AD9958 - VCO Gain Control not Set At High Frequency, Wrong Values?

I am using an AD9958 and trying to run my SYSCLK at 500MHz from a 100MHz source (M=5)

I just noticed that the VCO Gain Control is recommended to be set at this and I have been using it without setting it

My question is (before I go and set that bit), if I am attempting a 500MHz system clock without that bit set, is it possible for my values to get messed up? I am writing out the values for my start frequency in a linear sweep correctly (CFTW, R04), but in reality my starting frequency is half of what I've written out (i.e. I want 30MHz, I'm seeing 15MHz). Now I'm not ruling out an SPI error/hiccup, but I haven't had any problems so far with my SPI Controller. 

Would not setting that gain control bit result in a slower system clock, and in return a different SYSCLK that's used in calculations? I've noticed that the range is 255-500, and 255 is pretty close to half of 500

  • 0
    •  Analog Employees 
    on Sep 21, 2021 11:42 AM

    Hi,

    The VCO gain control bit defines two ranges (low/high) of frequency output. The VCO gain control bit defaults to low. When the VCO gain control bit is set to high, the internal VCO output frequency range is from 255 to 500 MHz. On the other hand, when the VCO gain control bit is set to low, the internal VCO output frequency range will be limited from 100 to 160 MHz. For the VCO frequency range of 160 Mhz to 255 MHz, there is no guarantee of operation.

    Best regards,

    Mark