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Oscillating noise on switching signal

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nodee

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Hi,

We have recently moved our design onto a PCB but have encountered problems with what looks to be noise on a switching signals from one IC, the IC is a 125kHz antenna driver for a RFID application. On a prototype board the 125kHz square wave is clean (see picture left) but when on the PCB the signal has a decaying oscillation after each switch point (see picture right).

The PCB is 4 layer with a ground plane in the layer below the switching signals. decoupling caps are probably not as close as they should be on the board but I have also soldered caps directly onto the pins producing no change in the signal noise.

Could anyone suggest what might be causing this and possibly offer a solution?

Thanks,
Nodee
 

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Why do you think that a problem is involved with the waveform?

In any case, without showing circuit details you can hardly expect helpful answers.
 

I Think Frequency fluctuation is cause of this problem, you have to do decoupling to this IC.just place capacitor to VCC and GND.
 

FvM, this oscillation on the switching frequency is detuning the attached antenna and reducing the read range of the device. There aren't many circuit details to give unfortunately, its just a simple uC controlling the on/off of the RFID IC that has the problem. power is supplied by a switching regulator but the output is smoothed with minimal voltage ripple.

honey 77, as I said in the OP I have tried your suggestion with no success.

Thanks for the quick replies.
Nodee
 

this oscillation on the switching frequency is detuning the attached antenna and reducing the read range of the device.
Click to expand...
How should an oscillation detune a circuit?

In fact I don't understand why a 125 kHz coil matching circuit has a multi MHz resonance. I rather expect mixed up component values, shorts or other circuit faults.
 

Ok well rather than detuning the antenna the oscillation is affecting the sine wave produced causing the amplitude of the sine to be reduced and ultimately the affecting the field produced.

The uC has a 10kHz clock signal output and a 4MHz ceramic resonator connected but neither of these seem to show the issue, I am beginning to think this is a grounding issue with the individual RFID IC, is there any way I can check and confirm this?

Nodee
 

The frequency of the noise imposed on the switching signal is the same frequency as my ceramic resonator used for both uC and RFID IC, when I probe with my o-scope the crystal the clock signal very quickly decays (<30 cycles of the clock) to a 2Vdc level. On my prototype I am able to probe the resonator and the clock signal remains. I have replaced the ceramic resonator but the problem persists.

I looks although I might have found something to do with the problem but I'm not sure what this is telling me?

Any ideas?


Nodee
 

Try using a decoupling cap with an SRF near the frequency you want to eliminate. From Murata's Simsurfing tool, an 0603 4.7 uF cap has an SRF near 4 MHz.
 

Ok well rather than detuning the antenna the oscillation is affecting the sine wave produced causing the amplitude of the sine to be reduced and ultimately the affecting the field produced.
Click to expand...
A sligtly different but rarely better interpretation of the waveform, I think.

Once again: There's most likely a fundamental circuit fault that causes both the about 2.3 MHz oscillation (not 4 MHz, just look sharp) and the amplitude degradation.

But without knowing details about the circuit and the measurement setup, it's not clear if there's an actual fault. The oscillation may be a trivial measurement artefact (unsuffcient probe grounding, 1:1 probe with high input capaitance) and the lower field strength caused by other problems.
 

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