Hi Since you have a handle on the Q (and thus the peaking width) you can come up with a pretty good mental picture of what a value shift will or won't do. For those who can't stand changing out / padding one cap (that's all that you would ever need to do in the is case) LTSpice is indeed your friend.
Bob On Aug 9, 2013, at 9:08 PM, Robert LaJeunesse <[email protected]> wrote: > 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. _______________________________________________ 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.
