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• How to assmble the circuit

  nobcha • 5 days ago • 0 comments

  Components and Circuitry

  The necessary components are the following circuit board components, components for the connection board (audio jacks, battery-related components, variable resistors and step-up transformer), wiring materials, and a case. The circuit diagram for the circuit board and an applied circuit diagram with added external components are shown. The step-up transformer is ST-45(600ohm:10ohm) of Sansui.

  Bill of Materials http://github.com/Nobcha/R909-VFO-ESP/blob/main/5531_esp_25_bom.pdf

  Circuit board schematics http://github.com/Nobcha/R909-VFO-ESP/blob/main/R909-VFO_ESP32_SCM.pdf

  Appliation circuit schematics　

  PCB and Assembly

  This PCB needed to be quite compact, so all RC components except the aluminum electrolytic capacitors were surface-mounted.

  I found a heat plate with PTC-controlled maximum temperature on AliExpress. I used this heat plate and solder paste for soldering the surface-mount RC/C components.

  The component density isn't high, so hand soldering is also possible.

  After surface-mounting the components, the DIP components—electrolytic capacitors, sensors, switches, and pin headers—were soldered by hand.

  Assembly Check

  After checking for short circuits between the power supply and ground, a diagnostic sketch for verifying the board's operation was prepared.

  This sketch displays the results of switch operations on the OLED screen.
  https://github.com/Nobcha/R909-VFO-ESP/blob/main/R909_VFO_esp_ol_SWRE_i2c_TEST.ino

  Sketch

  
  Launch the Arduino IDE (I used V2.3.5) and upload the sketch to the ESP32-C3 dev kit super mini via USB cable. Connect the USB cable, specify the CPU setting as ESP32C3 Dev Module, confirm that it is connected in "Tools" "port", load the following sketch, and compile and upload it. To get the Arduino IDE to recognize the ESP32 module, press the BOOT switch and RESET switch on the module simultaneously, and then release the RESET switch first.

  https://github.com/Nobcha/R909-SINAD/blob/main/try_7_3i_sinad_meter.ino

• I tried a digital SINAD indicator using an ESP32-C3.

  nobcha • 04/10/2026 at 11:40 • 0 comments

  In this setup, the SINAD value changes in real time as the signal level is adjusted.
  

  I measured the receiving sensitivity at 27MHz (AM 80% Mod) with the ATX-mini HF/FM receiver using the Si4732. The value was -104dBm. To set it to 12dB SINAD, the volume adjustment had to be maximized and a step-up using a transformer was necessary.

  ATX-mini 27MHz 1000Hz 80% AM -104dBm 12dB SINAD

  SINAD measurement in progress　　　FFT view of the test signal
  

  In JH1LHV-OM's measurement example, the reading was -100dBm at 26MHz (AM 30% Mod), so the difference is likely due to the modulation depth setting. At 30% modulation, the SINAD did not rise above 12dB at my case. 

  I also checked the R80 Chinese-made airband receiver. This receiver has a configuration of NE602-MC3361-TA7640-LM386. A problem arose. Even with volume adjustment, the SINAD did not rise above 8dB. When using a deeper modulation of 80% at 1000Hz and examining the FFT, a component was found at 500Hz. Could this be due to the MC3361 chip being an FM chipset?

  Therefore, the sensitivity of the R80 airband receiver is equivalent to a SINAD of 8dB.

  R80 118.1MHz 1000Hz 80% AM -105dBm 9.1dB SINAD

  SINAD measurement in progress
  

  The FFT display helps visualize noise and distortion components.
  

  Note: When measuring C401, the measurement was smooth.

• SG modulation depth and receiver audio volume setting

  nobcha • 04/08/2026 at 09:31 • 0 comments

  We have compiled a separate document outlining important points to consider when measuring receiver sensitivity using the SINAD indicator. (SG modulation depth and receiver audio volume setting)

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