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FM transmitter chip RDA5820 PLL unlocking?

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Oct 14, 2014
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I have built an FM transmitter circuit using the RDA5820. (It is one of those all in one FM stereo transmitter chips).

Anyway, it has a problem with what I think is the PLL unlocking after a few minutes of transmitting. It makes a loud popping noise and the carrier is shifting on the analyzer. I am pretty sure there is nothing wrong with my circuit. The only thing I am unsure of is the antenna matching. Right now the antenna output goes through 100p and into an op-amp.

I found that the problem is fixed by placing a small piece of metal on top of the chip. However, it only works once the problem has started. If I leave the metal on there before the problem, it doesn't fix it :bang: I know it seems like some type of interference, but I tried shielding everything and nothing changes.

I really don't know much about RF yet, so could you please give me some ideas of what to try? It's also possible that the module I am using is defective (I am not using the chip directly).

Here is what I am talking about:

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Your problem indicates the PLL is not stable. I think you should look for these possible reasons:

1. Injection pulling, due to high power matching frequency source nearby.
2. Unregulated power source with noise.
3. Defective chip.
4. Defective crystal.
5. Improper antenna size.
6. Error in register settings.
7. EM interference.

if I were to guess, you have some noise coming in on the SPI bus, and it resets the state of the synthesizers, or goes from transmit to receive, due to the corrupted register values.

Is there a good decoupling cap placed close to the chip for analog supply?
Mount on a small ground plane with insulation and connect to 0V with feedthru wires.

If running serial from floating laptop, with charger, ground it to reduce SMPS charger noise.
May need ferrite beads or small torroid on IO lines. Twist with ground for less stray emissions.

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