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ADAU1401 crystal oscillation

Question asked by keddy on Mar 18, 2014
Latest reply on Mar 23, 2014 by KJBob

The ADAU1401 is used in car infortainment project together with NXP tuner. The main problem is from the DSP Crystal frequency, as long as the crystal oscillates it will affect the FM freq at 98.3MHz. I am using 12.288MHz as MCLK. Any alternate way to resolve this? Just to avoid the FM scan frequency from false stop because of crystal harmonic.

 

We cannot change the threshold on this particular frequency, because certain location or state, some station frequency are using this
98.3MHz frequency. If increase the threshold then this station may not stop during scan.

 

Below Clock Oscillator can create harmonics frequencies to interference to FM:

1       3.072Mhz           x29th  x30th  x31th  x32th  x33th  x34th x35th (start up too slow)

2.      11.2896Mhz        x8th  x9th

3.      12.288Mhz          x8th

4.      18.432Mhz          x6th

5.      24.576Mhz          x4th

 

We have tried to add some RC in the board and have tried increasing the R to a value that the processor does notwork but there is no impact or improvement. Take note that the affected harmonics is roughly at 30uV level but our radio sensitivity is down to 10uV.There must be significant
reduction in the affected harmonics from the DSP itself to be able to bring down its level to very low so that our tuner will not lock onto it but onto the externally received broadcasting station at 98.3MHz.

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