a brief explanation about the uses of a PLL VCO.(from doepfer.)
2003-06-26 by Bakis Sirros
dieter says: "a PLL and it's application is not very easy to understand. I will try to write an explanation and publish it on our web site as soon as we publish the preliminary page for the PLL module (no A-100 number so far). Here is a simplified explanation: A PLL is a linear rectangle VCO with a frequency comparator that compares the frequency of the internal PLL-VCO with an external frequency (master frequency) - e.g. the rectangle output of "normal" A-100 VCO - and makes the the internal VCO frequency equal to the external frequency. Or better said: it tries to adjust the internal frequency so that it becomes equal to the external frequency. The problem is the frequency comparator - even called phase comparator - that generates a kind of a correction voltage that becomes zero if both frequencies are identical. Different versions with different advantages/disadvantages of frequency comparators are available. Some types of comparators e.g. will "lock" even at multiples of the frequencies. Another problem is the time that is necessary until the PLL-VCO is adjusted. It will always take some time until the new frequency of the PLL-VCO is reached (similar to portamento). The time constant of the internal low pass filter (not an audio low pass but a low pass/slew limiter for the correction CV) defines the time until the new frequency is reached. But this time has not to be too short otherwise the frequency of the PLL-VCO will "wobble" around the master frequency. These "disadvantages" lead to additional musical applications like portamento effects or wobbling frequencies according to the type of frequency comparator and time constant of the PLL low pass filter. Another application is frequency multiplication in combination with an external frequency divider. For this the output of the PLL-VCO is processed through a frequency divider (e.g. A-160, A-161, A-163) before it is fed to the PLL frequency comparator. In this case the frequency of the PLL-VCO will be a multiple of the master frequency. E.g if the the A-163 is used and adjusted to dividing factor 5 the frequency of the PLL-VCO will be 5 times the frequency of the master VCO. Consequently frequency division (A-163) leads to frequency multiplication with the PLL circuit. In combination with the PLL low pass frequency several effects can be realized (frequency multiplication with portamento or wobbling). The frequency multiplication can even be used to drive a graphic VCO. If your graphic VCO e.g. has 8 steps (e.g. A-155) and you use a frequency divider with factor 8 in the PLL feedback the output of the graphic VCO has the same frequency as the master VCO. Another application is the generation of pseudo-harmonics (not real harmonics as only rectangle waves are available). I hope this explanation will help and not cause more confusion. I know that the PLL is not very easy to understand. Best wishes Dieter Doepfer" INTERESTING???????OF COURSE,YES! bakis. ===== synthfreak(parallel worlds) athens-greece [Doepfer_a100] group owner __________________________________ Do you Yahoo!? SBC Yahoo! DSL - Now only $29.95 per month! http://sbc.yahoo.com