LMX2572LP: Unexpected Spurs in the LMX2572LP

Armagan Ergin

Part Number: LMX2572LP
Other Parts Discussed in Thread: LMX2594, LMX2572

Tool/software:

There are spurs I observe which isn’t predicted by PLLatinum Sim

There are some spurs that I couldn't find the root causes. I simulated on PLLatinum Sim, I used same registers as TICs Pro software says but I m getting unexpected results. For example, when I configured the PLL to output frequency is 240.02 MHz there are spurs at 100 kHz away from carrier frequency but PLLatium says there is a primary fractional spur at 160 kHz away from carrier frequency if MASH clock distortion is enabled.

My observation is that after the PLL, in the second stage, a spur appears at half of whatever is set after the decimal point.
For example, when it is set to 240.02 MHz, the spur shows up 100 kHz away; when it is set to 240.01 MHz, the spur shows up 50 kHz away — that is, at 5 times the value after the decimal point.
However, this is not the same for every frequency. For instance, at 210.01 MHz the spur is 100 kHz away, while at 215.01 MHz no spur appears. At 220.01 MHz more serious spurs appeared.

What I tried

I tried to see significant difference in PLLatium Sim for different output frequencies but I can't see any. 215.01 MHz and 220.01 MHz are almost the same configuration only the fractional N-div values are changing.
I tried different register configurations with different MASH orders, MASH seeds and different fractional N-div values. All gave the same result nothing has changed. Only lowering the charge pump current improved the spur level by 6 dBc.
Different LMX2572LP gave the same results too.

Thanks for helping.

3 months ago

0 Armagan Ergin 3 months ago

Prodigy 10 points

Register Configuration for 220.01 MHz calculated by Tics Pro software.

Reg[125]=0x0820;
Reg[124]=0x0000;
Reg[123]=0x0000;
Reg[122]=0x0000;
Reg[121]=0x0000;
Reg[120]=0x0000;
Reg[119]=0x0000;
Reg[118]=0x0000;
Reg[117]=0x0000;
Reg[116]=0x0000;
Reg[115]=0x0000;
Reg[114]=0x7802;
Reg[113]=0x0000;
Reg[112]=0x0000;
Reg[111]=0x0000;
Reg[110]=0x0000;
Reg[109]=0x0000;
Reg[108]=0x0000;
Reg[107]=0x0000;
Reg[106]=0x0007;
Reg[105]=0x4440;
Reg[104]=0x2710;
Reg[103]=0x0000;
Reg[102]=0x0000;
Reg[101]=0x0000;
Reg[100]=0x2710;
Reg[99]=0x8D89;
Reg[98]=0x0000;
Reg[97]=0x0000;
Reg[96]=0x0000;
Reg[95]=0x0000;
Reg[94]=0x0000;
Reg[93]=0x0000;
Reg[92]=0x0000;
Reg[91]=0x0000;
Reg[90]=0x0000;
Reg[89]=0x0000;
Reg[88]=0x0000;
Reg[87]=0x0000;
Reg[86]=0xFFF7;
Reg[85]=0xEDFF;
Reg[84]=0x0001;
Reg[83]=0xFFF7;
Reg[82]=0x3DFF;
Reg[81]=0x0000;
Reg[80]=0xCCCC;
Reg[79]=0x004C;
Reg[78]=0x0001;
Reg[77]=0x0000;
Reg[76]=0x000C;
Reg[75]=0x0940;
Reg[74]=0x0000;
Reg[73]=0x003F;
Reg[72]=0x0001;
Reg[71]=0x0081;
Reg[70]=0xC350;
Reg[69]=0x0000;
Reg[68]=0x03E8;
Reg[67]=0x0000;
Reg[66]=0x01F4;
Reg[65]=0x0000;
Reg[64]=0x1388;
Reg[63]=0x0000;
Reg[62]=0x00AF;
Reg[61]=0x00A8;
Reg[60]=0x03E8;
Reg[59]=0x0001;
Reg[58]=0x9001;
Reg[57]=0x0020;
Reg[56]=0x0000;
Reg[55]=0x0000;
Reg[54]=0x0000;
Reg[53]=0x0000;
Reg[52]=0x0421;
Reg[51]=0x0080;
Reg[50]=0x0080;
Reg[49]=0x4180;
Reg[48]=0x03E0;
Reg[47]=0x0300;
Reg[46]=0x07F0;
Reg[45]=0xC612;
Reg[44]=0x0863;
Reg[43]=0x04DB;
Reg[42]=0x0000;
Reg[41]=0x0000;
Reg[40]=0x0000;
Reg[39]=0x17F3;
Reg[38]=0x0000;
Reg[37]=0x8205;
Reg[36]=0x0023;
Reg[35]=0x0004;
Reg[34]=0x0010;
Reg[33]=0x1E01;
Reg[32]=0x05BF;
Reg[31]=0xC3E6;
Reg[30]=0x0CA6;
Reg[29]=0x0000;
Reg[28]=0x0488;
Reg[27]=0x0002;
Reg[26]=0x0808;
Reg[25]=0x0624;
Reg[24]=0x071A;
Reg[23]=0x007C;
Reg[22]=0x0001;
Reg[21]=0x0409;
Reg[20]=0x4848;
Reg[19]=0x27B7;
Reg[18]=0x0064;
Reg[17]=0x0096;
Reg[16]=0x0080;
Reg[15]=0x060E;
Reg[14]=0x1808;
Reg[13]=0x4000;
Reg[12]=0x5001;
Reg[11]=0xB018;
Reg[10]=0x10F8;
Reg[9]=0x0004;
Reg[8]=0x2000;
Reg[7]=0x00B2;
Reg[6]=0xC802;
Reg[5]=0x30C8;
Reg[4]=0x0A43;
Reg[3]=0x0782;
Reg[2]=0x0500;
Reg[1]=0x0808;
Reg[0]=0x211C;

0 Noel Fung 3 months ago in reply to Armagan Ergin

TI__Guru** 111700 points

Hi Armagan,

I also can see the 10kHz spurs, I don't know where are they came from.

At 240.02MHz output

This spurs can be reduced when we use a smaller output power setting. (OUTA_PWR = 5).

I measured the output from _P pin and has the _M pin 50Ω terminated.

Other frequencies you mentioned has similar spur level.

At 220.01MHz output, OUTA_PWR = 5

Here is the configuration file at 220.01MHz.

Fullscreen LMX2572LP_220_01M.txt Download

R125	0x7D0820
R124	0x7C0000
R123	0x7B0000
R122	0x7A0000
R121	0x790000
R120	0x780000
R119	0x770000
R118	0x760000
R117	0x750000
R116	0x740000
R115	0x730000
R114	0x727802
R113	0x710000
R112	0x700000
R111	0x6F0000
R110	0x6E0000
R109	0x6D0000
R108	0x6C0000
R107	0x6B0000
R106	0x6A0007
R105	0x694440
R104	0x682710
R103	0x670000
R102	0x660000
R101	0x650000
R100	0x642710
R99	0x630000
R98	0x620000
R97	0x610000
R96	0x600000
R95	0x5F0000
R94	0x5E0000
R93	0x5D0000
R92	0x5C0000
R91	0x5B0000
R90	0x5A0000
R89	0x590000
R88	0x580000
R87	0x570000
R86	0x560000
R85	0x55D800
R84	0x540001
R83	0x530000
R82	0x522800
R81	0x510000
R80	0x50CCCC
R79	0x4F004C
R78	0x4E0001
R77	0x4D0000
R76	0x4C000C
R75	0x4B0940
R74	0x4A0000
R73	0x49003F
R72	0x480001
R71	0x470081
R70	0x46C350
R69	0x450000
R68	0x4403E8
R67	0x430000
R66	0x4201F4
R65	0x410000
R64	0x401388
R63	0x3F0000
R62	0x3E00AF
R61	0x3D00A8
R60	0x3C03E8
R59	0x3B0001
R58	0x3A9001
R57	0x390020
R56	0x380000
R55	0x370000
R54	0x360000
R53	0x350000
R52	0x340421
R51	0x330080
R50	0x320080
R49	0x314180
R48	0x3003E0
R47	0x2F0300
R46	0x2E07F0
R45	0x2DC608
R44	0x2C05A3
R43	0x2B007E
R42	0x2A0000
R41	0x290000
R40	0x280000
R39	0x270271
R38	0x260000
R37	0x250205
R36	0x240023
R35	0x230004
R34	0x220010
R33	0x211E01
R32	0x2005BF
R31	0x1FC3E6
R30	0x1E0CA6
R29	0x1D0000
R28	0x1C0488
R27	0x1B0002
R26	0x1A0808
R25	0x190624
R24	0x18071A
R23	0x17007C
R22	0x160001
R21	0x150409
R20	0x144848
R19	0x1327B7
R18	0x120064
R17	0x110096
R16	0x100080
R15	0x0F060E
R14	0x0E1820
R13	0x0D4000
R12	0x0C5001
R11	0x0BB018
R10	0x0A10F8
R9	0x090004
R8	0x082000
R7	0x0700B2
R6	0x06C802
R5	0x0530C8
R4	0x040A43
R3	0x030782
R2	0x020500
R1	0x010808
R0	0x00211C

0 Armagan Ergin 3 months ago in reply to Noel Fung

Prodigy 10 points

Thank you for your reply. Your spur levels came out much better than mine. The problem is probably caused by VCO1. I’ll test it by obtaining the same output frequency with the other VCOs. I’ll post the results here.

0 Armagan Ergin 3 months ago in reply to Noel Fung

Prodigy 10 points

I’m trying to eliminate the spurs by feeding the output of the LMX2572LP into the LMX2594. However, these unexpected spurs are preventing me from doing so. Since the LMX2594 can only accept input up to 300 MHz, I can’t use the other VCOs on the LMX2572LP. The LMX2594 works fine, but the LMX2572LP is causing problems. What could I use instead of the LMX2572LP? Do you have any recommendations?

0 Noel Fung 3 months ago in reply to Armagan Ergin

TI__Guru** 111700 points

Hi Armagan,

I tried a couple of devices today but still cannot see the big 100kHz spurs. Do you have any 100kHz source in your system? For example, DC/DC converter.

Sounds like the function of LMX2572LP is to provide a reference clock to LMX2594. Depending on your phase noise requirement on LMX2594, this architecture is not ideal because the PLL noise from LMX2572 will be carried forward to LMX2594.

There are many high performance 1xxMHz XO available in the market. You can use the doubler to make fpd = 2xxMHz.

You may also consider to use LMK6 BAW oscillator (www.ti.com/.../lmk6c.pdf), if you want a 2xxMHz clock for LMX2594.

Clock & timing

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LMX2572LP: Unexpected Spurs in the LMX2572LP