🚀 go-pugleaf

In <1992Dec16.194706.27550@iitinc.uucp> sehat@iitinc.uucp writes:

> From article <1992Dec16.011918.25341@eng.ufl.edu>, by iqbal@omaha.eel.ufl.edu (M.Iqbal):
> 
> 
> > Anyways, one should be able see that square wave have harmonics all over 
> > the frequency band. What CPU is doing is picking up a harmonic of the input
> > clock frequecny. Therefore, second harmonic of 25 mhz is 50 mhz and similarly
> > second harmonic of 33 mhz is 66 mhz. Usually, a CPU has a Phase lock loop
> > to lock on the second harmonic of the clock frequency; thus, running 
> > the internal circuitry of the porcessor at 50 or 66 mhz. One can, however,
> > pick up second harmonic by using a filter, but, it is much easier to build 
> > a Phase lock loop on a silicon wafer than passive components like R, L, 
> > and C.
> > 
> 
> Actually, A CPU PLL normally does not pick up any harmonic component of the
> input signal. In fact, the way the CPU PLL works is to compare one edge (for
> example rising edge) of the input clock to the edges of the internal oscillator
> circuit. In order to get 2x internal clock, the internal oscillator is divided
> by 2 before the edge comparison. The frequency of the internal osc is then
> adjusted such that for every input edge there is corresponding edge of the
> divided internal osc (a phase-freq detector is normally used to do this).
> If you need to find more information on the circuit used in the 486DX2, pick
> up a copy of the latest JSSCC.
> -- 
> Sehat Sutardja,
> 
> sehat@iit.com				| INTEGRATED INFORMATION TECHNOLOGY
> 					| 
And aside from that, a square wave only has ODD harmonics, so filtering
for the 2nd harmonic wouldn't work too well...
Graham B