TPS62142: Radiated Spurious Emission test failing

Abishek,

That is the 560 th harmonic of the switching frequency.  If it were a harmonic effect, I would expect to see multiple spurs, not just a lone singleton as you show.  I cannot think what would cause a spur at 1.4 GHz.  I'll take a detailed look at your layout tomorrow, but I don't expect to see anything telling.  Do you have any lower frequency scans that may provide additional clues?

Abishek,

Also can you provide a waveform of your SW node?  Primarily I will be looking for ringing on the rising and falling edge.  It would be helpful if you can measure the frequency of any ringing present.

Hi John,

Sorry for late reply.

I am getting the waveform at the SW pin, should get you within a day or two.

Please do not close this ticket.

By the way , did you find anything in the layout ?

Regards

Abhishek

Hi John,

I am attaching the waveforms at the SW Pin, there are 3 waveforms, one at the time of Turning ON the device, another while Continuous ON and third at device turn OFF, with the name of the files in the same manner

1) Turn ON.

2) Continuous ON

3) Turn OFF

Abhishek,

I did not see anything disturbing with the layout, C1 could be placed closer to the IC, but I doubt that has any impact on your issue.

John,

Did you get chance to have a look at the waveforms I uploaded just now ?

Regards

Abhishek

Abhishek,

Those do not appear to be the SW node.  I would expect a 2.5 MHz rectangular pulse from approx. 0 V when low to approx Vin when high.  Duty cycle should be approx Vout/Vin. 

Hi John,

Attaching the waveform on the SW pin , the last waveforms I uploaded were on the Vos pin and not the SW pin.

Abhishek,

What yo will need to do is zoom in and measure the frequency of any ringing on the rising and falling edges.  I cannot make it out as your waveform is too grainy.  You should use a scope wit a lot wider BW than 100 MHz to have any chance of finding anything.

Jack,

Presently we do not have an oscilloscope with a higher bandwidth but will have to buy/rent one, how much bandwidth do you recommend to read this waveform ?

Meanwhile I can put a bypass capacitor of 100pf ~ 115pf  between the VOS and the gnd to filter this noise at 1409.96 MHz. Do you think this would help?

Regards

Abhishek 

Abhishek,

The higher the better.  500 MHz minimum.  If a small FH bypass cap, from VOS to GND will solve your issue, that is perfectly acceptable.  let me know if that does in fact fix your issue.  If so, there will be no need to rent an oscilloscope.

Jack,

When you say 'FH ' for the bypass capacitors what does that mean ?

Regards

Abhishek

Abhishek,

I have dyslexia pretty bad.  it should be HF "High Frequency", not FH.  My favorite word is "teh".

John,

No problem. I will try the high frequency bypass caps to resolve it. 

And my apologies for naming you incorrectly since last couple posts.

We can close this ticket for now. Will reopen if I have anything to question after I test with the caps on.

Regards

Abhishek

Hi John,

We tried with the 100pF ceramic cap on the baseboard and 110pF cap on the TPS62142 based daughter board, but it again failed the test.

Regards

Abhishek