Applications Information cked loops such as the LM565, the Lag-Lead Filter mportant parameters of interest are FREE RUNNING FREQUENCY C2 R2 R。C ay be used for wide closed loop band- deviation of the carrier is fairly high(greater than oop gaIn where wideband modulating signals must lowed K ity The natural bandwidth of the closed loop response may be found fron KD= phase detector sensitivity (radian The loop gain of the LM565 is dependent on supply voltage fn ound from: Associated with this is a damping factor. 33.6f6 2VR1c1K× and applications where a narrow noise band- f。= vCo frequency in Hz Vs total supply voltage to circuit Loop gain duced by connecting a resistor betwee pins 6 and 7; this reduces the load impedance on the output Iting in large overshoot and possibl mplifier and hence the loop gain. instability in the transient response of the loop. In this case, HOLD IN RANGE: the range of frequencies that the loop will the natural frequency of the loop may be found from remain in lock after initially being locked f f. free running frequency (R1+R2)C1 Ve total supply voltage to the circuit R2 is selected to produce a desired damping fa between 0.5 and 1.0. The damping factor is fou THE LOOP FILTER In almost all ations it will be desirable to filter the sic nal at the output of the phase detector(pin 7); this filter may These two equations are plotted for convenience. Filter Time Constant vs Natural Frequency Simple Lead Filter 1041031021011 w.national comApplications Information In designing with phase locked loops such as the LM565, the important parameters of interest are: FREE RUNNING FREQUENCY LOOP GAIN: relates the amount of phase change between the input signal and the VCO signal for a shift in input signal frequency (assuming the loop remains in lock). In servo theory, this is called the “velocity error coefficient.” The loop gain of the LM565 is dependent on supply voltage, and may be found from: fo = VCO frequency in Hz Vc = total supply voltage to circuit Loop gain may be reduced by connecting a resistor between pins 6 and 7; this reduces the load impedance on the output amplifier and hence the loop gain. HOLD IN RANGE: the range of frequencies that the loop will remain in lock after initially being locked. fo= free running frequency of VCO Vc= total supply voltage to the circuit THE LOOP FILTER In almost all applications, it will be desirable to filter the sig￾nal at the output of the phase detector (pin 7); this filter may take one of two forms: A simple lag filter may be used for wide closed loop band￾width applications such as modulation following where the frequency deviation of the carrier is fairly high (greater than 10%), or where wideband modulating signals must be fol￾lowed. The natural bandwidth of the closed loop response may be found from: Associated with this is a damping factor: For narrow band applications where a narrow noise band￾width is desired, such as applications involving tracking a slowly varying carrier, a lead lag filter should be used. In gen￾eral, if 1/R1C1 < Ko KD, the damping factor for the loop be￾comes quite small resulting in large overshoot and possible instability in the transient response of the loop. In this case, the natural frequency of the loop may be found from R2 is selected to produce a desired damping factor δ, usually between 0.5 and 1.0. The damping factor is found from the approximation: δ ) π τ2fn These two equations are plotted for convenience. Simple Lead Filter DS007853-11 Lag-Lead Filter DS007853-12 Filter Time Constant vs Natural Frequency DS007853-13 www.national.com 8