RF-Amp

From Land Boards Wiki
Jump to navigation Jump to search

RF-Amp Front.png

RF Amplifier Features

RF Amplifier Design

Schematic

RF Amp Schematic-4.PNG

LT Spice Simulation

RF-AMP-LTSPICE XFMRS.PNG

Charlie Morris Design

Beta DC

  • Geometric mean min/max beta at operating current
    • =sqrt(100*300) = 173

Beta AC

  • Gain bandwidth product divided by operating frequency
    • Assume operating frequency of 9 MHz (IF frequency)
    • = 300/9 = 33.3

DC Operating Point

  • Max HFE RF gain at CE current of 10 mA
    • If Vce = 6V, this is 60 mW power dissipation
  • Assume Ve (voltage across emitter resistor) = 1/10 Vcc = 12V/10 = 1.2V
    • R3 is Re (emitter resistor) = 1.2V/0.01A = 120 ohms
  • VCE = 0.7V (typical from data sheet)
  • V(emitter) at 10% of Vcc rule of thumb = 1.2V
  • V(base) = V(emitter) + VCE = 1.9V
  • Base current is collector current divided by Beta DC
    • Biasing resistors = 10x current needed by base current
    • 10 mA in C-E, beta DC less = 10 mA/173 = 48 uA
    • 10x the current in the biasing resistors = 480 uA (calculated)
  • R2 is 1.9V at 480 uA = 3.9K use 3.3K
    • Actual current will be 1.9V/3.3 ohms = 634 mA
  • R1 sources current to R2 and transistor base
    • Voltage = Vcc (12V) - 1.9V = 10.1V
    • Current = 576 uA + 57 uA = 634 uA
    • R1 = 10.1 / .634 mA = 15.9K, use 15K

Measured DC operating point

  • Measured with no input
  • Vcc = 11.96V
  • Current draw = 12 mA
    • Expected 11 mA - close enough
  • +BUFF = 11.84V
    • 0.12V which is 12 mA through R4 10 ohms - expected
  • V emitter = 1.41V
    • 12 mA through 120 ohm = 1.44V - close
  • V on input divider = 2.06V
    • Vbase + 0.7V - close

Input resistance

  • Xc for 0.1uF cap from emitter to ground
    • C=0.1uF
    • F=10MHz
    • 1/2*pi*F*C = 0.16 ohms
  • Parallel resistors R1, R2 paralleled with transistor input impedance
    • R1=15K, R2=3.3K
    • Transistor resistance = Beta AC (30) times re
      • re = 26 / Ie (10 mA in mA) = 26/10 = 2.6
      • SSDRA uses 25 as constant - close enough
      • Beta AC * re = 33.3*2.6 = 86.6 ohms - predominates
    • All in parallel are 83.9 ohms

Input/Output Transformers

FT37-43 10 Turns.PNG

Tracks

RF-Amp-tracks.PNG

Input Transformer

  • Input Transformer (T1 on Charlie's - T2 on this board)
  • 50:83.9 Ohms = 1 : 1.29 turns ratio
  • n = sqrt(Zout/Zin)sqrt(83.9/50) = 1.29
  • Turns choices
  • Want Xl (coil impedance smallest calue) to be least 4-5X the load
    • Load = 83.9 ohms
    • 5 * 83.9 ohms =

83.9** 8:10 = 1:1.25

    • 95:11 = 1:1.22
    • 8:11 = 1:1.38
    • 9 turns primary
      • 9 turns on FT37-43 = 38.3 uH
    • 11 turns on secondary
      • 11 turns on FT37-43 = 42.4 uH
    • 20 turns = 12 in

RF-Amp-T2.PNG

Output Transformer

  • Output transformer (T2 on Charlie's - T1 on this board)
  • T2 - different than Charlie's design since my Crystal filters are all 50 ohms in/out
  • 200:50 ohms
  • n = sqrt(200/50) = 2:1
  • 10:5 turns
    • 10 turns primary (on transistor collector)
      • 10 turns = 35 uH
    • 5 turns secondary (towards output)
      • 5 turns = 8.75 uH
  • 8 15 turns = 9.5 in

RF-Amp-T1.PNG

Charlie's Notes

IF Amp 0046A.jpg

IF Amp 0046B.jpg

IF Amp 0046C.jpg

IF Amp 0047A.jpg

IF Amp 0047B.jpg

IF Amp 0047C.jpg

NanoVNA Measurements

  • Goal: Measure RF-Amp performance using a NanoVNA running NanoSaver software on PC
  • S21 (gain) needs to be measured with a 40 dB attenuator on input to RF-Amp to avoid compression on the output
  • S11 (reflection) input impedance can't be measured with input 40 dB attenuator because S11 just ends up measuring the attenuator
    • Output should be terminated to 50 ohms for S11 measurement
  • DC current = 12 mA

Measure S21

  • Put 40 dB attenuator on RF-Amp input, measure S21 at output
    • NanoVNA provides 50 ohm load to RF-Amp to properly terminate output
  • Measure S21 with 9:11 input transformer
    • S21 @ 100 KHz = -8 dB dB
    • S21 @ 1.45 MHz = 35.4 dB (peak gain)
    • S21 @ 9.1 MHz = 24.3 dB
    • S21 @ 16 MHz = 20.1 dB
    • S21 @ 30 MHz = 12.7 dB
  • Peak gain justifies use of 40 dB attenuator to protect NanoVNA

RF-Amp S21 40dBAttenInput 1-30MHz.png

LTspice vs NanoVNA

  • LTspice simulation was pretty similar to NanoVNA results
    • -10 dB at 100 KHz
    • +32 dB at peak
    • Lower output at higher frequencies

RF-Amp S21 LTspice-vs-NanoVNA 1-30MHz.png

Measure Input Compression

  • Is there compression on lower frequencies if the NanoVNA drives the input directly?
  • Put external 40 dB RF Attenuators on output of RF-Amp to protect NanoVNA input
  • S21 shows lower gain
    • Was: 35 dB at 1.4 MHz
    • Is: 23.1 dB at 1.5 MHz
  • Due to compression can't accurately measure with attenuator at output

RF-Amp S21 40dBAttenOutput 1-30MHz.png

Measure Input Impedance

  • Is VSWR inaccurate due to input compression?
    • Shows VSWR at 14.4 MHz = 1.56:1
  • At 9 MHz
    • VSWR = 1.7:1
    • Impedance = 81-j10
    • Is this accurate?

RF-Amp AttenOutput VSWR 2021 1-30MHz.png

Video

Assembly Sheet