LMX2571: Calculating OSCin

Jingtai

Genius 4575 points

Part Number: LMX2571

According to

VCO = CHDIV1 x CHDIV2 x RFout

How is OSCin calculated suppose RFout is 25MHz?

How is VCO calculated suppose OSCin is 25MHz and RFout is 25MHz?

over 8 years ago

0 Noel Fung over 8 years ago

TI__Guru** 108550 points

Hi Jingtai,

Given RFout is 25MHz, you can adjust CHDIV1 and CHDIV2 until VCO is within 4300MHz and 5376MHz.
OSCin is an input parameter, you decide what is the frequency of OSCin.
Please check datasheet section 8.2.1 for an example how to do basic calculation.

0 Jingtai over 8 years ago in reply to Noel Fung

Genius 4575 points

Thanks for your support.

If OSCin=25MHz, I set Pre-Divider=1, MULT=5, Post-Divider=1, fpd=125MHz, RFout=26MHz, VOC=26*6*32=4992MHz,

therefore Ninteger=4992/25=39, Nfrac=0.936

For the time being I have no idea that how the spur strong is in my real circuit and I just suppose the value of MULT and Post-Divider and I will adjust relative values in future test. Is it ok?

0 Jingtai over 8 years ago in reply to Noel Fung

Genius 4575 points

In the datasheet 6.1 It is said "MULT>Pre-divider Not supported with Crystal reference input. " What dose it mean?

0 Noel Fung over 8 years ago in reply to Jingtai

TI__Guru** 108550 points

Hi Jingtai,

If you use passive crystal as the oscin, you cannot use MULT, i.e. MULT must be equal to 1.
If you worry about the spurs, you can use 26MHz OSCin. If you can only use 25MHz OSCin, I expect there will be a spurs at 1MHz offset.

0 Jingtai over 8 years ago in reply to Noel Fung

Genius 4575 points

Thanks for your support.

Alright, I'll select 26MHz passive crystal as OSCin and the RFout is set to 26MHz and the Ninteger=4992/26=192 and Nfrac=0

Is it ok?

0 Dean Banerjee over 8 years ago in reply to Jingtai

TI__Mastermind 28011 points

Jingtai,

Perhaps you picked a VCO frequency of 4992 because it is the only VCO frequency in range that is a multiple of 192 and 26.
Unfortunately, the output divider can not do 26. The output divider is a set of 2 cascated dividers. One can do 4,5,6,7 and the other can do 1,2,4,8,16,32,64. So there is no way to get 26 as the product of two numbers from these groups.

If you were curious if there were any other possibilities to use integer mode for a phase detector frequency of 26 and output frequecy of 192, there are not. LCM(26,192) = 1248, and 4992 is the only multiple of this that that fits in the VCO range. This means that you have to use fractional mode.

I recommend trying a VCO freuqency of 5376 MHz and dividing by 28 (28 = 7 x 4). You will be in fractional mode with a fraction of 5/13. This particular fraction is a good choice for low fractional spurs.

Regards,
Dean

0 Jingtai over 8 years ago in reply to Dean Banerjee

Genius 4575 points

Thanks for your support.

My RFout=26MHz

CHDIV1=6

CHDIV2=32

Ninteger=192

Is it possible?

0 Dean Banerjee over 8 years ago in reply to Jingtai

TI__Mastermind 28011 points

Yes, divide of 192 = 6*32 and VCO frequency is 4992.

Regards,

Dean

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LMX2571: Calculating OSCin