At 3:46 PM +0000 12/21/06, Peter Vince wrote: >David Forbes recently mentioned the Wikipedia item on dual-modulus >prescalers: > > http://en.wikipedia.org/wiki/Dual-modulus_prescaler > >I have looked at that article, and either my brain has slipped out of >gear, or there are some errors in both the diagram and the words:-( I >think I can see the sort of thing they are trying to achieve, but was >hoping someone here could provide a clearer explanation please? > >Thanks, Peter
Peter, I referred to that article having already understood how these things work, so it may be not very clear for a beginner. The basic task is to generate a lower frequency by dividing a higher one by a non-integer valuise. The method is to divide the input frequency by two different integers, M and M+1, in a controlled sequence. To use a basic example, say you wanted to divide a 53 Hz clock to make a 10 Hz clock. The division ratio is 5.3. Since you can't make a divide-by-five-point-three digital counter circuit, you have to make do with a divide-by-five and a divide-by-six and use them alternately to achieve an output that appears to divide by 5.3. Now we take the problem into the time domain rather than the frequency domain, since digital circuits are more easily understood in the time domain. Our original signal has 53 pulses in a second. We want to end up with 10 pulses in a second. That is, we will need to build a circuit which will put out 10 output pulses for exactly 53 input pulses. We could use a digital counter chip which produces one output pulse for every 5 input pulses, but that would give us 10 output pulses for only 50 input pulses. We need to absorb the extrea three input pulses. That's where the dual modulus divider comes in. Since we have three extra pulses to count in order to make our 10 output pulses, we use a counter that can be told to produce one output pulse for every six input pulses for a while, then it is switched to produce one output pulses for every five input pulses. The time to switch it is after it has gone through the 3 extra input pulses. The variables used in these devices are named M for the modulus, in this case 5, and A for the remainder, in this case 3. So the counter is told to divide by 6 for 3 output cycles, then to divide by 5 for the remaining 7 cycles. An illustration viewable in non-proportional font: 1 sec | | input 11111111112222222222333333333344444444445555 11111 pulse 1234567890123456789012345678901234567890123456789012312345678901234 count 1234561234561234561234512345123451234512345123451234512345612345612 output | | | | | | | | | | | | | pulse modulus 6 6 6 5 5 5 5 5 5 5 6 6 Notice that the output pulse spacing is irregular, which is why you can't use this method for dividing a frequency to make another frequency. You can only use it to make a precise number of pulses in a given period. Let me know if this clears it up for you. -- --David Forbes, Tucson, AZ http://www.cathodecorner.com/ _______________________________________________ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts