1. Rochester VHF Group 12 December 2008
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.
11/12/2018
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)
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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?
11/12/2018
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
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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
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Rochester VHF Group

7. Typical Receive Chain
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Rochester VHF Group

8. Down East Preamp – 144 MHz
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Rochester VHF Group

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

10. Noise Figure
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Rochester VHF Group

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

12. Noise Figure vs Sensitivity
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Rochester VHF Group

13. Y-Factor NF Measurement
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Rochester VHF Group

14. Excess Noise Ratio
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Rochester VHF Group

15. Using a Noise Source
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Rochester VHF Group

16. Y-Factor
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Rochester VHF Group

17. Noise Figure Measurement
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Rochester VHF Group

18. Noise Figure Measurement
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Rochester VHF Group

19. Linearity
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Rochester VHF Group

20. Effects of Non-Linearity
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Rochester VHF Group

21. Intercept Point
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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
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Rochester VHF Group

23. Resources Other Interesting Resources To Know About:
Noise Figure ; Microwaves 101
Principles of Semiconductor Devices – Bart Van Zeghbroeck
Tommy Henderson -
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Rochester VHF Group

24. Extras
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Rochester VHF Group

25. 432 Multimode With No Preamp
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Rochester VHF Group

26. 432 Multimode With Preamp in Shack
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Rochester VHF Group

27. 432 Multimode With Preamp at Antenna
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Rochester VHF Group

28. Noise Figure History
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Rochester VHF Group

29. Commercial Preamps
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Rochester VHF Group

30. Commercial Preamps
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Rochester VHF Group

31. Down East Preamp 10 GHz
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Rochester VHF Group

32. DB6NT Preamp 432 MHz
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Rochester VHF Group

33. Homebrew Designs
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Rochester VHF Group

34. Homebrew Designs
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Rochester VHF Group

35. Homebrew Designs
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Rochester VHF Group

36. Homebrew Designs
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Rochester VHF Group

37. Homebrew Designs
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Rochester VHF Group

38. Example Source of GaAs FETS
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Rochester VHF Group

39. 416B Triode
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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.
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Rochester VHF Group