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I'm designing a BA1404 Stereo FM transmitter and I found the following source: Source Link

You can find the component list, circuit scheme and the other stuff on this page. But when I look at the scheme, I couldn't find the 10uH Inductor on it. There are some images of the assembled circuit board and you can see the 10uH Inductor on the board.

I'm totally new at electrical circuits. Am I missing something at the scheme? By the way, are there some other sources on the internet where I saw the same circuit and similar assembled boards?

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It's definitely on the board (the green component on the top right), but not in the schematic. My guess, based on its positioning, is that it's part of the power supply filtering.

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  • \$\begingroup\$ Thank you for your help. I saw it on the circuit but I'm confused how to connect inductor to the circuit. On that site, they didn't put an image of the back side of the circuit. \$\endgroup\$ – Onur Alkan Oct 2 '14 at 17:28
  • \$\begingroup\$ @OnurAlkan: As the linked document shows, it forms a filter with one of the capacitors, specifically the 1nF C16 (which is the right 1nF capacitor along the top of the schematic). \$\endgroup\$ – Ignacio Vazquez-Abrams Oct 2 '14 at 18:12
  • \$\begingroup\$ @IgnacioVazquez-Abrams Green...? You mean orange right... \$\endgroup\$ – ACD Oct 2 '14 at 18:14
  • \$\begingroup\$ @ACD: No. The orange device is the 3.5T variable inductor. Look further down the page. \$\endgroup\$ – Ignacio Vazquez-Abrams Oct 2 '14 at 18:15
  • \$\begingroup\$ @IgnacioVazquez-Abrams Sneaky sneaky. Good call! \$\endgroup\$ – ACD Oct 2 '14 at 18:17
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When in doubt look at the data sheet and therein you can find a schematic that looks like this: -

enter image description here

From this I reckon the missing inductor may be associated with the antenna tuning on pin 7. As this is a transmitter, it's quite important to use a tuned circuit to remove unwanted harmonics that may spread RF "noise" (there are more appropriate words) into areas it shouldn't.

As usual I'd stick to the data sheet but if you want my best guess, it's in series with the 270R resistor and acts as an RF choke allowing the antenna signal to be a bit bigger but I'd follow the data sheet.

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I suspect the 10uH inductor is series with the power input to filter the incoming power.

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