AD9516-0 with passive loop filter and external VCO

Dear All,

I need a technical support about the following issue involving the AD9516-0.

I am using the AD9516 with external 820-870MHz VCO and passive loop filter. The PLL has been programmed to work at 860MHz and in this condition the VCO works at about 3V of tuning voltage.

Internally the 860MHz is divided by 4, downto 215MHz, and distributed to 3 outputs.

The Ad9516-0 VCP is set to +5V and CPRSET is set to 5.1Kohm.

I meet the following problem:

At ambient temperature the PLL is correctly locked, but the PLL loses lock when the temperature drops downto 0°C about. During the unlock condition, measuring the cp output voltage, I cannot measure more than 2.9-3V.

At page 19 of the datasheet there is a graph where the behaviour of Charge Pump current is reported, versus voltage: it shows that the AD9516 Charge Pump seems able to supply current (4.5mA when the CPRSET is set to 5.1Kohm) to loop filter until voltage as high as 4.5V. Therefore this voltage should be enough to grant phase lock to the VCO with 3V of tuning voltage.

Is this interpretation of the graph  correct? Where is the critical point? Is the PLL AD9516 able to stand tuning voltage as high as 3V or more? Is it possible that there is a problem in the component? Could it be a loop filter issue? Is there an internal register that could help me to solve this critical behaviour?

I know that I could solve the problem foreseeing an active loop filter in place of the passive loop filter but I would like to avoid it....

Thanks in advance for the support.

Giuliano

  • Hi Giuliano,

    Sorry for the delay because I have to get registration in this website to make any responses. AD9516-0's 5V mode charge-pump can support external VCO's tuning up to 4.5V (0.5V at low side), but Fig 12 on Page 19 of datasheets does not include full temperature effect, therefore (since the charge-pump switches and current sources were all made by CMOS transistors), I recommend you back up 300mV for each rail. For example, 0.5V at low side should add 300mV to it so the low side for full temperature range is 0.8V, and same for the high side (4.2V in the case). Please let me know if you have further questions. 

  • Hi yi2016wang,

    thanks for the answer.

    The PLL has been programmed to work at 860MHz and in this condition the VCO works at about 3V of tuning voltage. Therefore 4.2V should be ok for the control with a passive loop filter.

    Unfortunately it loses lock at low temperature (in the range 0-10 °C).

    Today I have changed the VCO with another one that can operate at 860MHz with a tuning voltage as low as 2.6V. In this condition the module can work at 0°C, namely the PLL at 860MHz locks once turned on.

    According to these measurements, it seems that there is an issue about the ability of the AD9516 to drive VCOs with tuning voltage higher than 3 V about.

    Let me know your feedback.

    Thanks

    Giuliano

  • Hi Giuliano,

    I don’t think it is AD9516 charge-pump issue because it is clear that AD9516 is specified for 0.8V~4.2V tuning operation over temperature range. I suggest you select the VCO with less temperature variation. If you have a VCO datasheets, I can check it for you (but you need to include temperature data in the VCO datasheets). The low frequency tuning voltage shifting is caused by VCO’s temperature coefficient, not the charge-pump on AD9516.

  • Hi yi2016wang,

    If the VCO worked with a tuning voltage higher than 4.2V at low temperature I should record a tuning voltage at the top of the AD9516 spec (0.8V~4.2V), therefore 4.2V. Instead, at low temperature, the tuning voltage measured with the multimeter does not exceed 3V. Furthermore, always at low temp, if I reprogram the AD9516 at a lower frequency, e.g. 850MHz, the PLL locks and the tuning voltage is about 2.7V. This latest test seems exclude also loop filter.

  • Hi Giuliano,

    Please confirm what I read from your email:

    You said “at low temperature, tuning voltage measured is <3V”, correct?

    If that’s true, then something else here is not right, because the charge pump should work at 3V at low temperature.

    Can you verify supply on all supply pins at low temperature? Can you measure the tuning voltage when AD9516 is unlocked at low temperature to see what is?

    Yi