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ADF4360 VCO frequency wobbles

Question asked by RoDe on Feb 1, 2012
Latest reply on Mar 2, 2012 by RoDe

Hello,

 

I have a space and cost critical aplication for 433/868 Mhz, the ADF4360-7 seems to be perfect for this.

 

I have a software routione to select the ideal combination of R and N so I can set every frequency in the ISM 434/868 Mhz band with 50 Hz resolution! Because needing high frequency resolution, the PDF frequency is 1,6 kHz and Band Wide must be low.

Low Band Wide is no problem / setting time (+-50 Hz) of 1 second is OK.

 

A big problem can´t be solved after two weeks:

The frequency is not constant. It is constantly wobbes/oszilates +-300...500Hz arround the desired value. Sometimes the oszilation is sinuid sometimes it is random. PLL has often problems to lock.

 

The problem is same with different loop filters (3/5 Element / 2....260 Hz Band Wide / 30.....70°)!

I am not sure, but without loop filter (free running VCO) it seems to be the same too.

Changing loop filter capacitor type, layout, charge pump current, register settings, core current, output power or bandwide has only little effect of the wobbling-frequency (10....40 Hz), but it did not make an improvement.

 

=> Leckage current in Loop/VCO and touching GND 30 mm arround VCO has great influence to phase noise and frequency(!).

=> R38 instead of R470 at the frequency setting inductor L2 improves wobbling  from +-400 to +-50 Hz.

 

Voltage regulator is ultra low noise. Cap VDD/GND 100µF X7R helps a little and lowers effect of PDF/charge pump to input voltage. Frequency source is 10 MHz TCXO +-0,5 ppm: Two pieces tested – same result / with LM2485 working perfect.

 

The PDF frequency modulates the output frequency. I can´t solve it by using extrem low Band Wide and filtering the VCO-power suply, there is maybe an other effect.

 

Questions:

 

What is the best layout for GND arround inductors/VCO?

 

Is it possible to compensate the leckage current of the VCO with an external circuit?

 

What oszilator is used by the chip? Maybe I can find mor/better ways to improve in the net or did somebody has an idea?

 

Thanks

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