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Original question:

LMX2594: LMX2594 Phase Noise

LMX2594: Some questions

Daisuke Higa
Genius 4315 points
Part Number: LMX2594

Tool/software:

Hi support team all.

My customer asked us some questions below.

We appreciate it if you could answer our questions.  

Q1:
Is it possible to output RFout B = RFout A / 3?
Example settings are as follows:
RFOUTA = 12.525 MHz
RFOUTB = 4.175 MHz

If this is possible, could you please tell me the settings?

Q2:
If dividing by 3 is not possible with Q1, it seems that harmonics of the set frequency are output from RFout B. Therefore, I am thinking of setting it to divide by 6 (extracting the RFout A / 6 component) and adding an external filter to extract twice the component of RFout B.
However, are there any concerns about device operation when using this method? For example, if I add a band-pass filter to RFout B and extract RFout B x 2, could the fundamental component of RFout B be reflected by the filter, causing the level to become unstable?

Q3:
The frequency I want to set is the setting for Q1. (There are three other settings.)
Since the reference signal Fosc is planned to be 25 MHz, I think I will have to set it to fractional, but I would like to know the recommended register settings to operate it under optimal conditions, such as minimum spurious, minimum phase noise, and maximum power output from RFout.
I would like to actually check this on an evaluation board, so it would be very helpful if you could tell me the settings in TICS Pro.

Best regards,

DH

  • 0
    TI__Guru** 107030 points

    Hi Higa-san,

    Div by 3 is not supported, you can use div by 6 and then extract the second harmonic. 

    I think we should first use a balun to turn the differential output to single-ended output and then go through a BPF to get the second harmonic signal. 

    If the reference clock frequency is just 25MHz, we cannot get very good performance because the loop bandwidth is limited and the N-divider value is high.

    Can we use 100MHz reference clock? We can get good phase noise and spurs performance with a 100MHz input.

  • 0 in reply to Noel Fung
    Genius 4315 points

    Hi Noel-san

    I have one concern.
    Is the reason that only even division can be selected because it outputs a signal with a 50% duty cycle?
    If so, wouldn't even-order spectrum not exist?

    I'm concerned that it will be difficult to extract the second-harmonic spectral components.

    Best regards,

  • 0 in reply to Daisuke Higa
    TI__Guru** 107030 points

    Hi Higa-san,

    Right, 50% duty cycle is the main reason for even division only. 

    In fact, div by 6 is obtained by div by 3 followed by div by 2. 

    I remember the 2nd harmonics of div by 6 is pretty strong, it should not be difficult to extract, I believe.