I am using the AD9851 in a scalar network analyzer and have some questions about the operation.
The frequency range I work in is ~2 MHz to 60-70 MHz and I am looking at LPF designs to put on the IOUT pin. I found the filter layout of the evaluation board:
Could someone explain the impedance termination? Does it not need a resistor in series with the signal as well, directly on the chip IOUT pin? The 200 ohm resistors R6 and R7 together form a 100 ohm DC to 200 ohm HF termination. If I wanted to use this DDS chip for 1Vpp do I just change these values to 100 ohm resistors?
I read that the elliptical LPF introduces ripple into both the pass band and stop band, but provides very steep attenuation. Can I simply take this LPF design or are there better options? I am planning on using a 180 MHz crystal oscillator as the REFCLK (so without 6x multiplier), but I am not sure if this has any benefits compared to a 30 MHz crystal with multiplier. From what I read in the datasheet of the AD9851 images can occur at the REFCLK +/- the frequency of interest. My maximum frequency is around 60 MHz, so that would mean that my first image is 120 MHz (correct?). I can imagine that if that is the case I can use a filter that has a slightly less steep drop off but no ripple in the pass band (e.g. Inverse Chebyshev). Does this make sense?
I am looking forward to your answer!
Many thanks in advance,