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Avoiding calibration on LMX2541

Other Parts Discussed in Thread: LMX2541, LMX2581

I am using an LMX2541 as a 65 tone Multifrequency FSK (MFSK) source for a transmitter    The D register is set to a value to give the required tone spacing (approximately 11Hz in this case) and the wanted tone number is added to a base value for F and Reg0 reprogrammed for each MFSK symbol, which occurs at a symbol rate of 2.7Hz

I noticed a wideband spectrum around the wanted signal every time the tone frequency changed, and assumed it was because the chip was going through its recal procedure.   Can you confirm this was actually the case and my assumpttion is correct.

I've mitigated the situation by redesiging the programming code to generate MFSK using the D register - not so easy to arrange but possible with a bit of care.     Is it possible to turn off the recal on Reg0 programming?  With a small frequency change like this no recal is  clearly necessary, and the ability to disable it would be really useful.

Andy Talbot

www.g4jnt.com

 

 

 

  • Andy,

    For the LMX2541, there is no way to turn off the VCO calibration.  If you program the N value or fractional numerator, even to the same value, this will activate the frequency calibration.

    The LMX2581 has a feature where you can disable this VCO calibration as well as another feature that causes it to calibrate based on where it was last.

     

    Regards,

    Dean