LMX2491: The PLL cannot be locked，

xiaofu wen

Part Number: LMX2491

The problem is this.

Circuit structure: LMX2491 + external loop filter+ VCO. Phase detector frequency is 40MHz.

I have verified the functionality of VCO. Verification method: Disconnect LMX2491, use a voltage source alone to supply power to the VCO, and adjust the voltage. The VCO can output the change frequency as the voltage changes, and the higher the voltage, the higher the frequency. It works as normal. Adjust the voltage range between 0.6 and 1.1V to correspond to my desired operating frequency.

After the two chips are connected together, the system does not work properly because the LMX2491 does not output the Vtune voltage signal properly, only about 0.01~0.03V. I checked all the registers and found nothing unusual. Then I tried setting the R28 register bit5 to 1 (the default is 0, and I think 0 is correct). The Vtune had a voltage signal, but it was too high for my default charge pump gain configuration of 3.1mA. In order to obtain the appropriate Vtune voltage value, I adjusted the charge pump gain. When it was set to 8, the Vtune voltage could work at about 0.9V. But the biggest problem is that PLL is not locked in any case. DLD detection is all low.

Attached are my 3 register configuration files, please help to check the other Settings are not correct. Thanks a lot.

Fullscreen HexRegisterValues_1121_singletone.txt Download

R141	0x008D00
R140	0x008C0F
R139	0x008BA0
R138	0x008A00
R137	0x008900
R136	0x008800
R135	0x008700
R134	0x008600
R133	0x00850F
R132	0x0084A0
R131	0x008300
R130	0x008200
R129	0x008100
R128	0x008000
R127	0x007F00
R126	0x007E0F
R125	0x007DA0
R124	0x007C00
R123	0x007B00
R122	0x007A00
R121	0x007900
R120	0x007800
R119	0x00770F
R118	0x0076A0
R117	0x007500
R116	0x007400
R115	0x007300
R114	0x007200
R113	0x007100
R112	0x00700F
R111	0x006FA0
R110	0x006E00
R109	0x006D00
R108	0x006C00
R107	0x006B00
R106	0x006A00
R105	0x006901
R104	0x006890
R103	0x00673F
R102	0x0066FF
R101	0x0065C2
R100	0x00648E
R99	0x006340
R98	0x006207
R97	0x0061D0
R96	0x006000
R95	0x005F00
R94	0x005E0C
R93	0x005D4A
R92	0x005C20
R91	0x005B00
R90	0x005AC8
R89	0x005900
R88	0x005800
R87	0x005700
R86	0x005600
R85	0x005500
R84	0x005400
R83	0x005300
R82	0x005201
R81	0x0051C0
R80	0x005000
R79	0x004F00
R78	0x004EFF
R77	0x004D40
R76	0x004C00
R75	0x004B00
R74	0x004A00
R73	0x004900
R72	0x004800
R71	0x004700
R70	0x004608
R69	0x004500
R68	0x004400
R67	0x0043C0
R66	0x004200
R65	0x004100
R64	0x004000
R63	0x003F00
R62	0x003EA0
R61	0x003D00
R60	0x003C00
R59	0x003B00
R58	0x003A00
R57	0x003900
R45	0x002D00
R44	0x002C00
R43	0x002B00
R42	0x002A00
R41	0x002900
R40	0x002800
R39	0x00275A
R38	0x002642
R37	0x00256A
R36	0x002492
R35	0x002341
R34	0x002284
R33	0x002120
R32	0x002000
R31	0x001F32
R30	0x001E06
R29	0x001D00
R28	0x001C28
R27	0x001B08
R26	0x001A00
R25	0x001901
R24	0x001800
R23	0x001707
R22	0x0016D0
R21	0x001500
R20	0x001403
R19	0x0013E8
R18	0x00122C
R17	0x001100
R16	0x001025
R15	0x000F00
R14	0x000E00
R13	0x000D00
R12	0x000C00
R11	0x000B00
R10	0x000A00
R9	0x000900
R8	0x000800
R7	0x000700
R6	0x000600
R5	0x000500
R4	0x000400
R3	0x000300
R2	0x000201
R1	0x000100
R0	0x000018

Fullscreen HexRegisterValues_1121_ramp_positive_31X.txt Download

R141	0x008D00
R140	0x008C0F
R139	0x008BA0
R138	0x008A00
R137	0x008900
R136	0x008800
R135	0x008700
R134	0x008600
R133	0x00850F
R132	0x0084A0
R131	0x008300
R130	0x008200
R129	0x008100
R128	0x008000
R127	0x007F00
R126	0x007E0F
R125	0x007DA0
R124	0x007C00
R123	0x007B00
R122	0x007A00
R121	0x007900
R120	0x007800
R119	0x00770F
R118	0x0076A0
R117	0x007500
R116	0x007400
R115	0x007300
R114	0x007200
R113	0x007100
R112	0x00700F
R111	0x006FA0
R110	0x006E00
R109	0x006D00
R108	0x006C00
R107	0x006B00
R106	0x006A00
R105	0x006901
R104	0x006890
R103	0x00673F
R102	0x0066FF
R101	0x0065C2
R100	0x00648E
R99	0x006340
R98	0x006207
R97	0x0061D0
R96	0x006000
R95	0x005F00
R94	0x005E0C
R93	0x005D4A
R92	0x005C20
R91	0x005B00
R90	0x005AC8
R89	0x005900
R88	0x005800
R87	0x005700
R86	0x005600
R85	0x005500
R84	0x005400
R83	0x005300
R82	0x005201
R81	0x0051C0
R80	0x005000
R79	0x004F00
R78	0x004EFF
R77	0x004D40
R76	0x004C00
R75	0x004B00
R74	0x004A00
R73	0x004900
R72	0x004800
R71	0x004700
R70	0x004608
R69	0x004502
R68	0x004400
R67	0x0043C0
R66	0x004200
R65	0x004100
R64	0x004006
R63	0x003F00
R62	0x003EA0
R61	0x003D00
R60	0x003C00
R59	0x003B00
R58	0x003A01
R57	0x003900
R45	0x002D00
R44	0x002C00
R43	0x002B00
R42	0x002A00
R41	0x002900
R40	0x002800
R39	0x002752
R38	0x00264A
R37	0x00253A
R36	0x002442
R35	0x002341
R34	0x002284
R33	0x002120
R32	0x002000
R31	0x001F32
R30	0x001E0A
R29	0x001D00
R28	0x001C1F
R27	0x001B08
R26	0x001A00
R25	0x001901
R24	0x001800
R23	0x001700
R22	0x001600
R21	0x001580
R20	0x001400
R19	0x001300
R18	0x00122C
R17	0x001100
R16	0x001025
R15	0x000F00
R14	0x000E00
R13	0x000D00
R12	0x000C00
R11	0x000B00
R10	0x000A00
R9	0x000900
R8	0x000800
R7	0x000700
R6	0x000600
R5	0x000500
R4	0x000400
R3	0x000300
R2	0x000201
R1	0x000100
R0	0x000018

Fullscreen HexRegisterValues_1121_ramp_negtive_8X.txt Download

R141	0x008D00
R140	0x008C0F
R139	0x008BA0
R138	0x008A00
R137	0x008900
R136	0x008800
R135	0x008700
R134	0x008600
R133	0x00850F
R132	0x0084A0
R131	0x008300
R130	0x008200
R129	0x008100
R128	0x008000
R127	0x007F00
R126	0x007E0F
R125	0x007DA0
R124	0x007C00
R123	0x007B00
R122	0x007A00
R121	0x007900
R120	0x007800
R119	0x00770F
R118	0x0076A0
R117	0x007500
R116	0x007400
R115	0x007300
R114	0x007200
R113	0x007100
R112	0x00700F
R111	0x006FA0
R110	0x006E00
R109	0x006D00
R108	0x006C00
R107	0x006B00
R106	0x006A00
R105	0x006901
R104	0x006890
R103	0x00673F
R102	0x0066FF
R101	0x0065C2
R100	0x00648E
R99	0x006340
R98	0x006207
R97	0x0061D0
R96	0x006000
R95	0x005F00
R94	0x005E0C
R93	0x005D4A
R92	0x005C20
R91	0x005B00
R90	0x005AC8
R89	0x005900
R88	0x005800
R87	0x005700
R86	0x005600
R85	0x005500
R84	0x005400
R83	0x005300
R82	0x005201
R81	0x0051C0
R80	0x005000
R79	0x004F00
R78	0x004EFF
R77	0x004D40
R76	0x004C00
R75	0x004B00
R74	0x004A00
R73	0x004900
R72	0x004800
R71	0x004700
R70	0x004608
R69	0x004502
R68	0x004400
R67	0x0043C0
R66	0x004200
R65	0x004100
R64	0x004006
R63	0x003F00
R62	0x003EA0
R61	0x003D00
R60	0x003C00
R59	0x003B00
R58	0x003A01
R57	0x003900
R45	0x002D00
R44	0x002C00
R43	0x002B00
R42	0x002A00
R41	0x002900
R40	0x002800
R39	0x002752
R38	0x00264A
R37	0x00253A
R36	0x002442
R35	0x002341
R34	0x002284
R33	0x002120
R32	0x002000
R31	0x001F32
R30	0x001E0A
R29	0x001D00
R28	0x001C28
R27	0x001B08
R26	0x001A00
R25	0x001901
R24	0x001800
R23	0x001700
R22	0x001600
R21	0x001580
R20	0x001400
R19	0x001300
R18	0x00122C
R17	0x001100
R16	0x001025
R15	0x000F00
R14	0x000E00
R13	0x000D00
R12	0x000C00
R11	0x000B00
R10	0x000A00
R9	0x000900
R8	0x000800
R7	0x000700
R6	0x000600
R5	0x000500
R4	0x000400
R3	0x000300
R2	0x000201
R1	0x000100
R0	0x000018

over 2 years ago

0 Noel Fung over 2 years ago

TI__Guru** 107590 points

Hi Xiaofu,

I reviewed the single tone text file, VCO related register setting is correct. If the VCO slope is positive, i.e. VCO frequency increase when Vtune increase, then R28[5] should be 0.

You have set TRIG1_MUX = output N/2, if the VCO is locked, you should see a 20MHz signal from the TRIG1 pin. You can set this register to Output R/2, you should also see a 20MHz signal if the 40MHz reference clock is available.

0 xiaofu wen over 2 years ago in reply to Noel Fung

Expert 2475 points

set TRIG1_MUX =Output R/2，I can see a 20MHz signal；set TRIG1_MUX =Output N/2, I can't see 20MHz, but 37.5MHz.

I know it's wrong. But i don't know what should I do. R28[5]=0, but charge pump voltage is too low, about 0.03V. SCH and registers configure are right. what can i do?

Reference clock is ok.

0 Julian Hagedorn over 2 years ago in reply to xiaofu wen

TI__Mastermind 20900 points

Hi Akanksh,

Because of the Thanksgiving holiday in the U.S., TI E2E design support forum responses may be delayed the week of Nov. 21. Thank you for your patience.

regards,

Julian

0 Noel Fung over 2 years ago in reply to Julian Hagedorn

TI__Guru** 107590 points

Hi Xiaofu,

What is the measured VCO frequency and amplitude? What is the 2nd harmonic (= 2 x fo) amplitude?

Clock & timing

Clock & timing forum

LMX2491: The PLL cannot be locked，