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.
