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LMK04828: Custom Board LMK04828 Phase noise level

KLN
Intellectual 320 points
Part Number: LMK04828

Hi 

We have a custom board with LMK04828 and RFSoC.

We are observing Phase noise of LMK output to be very less around -95 to 100 dBc/Hz at 1K to 100KHz offset

Due to this, RF DAC Outputs also have same Phase noise.

Below attached schematic of LMK Section:

We have tried changing Loop filter CP1 values of C622 to680nF, C623 to100nF and R402 to 39K. Still the same result of phase noise.

Please suggest us how we can improve the phase noise.

Below attached Phase noise for reference.

Phase noise of VCXO 100MHz (Y10)

Phase noise of LMK Output (DCLKOUT6)- 256MHz

Phase noise of DAC Output (440MHz)

  • 0
    TI__Mastermind 33620 points

    Hello Lakshminarayana,

    Can you provide the register programming? Your loop bandwidth looks very low on PLL2, and I'm not sure how PLL1 is configured. With the register programming, I should be able to check the loop filter behavior.

    Regards,

    Derek Payne

  • 0 in reply to Derek Payne
    Intellectual 320 points

    Hi Derek,

    Thanks for your immediate response,

    please find the attachment of configuration file.

    clkin0 =10MHz, external VCXO = 100MHz  

    Fullscreen Clkin0_10MHz_VCO2560MHz_final_txt.txt Download 
    R0 (INIT)	0x000090
R0	0x000004
R2	0x000200
R3	0x000306
R4	0x0004D0
R5	0x00055B
R6	0x000600
R12	0x000C51
R13	0x000D04
R256	0x01000A
R257	0x010155
R258	0x010255
R259	0x010301
R260	0x010422
R261	0x010500
R262	0x010670
R263	0x010755
R264	0x01086A
R265	0x010955
R266	0x010A55
R267	0x010BB1
R268	0x010C22
R269	0x010D00
R270	0x010E70
R271	0x010F11
R272	0x01106A
R273	0x011155
R274	0x011255
R275	0x0113B1
R276	0x011422
R277	0x011500
R278	0x011670
R279	0x011733
R280	0x01186A
R281	0x011955
R282	0x011A55
R283	0x011BB1
R284	0x011C22
R285	0x011D00
R286	0x011E71
R287	0x011F03
R288	0x012072
R289	0x012155
R290	0x012255
R291	0x0123B1
R292	0x012422
R293	0x012500
R294	0x012678
R295	0x012755
R296	0x012808
R297	0x012955
R298	0x012A55
R299	0x012B00
R300	0x012C02
R301	0x012D00
R302	0x012EF9
R303	0x012F00
R304	0x013016
R305	0x013155
R306	0x013255
R307	0x013300
R308	0x013402
R309	0x013500
R310	0x0136F1
R311	0x013700
R312	0x013805
R313	0x013903
R314	0x013A02
R315	0x013B80
R316	0x013C00
R317	0x013D0E
R318	0x013E03
R319	0x013F05
R320	0x014009
R321	0x014100
R322	0x014200
R323	0x014321
R324	0x0144FF
R325	0x01457F
R326	0x014618
R327	0x01470A
R328	0x014802
R329	0x014942
R330	0x014A02
R331	0x014B16
R332	0x014C00
R333	0x014D00
R334	0x014EC0
R335	0x014F7F
R336	0x015003
R337	0x015102
R338	0x015200
R339	0x015300
R340	0x01540A
R341	0x015500
R342	0x0156C5
R343	0x015700
R344	0x015896
R345	0x015900
R346	0x015A64
R347	0x015BD4
R348	0x015C20
R349	0x015D00
R350	0x015E00
R351	0x015F0B
R352	0x016000
R353	0x016105
R354	0x016245
R355	0x016300
R356	0x016400
R357	0x01650C
R369	0x0171AA
R370	0x017202
R380	0x017C15
R381	0x017D33
R358	0x016600
R359	0x016700
R360	0x016820
R361	0x016959
R362	0x016A20
R363	0x016B00
R364	0x016C00
R365	0x016D00
R366	0x016E13
R371	0x017300
R386	0x018200
R387	0x018300
R388	0x018400
R389	0x018500
R392	0x018800
R393	0x018900
R394	0x018A00
R395	0x018B00
R8189	0x1FFD00
R8190	0x1FFE00
R8191	0x1FFF53

  • 0 in reply to KLN
    Intellectual 320 points

    Hi @

    Attaching below TCXO 10MHz (Y9) Phase noise.

    Few experiment carried out:

    1. Made PLL1 PD1 at 0.1MHz and observed VCXO,LMK Output clock with charge pump gain set to 150uA and 450uA.
    2. As previously mentioned, the values for the CP1 filter were changed from C622 to 680nF, C623 to 100nF, and R402 to 39K. The phase noise result is still the same.
    3. Made PD2 as 100MHz for VCO0 of 2500MHz, LMK Output was 250MHz

    All of this resulted in similar phase noise at LMK output.

    Please advise on how we can reduce phase noise.

  • 0 in reply to KLN
    TI__Mastermind 33620 points

    Lakshminarayana,

    I do see that the register programming has FB_MUX_EN=1, which is driving the 4 MHz continuous SYSREF to the NCLK muxes - this may be introducing some noise in the feedback path, but I'd be surprised if this was the primary source of the noise.

    It's notable that the shape of the phase noise on the TCXO and the shape of the phase noise on the LMK are so similar. The 10 MHz TCXO phase noise looks astonishingly high, around 30 dB higher than what I would expect based on the part number in the schematic. The phase noise performance from the TCXO datasheet suggests you should be seeing -140 dBc/Hz at 1kHz offset; clearly your plot is well above this.

    I can think of a few possibilities:

    • TCXO is malfunctioning, and not performing to spec. The TCXO noise is so high that even the PLL1 loop filter is unable to completely suppress it, and reference noise is scaling through your whole system. This could be tested by validating a TCXO that performs according to your specs, and providing it from off-board to the other clock input.
    • You may have a switching supply or something similarly disruptive on the power supply and grounding for the TCXO or VCXO that is injecting broadband noise. This could be validated with a clean 3.3V supply if you suspect something like this is happening, but if you don't have switching noise, CMOS circuits switching in-band, or other sources that might be coupling broadband noise into your board, this might not be a likely problem.
    • The spectrum analyzer you are using may have a fundamental measurement limitation - check the datasheet for the analyzer and confirm that it can actually measure the phase noise you need to see at the frequencies you are measuring.

    Regards,

    Derek Payne

  • 0 in reply to Derek Payne
    Intellectual 320 points

    Hi 

    Thanks for your inputs.

     

    KLN said:
    1. Made PD2 as 100MHz for VCO0 of 2500MHz, LMK Output was 250MHz

    This was done using the on-board VCXO in PLL2 mode, and the phase noise was similar.

    We have tried with PLL2 mode and disabled PLL1, as well as an external VCXO of 100MHz (Part: CVSS-945-100.000).

    The phase noise are shown below, and  is still not good.

    VCXO Phase noise (CVSS-945-100.000)

    LMK Out Phase noise

    Derek Payne said:
    You may have a switching supply or something similarly disruptive on the power supply

    How do we remove the low frequency noise(~48KHz) on the supply?

  • 0 in reply to KLN
    Intellectual 320 points

    Hi 

    Awaiting for your feedback..

    Thanks.