JAJSVE4 September 2024 DDS39RF10 , DDS39RFS10
PRODUCTION DATA
For higher sample rates several options are available. First, a DRO can be selected that operates directly at the desired frequency and the multiplier chain and or reference frequency can be modified accordingly.
An alternative is to use dividers and mixers to translate the APLL output to a new higher frequency. Figure 8-11 shows an example for a 10GHz clock synthesizer. This uses the same reference multiplier chain and APLL as described above and adds a mixing stage to translate the DAC clock from 8 to 10GHz.
The LMX1204 can operate as a buffer, multiplier or divider. In this case the LMX1204 is used to divide the 8GHz APLL output by 4, which is then mixed with the input to translate the clock to 10GHz. A bandpass filter is required after mixing to remove the LO feedthrough and undesirable mixing products. Figure 8-12 shows the input 8GHz scaled to 10GHz and resulting 10GHz after mixing.
As with the reference multiplier chain, special care must be taken selecting the components and operating points for best phase noise. A slight improvement in noise floor was found by power combining two the of the LMX1204 outputs before feeding the IF input of the mixer.