LMK04828: Clock review

Keerthana U R

Intellectual 540 points

Part Number: LMK04828
Other Parts Discussed in Thread: LMK04832, LMX2820, LMX2594

Hi team,

May I request you to review the attached schematics that includes LMK04828/LMK04832 and LMX2820?

Specifications for LMX2820:

Output Frequency @ RFOUTA : 5Ghz or 10Ghz

Fosc = 100MHz

Fpd = 200MHz

Kpd = 15mA

Fvco = 10000MHz

Specifications for LMK04828/LMK04832:

PLL1 - VCO0

CLKinX = 10MHz

CP for PLL1 = 0.15mA

VCXO frequency = 122.88MHz

VCXO gain = 8kHz/V

2nd order filter

PLL2 - VCO0

CP for PLL2 = 3.2mA

VCO frequency = 2457.6MHz

VCO gain = 21.9MHz/V

4th order filter

Kindly let me know if you need any other information that would help your review.

Regards,

Keerthana

Clock Review.zip

over 4 years ago

0 Noel Fung over 4 years ago

TI__Guru** 110880 points

Hi Keerthana,

We will review and get back later.

0 Keerthana U R over 4 years ago in reply to Noel Fung

Intellectual 540 points

Thank you Noel.

In our design, we have used a buffer between LMK04828B and LMX2820. The buffer used is from a different manufacturer. Therefore we have not included buffer section in the schematics shared for TI review. Can I share the schematics that include the buffer section? So that TI can review the termination provided between the LMK04828 and LMX2820?

Regards,

Keerthana

0 Noel Fung over 4 years ago in reply to Keerthana U R

TI__Guru** 110880 points

Hi Keerthana,

Yes, please.

0 Keerthana U R over 4 years ago in reply to Noel Fung

Intellectual 540 points

Hi,

I have attached the schematics that include buffer section.

Regards,

KeerthanaClock Review (1).zip

0 Noel Fung over 4 years ago in reply to Keerthana U R

TI__Guru** 110880 points

Hi Keerthana,

here is my feedback.

1. suggest VCXO (U25) AC-couple to the 6-dB pad.

2. use a smaller resistors for R126, 127, 129, 131. 12kΩ is too big.

3. R2266, 2267, C3994 and 3995 are not needed, you can remove them from the schematic.

4. RFOUT of LMX2820 does not need external pull-up, R675, R676, L14, L15 are not required.

5. If you do not need RFOUTB of LMX2820, you can leave the pins floating.

0 Keerthana U R over 4 years ago in reply to Noel Fung

Intellectual 540 points

Hi Noel,

Thank you for the feedback. Kindly find the respective reply below.

1. We will add a 0.1uF capacitor for ac coupling VCXO (U25) output.

2. We will change R126, R127, R129, R131 to 1K.

3. We understand that R3, R4, C3 and C4 are internal resistors and capacitors for the fourth order filter. R2266, R2267, C3994 and C3995 are provided as optional. As per your suggestion we will remove them from the schematic.

4. We are following the clock104 board design which has LMX2594 in it . As per the Typical Application Schematic provided in LMX2594 datasheet RFOUT requires a external pull up resistors.

As LMX2820 have internal 50E pull to VCCBUFF we will remove the external pull option( R675, R676, L14, L15)

5. We will follow the recommendation from the datasheet .

Regards,

Keerthana

0 Vibhu Vanjari over 4 years ago in reply to Keerthana U R

TI__Genius 11900 points

Hello Keerthana,

Thanks for confirming the changes Noel suggested. Good luck with the rest of your design!

Thanks,

Vibhu

0 Keerthana U R over 4 years ago in reply to Vibhu Vanjari

Intellectual 540 points

Thank you Vibhu

0 Keerthana U R over 4 years ago in reply to Noel Fung

Intellectual 540 points

Hi Noel,

AC coupling capacitor was already added near the OSCIN pin of LMK04828. Do we need to add once again at the output of VCXO . Can you please let us know if it is required at the output of VCXO also.

Regards,

Keerthana

0 Vibhu Vanjari over 4 years ago in reply to Keerthana U R

TI__Genius 11900 points

Hello Keerthana,

That should not be necessary. I believe your schematic now matches "Figure 27. CLKinX and OSCin Single-Ended Termination" in the datasheet.

Thanks,

Vibhu

0 Keerthana U R over 4 years ago in reply to Vibhu Vanjari

Intellectual 540 points

Hi Vibhu,

I am little confused. I have attached the image of schematic update after TI review. May i know which capacitor to remove(C1 or C2).

Regards

Keerthana

0 Noel Fung over 4 years ago in reply to Keerthana U R

TI__Guru** 110880 points

Hi Keerthana,

I think Vibhu misread another datasheet.

Both C1 and C2 are required. The equivalent resistance as seem by the VCXO output is 50Ω, if without C2, the current flow will be very high and as a result, voltage will drop significantly.

0 Keerthana U R over 4 years ago in reply to Noel Fung

Intellectual 540 points

Thank you Noel.

0 Keerthana U R over 4 years ago in reply to Noel Fung

Intellectual 540 points

Hii team,

May I know what will happen if the 6dB attenuation is not provided at the VCXO output.

Regards,

Keerthana

0 Noel Fung over 4 years ago in reply to Keerthana U R

TI__Guru** 110880 points

Hi Keerthana,

What matter is the input voltage swing to the LMK device, it has to be 0.2V to 2.4V.

Output from the VCXO is 3.3V, we need to reduce the voltage swing to meet the above requirement.

Clock & timing

Clock & timing forum

LMK04828: Clock review