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LMX2594EVM: LMX2594EVM_Phasenoise is not met.

Part Number: LMX2594EVM
Other Parts Discussed in Thread: LMX2594


I am using LMX2594 EVM and Reference Pro module with TICSPRO software. While doing phase noise measurement of  frequencies in the range 3.5GHz to 5.5GHz it is noticed that there is a reduction in value of 6dB from what is expected. I tried with the following settings ;

fOSC   = 100MHz

FPD    =  200MHz

N div   =  40

fVCO  =  8 GHz

Ch  Div =2 

RF out  = 4 GHz

phase noise @ 10KHz offset from the carrier of 4GHz is -110dBc/Hz.

In LMX2594 datasheet page no:14 , it is given that the phase noise @10 KHz offset is -110.1dBc/Hz for fVCO of 8GHz. So the expected value of Phase noise at 10KHz offset is -116dBc/Hz for fVCO of 8GHz with channel divider of 2. But the measured value is -110dBc/Hz.

The phase noise measurement is done with R& S Spectrum Analyzer FSU 20Hz-8GHz.

Other observations are of the following;

     fOSC= 100MHz  : FPD= 200MHz : N Div  =55 :  RF out  = 5.5 GHz :  fVCO  =  11 GHz :  Ch  Div =  2   : measured value of  phase noise @ 10KHz offset = -107dBc/Hz but expected value is -113dBc/Hz.

     fOSC= 100MHz  : FPD= 200MHz  N Div =50 :  RF out  = 5 GHz    :  fVCO  =  10 GHz  : Ch  Div =  2  : measured value of  phase noise @ 10KHz offset = -109dBc/Hz but expected value is -115dBc/Hz.

     fOSC= 100MHz  : FPD= 200MHz  N Div =70 :  RF out  = 3.5 GHz :  fVCO  =  14 GHz :  Ch  Div =  4  : measured value of  phase noise @ 10KHz offset = -111dBc/Hz but expected value is -117dBc/Hz.

Please clarify....


  • Hello Rajeena,

    Please note we are looking into this and will respond shortly.

  • Hi Rajeena,

    The data we took in the datasheet was using a very clean reference clock from Wenzel. The 10kHz offset phase noise is 10dB better than the Reference Pro. 

    In the datasheet Figure 9, we have a plot at 3.5GHz, the 10kHz phase noise is -117dBc/Hz, which matches very well with the 6dB improvement when the frequency gets divided by 2.

  • Hi Aaron,

    Please share the Part number of the Wenzel oscillator.

    The phase noise requirement of Reference Oscillator given  in the "LMX2594 EVM Instructions" application note is -159dBc/Hz at 10kHz offset. This maybe arrived at the worst case requirement. In our application, the maximum frequency is 5.5GHz with divide by 2 operation at 11GHz VCO. The 11GHz synthesized phase noise at 10kHz offset is -107dBc/Hz. Reference oscillator phase noise requirement will be -107+20log(55)+10=-152dBc/Hz, for 0.4dB degradation. We tested using CVPD-922 oscillator, which is claiming a phase noise of -154dBc/Hz at 10kHz offset. But still we are not achieving the phase noise.

    The LMK61 Ref Pro oscillator is having only -142dBc/Hz at 10kHz offset.


    Vishnu R.

  • Hi Vishnu,

    The Wenzel clock is 501-4623G, unfortunately, it is discontinued. Here is the phase noise.

    3487.1157.100 MHz Wenzel 501-4623G.txt

    The best phase noise XO from Crystek I have ever seen is CVSS-945, it is a sine wave VCXO. 

  • Hi Noel,

    The Wenzel oscillator is having a phase noise of -160dBc/Hz at 10kHz offset.

    But based on the calculation the required phase noise is -152dBc/Hz ( -107+20log(55)+10=-152dBc/Hz) for the 11GHz VCO setting.

    I've given a 10dB margin considering a degradation of 0.4dB. But still the phase noise is off the requirement.


    Vishnu R.

  • Hi Vishnu,

    I did a simulation, if I use the datasheet phase noise specification of CVPD922, then the synthesizer output phase noise (at 5.5GHz) will be identical with a Wenzel reference clock.

    Did you measure your CVPD922 XO phase noise, isn't it same as the datasheet claimed?