LMK61PDEVM: Pre-programmed frequencies

Hello Mario, 

What types of frequencies are you looking to update to on the go? I ask this, because the specific frequencies matter. 

LMK61PD which allows pin select-ability of output frequency (and type) requires VCO to lock to a new frequency to support the specific output frequency, so in those cases it cannot be done "on the go". 

LMK61E2 EEPROM is fully programmable through I2C which allows for a known starting configuration. From there you can perform both fine frequency margining and coarse frequency margining. Both would involve I2C commands.

Coarse would be a simple output divider register update, for example 5 GHz VCO with output divider of 32, results in Fout of 156.25. Changing the output divider from 32 to 16, would then give you 156.25 MHz output frequency. This can be done on the go through I2C. Review datasheet section: 9.2.2.2

Fine frequency margining involves changing the load capacitance of the integrated oscillator. This would results in much smaller frequency steps, ppm. This again would require I2C commands to update the registers (R16 and R17). Review datasheet section: 8.3.5 and 9.2.2.1. 

So the key will be what frequencies do you need to change to on the go. 

Thanks and regards,

Amin 

Hello Amin,

thank you very much for your prompt and exhaustive reply. The number of required frequencies is very limited and they are coincident with the ones pre-programmed in the LMK61PD (that's why we selected that device). Actually, we even need less as 100, 125 and 156.25 MHz would be sufficient and, according to what you said, they should be obtainable by only changing the output divider.

It is definitely good to know that the LMK61E2 can be changed on the go. Thus, I'd like to ask you the following questions:

1. Would be possible to have the contents of all the  registers as per the pre programmed LMK61PD's frequencies (to be on the safe side and get the best in both noise and spurs performance without wasting time to optimize them)?
2. Supposing we use only the output divider, how long would the device take to quitely settle on the new frequency?

Thank you for your availability and shared expertise.

Mario

Hi Mario, 

If you look at the typical phase noise plots of 156.25 MHz on both datasheets (page 10 for LMK61PD and page 9 for LMK61E2), you'll notice its exactly the same. As the same typical performance is expected since the part is the exact same configurations. 

LMK61E2 comes programmed with VCO at 5 GHz (running integer N-PLL mode) and output divider of 32 for 156.25 MHz. The best performance of the device is achieved in integer mode for the PLL so there would be no need to optimize and 5 GHz can achieve all your needed frequencies, 100 (divide by 50) and 125 (divide by 40). 

Table 8.7 shows the output divider settings: 

20h corresponds to 32 decimal. So in order to go to 100 MHz, R23 needs to be updated to 50 (32h) and for 125 MHz, R23 needs to be updated to 40 (28h) 

With regards to the time question, it is essentially instantaneous. Connected to Oscilloscope, a Phase noise analyzer, or a frequency counter, if you update the output divider you will see it immediately change on the measuring equipment, there's no settling time visible to the eye.

Thanks and regards, Amin