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25th Dec 2014, 10:49 pm | #1 |
Octode
Join Date: May 2003
Location: Belper Derbyshire
Posts: 1,910
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Trying to make a logic decade divider box
Good evening,
I am trying to make a logic decade divider box for use with a 10MHz rubidium frequency standard. My idea is to have a box with the 10MHz reference sinewave input and then logic outputs each being divided by 10 from 10MHz down to 1Hz. I have been messing around with various circuits and have come across 2 problems. The first is trying to convert a 10 MHz sinewave to a square wave to go into a divider. I can put a sinewave directly into a logic gate and it does work in a fashion though the output is obviously not exactly very square!! The sinewave is nearly 5V P-P. The second is trying to get an equal mark space ratio on the decade divider itself. I can divide a square wave by 10 to give a tenth of the frequency out but it keeps coming out as a narrow pulse rather than 50% mark space ratio (i.e a square wave). Is there a logic gate which would do this task relativly easily? Many thanks, Christopher Capener
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Interests in the collection and restoration of Tefifon players and 405 line television |
25th Dec 2014, 11:42 pm | #2 |
Nonode
Join Date: Oct 2008
Location: Warsaw, Poland and Cambridge, UK
Posts: 2,669
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Re: Trying to make a logic decade divider box
Don't worry about the input to your divider being square. As long as the edges are clean (use a Schmitt trigger to square your sine wave) it will work fine.
To get a square wave output, do your division in two stages: divide first by five, and then by two. The division by five can be using any circuit you like, the output pulse shape doesn't matter. The final divide by two (one flip-flop of a binary counter) will guarantee you a 50% duty cycle, or as near as makes no difference, on the output. I hope that helps Chris
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25th Dec 2014, 11:45 pm | #3 |
Hexode
Join Date: Jul 2007
Location: Edinburgh, UK.
Posts: 344
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Re: Trying to make a logic decade divider box
Hi'
Have a look at this thread where oscilloscope time mark generators are discussed. Oscilloscope Calibrators: Bradley 192 vs. Heathkit IG-4505 https://www.vintage-radio.net/forum/...ad.php?t=54836 Having a 1:1 mark space ratio, while aesthetically pleasing, is not rally necessary. It is usually the leading edge of the pulses that are of interest for timing purposes. Last edited by Brian R Pateman; 26th Dec 2014 at 9:21 am. Reason: Link to thread added. |
26th Dec 2014, 11:55 am | #4 | |
Rest in Peace
Join Date: Jun 2006
Location: Chard, South Somerset, UK.
Posts: 7,457
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Re: Trying to make a logic decade divider box
Quote:
Al. / Dec. 26 |
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26th Dec 2014, 4:46 pm | #5 |
Octode
Join Date: Jul 2009
Location: Carmel, Llannerchymedd, Anglesey, UK.
Posts: 1,498
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Re: Trying to make a logic decade divider box
And if you are using TTL - try a 7413 schmitt trigger for sine/square pulse shape, or a 4093 in the CMOS world.
Les. |
26th Dec 2014, 4:50 pm | #6 |
Dekatron
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 13,953
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Re: Trying to make a logic decade divider box
In my days of doing this to provide marker-pips for amateur radio the usual thing was to use any easily-available set of TTL gates [7400] at 10MHz to oscillate the crystal at 10MHz, then put it through a Schmitt Trigger stage [7414] to get the waveform nice and square then a series of 7490 TTL divide-by-N stages.
There was also a 4-legged Motorola chip [MFC4040] intended for electronic organs, that did divide-by-two which was useful when you needed a UK VHF two-way-radio 25KHz or 12.5KHz alignment 'pip'. |
26th Dec 2014, 6:45 pm | #7 |
Dekatron
Join Date: Jul 2007
Location: Evesham, Worcestershire, UK.
Posts: 4,241
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Re: Trying to make a logic decade divider box
As well as TTL ICs, the CMOS 4017 might do the job (it's a bit marginal at 10MHz, but a 74HC4017 will be more reliable). It's a decade counter by default, but instead it could count to 5, and be followed by one half of a 4013 D-type latch to give a good square wave.
The 4017 has the advantage of decoded outputs, so you need only connect the 6th output to the reset pin to get a divide-by-5 circuit. Alternatively, rather than use the divide-by-two trick, you could use a 4078 - which is an 8-input OR gate. Feed this with the first 5 outputs of the 4017, and ground the other inputs. You'll need one of these per 4017, so this option might use more packages... For a Schmitt trigger input (to square up the sine wave), the 40106 is a hex inverter with Schmitt inputs. Alternatively, the 4093 is a quad 2-input NAND gate that also has Schmitt inputs. I'd be tempted to use a dedicated comparator, but that's only because I've got a drawer full of them Good luck, Mark |
26th Dec 2014, 7:45 pm | #8 |
Octode
Join Date: May 2003
Location: Belper Derbyshire
Posts: 1,910
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Re: Trying to make a logic decade divider box
Many thanks for all of your suggestions
I have after more experimenting managed to get a reliable circuit. I have used a 74HCT14N hex inverting schmitt trigger to convert the sine wave to a 10 MHz square wave. One of my issues was a poor square wave. This was caused by me not fitting enough decoupling capacitors across the devices I have then used 4 74HC390 ic's each of which give a divide by 10 and a divide by 100 output so I have cascaded them together to give outputs at 1MHz, 100KHz, 10KHz, 1KHz, 100Hz, 10Hz, and finally 1Hz with a bonus 0.1 Hz output as there was a spare divider left!! Each of the induvidual outputs shall go through a buffer stage before leaving the unit. Now I just need quite a few BNC panel sockets !! (10 of them!!) Many thanks again, Christopher Capener
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Interests in the collection and restoration of Tefifon players and 405 line television |