Hi
Rise and fall times are not the thing to worry about on the gates. Look at the propagation delay. That's what will vary. On Sep 27, 2013, at 2:11 AM, Tom Minnis <[email protected]> wrote: > Thanks for all your thoughts on the subject. Let me play back what I have > learned and how it may apply to my challenge. One of my first applications > is to use a 10MHz output to phaselock a VCXO master clock in a radio > transceiver. The VCXO is the Christek CVHD-950 which has a noise floor of > -164dBc and is -86dBc at 10Hz. The source I want to use is the Jackson Labs > GPSTCXO which has a noise floor of -155dBc and is -73dBc at 1Hz and 103dBc at > 10Hz. i did a quick survey of the phase noise specs on various Jackson > products that claim to be ultra low phase noise and found similar numbers. > One was -100dBc at 1Hz but only -145dBc at 100KHz. Another was down -90dBc > at 1Hz and -160dBc at 100KHz. It would appear that even the best parts I > could find quickly would not merit the fancy analog gizmo and that a good > stiff logic buffer would work. Next I went to IDT to find the best logic > buffer I could find. The phase noise out of a TBolt is roughly -165 to -170 floor and -155 to -160 at 100 Hz. (plus spurs of course) > I am looking at the IDT 74FCT38072 2 channel clock driver for PPS. It can > drive about 50mA if needed with 1nS rise and fall times. The one I am > looking at for 10MHz is the ICS553 4 channel clock driver. This one is good > for 25mA drive and they actually give a typical output impedance spec of 20 > Ohms. With a 3.3V supply, it has 1nS rise and fall times and a little faster > with a 5V supply, 0.7nS and 35mA drive. Rise and fall times are not the thing to worry about on the gates. Look at the propagation delay. That's what will vary. If a 3 ns delay varies 1% (30 ps) over 1,000 seconds that's going to give you 3x10^-14 in your ADEV. Are your sources good to 3x10^-14 at 1,000 seconds? If not, don't worry about it. > To make a sine wave should I use one of the 4 ports on the 4 port driver to > input to the filter or should I try to hook the filter input directly to the > clock driver input? > Are there tried and true 10MHz filter circuits or is that a non issue? After > the filter would come the video amp set up for a 50 Ohm drive and into a > splitter. That sound simple enough. What am I missing? Simply use a three element Tee on the output of a logic gate. Run one per output. Don't split for multiple output. That way you will have much better isolation (which very much does matter). Bob > > Tom > > On 9/26/2013 3:05 PM, Bob Camp wrote: >> Hi >> >> How clean is your clock source? If you have something that is -150 dbc at 1 >> Hz, then you probably need some fancy analog gizmos. If you can make do with >> "only" -110 to -120 dbc/Hz at 1 Hz, then properly driven LVC CMOS will do >> just fine. That's true for a square or a sine output. Since you pretty much >> can't find an OCXO better than -120 at 1 Hz, I'd bet you'll be ok. 5 volt >> logic will be a little more quiet than 3.3V. More or less faster is quieter >> as long as you stay with saturated silicon CMOS. Change materials and all >> bets are off. >> >> For square wave cable drive you can parallel up a couple of the '125 or '126 >> gates to get how ever much power you want to put into the cable. You can >> source or load terminate (or both). If you source and load terminate, your >> logic levels will be 1/2 the output. With either source only or load only >> termination you can get full swing logic levels. More drive will always be >> required with load termination (you are putting current into 50 ohms). >> >> Logic IC's are cheap, easy to use, and simple to find. A low voltage single >> supply drives them and they aren't current hogs unless heavily loaded. >> What's not to like? >> >> Bob >> >> >> On Sep 26, 2013, at 1:28 PM, Tom Minnis <[email protected]> wrote: >> >>> I am looking into various degrees of craziness. The source is CMOS and >>> there are plenty of 1 in to N out parts designed to drive clocks on a PCB >>> but not much is said about driving clocks on to a random length of coax to >>> another piece of equipment and what additional precautions that might >>> warrant. I am also considering making a sine wave output and maybe other >>> frequencies. >>> Tom >>> >>> On 9/26/2013 4:34 AM, Bob Camp wrote: >>>> Hi >>>> >>>> Standard high speed CMOS logic works pretty well. How crazy are you trying >>>> to get? >>>> >>>> Bob >>>> >>>> On Sep 26, 2013, at 1:48 AM, Tom Minnis <[email protected]> wrote: >>>> >>>>> I am working on a small clock distributor and wanted to get some ideas on >>>>> what works best for 10MHz and 1PPS driver circuits. I remember sifting >>>>> through the archives a year or so ago and tripped on some discussion of >>>>> this but I can't find it anymore. >>>>> Tom >>>>> _______________________________________________ >>>>> time-nuts mailing list -- [email protected] >>>>> To unsubscribe, go to >>>>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>>>> and follow the instructions there. >>>> _______________________________________________ >>>> time-nuts mailing list -- [email protected] >>>> To unsubscribe, go to >>>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>>> and follow the instructions there. >>>> >>> _______________________________________________ >>> time-nuts mailing list -- [email protected] >>> To unsubscribe, go to >>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>> and follow the instructions there. >> _______________________________________________ >> time-nuts mailing list -- [email protected] >> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. >> > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
