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I have a fairly simple circuit with an Arduino and a SA818 RF module, I'm providing 3v3 power to the SA818 via two series diodes (dirty I know) and am communicating using software serial, of which the TX line from the Arduino runs to the RX of the SA818 using a resistor divider to drop it to 3v3, I have a small speaker connected between the SA818 AF output and ground. everything works but the noise (pulses) when the TX line is transmitting completely swamps out the audio from the SA818. I have placed a capacitor between the AF out and the speaker but it has not helped. I know this is a bare minimum circuit but does anyone have ideas of how to isolate the UART from the audio?

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  • \$\begingroup\$ Could you give more details on your setup, is this a breadboard or PCB circuit? If PCB, can you share a screenshot of your layout? If breadboard, maybe a picture? \$\endgroup\$
    – eeintech
    Jan 14, 2020 at 12:37

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the TX line from the Arduino runs to the RX of the SA818

If you look closely at the block diagram for the SA818 (below)...

enter image description here

... you will see that the RX line is an output and this therefore means that you are connecting your UART output to the SA818 RX output. Even though you are doing this via resistor it doesn't make any sense at all.

Of course, I can't vouch for the datasheet being correct as it does appear to contradict itself later on when it says: -

 | 16 | RXD | RXD of the module and connect to external TXD |
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