Hello AD community!

I am using the AD9959 board. My problem: I can not generate RF signals in the range of >250 MHz. For instance, when I ask to the AD9959 to generate a 300 MHz frequency cosine wave it gives as output a cosine wave of 199 MHz frequency. Another example: if I ask for a 260 MHz frequency cosine wave, the resulting output is composed of a frequency peak (FFT) at 260MHz of intensity -55 dBm, but it is also present a sideband at 239 MHz of intensity -39 dBm, higher than the actual expected signal!

Thank you so much in advance for any input.

PS: The reference clock for all the above-mentioned examples was set to 500MHz (125 MHz Ref Clk and Multiplier = 4).

Hi,

What you are seeing is the image frequency of your 300MHz and 260MHz output frequency.

Using the formula for image frequencies, N*F

_{SYSCLK}+ M*F_{OUT}, where M=+/-1 and N = 1,2,3,…Your 300MHz output will have an image at 1*500MHz + (-1)300MHz = 200MHz

The same goes for your 260MHz output, the image will be at 1*500MHz + (-1)260MHz = 240MHz

The 300MHz and 260MHz is being filtered by the 200MHz low pass filter on the board

That's why your 260MHz output had lower power as compared to its 240MHz image.

Best regards,

Mark