Software PLL
in [DSP]
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From: HardySpicer on 16 Apr 2010 16:28 On Apr 16, 10:31 am, "smc123" <xmacleod(a)n_o_s_p_a_m.yahoo.com> wrote: > I'm looking for guidance on writing a software PLL for a signal acquired > from a data acquisition board: > The daq board will be sampling at 10kSPS. > The signals to which I'd like to lock on to are from 5Hz to 1kHz, sine or > square wave. > The output of the PLL will be used to multiply a signal acquired on an > adjacent channel of the daq board. Lock-in amplifier application. > The processing of the signals will be done on a PC-based software app. > > Thanks for your help %Phase Locked Loop %This simulates an analogue PLL %The VCO is sinusoidal but can be made a square wave %Suggested parameters: Sample at 10000Hz,Carrier freq 1000Hz = VCO free running freq % Make baseband freq 1Hz and frequency deviation 10Hz. % Make run time 1 second %When you run this you will see the demodulated sine wave (1Hz) as figure 1. % It will have 2fcarrier (2fc) superimposed on top of it (as in a real PLL) %First Generate Test Signal %Frequency Modulation of a sine wave is simplest! fs=input('Input Sampling Frequency (Hz)'); fb=input('Baseband Frequency (FM)'); fc=input('Carrier Frequency (FM)'); fset=input('Free running frequency for VCO(make this normally = carrier freq)'); delf=input('Frequency Deviation (Hz)'); %Compute FM Modulation Index beta=delf/fb; tmax=input('Run Time (Secs)'); npoints=tmax*fs; %%%%%%%%%%%%%%%%%%%%%%%%%% %Some arrays inialised pout=zeros(npoints,1); fm=zeros(npoints,1); vco=zeros(npoints,1); ff1=zeros(npoints,1); ff2=zeros(npoints,1); %%%%%%%%%%%%%%%%%%%%%%%%% %Generate FM for i=1:1:npoints fm(i)=cos(2*pi*i*(fc/fs) + beta* sin(2*pi*i*(fb/fs) ) ); end %Unity Gain Crossover Frequency (Hz) for Bode Plot fcp=(2*fc)/10; %Make span ratio 10:1 gives a phase margin of around 57 degrees %Change this and you change stability phase margin is arcsin[(span-1)/ (span+1)] spanp=10; %This function computes phase advance parameters [ap,bp,delp]=plead(spanp,fs,fcp); %Compute Overall Gain gainp=2*(1-cos (2*pi*(fcp/fs) ) )/sqrt(spanp); %Divide up between Integrators gainip=sqrt(gainp); %Initialise just in case.. p0=0.0; p1=0.0; q0=0.0; q1=0.0; r0=0.0; r1=0.0; fu0=0.0; %Main Loop for Phase Locked Loop for i=1:1:npoints %Controller (Filter) %First Integrator p1=p0; p0=p1+fu0*gainip; %fu0 is Phase Detector Output %Second Integrator q1=q0; q0=q1+gainip*p0; %Phase Advance r1=r0; r0=-r1*bp + delp*(q0+ap*q1); %VCO Output vco(i)=cos( (2*pi*i*(fset/fs) ) + r0 ) ; %Now Limit VCO Output to +1.0 or - 1.0 %For square wave VCO uncomment the following %if(vco(i)>0)vco(i)=1.0;end %if(vco(i)<0)vco(i)=-1.0;end %Phase Detector (A simple multiplier) fu0=vco(i)*fm(i); %PLL Output is first Integrator Output pout(i)=p0; end figure(1) plot(pout) function [a,b,del]=plead(span,fs,fc) %Phase Lead % Returns zero coefficient a % Pole coefficient b % and gain del %span ratio is span %fs is sampling frequency %fc is frenquency for maximum phase advance thetac=2*pi*(fc/fs); sqs=sqrt(span); x=cos(thetac); y=sin(thetac); num1=sqs*(1+span)*y-2*span; den1=sqs*(1-span)*y + 2*span*x; a=num1/den1; beta=(1-a)/(1+a); b=(span-beta)/(span+beta); del=(1+b)/(1+a); %Th Thats all folks.... |