Hi Bob, On 09/27/2013 01:44 PM, Bob Camp wrote: > 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 <tom_min...@att.net> 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. Indeed. I would assume that temperature and power supply voltage be the major contributors, and both can be handled if you care 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). Isolation is indeed something to care about. Just consider the effect of someone connecting or disconnecting a cable. The unconnected output will see the energy bounce back while the connected (and destination loaded) output will transmit energy out. As you swap between these, the isolation steers how well that is hidden from the other outputs. In a precision environment, bad isolation essentially means you can't touch it when it is operating. With isolation you can connect and unconnect more freely even if the other outputs is operational.
A customer once asked if they could passively divide a signal (which is wide-band) using a BNC-T. For the production environment they where going to install, I more or less forbid them to do that. They where thinking MTBF and cheap solution, but the lack of isolation means that *any* fault will potentially kill both legs and there would be no maintenance possible. Their counter-argument was that "well, if you cut the cables to the wavelength" and I then pointed out that it only works for sine and narrow-band signals, but this wide-band signal will not handle it well. They took my advice to the best of my knowledge. Cheers, Magnus _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
