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ADF4158 PLL strange 'moving' spurs

Hi there,

I've made a PLL design going from 900 to 1000 Mhz using the ADF4158 and I'm experiencing strange results.

The VCO is Minicircuit ROS-1000V (850-1050 Mhz), the loop filter is an active inverting low noise opamp (OP184). Components values from ADIsimPLL

When generating 890 000 000 Hz with no modulation, I see my carrier and two side carriers at +/-315 KHz and -65dB approx.

But what is strange, I see two other carriers which are moving quite fastly, symetrically at each side of the main carrier and going into opposite direction like if it was starting from the carrier, and one is going down in frequency whereas the second is going high... and then it's comes back and return and so on...

Replacing the REFin from a TCXO (with multiple harmonics) to an externa DDS function generator doesn"t change anything.

If I change the frequency setting to for instance 890 000 005 Hz I change the speed of these moving carriers. If  I set 890 001 250, there is no longer these carriers.

The power supply is supposed to be quite clean and nothing serious appears on the oscilloscope (noise of about 2-5mV in x1 probe). On each RF device there are 2 X7R caps (10pf, 100nF) following a bead ferrite. PCB layout has proper grounding.

On the same board I have a CPU running at 18.668 MHz from a 4.9152 MHz crystal and I'm wondering if these carriers could be a beat frequency from a mixing between any of the CPU clocks and the PLL. Does anyone has heard about a such thing ?

Does anyone has already experienced a such behaviors on these 'moving' spurs. Any idea on how to investigate ?

Thanks

Regards

Stephane

Parents
  • I've swept all the CP values from 5.00 to 0.31mA : the spurs at 10 and 20 MHz have decreased from 5 to 2.81 mA (only two values were showing no apparent effect). Then at 2.5 mA it came back to the level of 5mA and decreased again down to 0.31mA.

    The two lowest spurs values are at 2.81 and 0.31mA but they do not exhibit the same phase noise. see attached picture. The 0.31 has a higher close phase noise but which decreases faster than the 2.81mA.

    The 'moving' spurs are here at any CP value but their amplitude depend on CP values.

    At that point the best value sounds to be 0.31mA but I would be please to improve the close phase noise

    Few questions :

    1. Will the CP setting have an effect an all frequencies or may it be possible that CP needs do be adjusted according to the desired frequency (which would be rather impossible to achieve) ?

    2. Will the CP value depend on the VCO used ?

    3. In fact, how this CP setting acts on the phase noise and spurs ? Is there a way to predict that or shall it be tested for all values in a design from experimentation before choosing the correct value ?

    Regarding my requirement, I need to have of course the best phase noise and the lowest spurs as possible with the the maximum frequency resolution. From that, everything is compromise but I would like to get a close phase noise better than what I have, without degrading too much the far phase noise and reducing spurs at maximum. Frequency resolution could be decreased but should remain in the range of 1 KHz. The moving spurs are not acceptable.

    If you have any suggestion this would be interresting but get your feedback.

    Stephane

    attachments.zip
Reply
  • I've swept all the CP values from 5.00 to 0.31mA : the spurs at 10 and 20 MHz have decreased from 5 to 2.81 mA (only two values were showing no apparent effect). Then at 2.5 mA it came back to the level of 5mA and decreased again down to 0.31mA.

    The two lowest spurs values are at 2.81 and 0.31mA but they do not exhibit the same phase noise. see attached picture. The 0.31 has a higher close phase noise but which decreases faster than the 2.81mA.

    The 'moving' spurs are here at any CP value but their amplitude depend on CP values.

    At that point the best value sounds to be 0.31mA but I would be please to improve the close phase noise

    Few questions :

    1. Will the CP setting have an effect an all frequencies or may it be possible that CP needs do be adjusted according to the desired frequency (which would be rather impossible to achieve) ?

    2. Will the CP value depend on the VCO used ?

    3. In fact, how this CP setting acts on the phase noise and spurs ? Is there a way to predict that or shall it be tested for all values in a design from experimentation before choosing the correct value ?

    Regarding my requirement, I need to have of course the best phase noise and the lowest spurs as possible with the the maximum frequency resolution. From that, everything is compromise but I would like to get a close phase noise better than what I have, without degrading too much the far phase noise and reducing spurs at maximum. Frequency resolution could be decreased but should remain in the range of 1 KHz. The moving spurs are not acceptable.

    If you have any suggestion this would be interresting but get your feedback.

    Stephane

    attachments.zip
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