Thanks. Might end up more useful than the Pi-network approach I've used a few times before. I appreciate knowing of more tools that can be called upon to help with a design. I just wish the calculators had some way to deal with standard values (like TI's FilterPro). Its frustrating getting a 79pF result and wondering how an 82pF part works. Well, I guess that's what Spice is for...
Bob LaJeunesse >________________________________ > From: Bob Camp <[email protected]> >To: Discussion of precise time and frequency measurement <[email protected]> >Sent: Friday, August 9, 2013 6:41 PM >Subject: Re: [time-nuts] Modified Extron DA [WAS: Rb video] > > >Hi > >The simplest way to design it is to do a T matching network. Two inductors in >the top of the T and one cap to ground. Weather it's a filter or a match, it's >the standard three element T lowpass. > >The logic gate wants to "see" an inductor at high frequency. The T has an >input inductor and that keeps it happy (so would a step up L). Since it's a >three element match, you get to pick Z in, Z out, and Q. (with an L network >you just would get Z in and Z out). Simply design it for a low Q. Q of three >isn't a bad number. Anything up to 5 is practical with rational parts (no >tuning). The narrower bandwidth of the higher Q design will increase it's >sensitivity to temperature. The lower Q will have a smaller coil / lower >impedance above cutoff. If you have 18 to 20 dbm out, you can put a 6 to 8 db >pad on it. That will improve the broadband match into the cable. > >If you want to design it as a filter, everything still works pretty much the >same. It's still Zin / Zout and one other number with a three element network. >If you want to go to more elements, you can indeed get better filtering at the >cost of higher temperature sensitivity. With three elements the harmonics are >down > 60 db. That's plenty good enough…. > >LC match calculators (there are many others): > >http://www.changpuak.ch/electronics/calc_18.php >http://home.sandiego.edu/~ekim/e194rfs01/jwmatcher/matcher2.html > >Filter calculator: > >http://www.calculatoredge.com/electronics/bw%20tee%20low%20pass.htm > >If you plug the numbers into the calculators you can see what the match does >for you in terms of the inductor value. > >Why not design a flat passband filter? You are only interested in passing 10 >MHz. Attenuating other frequencies is not a problem and may be beneficial. The >bandwidth is not going to be small enough (with a low Q) to give you trouble. >The peaking of the filter gives you a steeper cutoff at harmonic frequencies. >It rolls off just like any filter, but it starts from a higher peak. > >With the T you can do any Zin / Zout ratio provided the Q is high enough. If >you want to do low power, set it up as a 100 ohm to 50 ohm or 200 ohm to 50 >ohm match. It's a pretty simple solution to the problem that is flexible >enough to get the job done. > >Bob > > >On Aug 9, 2013, at 5:24 PM, Robert LaJeunesse <[email protected]> >wrote: > >> Bob, I need some education. For a low-pass filter I think series L and shunt >> C. For two inductors that normally means 2-3 capacitors. If you use only >> one shunt capacitor is the second L in series with it (as a harmonic trap)? >> Can you point me to a design tool (or equation set) somewhere that shows how >> to choose values best to match the impedances? >> >> thanks, >> >> Bob LaJeunesse >> >> >> >>> ________________________________ >>> From: Bob Camp <[email protected]> >>> To: Discussion of precise time and frequency measurement >>> <[email protected]> >>> Sent: Friday, August 9, 2013 4:57 PM >>> Subject: Re: [time-nuts] Modified Extron DA [WAS: Rb video] >>> >>> >>> I still think that a distribution amp based on logic ic's is cheaper / >>> simpler / lower power / higher performance. A pair of NC7SZ125's will dump >>> 20 dbm into 50 ohms all day long running at 5.5 volts. Good isolation as >>> well. Do the lowpass filter right and the harmonics are not an issue. Two >>> coils / one cap plus dc blocking does it quite nicely. >>> >> _______________________________________________ >> 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.
