I noticed that some DDS evaluation boards make use of a transfomer-coupled output. Is there any available information that can help me with designing a transformer coupled output for my specific application?
See application note AN-912 for detailed coverage of this subject. Also, an Excel spreadsheet is available (see attachement) that performs the calculations covered in AN-912 and greatly simplifies the task of including a transformer in your design.
In the evaluation board they have used two 50E at the DDS O/Ps and used a 1:1 transformer.
In this way the output impedance at connector J3 will be 100E. But we usually use a spectrum analyser, which will have a 50E, I/P impedance.
I believe we need to use a transformer with 2:1 impedance ratio in the place of T1. Am I correct ?
Having the two 50 ohm resistors at the DDS output with a 1:1 transformer does make a mismatch. The easiest way to match to the 50 ohm input impedance of the spectrum analyzer is to change the resistors at the DDS output to 25 ohms. However, since the DAC output is a current source, you actually get more power into the spectrum analyzer using 50 ohm resistors. Else, you need a 0.707 transformer turns ratio to match.
The Transformer T1 secondary is connected to the DDS O/P, so I'll have to use a 2:1 impedance ratio (or 1.414 turns ratio) transformer. Otherwise I'll loose power at the O/P if I use 25E terminations.
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