Hi If you are buying NPO caps that are +/- 20%, get another supplier….
Bob On Aug 9, 2013, at 10:27 PM, briana <als...@nc.rr.com> wrote: > A cap marked 82pf might indeed be 79pf or any value 15-20% either side of the > marked value. > Depends upon what cap type you use. If you really need 79pf, buy a couple > dozen 82 pf caps and select one based upon measurement. Be aware that the > measure may be off by 10% too. > > Regards, > Brian > > On 8/9/2013 8:08 PM, Robert LaJeunesse 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 <li...@rtty.us> >>> To: Discussion of precise time and frequency measurement >>> <time-nuts@febo.com> >>> 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 <rlajeune...@sbcglobal.net> >>> 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 <li...@rtty.us> >>>>> To: Discussion of precise time and frequency measurement >>>>> <time-nuts@febo.com> >>>>> 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 -- time-nuts@febo.com >>>> To unsubscribe, go to >>>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>>> and follow the instructions there. >>> _______________________________________________ >>> 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. >>> >>> >>> >> _______________________________________________ >> 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. >> >> ----- >> No virus found in this message. >> Checked by AVG - www.avg.com >> Version: 2012.0.2242 / Virus Database: 3211/6064 - Release Date: 08/09/13 > > _______________________________________________ > 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. _______________________________________________ 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.
