Hi All, I have built and tested a PLL circuit that will be used to generate a 1 MHz signal locked to a 0.5 HZ signal from a pendulum. (Details available upon request.)
The circuit is a classic 4046 generating the 1 MHz signal which is fed into a 2e6 digital divider which outputs 0.5 Hz which is fed back to the 4046 phase comparator (PC). I take a 1 MHz signal from an HP 107A run through another 2e6 divider to generate a reference 0.5 Hz signal for the other 4046 PC input. I tested this by feeding the 0.5 Hz output of the PLL into a "time-stamp counter" board which I built to go into an HP 3582A Data Acquisition unit. The TSC uses the 5 MHz signal from the HP 107A to feed a free-running 32-bit binary counter. The 0.5 Hz input latches the count value (on the rising edge of the signal), which is then logged. See the attached diagram. The PLL under test is in the red box. (Not sure what the policy is here for attachments?) If all was perfect I would get a string of values of 10,000,000 counts each, one every 2 seconds. Over the course of one day the average reading is, in fact, 10e6, so the PLL looks to be working over "long" time scales. The attached histogram plot shows the actual data for the 0.5 Hz signal, showing the distribution of deviations from 10e6 counts. This is almost a full day of data, about 40,000 readings. The standard deviation for the data is about 55 counts. The plot looks to my eye to be a nice Gaussian shape, so I assume that the deviations are caused mainly by (white?) noise. There does not look to be much other structure in the shape of the data. (Comments welcome.) Sorry for the long introduction, there are some questions coming! I have looked for information on the web about others who may have done this kind of PLL, but did not find much. Does anyone know of any articles related to this? If so, do you know what kind of performance they got? What kind of statement could I make about the 'stability' of this circuit? Simplistically: a 'stability' of ~50 counts in 10e6 is ~5e-7? By the way, this performance is WAY WAY beyond what I was expecting.... Cheers, Scott
PLL0.pdf
Description: Adobe PDF document
histogram-utcday21613x.pdf
Description: Adobe PDF document
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