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# 锁相环噪声

• Hi,

To make the calculation easier, I'll use your output frequency as 1 GHz and I 'll ignore the noise coming from the reference for now.

To get -70 dBc at 5 Hz offset for 1 GHz output frequency, the PLL should have 1/f FOM of at least -243 dBc. Datasheets usually provides normalized 1/f noise spec for 1 GHz output at 10 kHz offset. If you do the math, it corresponds to -103 dBc normalized 1/f noise.

Below is the selection table for our PLLVCO parts sorted by normalized 1/f noise

As it can be seen, all the PLLs can meet this spec except ADF4360-X (1/f noise of ADF4360-X family is not specified in the datasheet).

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 20 dB higher than the value that you are given in the reference oscillator datasheet.

The 1/f noise and the reference noise will be combined with square root of sums of squares relation, and you'll get the final PN.

Since most of the PLLs in our portfolio has much better specs than this, you can only consider the noise coming from your reference. 1/f noise contribution of the PLL will be negligible.

So, the phase noise of your reference must be better than -70 - 20*log (fout/fref).

Regards,

Kudret

• Edited the last equation. -70 - 20*log (fout/fref).

Kudret

• hello sir,I have a question,how get -243this answer(To get -70 dBc at 5 Hz offset for 1 GHz output frequency, the PLL should have 1/f FOM of at least -243 dBc.),I have seen HMC704and ADF4377datasheet,those show some equations like floor noise,flick noise,FOM...and so on

• hello sir,I have a question,how get -243this answer(To get -70 dBc at 5 Hz offset for 1 GHz output frequency, the PLL should have 1/f FOM of at least -243 dBc.),I have seen HMC704and ADF4377datasheet,those show some equations like floor noise,flick noise,FOM...and so on

• hello sir,I have a question,how get -243this answer(To get -70 dBc at 5 Hz offset for 1 GHz output frequency, the PLL should have 1/f FOM of at least -243 dBc.),I have seen HMC704and ADF4377datasheet,those show some equations like floor noise,flick noise,FOM...and so on