LMK61E07: I2C communication interfered with clock output

Weida Zhang

Part Number: LMK61E07
Other Parts Discussed in Thread: LMK61E2EVM, LMK61E2

Hi there,

I've noticed that the phase noise of the LMK61E07 is not as good as it was when I2C operations taken place.

I've tried periodically sending commands to change the LMK61E07 frequency or sending command just to read the product ID, or sending non-I2C PRN data to the I2C ports on the chip.

All the above three significantly worsen the phase noise of the LMK61E07 at the sideband related to the I2C port's communication.

Please see the following figure attached.

Thanks.

Regards,

Wade

over 5 years ago

0 Shawn Han over 5 years ago

TI__Genius 9920 points

Hi Weida,
Can you share your LMK61E07 schematic with power supply ? Just want to check if there are other causes.
I can expect there is some spurs related with I2C clock, but do not expect there is such worse phase noise degraded.

Regards,
Shawn

0 Madhu Balasubramanian over 5 years ago

TI__Expert 4400 points

Hi, please check the attachment for what we've seen in our EVM when we do continuous write on I2C bus. As Shawn recommends, please share the schematics for us to review. Thanks a lot.

LMK61xx I2Cwrite phase noise.pptx

0 Weida Zhang over 5 years ago in reply to Madhu Balasubramanian

Prodigy 90 points

Thanks both!

I've been using silicon labs evaluation board, Si5xxUC-EVB.

www.silabs.com/.../ug298-si5xxuc-evb-ug.pdf

The microcontroller has been disabled.

Three wires, SDA, SCL and GND are connected from Xilinx KC705.

Is that just because the evm is from a wrong company...?

There doesn't seem to have significant defects and I've added quite a few decouple caps as well.

Thanks a lot.

0 Madhu Balasubramanian over 5 years ago in reply to Weida Zhang

TI__Expert 4400 points

Even without I2C active, the phase noise doesn't look as clean as what we measure. Did you do hand solder on the Si Labs board? Did the reflow profile follow what's given in JEDEC-STD-020?

0 Weida Zhang over 5 years ago in reply to Madhu Balasubramanian

Prodigy 90 points

We didn't use reflow. It was hand soldered.

Do you mean this may due to mechanical stretching/tension?

We can certainly buy another eval board and chip to do a proper reflow if the soldering is likely the cause to the fault.

Thanks for your feedback!

Regards,

Weida

0 Madhu Balasubramanian over 5 years ago in reply to Weida Zhang

TI__Expert 4400 points

I suspect this could be due to mechanical tension from the solder profile exceeding recommended reflow temperatures. Please do try machine solder with right profile and let us know the results.

0 Weida Zhang over 5 years ago in reply to Madhu Balasubramanian

Prodigy 90 points

Thank you for your insight! We will try it and post the result here.
Regards

0 Weida Zhang over 5 years ago in reply to Weida Zhang

Prodigy 90 points

Hi guys,

We have bought the LMK61E2EVM and tested it.

It appears the default frequency is as the datasheet specified, however, when set to a different frequency (125MHz is the frequency we wanted to use it) the performance is much worse.

Please see the attached figure.

I tried to alter the internal PLL setting, however, the Ko (VCO gain) seems not mentioned anywhere.

Also, I found that when the EVM is powered with ext 5V, and let the on board LDO reg to provide the 3.3V, the noise is much worse than using an external 3.3V to power the chip directly.

Any suggestions?

Thanks.

Regards,

Weida

0 Alan O over 5 years ago in reply to Weida Zhang

TI__Mastermind 32575 points

Is this issue still open? If so, can you please share your LMK61E2 programming config file (.tcs) that you used for your testing? Looking at the yellow trace, it appears the registers settings may not have been correctly programmed.

In the LMK61E2 programming tool, did you enter 125 (MHz), select "LVPECL" output type, click "Generate Configuration", write all registers, then measure the output clock phase noise (preferably using a balun for differential-to-single-ended conversion).

Alan

0 Weida Zhang over 5 years ago in reply to Alan O

Prodigy 90 points

Please find the updated result attached. I've tried to upload the TCS file, however the website avoids me from doing so. The registers values are provided following.

R0 0x0010
R1 0x010B
R2 0x0233
R8 0x08B0
R9 0x0901
R16 0x1000
R17 0x1180
R21 0x1502
R22 0x1600
R23 0x172A
R25 0x1900
R26 0x1A34
R27 0x1B20
R28 0x1C00
R29 0x1D01
R30 0x1E3F
R31 0x1FFF
R32 0x20FF
R33 0x2103
R34 0x2224
R35 0x2327
R36 0x2424
R37 0x2502
R38 0x2600
R39 0x2707
R47 0x2F00
R48 0x3000
R49 0x3110
R50 0x3200
R51 0x3300
R52 0x3400
R53 0x3500
R56 0x3800
R72 0x4802

0 Timothy T over 5 years ago in reply to Weida Zhang

TI__Mastermind 28550 points

Based on your config above, it looks like...
You are using LVDS instead of LVPECL, and the PLL settings are what I produced with the LMK61E2 GUI. Particularly using a different VCO frequency, fractional mode, and 1.6 mA instead of 6.4 mA.

Please try this configuration attached, registers for reference:

R0   0x0010
R1   0x010B
R2   0x0233
R8   0x08B0
R9   0x0901
R16   0x1000
R17   0x1180
R21   0x1501
R22   0x1600
R23   0x1728
R25   0x1900
R26   0x1A32
R27   0x1B00
R28   0x1C00
R29   0x1D00
R30   0x1E00
R31   0x1F00
R32   0x2001
R33   0x210C
R34   0x2228
R35   0x2303
R36   0x2408
R37   0x2500
R38   0x2600
R39   0x2700
R47   0x2F00
R48   0x3000
R49   0x3110
R50   0x3200
R51   0x3300
R52   0x3400
R53   0x3500
R56   0x3800
R72   0x4802

LMK61E2_125.tcs

Clock & timing

Clock & timing forum

LMK61E07: I2C communication interfered with clock output