AD9951/AD9859 temperature-depended stability issue

Hello everyone,

I'm working on a project containing the AD9951 as frequency generator for 145 MHz. Previously to that, the AD9859 was used, but was swapped for the pin-compatible AD9951 because of availability reasons. The issue described in the following post applied to both devices.

The DDS chip is used to generate an approx. 145 MHz carrier (HAM-radio band). When turned on, it works nicely and generates a clean signal on the output transformer when viewed on the spectrum analyzer.

This trace, however, suddenly becomes much broader and unusable when the chip and the board heats up. And by heating I do not mean hundreds of degrees, it is not at all uncomfortable to touch by hand.

It is important to state that the whole setup was not touched at all between good and bad signal. The behaviour can be reverted to the first picture by placing a small heatsink on the AD9951. If however, a hot soldering iron is placed approx. 1cm above the heatsink (without touching, just to raise the temperature by a couple of degrees), the issue re-emerges.

First I was suspecting some oscillation issues with the 145MHz power amplifier on the same board, but switching off the power and disconnecting the amplifier from the DDS does not change the behaviour at all. The issue seems to be coming from the DDS itself.

The used schematic and board layout are shown here:

The spectrum analyzer is connected directly to the output of the transformer with a 4n7 coupling capacitor and a 20dB attenuator (for safety). Attempts were made to amplitude-modulate the DDS via the DAC_RSET pin, but this feature is currently disabled, so the 3k9 resistor is just connected to ground.

Furthermore, the issue occured on two different PCBs, one with the AD9951 and one with the AD9859. The layout, however, is almost the same.

Now it is clear to me that it is probably hard to tell exactly what is happening on my board from a distance via a couple of pictures, but I just wanted to ask if this has been observed bevore in some situations or maybe I've left something obvious out. In case more information helps, I could also upload more design data, Settings, microcontroller code etc.

Thank you very much for your time in advance!

Best regards,

Josef

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  • Eventually figured it out!

    There were two issues in the design: First, the CLKMODESELECT pin was connected to 3.3V and not not 1.8V. This is mentioned in the datasheet of the AD9951 but not for the AD9859, thats why I overlooked it.

    But the second, more important issue was that the DDS was in fact run at 500MSPS and therefore overclocked. I did not know that (Project is years old and software was not done by me), but thanks for your tip regarding the clock system, I checked all the registers once more and found out that it was indeed overclocked. When reducing to the specified 400MSPS, I can now almost touch the chip with the soldering iron and it still runs nice and stable.

    In some ways I want to apologize for demanding your help for such a trivial issue, but it might be a nice showcase to demonstrate what happens if it is overclocked because I read that it is quite common to overclock this chip and I now find it hard to imagine how people can get away with it.

    Thanks again, topic is closed!

Reply
  • Eventually figured it out!

    There were two issues in the design: First, the CLKMODESELECT pin was connected to 3.3V and not not 1.8V. This is mentioned in the datasheet of the AD9951 but not for the AD9859, thats why I overlooked it.

    But the second, more important issue was that the DDS was in fact run at 500MSPS and therefore overclocked. I did not know that (Project is years old and software was not done by me), but thanks for your tip regarding the clock system, I checked all the registers once more and found out that it was indeed overclocked. When reducing to the specified 400MSPS, I can now almost touch the chip with the soldering iron and it still runs nice and stable.

    In some ways I want to apologize for demanding your help for such a trivial issue, but it might be a nice showcase to demonstrate what happens if it is overclocked because I read that it is quite common to overclock this chip and I now find it hard to imagine how people can get away with it.

    Thanks again, topic is closed!

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