[SOLVED] Radiation issue on one frequency

[dharithothi]

Hello Guys,

In my board I am facing issue of radiation emission of particular 74.25MHz frequency harmonics of 7th number and getting maximum radiation on 520Mhz. Trace length is about 2168 mils and having ground plane on second layer. I have tried by putting RC filter as well putting bead also. But I cant get benefit. Help me to reduce the radiation level.

Dharit.

 

[FvM]

How do you know that the said trace is radiating the unwanted signal and not e.g. the power and ground nets of the driving IC? The fact that you "see" a signal with a field probe in one place doesn't mean that you have found the major radiation source.

For a serious discussion, we need to see circuit schematic and layout.

 

[jiripolivka]

Open structures like yours will always radiate , also harmonics. Enclose your whole structure in a metal envelope, use good DC and low-frequency feedthroughs blocked with capacitors, and a RF connector for RF output. Then you may need a low-pas filter for the RF output to cut down harmonics.

 

[chuckey]

Have you considered what is generating this harmonic?
Frank

 

[criterion456]

If the trace is impedance-controlled.
The resistance of RC filter and internal resistance of bead will affect the impedance of the trace.
And the radiation emission perhaps becomes more serious.....

 

[biff44]

like they said, you have to solve this at the source, not along some 2" long trace.

Start with bypass caps right a the chip that is making the signal. 1000 PF in Parallel with 0.1 uF, as close as possible to the power supply pin and the ground pin. that will keep the circulating ground currents to a minimum. If you do not have a decent ground under the chip (such as a ground plane), or if your ground plane is cut under the chip, then stich over the cut with short soldered on wires.

After that, maybe near the chip output, you put a series L-C circuit to ground that is tuned to 520 MHz.

 

[criterion456]

In addition to L-C filter, you can also make use of pi-type filter, i.e. C-L-C, to reject the noise.

Because pi-type filter has better noise rejection than L-type.

However, for pi-type filter,

there is one important thing you should pay attention to.

You can NOT make the 2 capacitors share the same ground on top layer.

Otherwise, the pi-type filter will nearly be unable to reject noise.