Antenna94
Junior Member level 3
Hi!
I have designed and built a VCO with a 12...25V tuning range and I would like to design an active loop filter for an integer PLL IC to control the VCO frequency. I would need some advice, help, or just verification that my train of thought is not wrong.
The charge pump output can produce a voltage between 0 and 5V (more likely 0.5 and 4.5V), so I would need an active loop filter with positive supply voltage say 28...32V and a grounded negative supply as the charge pump minimum output voltage is 0V as well. The topology would be a second-degree passive filter (parallel C1 and R2C2), then a non-inverting amplifier (with ~6 voltage gain to produce ~30V from 5V maximum charge pump voltage), then a third RC pole. (**broken link removed**
My question is the following: does it cause any problems that the minimum VCO tuning voltage is higher than the minimum voltage that the filter can produce? Far below the minimum tuning voltage the VCO doesn't oscillate or spuriously oscillates. Or should I use another active filter topology in which half of the charge pump supply is lead on the non-inverting input of the amplifier (so called type A active filter)? To be honest, I don't exactly understand what happens because of that (I just read that it is optimal for spurs), am I right in thinking that the "initial" tuning voltage will be half of the charge pump supply? (So my problem would be solved because upon power-up the tuning voltage would be 2.5V*gain of the opamp?)
Thanks in advance for any insights!
I have designed and built a VCO with a 12...25V tuning range and I would like to design an active loop filter for an integer PLL IC to control the VCO frequency. I would need some advice, help, or just verification that my train of thought is not wrong.
The charge pump output can produce a voltage between 0 and 5V (more likely 0.5 and 4.5V), so I would need an active loop filter with positive supply voltage say 28...32V and a grounded negative supply as the charge pump minimum output voltage is 0V as well. The topology would be a second-degree passive filter (parallel C1 and R2C2), then a non-inverting amplifier (with ~6 voltage gain to produce ~30V from 5V maximum charge pump voltage), then a third RC pole. (**broken link removed**
My question is the following: does it cause any problems that the minimum VCO tuning voltage is higher than the minimum voltage that the filter can produce? Far below the minimum tuning voltage the VCO doesn't oscillate or spuriously oscillates. Or should I use another active filter topology in which half of the charge pump supply is lead on the non-inverting input of the amplifier (so called type A active filter)? To be honest, I don't exactly understand what happens because of that (I just read that it is optimal for spurs), am I right in thinking that the "initial" tuning voltage will be half of the charge pump supply? (So my problem would be solved because upon power-up the tuning voltage would be 2.5V*gain of the opamp?)
Thanks in advance for any insights!