RF Noise Generator

From Land Boards Wiki
Jump to navigation Jump to search

NoiseGen-P783-720px.jpg

Usage[edit]

  • Useful as a filter test signal source
    • Pass filters
    • Crystal filters
  • Would like to use as noise source for tinySA

Others[edit]

Design #1[edit]

  • From Experimental Methods in RF Design
    • Fig 7.72
    • Described as "not flat"
  • "Junk box" parts
    • (2) 2N3904 transistors
    • (2) 5V (nominal) zener diodes
  • Noise figure -50 dBm @ 10 MHz

Schematic[edit]

Ng-sch-prototype.PNG

First Prototype[edit]

Build / Enclosure (Original board design)[edit]

  • Used very old, but high quality chassis mount BNC connector
    • Silver connector has a nice patina
  • Built onto Single Side PCB material
    • Acts as ground plane
    • Pads cut from PCB material and superglued down onto the Base PCB
  • Hot glued down into plastic enclosure
  • Ferrite toroids at top are not used

NoiseGen-P783-720px.jpg

NoiseGen-P790-720px.jpg

NoiseGen-P787-720px.jpg

  • Unique nodes in red
  • Prototype on single sided copper clad PCB
  • KiCAD zener packages have backwards silkscreen (will need to fix on OshPark PCBs)

NG-layout.PNG

  • OSHPark PCB

NoiseGen P1855-720px.jpg

LTSpice Simulation[edit]

  • Amplifier looks flat but the result wasn't

NoiseGen Design1.PNG

Measurements[edit]

  • 12V, 43mA current draw

tinySA Measurements[edit]

  • Measure from 1-30 MHz
  • Power Off
  • -90 dB noise floor

NoiseGenOnTinySA Off.PNG

  • Power On
  • Measure from 1-30 MHz

NoiseGenOnTinySA On.PNG

  • 100 KHz-1 MHz
  • Power Off

NoiseGenOnTinySA Off 100KHz-1MHz.PNG

  • Power On
  • Lot more energy at ~400 KHz
  • -30 dB
  • Not too useable for filter testing
  • Might be OK for receiver noise source

NoiseGenOnTinySA On 100KHz-1MHz.PNG

Rigol DS1054Z Measurements[edit]

  • 50 MHz scope with mods
  • UltraScope settings
    • SCAL 1.00 V
    • H 200nS
  • FFT settings
    • CF = 16 MHz
    • 2 MHz start, 50 MHz stop
  • RBW 416.6 KHz
  • Background noise
  • Generator off
  • Average ~ -64 dB noise floor too high

NG OFF.PNG

  • Noise Generator On
  • Falls off with frequency, but better than -50 dB

NG ON.PNG

  • Measured performance per EMiRFD (predicted - 50 dB)

Better Design #2[edit]

  • Replace Zener as noise source with 2N3904 B-E junction reverse biased
  • C1 is connected Emitter of Q1 to GND instead of +12V to reduce power supply noise
  • Higher noise than EMiRFD design
    • Usable from 1-10 MHz
    • -40 dBm at 1 MHz
    • -80 dBm at 30 MHz
  • Nearly zero power supply current

Better Noise Gen schematic.png

NoiseGen V2-P1852-720px.jpg

Better Design #2 - LTSPice Simulation[edit]

  • Falls off in a straight line (in dBm) with frequency
    • 10 dB at 1 MHz
    • 0 dB at 10 MHz
    • -10dB at 100 MHz

LTSpiceSim Better2.PNG

tinySA Measurements[edit]

  • Off

V2-NoiseGenOnTinySA OFF 1MHz-10MHz.PNG

  • On, 1-30 MHz
    • Consistent with the LTSpice simulation

V2-NoiseGenOnTinySA On 1MHz-30MHz.PNG

Better Design #3 - Cascode Amp with output buffer[edit]

  • Previous design is probably good into receiver but not as good for tinySA tracking generator
    • Want to be able to drive passive/crystal filters for testing
    • Can already use NanoVNA but I'd prefer the tinySA due to lower noise floor
  • Goal
    • Flat response across 1-30 MHz
    • Good parts of Design #2
    • Transistor seems to be a better noise source than a zener
  • Idea - replace amp stage with cascode output stage followed by emitter follower for better match to 50 ohm load

Example cascode design[edit]

CascodeExample-720px.jpg

  • Replace R1 with 0.1uF in parallel with 470K (from the the previous)
  • Remove R2

Better Design #3 - LTSpice Simulation[edit]

  • Output stage is high gain, but limits bandwidth

Cascode2.PNG

Better Design #4[edit]

  • #3 output stage was limiting the bandwidth
    • Remove output stage

Cascode2 Sch.PNG

  • PCB design

NoiseGen V3 3D.png

RF Noise Cascode2 PCB.PNG

Qty Value RefDes Qty Value RefDes
2 100K R2,R5 3 0.1uF C1-C3
1 150K R3 3 2N3904 Q1-Q3
1 470K R1 4 TestPoint TP1-TP4
1 560 R4
  • < 3 dB down at 100 MHz

RF Noise Cascode2.PNG

Cascode Design Videos[edit]

Assembly Sheet[edit]

RF Noise Generator Rev 1