How to choose a high-performance clock device?

Hi,

Phase noise value at 1kHz offset will be completely dependent on 1/f noise of the PLL and the reference source noise. If we ignore the noise coming from the reference and only encounter 1/f noise of the PLL, you need to find a part that has 1/f noise at least -272dBc/Hz (not normalized) according to the formula: 

PN_1kHz = FOM_1f + 20*log10(fout) - 10* log10(f_offset)

-130 = FOM_1f + 20*log10(400 x 10^6) - 10*log10(10^3)   ---->   FOM_1f = -272 dBc 

1/f FOM of HMC7044 is -252 which clearly shows that this spec cannot be met with HMC7044. 

LTC6952 has 1/f FOM of -281 dBc and the corresponding 1/f noise for 400 MHz output at 1 kHz offset will be ~ -139 dBc. So, it can provide good margin. 

Now, you have to consider the noise coming from the reference. Reference noise will translate with 20* log10(fout/fref) relation (let's assume you are using 100 MHz reference clock), meaning that the noise contribution of the reference will be 12 dB higher than the value that you are given in the reference oscillator datasheet. Your aim is to get -130 dBc at 1kHz, so you have to find a 100 MHz crystal oscillator that has a PN value of -142 dBc at 1 kHz. (Note that 1/f noise contribution is ignored as the total noise will be dominated by the reference, the 1/f noise contribution might be considered as negligible here.) With a few dB margin, you need to aim a crystal that has ~-145 dBc PN at 1kHz offset. 

CCSS-945-100 crystal oscillator from Crystek is a good crsytal oscillator which can achieve this goal. Datasheet link is below. 

CCSS-945.pdf (crystek.com)

The only remaining thing is to find an external VCO that works within the range of LTC6952 because LTC6952 does not have an internal VCO. 

Regards,

Kudret

Can the LTC6946-2 be used as a VCO for the LTC6952?

Hi,

Yes, you can use LTC6946-X series as external VCO to LTC6952. The only thing that you need to consider is that max input freq of LTC6952 is 4.5GHz. 

Kudret

Hi,

Yes, you can use LTC6946-X series as external VCO to LTC6952. The only thing that you need to consider is that max input freq of LTC6952 is 4.5GHz. 

Kudret

Hi，

ADI's official website shows that the LTC6946-X is the Last Time Buy?

I'm worried about whether I can buy this device.

Are there any other VCO options?

Sorry, for missing that. You can use LTC6948. 

Regards,

Kudret

Hi，

Does the phase noise of the LTC6948 output affect the output phase noise of the LTC6952?

That depends on your requirement from the output of LTC6952. Are you interested with 1kHz offset only of wideband PN/jitter? 

yes， phase noise of the clock can reach -130dBc/Hz@1kHz  offset.

If your only requirement is 1kHz offset PN value, then VCO noise does not have any effect because it will be suppressed by loop filter. Loop filter works as LPF to reference but HPF to VCO.

Kudret 

Hi,

"Loop filter works as LPF to reference but HPF to VCO." how to understand?

Add HPF to the back end of the VCO and then to the LTC6952?

No, I am saying that VCO noise within the loopbandwidth will have no contribution to the total noise because it will be suppressed by loop filter

ok,thanks very much.