LMX2571: Lock detect with external VCO

Ghyslain NADAL

Part Number: LMX2571

The LOCK DETECT signal (Muxout) is not responding in using LMX2571 External VCO mode. Is the function operational only for Internal VCO Mode?

over 2 years ago

0 Noel Fung over 2 years ago

TI__Guru** 108480 points

Hi Ghyslain,

Lock detect works in both internal and external VCO mode.

0 Ghyslain NADAL over 2 years ago

Prodigy 10 points

Hi Noel,

However, we have no activity on the LOCK detect signal in external VCO mode. Below is the configuration of all the PLL registers, in particular with register 39 which allows the configuration of the LOCK DETECT function to be modified.

Adress Data in hexadecimal

3C A000
3A 8C00
35 7806
2F 0000
2A 0202
29 0100
28 001C
27 11FB
23 0004
22 C000
21 0000
20 0000
1F 0000
1E 0000
1D 0000
1C 0000
1B 0000
1A 0000
19 0000
18 0020
17 0000
16 0061
15 0501
14 3199
13 0005
12 0003
11 0000
10 0000
F 0000
E 0000
D 0000
C 0000
B 0000
A 0000
9 0000
8 0020
7 0000
6 0061
5 0501
4 3199
3 0005
2 0003
1 0000
0 0002

0 Noel Fung over 2 years ago in reply to Ghyslain NADAL

TI__Guru** 108480 points

Hi Ghyslain,

Here is my register setting with 20MHz input and 480MHz VCO. What is your input and output frequency in your configuration?

Fullscreen 2571HexRegisterValues.txt Download

R60	0x3CA000
R58	0x3A8C00
R53	0x357806
R47	0x2F6000
R46	0x2E001A
R42	0x2A0202
R41	0x290110
R40	0x28101C
R39	0x2711FB
R35	0x230C83
R34	0x221000
R33	0x210000
R32	0x200000
R31	0x1F0000
R30	0x1E0000
R29	0x1D0000
R28	0x1C0000
R27	0x1B0000
R26	0x1A0000
R25	0x190000
R24	0x18000E
R23	0x170E84
R22	0x168584
R21	0x150101
R20	0x14301B
R19	0x1303E8
R18	0x120000
R17	0x110000
R16	0x100000
R15	0x0F0000
R14	0x0E0000
R13	0x0D0000
R12	0x0C0000
R11	0x0B0000
R10	0x0A0000
R9	0x090000
R8	0x08002E
R7	0x070E04
R6	0x068481
R5	0x050101
R4	0x042018
R3	0x031200
R2	0x020000
R1	0x017A00
R0	0x000083

0 Ghyslain NADAL over 2 years ago in reply to Noel Fung

Prodigy 10 points

Hi Noel,

Input frequency is 10MHz and the output is 819.2MHz. The external VCO is a VCXO with a slope close to 11kHz/Volt.

0 Noel Fung over 2 years ago in reply to Ghyslain NADAL

TI__Guru** 108480 points

Hi Ghyslain,

Is the 10MHz TCXO? If this is the case, you need to make R34[15] = 0, otherwise, there is not sufficient input to the PLL.

Since you are using a VCXO, use a scope to check the Vtune voltage to see if it is a flat trace or triangle waveform. If you see triangle waveform, that means the input impedance of the VCXO Vtune pin is low, you need to either increase fpd or charge pump current to compensate for the leakage current.

0 Ghyslain NADAL over 2 years ago in reply to Noel Fung

Prodigy 10 points

Hi Noel,

No action by modifying the R34 register (R34[15]=0) but when we make CAL_EN (REG0[0]) bit to 1 and return to 0 then the detection function works. Do you confirme that it is necessary to make thos to activate or refresh the function?

Also, the input impedance of the VCXO Tune is probabaly too low regarding the charge pump current. We work about this.

0 Noel Fung over 2 years ago in reply to Ghyslain NADAL

TI__Guru** 108480 points

Hi Ghyslain,

It seems that we need to make R0[7]=1 after a Vcc power up or a reset, otherwise, we need to toggle the FCAL_EN bit to make lock detect function. I didn't know R0[7] has this function.

In summary, you can make R0[7]=1 all the time, you don't need FCAL_EN=1 as you are not using the internal VCO. Lock detect works in PLL mode.

0 Ghyslain NADAL over 2 years ago in reply to Noel Fung

Prodigy 10 points

Hi Noel,

for our application it is more simple to toggle the FACL_EN bit but set the R0[7] to 1 works also.

Thank you.

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LMX2571: Lock detect with external VCO