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Questions about my PFD/Charge pump output

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firasgany7

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Hello guys,
I have built a Sequential NAND based phase detector with a charge pump that includes a differential amplifier and I want your help make some conclusions about the results I got running all the circuits together, and also to find an answer for some questions that came up to me.

Note: the PFD works on Falling edge.


PFD Schematic:

Capture.PNG

Charge Pump Schematic:

chargepump_wiith_opamp.png

all circuit schematic:
allcuircit.png


Simulation Results:
charging and uncharging.png

I have a few questions about the simulation result:

1. Do you think the circuit works properly? I think its fine, since as we see that the dominant pulses are up, we see that the charge pump is charging the capacitor on its output, opposite happens when the down is dominant.

2. how do you suggest me to calculate the dead zone period? can I figure out the dead zone only from this graph at all? to me, dead zone occurs when we have small phase difference that makes very short down/up pulses that are not sufficient for the charge pump to charge/uncharge its output, the question is, what determines how much of a small change is considered a dead zone?

3. in order to plot the PFD characteristic plot, I need to plot the average current Icp as a function of (Up - Down ) signal. looks like the dead zone in my graph is around 26 ns, were both up and down pulses are so tiny. so there I should expect a small change in voltage and current. do you agree with this analysis?

4. what do you suggest me to do in order to assure a proper work of the charge pump? after reading some articles, I understood that I need to make sure that the charging and the unchanging current should be nearly the same (correct me if i'm wrong) , I don't see how this is happening in my circuit, looks like the current can't be stable for any period of time.

5. why is the current icp is the opposite/reverse of the current that I should expect from the derivative of the charging/unchanging voltage, CP_vout?
 
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