Bypass capacitor values at RF

[Paul Burridge]

Hi chaps,

I'm just starting to build Frank's circuit for the VCO he kindly
designed for me for 40Mhz. Frank's chosen values for bypassing are all
22nF caps; but I'd like to use 10nF ones if possible, since I've got
200 of them and no 22s whatsoever. I can't see a problem with this but
thought I'd best check with the Panel first. Can any far-sighted
individuals see a difficulty in this expedient?

Thanks awfully,

p.

 

[Active8]

Hi chaps,

I'm just starting to build Frank's circuit for the VCO he kindly
designed for me for 40Mhz. Frank's chosen values for bypassing are all
22nF caps; but I'd like to use 10nF ones if possible, since I've got
200 of them and no 22s whatsoever. I can't see a problem with this but
thought I'd best check with the Panel first. Can any far-sighted
individuals see a difficulty in this expedient?

Thanks awfully,

p.

they'll be half as effective. parallel two of your 10nF caps.

mike

 

[Michael Black]

Hi chaps,

I'm just starting to build Frank's circuit for the VCO he kindly
designed for me for 40Mhz. Frank's chosen values for bypassing are all
22nF caps; but I'd like to use 10nF ones if possible, since I've got
200 of them and no 22s whatsoever. I can't see a problem with this but
thought I'd best check with the Panel first. Can any far-sighted
individuals see a difficulty in this expedient?

Thanks awfully,

p.

I wasn't following the thread at all, but it seems like something smaller
has been traditionally common at 40MHz. More like .001uf aka 1nF.

Maybe there's something special about this design, but even if you stick
with the larger values I can't see a lot of different between 22nF
and 10nF. Likely more important is making sure the lead lengths are
small, and picking capacitors which don't have a lot of built in inductance.

Michael

 

[Paul Burridge]

I wasn't following the thread at all, but it seems like something smaller
has been traditionally common at 40MHz. More like .001uf aka 1nF.

That's the kind of value I'd have expected to see, but no doubt Frank
had good reason for specifying the larger ones.

Maybe there's something special about this design, but even if you stick
with the larger values I can't see a lot of different between 22nF
and 10nF. Likely more important is making sure the lead lengths are
small, and picking capacitors which don't have a lot of built in inductance.

The ones I've got are all Mylars. I've no idea how much paracitic
inductance they have and without access to a network analyser I'm
unable to tell what their SRF is. But I should be okay with Mylars,
surely?

 

[Watson A.Name - 'Watt Sun']

Hi chaps,

I'm just starting to build Frank's circuit for the VCO he kindly
designed for me for 40Mhz. Frank's chosen values for bypassing are all
22nF caps; but I'd like to use 10nF ones if possible, since I've got
200 of them and no 22s whatsoever. I can't see a problem with this but
thought I'd best check with the Panel first. Can any far-sighted
individuals see a difficulty in this expedient?

What often happens to RF byp caps is the inductance of the leads
resonates with the cap to form a series resonant circuit. Works great
at the Fr, but above or below it becomes less of a bypass. So it
depends on the capacitor, lead length, etc. Ceramic disks make good
bypass caps. You can find out by dipping it with the leads shorted.

Sso post the URL of the circuit so we can check it out. Sorry, no, I
haven't read other followups.

Thanks awfully,

p.

--
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[Watson A.Name - 'Watt Sun']

they'll be half as effective. parallel two of your 10nF caps.

Not necessarily. See my other followup.

mike

--
@@F@r@o@m@@O@r@a@n@g@e@@C@o@u@n@t@y@,@@C@a@l@,@@w@h@e@r@e@@
###Got a Question about ELECTRONICS? Check HERE First:###
http://users.pandora.be/educypedia/electronics/databank.htm
My email address is whitelisted. *All* email sent to it
goes directly to the trash unless you add NOSPAM in the
Subject: line with other stuff. alondra101 <at> hotmail.com
Don't be ripped off by the big book dealers. Go to the URL
that will give you a choice and save you money(up to half).
http://www.everybookstore.com You'll be glad you did!
Just when you thought you had all this figured out, the gov't
changed it: http://physics.nist.gov/cuu/Units/binary.html
@@t@h@e@@a@f@f@l@u@e@n@t@@m@e@e@t@@t@h@e@@E@f@f@l@u@e@n@t@@

 

[John Popelish]

Paul Burridge wrote:
(snip)

The ones I've got are all Mylars. I've no idea how much paracitic
inductance they have and without access to a network analyser I'm
unable to tell what their SRF is. But I should be okay with Mylars,
surely?

Not so surely. The cheap (usually green) mylars from China have their
wire leads attached to the ends of two metalized mylar strips that are
then wound around the pair of lead ends. They are more of a wound
transmission line than a lumped capacitor (Arrrggg!). Good quality
mylars have the metalization or foil extended from both ends of the
roll, and the leads attached such that there is a very short
conductance distance and low resistance from the lead to any part of
the capacitor. Crush one of yours, and see how it is made.

 

[Paul Burridge]

Paul,

The 8 MHz portion of the circuit (up through R5) is built on a 0.6
inch by 1.1 inch piece of double-sided FR-4 I snipped and modified
from a radio prototype that used a similar oscillator. It has four
connections: power and gnd, tuning input and 8 MHz out. There is
enough room to *dead-bug* an HC04 to the topside, and maybe build the
5x multiplier on the backside, which is mostly ground plane. The 5x
multiplier and HC04 were built on a separate *proto* board. The
circuit is the first link at:

http://www.aomwireless.com/resources/utilities.htm

The 8 MHz VCO PCB assembly has just been taking up space in my lab, so
I would be pleased to give it a good home. Just e-mail me with postal
info.

That's very good of you, Frank, but I'm determined to press ahead and
get this thing built over the next day or so as time is getting very
short now. Posting it would take too long, I'm afraid and I've done
all the layout stuff now anyway so am most of the way there.

I don't have an elegant answer to the capacitor question, other than I
have lots of 22 nF *0603* caps. If I had 10 nF (surface mount) those
would probably serve as well. I don't normally consider self-resonance
for *0603* SMT capacitors at 40 MHz and below. The series inductance
and transmission delay seem low enough to ignore for many HF

I'm sure they're the best solution from the performance side of
things, but I have to admit I'm extremely reluctant to get involved
with SMDs unless it's totally unavoidable!

 

[Reg Edwards]

"I believe history will be kind to me, since I intend

to write it." - Winston Churchill

---------------------------------------------

Winston admitted to being one of the most proficient warmongers of the
century.

It's time this sort of signature was abolished from the network.

 

[John Popelish]

---------------------------------------------

Winston admitted to being one of the most proficient warmongers of the
century.

It's time this sort of signature was abolished from the network.

Are you volunteering to be the standard maker and censor?

 

[Roger Johansson]

Are you volunteering to be the standard maker and censor?

Speaking of standard makers, does somebody know what has happened to
John Woodgate? I miss him.

 

[Paul Burke]

John Popelish wrote:




Speaking of standard makers, does somebody know what has happened to
John Woodgate? I miss him.

He seems to have taken up archaeology in a big way: his posts constitute
about a quarter of the Britarch mailing list. Cantankerous and sensible
as always.

Paul Burke

 

[Don Lancaster]

That's the kind of value I'd have expected to see, but no doubt Frank
had good reason for specifying the larger ones.


The ones I've got are all Mylars. I've no idea how much paracitic
inductance they have and without access to a network analyser I'm
unable to tell what their SRF is. But I should be okay with Mylars,
surely?

The frequency response of a bypass capacitor drops to near zero where it
resonates with its lead inductance, and then RISES with incresing
frequency after that.

You have to plot or RX bridge the capacitor to see what it does at the
frequencies of interest.

"Larger" capacitors are usually "worse".

--
Many thanks,

Don Lancaster
Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
voice: (928)428-4073 email: [email protected] fax 847-574-1462

Please visit my GURU's LAIR web site at http://www.tinaja.com

 

[Roger Johansson]

He seems to have taken up archaeology in a big way: his posts constitute
about a quarter of the Britarch mailing list. Cantankerous and sensible
as always.

Signs of a really alert brain, can change field of interest.

I'm thinking of taking up oil painting myself, after seeing some
painters course (by Bob Ross) on tv. It made me look at the world in a
new way, I see things I never saw before and I often think about how I
could paint things I see.

Bob Ross has hi-jacked my brain. And I like it.

 

[Paul Burridge]

---------------------------------------------

Winston admitted to being one of the most proficient warmongers of the
century.

It's time this sort of signature was abolished from the network.

People of your age should know better. You're talking like a Taffy,
which I assume you're not by your callsign. So you've no mitigation.

 

[Michael Black]

The frequency response of a bypass capacitor drops to near zero where it
resonates with its lead inductance, and then RISES with incresing
frequency after that.

You have to plot or RX bridge the capacitor to see what it does at the
frequencies of interest.

"Larger" capacitors are usually "worse".

And that's why in the old days, it was common in hobby circuits to
show .o1uFs up to a certain frequency, and then the switch to .001uFs
for the next range. Theoretically the .01uFs were still fine, indeed
they'd be even better the higher you went in frequency, with the
reactance getting ever smaller. But since they tended to be physically
larger, a switch to a lower value bypass was the norm, because the
inductance from it's leads would be (hopefully) lower.

Michael