[time-nuts] Wikipedia Dual-modulus prescaler item

David Forbes dforbes at dakotacom.net
Thu Dec 21 13:06:27 EST 2006


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/



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