dB Compression is typically easier to measure and shows trend
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Resources
Other Interesting Resources To Know About:
• Noise Figure ; Microwaves 101
http://www.microwaves101.com/encyclopedia/noisefigure.cfm
• Principles of Semiconductor Devices – Bart Van Zeghbroeck
http://ece-www.colorado.edu/~bart/book/contents.htm
• http://www.dxzone.com/catalog/Technical_Reference/Preamplifiers/
• Tommy Henderson - [email protected]
• http://www.g0mrf.freeserve.co.uk/432LNA.htm
• http://www.frenning.dk/OZ1PIF_HOMEPAGE/144_and_432MHzLNA.htm
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Extras
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432 Multimode With No Preamp
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432 Multimode With Preamp in Shack
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432 Multimode With Preamp at Antenna
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Noise Figure History
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Commercial Preamps
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Commercial Preamps
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Down East Preamp 10 GHz
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DB6NT Preamp 432 MHz
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Homebrew Designs
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Homebrew Designs
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Homebrew Designs
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Homebrew Designs
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Homebrew Designs
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Example Source of GaAs FETS
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416B Triode
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Parametric Amplifier
• Uses a diode pumped with microwave power in a “negative
resistance” region of it’s transfer curve to amplify signal energy.
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