active filter design
Bipolar amplifiers noise performance is good, like NE5534, OP27 etc. Their noise
are all less than 4nV/(Hz)^0.5. FET input amplifiers have very low input bias current, but noise performance is bad.
For VCO with very low sensitivity(20Hz/V), which kind of amp should be selected?
thanks
Usually it is not a problem, but it depends by your configuration, i.e. if your phase comparator topology is charge pump (you charge/discharge a capacitor or leave it substain the imposed voltage) you should use a FET input amplifier, to reduce leakage effects. If you need an OPAmp with very large bandwidth, probably it is better to use a bipolar device.
Since there are hundreds of different OPAmps (also mixed FET input/Bipolar output) you have to trade off costs vs. performance.
Some advices:
1) Be careful to have enough OPAmp bandwidth to assure the active filter do not stop working when you really need it (i.e. at the output frequency of the phase comparator).
2) Depending your topology, consider using rail-to-rail input devices, to prevent unwanted output inversion.
3) Foresee an R-C LP filter after the OPAmp to cut its noise and everything else.
4) Assure a very clean power supply for the VCO.
5) Assure a very stable and quiet voltage reference at the circuit (if you have it).
6) Analyze carefully the PLL loop gain.
Only for curiosity:
what is the VCO output frequency?
Why are you worried about noise?
How do you use this PLL (for clock recovery, or something else)?
To help you about the OPAmp, it is useful to know also the phase comparator output frequency, the gain you need and the PLL loop bandwidth.
hi, Smanetto
Thank you for so many useful advice.
I had post a topic to ask help on design PLL :
https://www.edaboard.com/ftopic114064.html
Many friends had give me worthful help. The topic about op amp selection is different from that one, i think, although the PLL is the same.
The reference is 10MHz, the VCXO output is 70MHz. The FPD is charge pump type--ADF4001 from analog device. The 70MHz will be used as another PLL's reference.
Regards