LMK0828B

Ignacio Linares said:

I have a customer asking the following questions about the LMK0828B:

I believe that you are asking about the LMK04828.

Ignacio Linares said:

What is the fundamental frequency of the two internal VCOs in LMK0828B device?

This VCOs do operate at fundamental frequencies of 2.5 GHz / 2.9 GHz.

Ignacio Linares said:

Are these VCOs tuned to their fundamental frequencies?

Yes.

Ignacio Linares said:

Do these VCOs have sub-harmonics? How low are sub-harmonics in comparison with the fundamental?

It is possible to get sub-harmonics by crosstalk.  Since dividers can be used to reduce frequency, if an output with a /4 is adjacent to an output with a /2, there could be some crosstalk at fout/2.  The amount depends upon physical proximity, PCB layout, and the output format selected.  For instance LVDS/LVPECL/HSDS.

The VCXO reference or phase detector frequency may also cause some crosstalk.

Is there a particular specification you seek to achieve?

73,

Timothy

Customer's goal is to get a 400MHz clock with 50-50 duty cycle, 144-femto RMS jitter, and no sub-harmonics to use as a sampling clock for the TI ADS5474 device

More questions from my customer:

1. Is LMK04816 tuning to its fundamental frequency 2.4GHz, without sub-harmonics?
2. Can VCXO be 400MHz? The datasheet states that the maximum frequency is 500MHz, but the TI clock simulation tool said 400MHz is too high.

1. Yes, per information that I have.

2. Yes, the Data Sheet spec is correct.

I'll ask the LMK device folks to verify.

For LMK04816 device, can the power of a clock group be connected to ground or float if the clock group is not used? For example, if Clock Group 3 (CLKout6 and CLKout7) is not used, can VCC10 pin be connected to ground or left floating, no connection?

From the Data Sheet for the device:

9.5.1 Vcc Pins and Decoupling

All Vcc pins must always be connected.

I take that to mean "connected to Vcc supply".

For LMK04816, if an external VCO is used by connecting the VCO to Fin/Fin* pins, are the integrated loop filter resistors (R3 and R4) and capacitors (C3 and C4) for PPL2 still available for use with the external VCO? Or, are the integrated loop filter capacitors and resistors only for the internal VCO?

R3/R4 and C3/C4 are only for internal VCO.  However they could slightly impact external loop filter.  Since the values/caps are so small, I expect any impact to be very minimal, but best practice for external VCO design would be to maximize R3/R4 and minimize C3/C4.  Then if a 3rd or 4th order filter is desired with external VCO, use R3e/C3e/R4e/C4e.  See image below:

73,

TImothy

In regard to maximum OSCin frequency of 400 or 500 MHz.  The datasheet value of 500 MHz is correct.  The clock design tool of 400 max is in error.  It will be updated.  In the meantime, it is still possible to simulate with up to 500 MHz, but the part selection will need to use a lower value to trick it.

73,

Timothy

When LMK04816 is in External VCO Mode, is the Internal VCO powered down? In the External VCO Mode, if the Internal VCO is free running, how much cross-talk can the output from the internal VCO impact on the clock distribution?

Yes, the internal VCO is powered down during external VCO mode.  Page 113 of datasheet, section 9.11.1 shows the various Icc draw of the various modes.

73,

Timothy

For LMK04816, on the data sheet Section 3.4 shows that the OSCin maximum frequency (fOSCin)is 500MHz. But, Table 8-89 in Section 8.21.1 appears to indicate that the OSCin maximum frequency is 400MHz. Which value is correct 400MHz or 500MHz?

The electrical specs, Section 3.4, is proper for fOSCin max.  Setting 0x04 is valid for 500 MHz.

73,

Timothy

When LMK04816 is switched from External VCO mode to PLL2 External VCO mode (single PLL mode), what impacts by PLL1 settings will happen? Is there any need to change all settings related to PLL1 when switching to the single PLL mode?

Changing from MODE = 3 (Dual PLL, External VCO) to MODE = 6 (PLL2, Internal VCO) - there is no need to make any changes to PLL1 since PLL1 functionality is powered down automatically by the MODE control field.

73,

Timothy