AD9959 UNLOCK Low Frequency Output.
To remove the lower frequency limit of 100kHz. For example, take RF OUT2 DDS9959. To do this, you need to take the following steps:
1. Remove (unsolder) transformer T2.
2. In place of the Footprint T2, you need to install the capacitor C1 = 10uF 10…16V 0805 and 2pcs Resistor 51 Ohm 0603. Here is a draft of the diagram below and the location of the components on the board.
3. If you need to get a low frequency on another channel, for example, on RF OUT0, then you need to remove (unsolder) the transformer T0 and do steps 1 and 2.
What will we get?
1. The lower frequency limit is about 3-5Hz (If you want to get a lower frequency, for example, 1Hz, you need to increase the capacitor C1 to 47uF.)
2. The level is 3 dB lower since we use one arm of the DDS current mirror.
3. The level of harmonics will become higher since we do not use a current mirror and a balanced transformer.
If you want all these characteristics not to deteriorate, then instead of T2 you need to put a transformer that will have a large inductance that will allow it to work at lower frequencies. This transformer must be calculated and manufactured independently.