Hi John: When I use 3 x 47 Ohm resistors and 3 gates the rise time seems about the same, but the voltage into a 50 Ohm load increases from 2.5 V p-p to 3.6 v p-p, so I tend to agree that's the way to go.
Have Fun, Brooke John Ackermann N8UR wrote: >Hi Brooke -- > >I meant to say that there was a separate 47 ohm resistor on each of the >inverter outputs, with the far end of those resistors tied together. > >Last time this came up, there was an argument about why you wouldn't use >resistor values that in parallel would equal 50 ohms (e.g., 150 ohm if >three gates were used), but Tom Clark jumped in to explain the basis for >using the lower values -- I think it was mainly to ensure that you still > have a valid TTL level into 50 ohms, coupled with looking at the >internal resistance of the gate, and minimizing the ringing of the edge. > >In any event the scheme I described has been used with good success in >the TAC-2 for a bunch of years. When I get home tonight, I'll post >photos of the risetime that I'm seeing. > >John > >Brooke Clarke wrote: > > >>Hi John: >> >>I think you mean by "-- paralleling several sections of a 74AC04 hex >>inverter through 47 ohm resistors" directly connecting the outputs then >>using a series 47 Ohm resistor. But the post by Normand makes the good >>point that by using a seperate resistor on each gate the loading on the >>gate is reduced. So I could use a single 74AC04 chip with 150 Ohm >>resistors on each gate. I'll try that when I'm finished reading the mail. >> >>Have Fun, >> >>Brooke >> >>John Ackermann N8UR wrote: >> >> >> >> >>>Brooke Clarke wrote: >>> >>> >>> >>> >>> >>>>Hi: >>>> >>>>I've got the TVB 1 PPS PIC circuit working and would like to add an >>>>output IC to drive 50 a couple of Ohm cables. >>>>Need two outputs, the normal 1 PPS and also the 10 kHz output since I >>>>think it would work with the SRS app note for making 1,000 TI >>>>measurements in one second to better see small offsets. >>>> >>>>I seem to remember that there may be an internal resistance already in >>>>some chips so putting a 47 or 50 Ohm resistor in series may result in an >>>>impedance above 50 Ohms. Any thoughts? >>>> >>>>Thanks, >>>> >>>>Brooke Clarke >>>> >>>> >>>> >>>> >>>> >>>Hi Brooke -- >>> >>>The circuit I use is stolen from Tom Clark's TAC -- paralleling several >>>sections of a 74AC04 hex inverter through 47 ohm resistors. For the >>>current TADD projects, I'm using three sections that way, so each chip >>>gives me two outputs, and I use a 47 ohm SIP resistor pack to keep the >>>board space down. >>> >>>Tom came up with the 47 ohm value (though IIRC he used four sections in >>>parallel) based on compromising for maximum voltage into a 50 ohm >>>termination, and best rising edge. >>> >>>The 74AC chips don't have internal source resistors, but I think some of >>>the op amp distribution amp chips might. For square waves at these >>>frequencies, I'd use 74AC parts over an analog circuit. >>> >>>John >>> >>> >>>_______________________________________________ >>>time-nuts mailing list >>>time-nuts@febo.com >>>https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>> >>> >>> >>> >>> >>> >> >> > > >_______________________________________________ >time-nuts mailing list >time-nuts@febo.com >https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > > > > -- w/Java http://www.PRC68.com w/o Java http://www.pacificsites.com/~brooke/PRC68COM.shtml http://www.precisionclock.com _______________________________________________ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
