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# what is the max output power of the AD9959EVB

Hi all,

i started to test his max output power when our work mode is single tone in 200MHz with full scale output.

we expected to get around the +5dBm (chip's DAC current is 10mA with an impedance of 50 ohm + some adapters and cable loss)

what we see in reality is around -6 to -7 dBm.

we checked and the Rset is 1.91Kohm, we simulated the on board low pass filter and it shouldn't give more then 1-2 dB attenuation

we checked the cables and setup with another reference and it seems accurate (1 dB difference)

below you can see 100MHz maximum power in spectrum and below it the right side is a 10 MHz signal in maximum power measured in a scope

can somebody help?

Parents
• The EVB couples to the DAC output of the AD9959 via a center-tapped transformer (1:1 turns ratio). In addition, there is a 50Ω termination on each DAC output pin. Assuming a 50Ω termination (RL) at the output of the transformer, the ideal power into RL is -5.6dBm (see Application Note AN-912 for details involving transformer coupling to a DAC). However, the sinc response (sinx/x) of the DAC incurs another 0.5dBm of loss (assuming a system clock frequency of 500 MHz with the DDS set for 100 MHz out). Thus, the actual power into RL is -6.1dBm, which is in agreement with your measurement results.

• The EVB couples to the DAC output of the AD9959 via a center-tapped transformer (1:1 turns ratio). In addition, there is a 50Ω termination on each DAC output pin. Assuming a 50Ω termination (RL) at the output of the transformer, the ideal power into RL is -5.6dBm (see Application Note AN-912 for details involving transformer coupling to a DAC). However, the sinc response (sinx/x) of the DAC incurs another 0.5dBm of loss (assuming a system clock frequency of 500 MHz with the DDS set for 100 MHz out). Thus, the actual power into RL is -6.1dBm, which is in agreement with your measurement results.

Children
• Hi KennyG,

Thanks for the information, it's clear now.

I have one question, assuming i will increase the Pull up resistors at the DAC output to be >> from RL,

I'm guessing i will suffer from unmatched impedance with the Filter after the transformer and maybe ending losing in

both power and some harmonics due to some VSWR/return loss effects?

Thanks

• A few considerations:

• Increasing RO (the DAC load resistors) increases the voltage at the DAC output (assuming the other circuit components are unchanged). So, you must ensure that the circuit as a whole does not cause the voltage at the DAC output to violate the voltage compliance specification.
• VSWR is a transmission line effect. So, if you keep the circuit compact (relative to the shortest expected signal wavelength), you should be able to ignore VSWR.
• Return loss comes into play when you have a source with a defined impedance. Return loss is a measure of the matching between the load and the given source impedance. In your case, the source impedance is variable as defined by the component values chosen. That said, if the goal is to maximize power to the load, then return loss is immaterial.
• Spurious performance (i.e., harmonics) is somewhat unpredictable. You may need to experiment with the value of RO in order to minimize harmonic spurs. That is, you may need to settle for a smaller value than expected to get the best spurious performance at the expense of less output power.