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Preamps & Noise Figure

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.

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Preamps & Noise Figure

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  1. Preamps & Noise Figure Rochester VHF Group 12 December 2008

  2. 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. Rochester VHF Group

  3. 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) Rochester VHF Group

  4. 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? Rochester VHF Group

  5. 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 Rochester VHF Group

  6. 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 Rochester VHF Group

  7. Typical Receive Chain Rochester VHF Group

  8. Down East Preamp – 144 MHz Rochester VHF Group

  9. Noise and Gain • Noise is amplified along with the signal • Both S + N eventually presented to the detector Rochester VHF Group

  10. Noise Figure Rochester VHF Group

  11. Thermal Noise Power Noise Power of a 3 dB NF preamp = 1.2E-17 Watts = -139.2 dBm Rochester VHF Group

  12. Noise Figure vs Sensitivity Rochester VHF Group

  13. Y-Factor NF Measurement Rochester VHF Group

  14. Excess Noise Ratio Rochester VHF Group

  15. Using a Noise Source Rochester VHF Group

  16. Y-Factor Rochester VHF Group

  17. Noise Figure Measurement Rochester VHF Group

  18. Noise Figure Measurement Rochester VHF Group

  19. Linearity Rochester VHF Group

  20. Effects of Non-Linearity Rochester VHF Group

  21. Intercept Point Rochester VHF Group

  22. 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 Rochester VHF Group

  23. 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 - wd5ago@hotmail.com • http://www.g0mrf.freeserve.co.uk/432LNA.htm • http://www.frenning.dk/OZ1PIF_HOMEPAGE/144_and_432MHz-LNA.htm Rochester VHF Group

  24. Extras Rochester VHF Group

  25. 432 Multimode With No Preamp Rochester VHF Group

  26. 432 Multimode With Preamp in Shack Rochester VHF Group

  27. 432 Multimode With Preamp at Antenna Rochester VHF Group

  28. Noise Figure History Rochester VHF Group

  29. Commercial Preamps Rochester VHF Group

  30. Commercial Preamps Rochester VHF Group

  31. Down East Preamp 10 GHz Rochester VHF Group

  32. DB6NT Preamp 432 MHz Rochester VHF Group

  33. Homebrew Designs Rochester VHF Group

  34. Homebrew Designs Rochester VHF Group

  35. Homebrew Designs Rochester VHF Group

  36. Homebrew Designs Rochester VHF Group

  37. Homebrew Designs Rochester VHF Group

  38. Example Source of GaAs FETS Rochester VHF Group

  39. 416B Triode Rochester VHF Group

  40. Parametric Amplifier • Uses a diode pumped with microwave power in a “negative resistance” region of it’s transfer curve to amplify signal energy. Rochester VHF Group

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