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Design >> RF Design >> PLL/TX debugging at the board level
https://designers-guide.org/forum/YaBB.pl?num=1432290276

Message started by aaron_do on May 22^nd, 2015, 3:24am

Title: PLL/TX debugging at the board level
Post by aaron_do on May 22^nd, 2015, 3:24am

Hi all,

I am working on an RF transmitter board with an on-chip PLL and TX. I did not design the chip, and the data sheet is extremely sketchy on the details inside. Unfortunately, the TX output shows a spur at approximately 1.33x the output channel frequency, and the amplitude is independent of the signal level (actually if I back off the TX power by 20 dB, the spur level drops, but above that it is fairly constant).

My question is, what would be a good technique to isolate the source of the problem. I am thinking it is likely to be a supply/grounding issue. I can probe the supply caps for the TX or the PLL, and if absolutely necessary, I could probably change some of the capacitor values here and there. I have tried probing the PLL supply caps hoping that the small disturbance of the probe would cause a change in the spur level, but so far no luck.

Any suggestion is appreciated. BTW, as we are working on small quantities, its unlikely that the chip manufacturer will give us much support.

thanks,
Aaron

Title: Re: PLL/TX debugging at the board level
Post by loose-electron on May 27^th, 2015, 8:12am

if you change the TX frequency does it move with the TX freq?

If so, probably inherent to the design of the IC.

Title: Re: PLL/TX debugging at the board level
Post by aaron_do on May 27^th, 2015, 5:39pm

Yes it does. Problem is the chip is failing FCC. I have a PA after the chip to boost the output power. Perhaps the chip isn't designed to output that much power......

thanks,
Aaron