RF-Amp

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RF-Amp P1943-720px.jpg

RF Amplifier Features

  • Useful as an IF or Antenna Amplifier
  • From Charlie Morris' (ZL2CTM) Go QRP Portable SSB Rig
    • Charlie references Solid State Design for the Radio Amateur (pp 19-20)
  • Single 2N3904 NPN transistor
    • Ft = 300 MHz (Gain Bandwidth Product)
      • Theoretical gain
        • +20 dB at 30 MHz
        • +29.5 dB at 10 MHz
        • Reality is lower due to capacitance, etc.
  • Measured +22 dB gain @12V, +25dB gain @14V
  • Input connectors: SMA or BNC
  • +12V nominal power
  • 49x49mm card
  • 4x 4-40 mounting holes

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 = 58 uA
    • 10x the current in the biasing resistors = 580 uA (calculated)
  • R2 is 1.9V at 580 uA = 3.29K use 3.3K
  • R1 sources current to R2 and transistor base
    • Voltage = Vcc (12V) - 1.9V = 10.1V
    • Current = 577 uA + 58 uA = 635 uA
    • R1 = 10.1 / .635 mA = 15.9K, use 15K

Measured DC operating point

  • Measured with no input
  • Vcc = 11.96V
  • Current draw = 12 mA
    • Quick test for wiring and more or less correct parts
    • Expected 11 mA - close enough
  • +BUFF = 11.84V
    • 0.12V which is 12 mA through R4 10 ohms - expected
  • V emitter = 1.41V
    • 1.41V/12 Ohms = 11.75 mA close to 12 mA total measured current
  • V on input divider = 2.06V
    • Vbase + 0.7V - close
    • Measured Vbe = 2.06-1.41 = 0.65 - 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 (33.3) times re
      • re = 26 / Ie (10 mA in mA) = 26/10 = 2.6
      • SSDRA uses 25 as constant - close enough
        • 26 comes from Ebers-Moll approximation
      • Beta AC * re = 33.3*2.6 = 83.3 ohms - predominates
    • All in parallel are 80.8 ohms

Gain calculation

  • Approximation
  • Ic = 0.01A
  • Rc = 200
  • Vrc = 2V
  • Gain = Vrc / vt
    • vt = 26 mV at room temperature
    • Gain = 2V / .026V = 79.2 V/V
    • Gain = +37 dB

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)
  • Need to calculate turns ratio
  • 50:80.8 Ohms
  • n = sqrt(Zout/Zin)sqrt(80.8/50) = 1.27 turns ratio
  • Turns choices
  • Minimum number of turns
  • Rule of thumb - want Xl (coil impedance smallest value) to be least 4-5X the load
    • Load = 80.8 ohms
    • 5 * 80.8 ohms = 404.2 ohms minimum
      • More turns = larger capacitance and drops bandwidth
    • Toroid is FT37-43
    • From Toroid page
      • Xl = 404.4 at 9 MHz is 4.5 turns, round up to 5
    • Try nearest integer numbers turns ratios
      • 5:6 = 6% error
      • 6:8 = -4.6%
      • 7:9 = -1.1% << good choice
      • 8:10 = +1.7%
      • 9:11 = +4.0%
      • 10:13 = -2.19%
  • Use 7:9 turns ratio for optimal input transformer

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
  • SSDRA suggest presenting 200 ohm load to the collector
    • Can't find reference in SSDRA
    • Reflecting back 50 ohms load to 200 ohm collector...
  • 200:50 ohms
  • n = sqrt(200/50) = 2.0:1 turns ratio
  • 10:5 turns
    • 10 turns primary (on transistor collector)
      • 10 turns = 35 uH
    • 5 turns secondary (towards output)
      • 5 turns = 8.75 uH
    • 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 if the NanoVNA drives the input directly?
    • Test by driving directly from NanoVNA set to CW = 9 MHz
    • Measured output with scope - not clipped at 9 MHz
      • Approx. 1Vpp input = +22.1 dBm gain which matches the S21 with the attenuator on the input
      • Vpp = 12.4V with 50 Ohm load resistor
    • Starts clipping at 7 Mhz and down
  • Therefore, can measure input impedance at 9 MHz
  • Other evidence of compression
    • Compare S21 gain with no input attenuator, put external 40 dB RF Attenuators on output of RF-Amp to protect NanoVNA input
    • S21 shows lower gain in lower frequencies so clipping/compression is happening
    • Was: 35 dB at 1.4 MHz
    • Is: 23.1 dB at 1.5 MHz
  • Due to compression can't accurately measure lower frequencies with attenuator at output
  • Compression below 7 MHz matches what was on scope

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

Measure Input Impedance

  • Shows VSWR at 14.4 MHz = 1.56:1
  • At 9 MHz
    • VSWR = 1.7:1
    • Impedance = 81-j10

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

Change Input Transformer turns ratio

  • Above had 9:11 turns ratio
  • Change to 7:9 turns ratio
  • Slightly better gain at higher frequencies
  • Was: S21 @ 30 MHz = 12.7 dB
  • After: S21 @ 30 MHz = 15.3 dB
  • Small additional gain at 8 MHz
    • Was: S21 @ 9.1 MHz = 24.3 dB
    • After: S21 @ 9.1 MHz = 24.8 dB

RF-Amp S21 40dBAttenInput Turns7to9 1-30MHz.png

  • New turns improved the input VSWR slightly
  • Was: At 9 MHz, VSWR = 1.7:1, Impedance = 81-j10
  • After: At 9 MHz, VSWR = 1.6:1, Impedance = 76.7-j12

RF-Amp vswr 40dBAttenInput Turns7to9 1-30MHz.png

Tune input transformer

  • Isolate output by replacing output transformer with 200 resistor
  • Add one more output winding to input transformer T2 (7:10)
  • VSWR nearly 1.04:1 at 11.1 MHz
  • -19 dB return loss at 9 MHz VSWR = 1.249:1

RF-Amp VSWR 1-30MHz 7to10Turns.png

  • With output transformer
  • Slightly better with 1 extra winding

RF-Amp VSWR 1-30MHz 7to10Turns-2.png

W2AEW Measurement Method

  • Calibrate NanoVNA using External 30 dB Attenuator
  • Scan 1-30 Mhz
    • Overdriven at 1 MHz which "swamps" the RF Amp
  • Re-calibrate at 1.5-31.5 MHz
    • Peak gain at 1 MHz = 32 dB
      • Does not overdrive the Amp or NanoVNA
      • Downsize is a lot of noise in the return loss

Unit 1 - 9 MHz measurement

    • VSWR = 1.172
    • S11 (Return Loss) = -22.014 dB
    • S21 (Gain) = +23.624 dB

RF-Amp W2AEW S21 1-30MHz.png

RF-Amp W2AEW S11 1-30MHz.png

Unit 2 - 9 MHz measurement

    • VSWR = 1.182
    • S11 (Return Loss) = -21.565 dB
    • S21 (Gain) = +24.656 dB

RF-Amp U2 W2AEW S21 1-30MHz.png

RF-Amp U2 W2AEW S11 1-30MHz.png

Video

Assembly Sheet