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− | [[File:RF-Amp_P1943-720px.jpg]]
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− | == RF Amplifier Features ==
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− | * Useful as an IF or Antenna Amplifier
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− | * From [https://zl2ctm.blogspot.com/2020/11/go-qrp-portable-ssb-rig.html Charlie Morris' (ZL2CTM) Go QRP Portable SSB Rig]
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− | ** Charlie references Solid State Design for the Radio Amateur (pp 19-20)
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− | * Single 2N3904 NPN transistor
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− | ** Ft = 300 MHz (Gain Bandwidth Product)
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− | *** Theoretical gain
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− | **** +20 dB at 30 MHz
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− | **** +29.5 dB at 10 MHz
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− | **** Reality is lower due to capacitance, etc.
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− | * Measured +22 dB gain @12V, +25dB gain @14V
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− | * Input connectors: SMA or BNC
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− | * +12V nominal power
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− | * 49x49mm card
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− | * 4x 4-40 mounting holes
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− |
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− | == RF Amplifier Design ==
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− |
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− | === Schematic ===
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− | [[file:RF_Amp_Schematic-4.PNG]]
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− | == LT Spice Simulation ==
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− | * [https://github.com/land-boards/lb-boards/blob/master/HamRadio/RF-Amp/LTSpice/2n3904%20amp.asc LTspice Simulation] - GitHub source file
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− | ** +28.4 dB at 9 MHz
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− | [[File:RF-AMP-LTSPICE_XFMRS.PNG]]
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− |
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− | == Charlie Morris Design ==
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− |
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− | * From Charlie's notes with mods for my use
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− | ** [https://zl2ctm.blogspot.com/2020/11/go-qrp-portable-ssb-rig.html Charlie Morris' (ZL2CTM) Go QRP Portable SSB Rig]
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− | ** Charlie describes the design in detail in his video [https://www.youtube.com/watch?v=CHdtoupH2Vg Simple SSB Rig: Part 6 - IF Amplifiers] (Feb 2021)
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− | ** Based on the Class A RF Amplifier in [https://www.amazon.com/Solid-State-Design-Radio-Amateur/dp/0872590402 Solid State Design for the Radio Amateur] pp 19-20
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− | * [https://www.mouser.com/datasheet/2/308/1/2N3903_D-2310199.pdf 2N3904 data sheet]
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− | * [https://www.electronics-tutorials.ws/amplifier/emitter-resistance.html Emitter Resistance] - helpful paper
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− |
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− | === Beta DC ===
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− | * Geometric mean min/max beta at operating current
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− | ** =sqrt(100*300) = 173
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− |
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− | === Beta AC ===
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− | * Gain bandwidth product divided by operating frequency
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− | ** Assume operating frequency of 9 MHz (IF frequency)
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− | ** = 300/9 = 33.3
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− |
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− | === DC Operating Point ===
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− | * Max HFE RF gain at CE current of 10 mA
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− | ** If Vce = 6V, this is 60 mW power dissipation
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− | * Assume Ve (voltage across emitter resistor) = 1/10 Vcc = 12V/10 = 1.2V
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− | ** R3 is Re (emitter resistor) = 1.2V/0.01A = 120 ohms
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− | * VCE = 0.7V (typical from data sheet)
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− | * V(emitter) at 10% of Vcc rule of thumb = 1.2V
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− | * V(base) = V(emitter) + VCE = 1.9V
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− | * Base current is collector current divided by Beta DC
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− | ** Biasing resistors = 10x current needed by base current
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− | ** 10 mA in C-E, beta DC less = 10 mA/173 = 58 uA
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− | ** 10x the current in the biasing resistors = 580 uA (calculated)
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− | * R2 is 1.9V at 580 uA = 3.29K use 3.3K
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− | * R1 sources current to R2 and transistor base
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− | ** Voltage = Vcc (12V) - 1.9V = 10.1V
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− | ** Current = 577 uA + 58 uA = 635 uA
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− | ** R1 = 10.1 / .635 mA = 15.9K, use 15K
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− |
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− | ==== Measured DC operating point ====
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− | * Measured with no input
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− | * Vcc = 11.96V
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− | * Current draw = 12 mA
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− | ** Quick test for wiring and more or less correct parts
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− | ** Expected 11 mA - close enough
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− | * +BUFF = 11.84V
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− | ** 0.12V which is 12 mA through R4 10 ohms - expected
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− | * V emitter = 1.41V
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− | ** 1.41V/12 Ohms = 11.75 mA close to 12 mA total measured current
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− | * V on input divider = 2.06V
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− | ** Vbase + 0.7V - close
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− | ** Measured Vbe = 2.06-1.41 = 0.65 - close
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− |
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− | === Input resistance ===
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− | * Xc for 0.1uF cap from emitter to ground
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− | ** C=0.1uF
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− | ** F=10MHz
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− | ** 1/2*pi*F*C = 0.16 ohms
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− | * Parallel resistors R1, R2 paralleled with transistor input impedance
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− | ** R1=15K, R2=3.3K
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− | ** Transistor resistance = Beta AC (33.3) times re
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− | *** re = 26 / Ie (10 mA in mA) = 26/10 = 2.6
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− | *** SSDRA uses 25 as constant - close enough
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− | **** 26 comes from Ebers-Moll approximation
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− | *** Beta AC * re = 33.3*2.6 = 83.3 ohms - predominates
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− | ** All in parallel are 80.8 ohms
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− |
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− | === Gain calculation ===
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− | * Approximation
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− | * Ic = 0.01A
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− | * Rc = 200
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− | * Vrc = 2V
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− | * Gain = Vrc / vt
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− | ** vt = 26 mV at room temperature
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− | ** Gain = 2V / .026V = 79.2 V/V
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− | ** Gain = +37 dB
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− |
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− | === Input/Output Transformers ===
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− | * Using [http://toroids.info/FT37-43.php FT37-43 Toroid]
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− | [[file:FT37-43_10_Turns.PNG]]
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− | ==== Tracks ====
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− | [[file:RF-Amp-tracks.PNG]]
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− | ==== Input Transformer ====
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− | * Input Transformer (T1 on Charlie's - T2 on this board)
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− | * Need to calculate turns ratio
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− | * 50:80.8 Ohms
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− | * n = sqrt(Zout/Zin)sqrt(80.8/50) = 1.27 turns ratio
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− | * Turns choices
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− | * Minimum number of turns
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− | * Rule of thumb - want Xl (coil impedance smallest value) to be least 4-5X the load
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− | ** Load = 80.8 ohms
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− | ** 5 * 80.8 ohms = 404.2 ohms minimum
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− | *** More turns = larger capacitance and drops bandwidth
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− | ** Toroid is FT37-43
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− | ** From [http://toroids.info/FT37-43.php Toroid page]
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− | *** Xl = 404.4 at 9 MHz is 4.5 turns, round up to 5
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− | ** Try nearest integer numbers turns ratios
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− | *** 5:6 = 6% error
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− | *** 6:8 = -4.6%
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− | *** 7:9 = -1.1% << good choice
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− | *** 8:10 = +1.7%
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− | *** 9:11 = +4.0%
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− | *** 10:13 = -2.19%
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− | * Use 7:9 turns ratio for optimal input transformer
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− | [[file:RF-Amp-T2.PNG]]
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− | ==== Output Transformer ====
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− | * Output transformer (T2 on Charlie's - T1 on this board)
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− | * T2 - different than Charlie's design since my Crystal filters are all 50 ohms in/out
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− | * SSDRA suggest presenting 200 ohm load to the collector
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− | ** Can't find reference in SSDRA
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− | ** Reflecting back 50 ohms load to 200 ohm collector...
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− | * 200:50 ohms
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− | * n = sqrt(200/50) = 2.0:1 turns ratio
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− | * 10:5 turns
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− | ** 10 turns primary (on transistor collector)
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− | *** 10 turns = 35 uH
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− | ** 5 turns secondary (towards output)
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− | *** 5 turns = 8.75 uH
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− | ** 15 turns = 9.5 in
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− | [[file:RF-Amp-T1.PNG]]
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− | === Charlie's Notes ===
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− | [[FILE:IF Amp_0046A.jpg]]
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− | [[FILE:IF Amp_0046B.jpg]]
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− | [[FILE:IF Amp_0046C.jpg]]
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− | [[FILE:IF Amp_0047A.jpg]]
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− | [[FILE:IF Amp_0047B.jpg]]
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− | [[FILE:IF Amp_0047C.jpg]]
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− | == NanoVNA Measurements ==
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− | * '''Goal''': Measure RF-Amp performance using a [[NanoVNA]] running [https://nanovna.com/?page_id=90 NanoSaver software on PC]
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− | * S21 (gain) needs to be measured with a [[RF_Attenuators#40_dB_Attenuator|40 dB attenuator]] on input to RF-Amp to avoid compression on the output
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− | * S11 (reflection) input impedance can't be measured with input [[RF_Attenuators#40_dB_Attenuator|40 dB attenuator]] because S11 just ends up measuring the attenuator
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− | ** Output should be terminated to 50 ohms for S11 measurement
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− | * DC current = 12 mA
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− | === Measure S21 ===
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− | * Put [[RF_Attenuators#40_dB_Attenuator|40 dB attenuator]] on RF-Amp input, measure S21 at output
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− | ** [[NanoVNA]] provides 50 ohm load to RF-Amp to properly terminate output
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− | * Measure S21 with 9:11 input transformer
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− | ** S21 @ 100 KHz = -8 dB dB
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− | ** S21 @ 1.45 MHz = 35.4 dB (peak gain)
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− | ** S21 @ 9.1 MHz = 24.3 dB
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− | ** S21 @ 16 MHz = 20.1 dB
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− | ** S21 @ 30 MHz = 12.7 dB
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− | * Peak gain justifies use of 40 dB attenuator to protect [[NanoVNA]]
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− | [[file:RF-Amp_S21_40dBAttenInput_1-30MHz.png]]
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− | ==== LTspice vs NanoVNA ====
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− | * LTspice simulation was pretty similar to [[NanoVNA]] results
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− | ** -10 dB at 100 KHz
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− | ** +32 dB at peak
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− | ** Lower output at higher frequencies
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− | [[file:RF-Amp_S21_LTspice-vs-NanoVNA_1-30MHz.png]]
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− | === Measure Input Compression ===
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− | * Is there compression if the [[NanoVNA]] drives the input directly?
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− | ** Test by driving directly from NanoVNA set to CW = 9 MHz
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− | ** Measured output with scope - not clipped at 9 MHz
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− | *** Approx. 1Vpp input = +22.1 dBm gain which matches the S21 with the attenuator on the input
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− | *** Vpp = 12.4V with 50 Ohm load resistor
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− | ** Starts clipping at 7 Mhz and down
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− | * Therefore, can measure input impedance at 9 MHz
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− | * Other evidence of compression
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− | ** Compare S21 gain with no input attenuator, put external [[RF Attenuators|40 dB RF Attenuators]] on output of RF-Amp to protect [[NanoVNA]] input
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− | ** S21 shows lower gain in lower frequencies so clipping/compression is happening
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− | ** Was: 35 dB at 1.4 MHz
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− | ** Is: 23.1 dB at 1.5 MHz
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− | * Due to compression can't accurately measure lower frequencies with attenuator at output
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− | * Compression below 7 MHz matches what was on scope
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− | [[file:RF-Amp_S21_40dBAttenOutput_1-30MHz.png]]
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− | === Measure Input Impedance ===
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− | * Shows VSWR at 14.4 MHz = 1.56:1
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− | * At 9 MHz
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− | ** VSWR = 1.7:1
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− | ** Impedance = 81-j10
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− | [[file:RF-Amp_AttenOutput_VSWR_2021_1-30MHz.png]]
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− | === Change Input Transformer turns ratio ===
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− | * Above had 9:11 turns ratio
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− | * Change to 7:9 turns ratio
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− | * Slightly better gain at higher frequencies
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− | * Was: S21 @ 30 MHz = 12.7 dB
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− | * After: S21 @ 30 MHz = 15.3 dB
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− | * Small additional gain at 8 MHz
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− | ** Was: S21 @ 9.1 MHz = 24.3 dB
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− | ** After: S21 @ 9.1 MHz = 24.8 dB
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− | [[file:RF-Amp_S21_40dBAttenInput_Turns7to9_1-30MHz.png]]
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− | * New turns improved the input VSWR slightly
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− | * Was: At 9 MHz, VSWR = 1.7:1, Impedance = 81-j10
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− | * After: At 9 MHz, VSWR = 1.6:1, Impedance = 76.7-j12
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− | [[file:RF-Amp_vswr_40dBAttenInput_Turns7to9_1-30MHz.png]]
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− | ==== Tune input transformer====
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− | * Isolate output by replacing output transformer with 200 resistor
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− | * Add one more output winding to input transformer T2 (7:10)
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− | * VSWR nearly 1.04:1 at 11.1 MHz
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− | * -19 dB return loss at 9 MHz VSWR = 1.249:1
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− | [[file:RF-Amp_VSWR_1-30MHz_7to10Turns.png]]
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− | * With output transformer
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− | * Slightly better with 1 extra winding
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− | [[file:RF-Amp_VSWR_1-30MHz_7to10Turns-2.png]]
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− | == W2AEW Measurement Method ==
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− | * Calibrate NanoVNA using External 30 dB Attenuator
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− | ** See [https://youtu.be/7TtKE39TWpI W2AEW #337 video] below
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− | ** Insert 30 dB attenuator and calibrate with attenuator installed
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− | ** Open/sort/thru at the output side of the attenuator using [[NanoVNA#RF_Demo_Kit|NanoVNA RF Demo Kit]]
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− | * Scan 1-30 Mhz
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− | ** Overdriven at 1 MHz which "swamps" the RF Amp
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− | * Re-calibrate at 1.5-31.5 MHz
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− | ** Peak gain at 1 MHz = 32 dB
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− | *** Does not overdrive the Amp or NanoVNA
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− | *** Downsize is a lot of noise in the return loss
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− | === Unit 1 - 9 MHz measurement ===
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− | ** VSWR = 1.172
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− | ** S11 (Return Loss) = -22.014 dB
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− | ** S21 (Gain) = +23.624 dB
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− | [[file:RF-Amp_W2AEW_S21_1-30MHz.png]]
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− | [[file:RF-Amp_W2AEW_S11_1-30MHz.png]]
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− | === Unit 2 - 9 MHz measurement ===
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− | ** VSWR = 1.182
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− | ** S11 (Return Loss) = -21.565 dB
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− | ** S21 (Gain) = +24.656 dB
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− | [[file:RF-Amp_U2_W2AEW_S21_1-30MHz.png]]
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− | [[file:RF-Amp_U2_W2AEW_S11_1-30MHz.png]]
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− | == Video ==
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− | <video type="youtube">7TtKE39TWpI</video>
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− | <video type="youtube">CHdtoupH2Vg</video>
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− | <video type="youtube">YJTsWV2kzFY</video>
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− | <video type="youtube">xPFzFhM0ojE</video>
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− | == Assembly Sheet ==
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− | * [[RF Amplifier Assembly Sheet]]
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