Using nanoVNA to find cold solder joints in the RF board

[va3rr]

As recommended by Manuel, I've been using my nanoVNA to tweak the Lx2 inductors to tune the 2nd harmonic 'trap'. The RF boards I purchased for our group's 'boards only' buy did not have 1nF capacitors available when i ordered the boards, so I bought some GRM2195C2A102JA01D capacitors.

When I was adjusting the 7 MHz filter with the nanoVNA, I noticed that the filter didn't 'roll off' on the 3rd harmonic like it did on my 2nd RF board.



On closer inspection, it looked like C35 wasn't soldered correctly. After I touched up the capacitor, here's what the filter output should look like: (Apologies as the scale used isn't quite the same)



This will probably provide better suppression of the 3rd harmonic on 21 MHz

Edit; As an added bonus, the power output went from .61W to .89W when powered with 5Vdc via the USB port. Efficiency is still 88%

[DL2MAN]

Good finding ! Thanks for sharing with us !

73 Manuel; DL2MAN