The Phase Locked Loop PLL Zero Order view of PLL and role of VCO Key Objective of Phase Locked Loop Use the phase comparator block X to keep red VCO doing exactly what the incoming signal is doing This can either a guarantee clock synchronization or b demodulate FM signals initially coming from green A word about units Phase is the working units in the feedback equations Transfer Function vo i i to osc A a 1 af VCO1 a o f VCO2 Input voice signals i i o Output FM modulated signals This slide and the next one are intended give conceptual intuitive views of how the PLL works The words may actually be very similar between the two slides VCO FM Modulation Lab 2 Multiplier X Lab 1 Feedback A a 1 af Lab 3 different target application Putting them together with a slightly strange twist we can create the Phase Locked Loop PLL That is IF you may have to read a lot more to totally understand this in detail we can achieve a feedback function such that A 1 f then the vo that drives that feedback function IS the original modulating signal A key again that will require work in terms of reading and digesting the derivation etc is to understand that the UNITS of feedback and to use them properly 1 The Analogy between O A feedback and PLL o i i VCO1 R1 R2 iin ifb o i to Virtual ground Requires that iin ifb i o osc VCO2 This is a very overly simple view of how the PLL might be thought of in the context of a feedback loop That is for the inverting O A the summing node into the O A results in the driven node at the output to have a voltage such that the currents match at the input For the PLL the X phase comparator block takes the two FM signals one from our modulating source the other created by the VCO within the PLL and creates a output Vo such that the two incoming signals exactly match each other frequency and a welldefined phase relationship If this seems too simplistic that s OK the following math and details provided by text information from Mayaram and Grebene should flesh out the story The key points include a this is a feedback problem and b the feedback function basically generates VCO 1 the inverse of what is coming into the system which was used to encode the information via the VCO block on the transmitter side 2 Grebene This is the first of the figures from Alan Grebene s book same one I used in explaining the 566 that give details about both the internal components of the PLL and the external behavior of PLL performance TOP figure this is the same block diagram as the previous slides but now adds loop filter and amplifier The loop filter is a BIG DEAL It and the amplifier is needed to condition the signal in order for it to NOT confuse the VCO TBD BOTTOM figure this adds some terminology about KD F s A KO and maybe most importantly the 1 s term As we will see this is the step that in fact gets us to as the feedback variable The next few slides show how the error signal Ve t looks in time what the meaning of the ranges of capture and hold for the PLL getting staying in LOCK as in Phase LOCKED 3 Grebene This shows error voltage Ve versus frequency and key PLL performance parameters of capture and tracking locked ranges Stated simply if Ve goes up V so does the frequency of the VCO f0 is the free running frequency of oscillation Similarly if Ve goes down V the frequency of oscillation does as well There is a limit to how much the VCO can be tuned As we will see from the SPICE modeling this has to do with how much phase change coming out of the VCO can be accommodated The next slide shows a bit more of a dynamic picture of the process and in fact more representative of how YOU will be working with the PLL in some of your testing 4 Grebene This pair of figures shows how the error signal looks when TOP the incoming frequency is swept from much below fo to well above it From the low end of frequency Ve 0 meaning that the VCO is free running and doesn t really care about the input When the error signal jumps to a negative value now the VCO is with the program and it has captured the incoming signal Namely it has adjusted it s Ve and phase of it s output such that it has found and locked onto the input As frequency increases from the input the error voltage tracks the input signal adjusting the VCO frequency and phase to stay in lock with the input Finally it looses LOCK at some frequency much higher than fo The BOTTOM trace corresponds to going from input frequencies much higher than fo towards lower frequencies Putting the information from both these traces together is what allows us to define capture range and tracking range the latter being much broader than the former 5 Grebene This is the transient view of the error voltage as it tries to find the lock condition for a new incoming frequency Basically in the process of trying to adjust both frequency and phase of the VCO the control voltage Ve goes back and forth from too high a Ve and frequency to too low a Ve and resulting frequency until it damps to the just right control voltage Reminder the loop filter has a lot to do with this process and is a critical and non trivial part of designing PLLs that quickly capture incoming signals and their changes in frequency We ll see more about this kind of figure in the discussion related to the simulation results ala the Mayaram text 6 Block Diagram of PLL i s comp s p s F s f s A o s osc s VCO Notation per K Mayaram book see slides at end of these notes AND second half of notes in course reader This block diagram and detailed discussion including SPICE model for closed loop time domain simulations provide the basic feedback view of operation of the Phase Locked Loop PLL Starting from the input side the phase comparator is the summing node from OpAmp terminology which generates an error signal that after filtering F s and amplification A alters the frequency and phase of the VCO The condition of lock in the loop corresponds to both the frequency and phase of external input and VCO output matching If as we expect for an incoming FM signal the frequency is varying then the error voltage that drives the VCO must track the modulating information That is if we think of how you created FM using a VCO now imagine that this overall block puts a VCO in the feedback loop such that the output of the overall system …