That Field-Strengh Meter again...

[John Woodgate]

I read in sci.electronics.design that mike <spamme0@juno.com> wrote (in
<400841E2.4030400@juno.com&gt about 'That Field-Strengh Meter again...',
on Fri, 16 Jan 2004:

The reason I'd do this is that a very long (unterminated) cable is NOT a
cpacitive load.
It can be APPROXIMATED to first order by a capacitor as long as the
cable is SHORT in relation to the highest frequency components to be
transmitted. A terminated cable can be approximated by a RESISTOR.

The terms of the question were that it IS a capacitive load. You have to
infer from that facts about the frequency etc. You may not challenge the
question, except on ambiguity or insufficiency.

Your move.

No, it's got boring.
--
Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
Interested in professional sound reinforcement and distribution? Then go to
http://www.isce.org.uk
PLEASE do NOT copy news posts to me by E-MAIL!

 

[James Meyer]

On Fri, 16 Jan 2004 05:51:46 -0800, mike <spamme0@juno.com> posted this:

So, come back from the deep end. The more you know about such things,
the more you instinctively create circuits without these kinds of
problems. It's stuff you don't learn in SPICE class.
mike

What I was trying to point out is that there are no general purpose
rules of thumb that can be followed without thinking.

For example:

We all know how important bypass capacitors are. Yet, I have seen
circuits that function perfectly with no bypass capacitors at all in evidence.
And I have seen circuits where bypass capacitors were employed with almost
religious fervor, sprinkled all over the design like holly water or garlic to
ward off vampires.

Not everybody is aware of the fact that bypass capacitors can look like
low impedance reactances, series resonant circuits, parallel resonant circuits,
or even inductors, depending on the particular capacitor and its application in
a particular circuit.

So a rule of thumb that every circuit should have power supplied through
bypass capacitors is useless unless the designer recognizes that a .1uF cap
might just be *worse* than no capacitor at all.

Jim

 

[Paul Burridge]

On Sat, 17 Jan 2004 00:45:08 GMT, James Meyer <jmeyer@nowhere.com>
wrote:

What I was trying to point out is that there are no general purpose
rules of thumb that can be followed without thinking.

For example:

We all know how important bypass capacitors are. Yet, I have seen
circuits that function perfectly with no bypass capacitors at all in evidence.
And I have seen circuits where bypass capacitors were employed with almost
religious fervor, sprinkled all over the design like holly water or garlic to
ward off vampires.

Not everybody is aware of the fact that bypass capacitors can look like
low impedance reactances, series resonant circuits, parallel resonant circuits,
or even inductors, depending on the particular capacitor and its application in
a particular circuit.

So a rule of thumb that every circuit should have power supplied through
bypass capacitors is useless unless the designer recognizes that a .1uF cap
might just be *worse* than no capacitor at all.

I've used mylars for the RF ground return in this circuit (and at
other points in the circuit too). I guess they're far from ideal in
that role, being quite inductive IIRC. Maybe I should have used 10n
plate ceramics which I do have *somewhere* about the place.
Once again, I have to apologise for the lack of any pictures of this
wonderful multiple oscillator. My ISP has been totally useless in
tracking down the problem but I've thought of a way around it which
involves re-photographing the board, printing out the pictures in hard
copy, scanning the pictures at a lower resolution with a desktop
scanner, transferring the files onto floppies, reloading them onto a
laptop and uploading the scans of the pictures instead of the pictures
themselves. What a performance! :-(
--

My opinion is worth what you've paid for it.

 

[Leon Heller]

Paul Burridge wrote:

On Sat, 17 Jan 2004 00:45:08 GMT, James Meyer <jmeyer@nowhere.com
wrote:


What I was trying to point out is that there are no general purpose
rules of thumb that can be followed without thinking.

For example:

We all know how important bypass capacitors are. Yet, I have seen
circuits that function perfectly with no bypass capacitors at all in evidence.
And I have seen circuits where bypass capacitors were employed with almost
religious fervor, sprinkled all over the design like holly water or garlic to
ward off vampires.

Not everybody is aware of the fact that bypass capacitors can look like
low impedance reactances, series resonant circuits, parallel resonant circuits,
or even inductors, depending on the particular capacitor and its application in
a particular circuit.

So a rule of thumb that every circuit should have power supplied through
bypass capacitors is useless unless the designer recognizes that a .1uF cap
might just be *worse* than no capacitor at all.


I've used mylars for the RF ground return in this circuit (and at
other points in the circuit too). I guess they're far from ideal in
that role, being quite inductive IIRC. Maybe I should have used 10n
plate ceramics which I do have *somewhere* about the place.

That's probably at least one cause of the problems. Mylars are OK for AF
but quite useless for RF.

Leon
--
Leon Heller, G1HSM
Email: aqzf13@dsl.pipex.com
My low-cost Philips LPC210x ARM development system:
http://www.geocities.com/leon_heller/lpc2104.html

 

[John Popelish]

Leon Heller wrote:

That's probably at least one cause of the problems. Mylars are OK for AF
but quite useless for RF.

That is quite a generalization. I think the construction method makes
a lot of difference. Mylars that have a lead contact at one end of a
wound pair of foils are hardly useful at the upper end of the audio
range (Greenies made in China). Stacked metalized films work well in
the MHz region. This is my favorite film capacitor:
http://www.maco.panasonic.co.jp/www-data/pdf/ABD0000/ABD0000CE8.pdf

--
John Popelish

 

[Active8]

On Sat, 17 Jan 2004 12:03:03 +0000, pb@osiris1.notthisbit.co.uk
said...

On Sat, 17 Jan 2004 00:45:08 GMT, James Meyer <jmeyer@nowhere.com
wrote:

What I was trying to point out is that there are no general purpose
rules of thumb that can be followed without thinking.

For example:

We all know how important bypass capacitors are. Yet, I have seen
circuits that function perfectly with no bypass capacitors at all in evidence.
And I have seen circuits where bypass capacitors were employed with almost
religious fervor, sprinkled all over the design like holly water or garlic to
ward off vampires.

Not everybody is aware of the fact that bypass capacitors can look like
low impedance reactances, series resonant circuits, parallel resonant circuits,
or even inductors, depending on the particular capacitor and its application in
a particular circuit.

So a rule of thumb that every circuit should have power supplied through
bypass capacitors is useless unless the designer recognizes that a .1uF cap
might just be *worse* than no capacitor at all.

I've used mylars for the RF ground return in this circuit (and at
other points in the circuit too). I guess they're far from ideal in
that role, being quite inductive IIRC. Maybe I should have used 10n
plate ceramics which I do have *somewhere* about the place.
Once again, I have to apologise for the lack of any pictures of this
wonderful multiple oscillator. My ISP has been totally useless in
tracking down the problem but I've thought of a way around it which
involves re-photographing the board, printing out the pictures in hard
copy, scanning the pictures at a lower resolution with a desktop
scanner, transferring the files onto floppies, reloading them onto a
laptop and uploading the scans of the pictures instead of the pictures
themselves. What a performance! :-(

Use the Bollocks function. If you cyber transpose the juxtaposition

of the weenie wanker factor and subtract the fudge packer packer
factor, you'll see that the flippin' floppy ain't flippin' the
flappers fast enough to stay out of flaturation and yer wastin'
effort and Jack time.

--
Best Regards,
Mike

 

[Leon Heller]

John Popelish wrote:

Leon Heller wrote:


That's probably at least one cause of the problems. Mylars are OK for AF
but quite useless for RF.


That is quite a generalization. I think the construction method makes
a lot of difference. Mylars that have a lead contact at one end of a
wound pair of foils are hardly useful at the upper end of the audio
range (Greenies made in China). Stacked metalized films work well in
the MHz region. This is my favorite film capacitor:
http://www.maco.panasonic.co.jp/www-data/pdf/ABD0000/ABD0000CE8.pdf

I was thinking of those green things Paul is using.

Leon
--
Leon Heller, G1HSM
Email: aqzf13@dsl.pipex.com
My low-cost Philips LPC210x ARM development system:
http://www.geocities.com/leon_heller/lpc2104.html

 

[John Woodgate]

I read in sci.electronics.design that Active8 <mTHISREMOVEcolasono@earth
link.net> wrote (in <MPG.1a73ae75c53c981f9898a1@news.west.earthlink.net>
) about 'That Field-Strengh Meter again...', on Sun, 18 Jan 2004:

Use the Bollocks function. If you cyber transpose the juxtaposition
of the weenie wanker factor and subtract the fudge packer packer
factor, you'll see that the flippin' floppy ain't flippin' the
flappers fast enough to stay out of flaturation and yer wastin'
effort and Jack time

A poor emulation of Genome. He should sue. (;-)
--
Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
Interested in professional sound reinforcement and distribution? Then go to
http://www.isce.org.uk
PLEASE do NOT copy news posts to me by E-MAIL!