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DDS Own IMD

DOUBLE DDS
It will be important to know DDS own OIP3. We remember that N2PK VNA has abt. +44 dBm its own OIP3. DDS output level is variable from abt -6 to abt. +6 dBm. For testing purpose we will use known connection:

                                                                      DDS - BRIDGE - ATT - SA - PC

We can not forget that Reflection bridge has 6 dB of attenuation. First we will check IM features for minimum output level, then for maximum output level. See the following pictures.


Signal level at the output of the combiner is -11.6 dBm (SA input attenuator = 30dB, marker = -41.6 dBm). Noise level of the Spectrum analyzer is approx. at -155dBm. NO INTERMODULATION PRODUCTS ARE VISIBLE!  We can NOT calculate OIP3.

Signal level at the output of the combiner is -0.3 dBm (SA attenuator = 30dB, marker = -30.3 dBm). Noise level of the Spectrum analyzer is approx. at -150dBm. There are IM products visible at -135 dBm, so we can calculate OIP3:

                                                             IMD3 = Po - Po3 
                                                             IMD3 = 135 - 30.3 = 104.7 dB
                                                             IMD3/2 = 104.7/2 = 52.35 dB
                                                             Po = -0.3 dBm
                                                             OIP3 = IMD3/2 + Po =52.35 - 0.3  =  +52.05 dBm 
Conclusion. Separate Double DDS generator has really an excellent OIP3. Its value is abt. +52 dBm @ 0 dBm output level from the Reflection bridge. This corresponds to a maximum output level of the DDS. At the minimum DDS output  level OIP3 is not measurable in my conditions. Eventual IM products dropped down below the noise floor.

 
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