ADF7020_FM by using TEST DAC

Document created by analog-archivist Employee on Feb 23, 2016
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I use ADF7020 in wireless audio project.I hope to use the TEST DAC to do the FM
modulation/demodulation. I did the test by following the AN-852, and the FM can
work. But the output SNR is only about 10dB. So do you have any test result of
the FM using ADF7020, is there any detail app note about it, inclding the
example circuit and register configuration

 

An important point is to use as high as possible CDR clock frequency as this is
the oversampling clock for the sigma delta test DAC. If using analog FM you do
not need to set the CDR_CLK = 32 x data-rate as you are not using the CDR
circuit. this is described in the AN-852 note.
Some suggestions.
- Ensure that the CDR divider is set to one (i.e. so the  CDR clock is set to
maximum), as indicated by Austin.
- Ensure that the lowpass RC filter bandwidth is optimally set. Note that the
Test DAC app note (AN-852, Page 1) states that the effective 3dB bandwidth of a
3 pole RC filter is 0.51*Single pole bandwidth. It is only true if the RC
stages have an analog buffer between them. The correct calculation for the 3dB
bandwidth of 3 pole passive RC filter is = 0.194*Bandwidth of a Single Pole.
- you may also need to widen the post demod bandwidth to between 1x and 2x the
highest FM frequency content to give highest SNR.

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