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LMX2820: LOOPBACK mode doesn't work as expected.

Doron Gamliel
Prodigy 31 points
Part Number: LMX2820
Other Parts Discussed in Thread: LMX2594, LMX2594EVM,

Hello team.

I'm working with the LMX2820.

When I'm working in standard mode everything works just fine.

I'm trying to improve its phase noise, so I operated the LOOPBACK_EN.

The wanted frequency is 10GHz.

At first, I'm locking the LMx2820 to 10GHz in standard mode with no issue. RFOUTB is applied to Mini-Circuits mixer.

I'm using the LMX2594 at 7.6GHz as an LO and apply it to the Mini-Circuits mixer and apply its IF 2.4GHz output to the RFin of the LMX2820 while enabling the LOOPBACK_EN. 

I'm changing R10 to 0X1000 ; R36 to 0x37 and R34 to 0x810.

The problem I'm having is that the LMX2820 loose lock and jumps to 10.1GHz.

Please help.

  • 0
    TI__Guru** 110330 points

    Hi Doron,

    Could you provide your configuration in TICS Pro format?

  • 0 in reply to Noel Fung
    Prodigy 31 points

    Hello,

     

    Fullscreen 4274.HexRegisterValues.txt Download 
    R122	0x7A0000
R121	0x790000
R120	0x780000
R119	0x770000
R118	0x760000
R117	0x750000
R116	0x740000
R115	0x730000
R114	0x720000
R113	0x710000
R112	0x70FFFF
R111	0x6F0000
R110	0x6E001F
R109	0x6D0000
R108	0x6C0000
R107	0x6B0000
R106	0x6A0000
R105	0x69000A
R104	0x680014
R103	0x670014
R102	0x660028
R101	0x6503E8
R100	0x640533
R99	0x6319B9
R98	0x621C80
R97	0x610000
R96	0x6017F8
R95	0x5F0000
R94	0x5E0000
R93	0x5D1000
R92	0x5C0000
R91	0x5B0000
R90	0x5A0000
R89	0x590000
R88	0x5803FF
R87	0x57FF00
R86	0x560040
R85	0x550000
R84	0x540040
R83	0x530F00
R82	0x520000
R81	0x510000
R80	0x5001C0
R79	0x4F001E
R78	0x4E0001
R77	0x4D0608
R76	0x4C0000
R75	0x4B0000
R74	0x4A0000
R73	0x490000
R72	0x480000
R71	0x470000
R70	0x46001E
R69	0x450011
R68	0x440000
R67	0x431000
R66	0x42003F
R65	0x410000
R64	0x400080
R63	0x3FC350
R62	0x3E0000
R61	0x3D03E8
R60	0x3C01F4
R59	0x3B1388
R58	0x3A0000
R57	0x390001
R56	0x380001
R55	0x370002
R54	0x360000
R53	0x350000
R52	0x340000
R51	0x33203F
R50	0x320080
R49	0x310000
R48	0x304180
R47	0x2F0300
R46	0x2E0300
R45	0x2D0000
R44	0x2C0000
R43	0x2B0000
R42	0x2A0000
R41	0x290000
R40	0x280000
R39	0x2703E8
R38	0x260000
R37	0x250500
R36	0x24000C
R35	0x233080
R34	0x220810
R33	0x210000
R32	0x201001
R31	0x1F0401
R30	0x1EB18C
R29	0x1D318C
R28	0x1C0639
R27	0x1B8001
R26	0x1A0DB0
R25	0x190624
R24	0x180E34
R23	0x171102
R22	0x16E2BF
R21	0x151C64
R20	0x14272C
R19	0x132120
R18	0x120000
R17	0x1115C0
R16	0x101712
R15	0x0F2001
R14	0x0E3007
R13	0x0D0038
R12	0x0C0408
R11	0x0B0602
R10	0x0A1000
R9	0x090005
R8	0x08C802
R7	0x070000
R6	0x060A43
R5	0x050032
R4	0x044204
R3	0x030041
R2	0x02B1F4
R1	0x0157A1
R0	0x004430
    Please find attached.

  • 0 in reply to Doron Gamliel
    TI__Guru** 110330 points

    Hi Doron,

    Could you try disable INSTCAL and set PFD_DLY = 1? Remember to program all registers after change.

  • 0 in reply to Noel Fung
    Prodigy 31 points

    I made the changes according to your suggestions and it solved the locking problem.

    However, I didn't see any improvement in phase noise.

    Please find attached register maps for 20GHz. One is direct and the other is with LOOPBACK enabled using a mixer.

    RFoutA =20GHz ; RFoutB=10GHz, LO=7.6GHz, OSCIN=1400MHz (divided by 7).

    I'm using LMX2594 as an LO source with the same OSCIN as the LMX2820

    20GHz Lock.xlsx20GHz Lock_Mixer.xlsx

    At first I make a RESET by programming Reg 0 with 0x6472 and then I'm programming the registers which are marked in yellow at the Notes column. 

  • 0 in reply to Doron Gamliel
    TI__Guru** 110330 points

    Hi Doron,

    The phase noise quality of the LO is important, see section 5 of this appnote for details: https://www.ti.com/lit/pdf/snaa361

    In fact, phase noise of LMX2594 at 7.6GHz is poor than LMX2820 at 10GHz.

  • 0 in reply to Noel Fung
    Prodigy 31 points

    I did the same setup using the same components using 2 TI evaluation boards and I got a distinct 6dB improvement of phase noise.

    the one eval board is using LMX2820 main synthesizer and LMX2594 is the secondary synthesizer that is input to the mixer. 

    I should get 12dB theoretical improvement of phase noise but obviously because of the extra noise from the secondary LO this is reduced to 6db.

    but now, on a much better layout board with the above 2 synthesizers - I do not get any improvement. 

    I see that the phase noise plot just stays the same - as if nothing happened.

    I assume that this is not a board, synthesizer issue (secondary or main) but more to do with the setting of the registers in the main synthesizer.

    Something that you set in your eval board TICS PRO GUI SW makes this happen. 

    please check carefully again the registers - the reserved ones as well to make sure that the registers are the same as you set in your SW.

    if you have any suggestion as to why this happens I would be most grateful

  • 0 in reply to Doron Gamliel
    TI__Guru** 110330 points

    Hi Doron,

    Your configuration did have some errors, see sheet 2 for details.

    20GHz Lock_Mixer (2).xlsx

  • 0 in reply to Noel Fung
    Prodigy 31 points

    Hello Noel,

    Thank you for your support.Phase Noise.pdf

    I attached the phase noise plots after applying your suggestions.

    Page1 is the Mixer output phase noise at 2.4GHz

    Page 2 is the phase noise at 40GHz (with external doubler) with and without LOOPBACK_EN. As can be seen there is no change.

    Page 3 is the phase noise at 7.6GHz of the local oscillator.

    Can you advice why I'm not getting any improvement?

  • 0 in reply to Doron Gamliel
    TI__Guru** 110330 points

    Hi Doron,

    At a glance I am not sure but I am not able to identify the loop bandwidth from the plots, could you set the end point to 100MHz offset?

  • 0 in reply to Noel Fung
    Prodigy 31 points

    Hello Noel,

    Please find attached the design files4073.LMX2594.docxLMX2820.docx

    Both files are TI PLLatinum Sim files, however I couldn't upload the .sim files, so I changed it to .docx.

    Please change it back to .sim.

  • 0 in reply to Doron Gamliel
    TI__Guru** 110330 points

    Hi Doron,

    The loop filters in the sim files are different from the EVM default, in your early system verification experiment using individual EVMs, did you change the loop filters? Could you verify the performance again using individual EVMs with the latest register values you use in the customized board?

  • 0 in reply to Noel Fung
    Prodigy 31 points

    Hello Noel,

    I built a full setup with LMX2820EVM and LMX2594EVM and found a small improvement.

    I then changed the LO to the mixer to Keysight generator instead of the LMX2594EVM and i got a 6dB improvement in phase noise.

    Thank for your patience and help.