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 <[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.
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