effect of NF to VCO phase noise
I've figured out that amplifier noise voltage can be converted to phase noise using vco sensivity and a few math operations but i am confused how to deal with mixer (used as FD). The only noise property is noise figure.
Can anybody give me idea how to convert mixer's NF to PN? I need to calculate affect of NF to VCO PN.
what does "used as FD" mean?
oops. Phase Detector i mean. So it is double balanced mixer.
Well....when you use a passive mixer as a phase detector, there are two effects. The NF tells you the equivalent noise floor level at the input to the mixer. For example, an 8 dB nf means the equivalent noise at the mixer front end is going to be -174 dBm/Hz + 8 dB = -162 dBm/Hz. If you know the power level of the signal hitting the mixer, you can determine the noise figure of that one part, for example if you have a -10 dBm signal, that mixer alone will cause -152 dBc/Hz phase noise floor where the PLL loop filter still has open loop gain.
In addition, mixers have 1/f noise also, so close to the carrier there is that addative noise. It is not unusual to find some mixers that are significantly better than others for 1/f noise.
Of course, the op amp following the mixer phase detector adds its own noise. Even a very good one a 1 nv/root hz will add a lot of phase noise.
thanks for answer. Forgive me for my English - i have lack of practice in writing
In practice only OPAMP determines PN floor of analog PLL. But the PNF level depends on VCO tuning sensitivity. Less sensitivity - less PNF. Often an mixers with P1>0 dBm are used to minimise it's noise floor. So PN at small offsets from carrier depends on reference and integrator. Try to build an noise model of PLL. PD sensitivity is (signal level in Volts)/(mixer's conversion loss).
VCO->AMP->Mixer
VCO with +10dBm output power, amplifier with also +10dBm so at the mixer input i have signal level with 20dBm ( i hope it is so).
So -174 dBm/Hz+7.5(mixer NF)+20 =-146,5 dBc/Hz at thr mixer output - am i right ?
What is: VCO with 10 dBm and amplifier with 10 dBm? At one port mixer must have 10-15 dBm (LO) and at RF port must have 0-6 dBm. Mixer (8 dB conv loss) noise floor will be -168...-162 dBc/Hz. But you can't see this.
When you build an PLL with 100 kHz BW using ADA4898, AD829, AD797 and VCO with tuning sensitivity is 4 MHz/V, the noise floor will be only -143...-140 dBc/Hz. Thanks to OpAmp and VCO...