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Original question:

LMX2572LPEVM: Side bands at modulating sampling rate

LMX2572LP: spurs at modulating sampling rate!

Muharrem Kara
Prodigy 110 points
Part Number: LMX2572LP

Hi TI Experts,

we are using LMX2572LP in our Digital Mobile Radio at 136-174 MHz frequencies.

We are struggling to solve the spur issues of LMX2572LP at 192 kHz modulating sampling rate. 

However, we observed that the spur level is changing wrt the output frequency. as follows: 

frequency : 136 to 154 MHz spur level: -90 dBc

frequency : 155 to 161 MHz spur level: -65 dBc (this is an issue for our DMR)

frequency :162 to 174 MHz spur level: -90 dBc

could you check and advise please, why the spur level between 155 and 161 MHz is higher than the other frequencies? 

Reference frequency is 48 MHz CMOS.

Thanks and kind regards,

kara

  • 0
    TI__Mastermind 28011 points

    Muharrem,

    Perhaps you are modulating near an integer boundary channel. 

    I do not know the input reference rate, but if the VCO frequency is near an integer multiplie of this, maybe this is related to the integer boundary spur.

    Regards,

    ean

  • 0 in reply to Dean Banerjee
    Prodigy 110 points

    Hello Dean,

    Many thanks for your reply and interest.

    input reference clock is 48 MHz. 

    Modulating sampling rate is 192 kHz. We observe the spurs at 192 kHz and at its multiplies.

    The problem is that, we do not see 192 kHz spurs in all frequencies of 136 to 174 MHz, but at a limited frequency band of 154 to 161 MHz. Other frequencies are almost clear. 

    So, we couldn't understand why some frequencies are clear, but some are not?

    Best Regards,

    Muharrem Kara

  • 0 in reply to Muharrem Kara
    TI__Guru** 108550 points

    Hi Muharrem,

    Looks like these channel frequencies fall into VCO4, we also observed higher spurs with this VCO.

    Are you using SPI Fast mode or I2S mode? Looks SPI Fast mode has better spurs. Also suggest use 1.8V logic, this will slightly improve the spurs as well.