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Using ADF4360-7 to an audio FM transmitter

Question asked by carlos777 on Jun 9, 2015
Latest reply on Jun 16, 2015 by carlos777

Hi There,

Sorry to explain this issue in poor English.


I want to using ADF4360-7 to design a FM audio transmitter in sub-G band.
The channel band width is 200kHz, PFD frequency is 25kHz, and deviation is 38kHz.

 

But I find some problems in my PCB,

 

When I using 0 ohm short TP_Signal_IN to GND,

and check the spectrum,
the signal looks not correctly,
it looks like have many spurs and very strong.
After 15s,
the spurs will decrease to noise floor,
but when I using average mode,

these spurs still exist.

 

Then,
I using audio signal generator produce 1kHz signal connect to TP_Signal_IN and set output off,
this problem still exist.

When I set audio signal generator output on,

the spectrum looks like the audio signal modulation to each spurs.

 

I had check the phase noise first(remove C20 and R18, the audio ac coupling parts).

It don't have this issue.

 

 

I'm so confuse,

which condition is wrong?

Does any body can provide any suggestion for me?

 

Thanks.

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