divide by 25/16
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From: Joop on 19 Aug 2006 07:45 On Fri, 18 Aug 2006 14:01:05 GMT, "colin" <no.spam.for.me(a)ntlworld.com> wrote: > >"Joop" <jojo(a)chello.nl> wrote in message >news:tss9e2d8q3dtl6587k1lsvkgp1jshmsihg(a)4ax.com... >> >> > >> >aha yes that would do, many thanks, shld be able to do in 2 chips a >74'390 >> >dual decade counter and a 74'00. >> > >> >I had written down the numbers in base 5, and had a big page of possible >> >sequences looking for easy decodes, I think I just about had this >sequence >> >but didnt see the wood for the trees and overlooked its simplicity, im >> >impresed at ariving at this so quickly :) >> > >> >Colin =^.^= >> > >> I actually have such a device. It is a :100 prescaler using the >> 74LS390 after a U664 1.2GHz :64 counter. >> >> Apparently it was based on a Siemens appnote. Basically there are two >> 5/4 counters cascaded. >> An LS02 is used next to the LS390. The input at pin 12 is also fed via >> an inverter and norred with pin 9. The output goes into pin 4. Pin 4 >> is inverted and norred with pin 7. This nor output is your :100 >> signal. >> >> Cheers. >> >> Joop > >Cool thanks, I tried to find the appnote with google but no luck > >Colin =^.^= > I could not find it either, but here is something similar to my complete circuit. In mine the last two ls02 ports are left out. http://www.ure.es/gpa/modules.php?name=Downloads&d_op=getit&lid=169#search=%22prescaler%20de%201300%20MHz%2074ls390%22 The original idea was using the 74167, but that chip seems difficult to get these days. It is also used in another version in the above article by the way. Anyway I am not sure if 50MHz input is a bit too high. You might need an additional :10 counter in front of the LS390. That is something I would not have mind either, since it would put the decimal point of my frequency counter back a normal spot. Joop
From: Joop on 19 Aug 2006 08:02 >Anyway I am not sure if 50MHz input is a bit too high. You might need >an additional :10 counter in front of the LS390. That is something I >would not have mind either, since it would put the decimal point of my >frequency counter back a normal spot. > Or use a :256 prescaler and another 4/5 section followed by a /2. That also gives you a :1000 total and bring the LS390 input within range.
From: colin on 19 Aug 2006 08:44 "Joop" <jojo(a)chello.nl> wrote in message news:btude2ppkf5u5slmad0b88l0a22gqberu3(a)4ax.com... > > >Anyway I am not sure if 50MHz input is a bit too high. You might need > >an additional :10 counter in front of the LS390. That is something I > >would not have mind either, since it would put the decimal point of my > >frequency counter back a normal spot. > > > Or use a :256 prescaler and another 4/5 section followed by a /2. That > also gives you a :1000 total and bring the LS390 input within range. yes 50mhz is a bit much for a ripple clounter, maybe the synchronous ones would be better. I just found the mc12080 will divide by 10,20,40,80 at 1.1ghz and the mc100lvel33 will divide by 4 at 4ghz, leaves quite a few good possibilities. such as /4 followed by /20, then only 1 4/5 stage needed for divide by 100 Would be nice to have as many digits of resolution left as possible as im looking at RF and LO freqs >2ghz but only some khz apart. I did think about gating off the input to the prescaler at the end of the period, then pulsing the input relativly slowly to till it reaches terminal count again to determine the count reached. of course it will probably need a microcontroller to do this, wich might as well make a complete freq counter, but this is then hardly the simple add on it was suposed to be ! Colin =^.^=
From: Joop on 19 Aug 2006 21:28
On Sat, 19 Aug 2006 12:44:17 GMT, "colin" <no.spam.for.me(a)ntlworld.com> wrote: > >"Joop" <jojo(a)chello.nl> wrote in message >news:btude2ppkf5u5slmad0b88l0a22gqberu3(a)4ax.com... >> >> >Anyway I am not sure if 50MHz input is a bit too high. You might need >> >an additional :10 counter in front of the LS390. That is something I >> >would not have mind either, since it would put the decimal point of my >> >frequency counter back a normal spot. >> > >> Or use a :256 prescaler and another 4/5 section followed by a /2. That >> also gives you a :1000 total and bring the LS390 input within range. > >yes 50mhz is a bit much for a ripple clounter, maybe the synchronous ones >would be better. > >I just found the mc12080 will divide by 10,20,40,80 at 1.1ghz >and the mc100lvel33 will divide by 4 at 4ghz, >leaves quite a few good possibilities. >such as /4 followed by /20, then only 1 4/5 stage needed for divide by 100 > >Would be nice to have as many digits of resolution left as possible as im >looking at RF and LO freqs >2ghz but only some khz apart. >I did think about gating off the input to the prescaler at the end of the >period, then pulsing the input relativly slowly to till it reaches terminal >count again to determine the count reached. >of course it will probably need a microcontroller to do this, wich might as >well make a complete freq counter, but this is then hardly the simple add on >it was suposed to be ! > >Colin =^.^= > /80 does not bring the frequency much lower than /64. So I guess if you really want a lot of resolution (not the same as accuracy), then you probably need to think about gating in the early stages. And thus incorporate this into a counter. So not having a seperate prescaler. And about making a frequency counter. It can be as simple as this if you use a PC: http://home.planet.nl/~wvh/flogger.htm I build a modified flavor to unattended monitor VFO stability over time. Great simple gadget. The shown software limits it to 16 MHz (24bit) but the PIC hardware is often used to 50MHz. Joop |