Paul,
If you are trying to keep things small, have you considered one of the high
speed versions of the 4046 PLL/OSC? Some manufacturers spec these up to 20
MHz. No coils. If you go with the LC, and you have any fixed capacitors in
parallel with the inductor, use decent capacitors, like mica or RF approved
ceramic. I have seen cheap ceramic caps meant for bypassing just not work in
applications like yours.

Tam
"Paul Burridge" wrote in message
...
On Fri, 19 Mar 2004 17:46:43 GMT, "Harold E. Johnson"
wrote:


I don't think you can get away
with short cuts at RF, especially if you're a non-expert.

3 MHz is practically DC. Why don't you ask one of your friends at the BBC
to
build it for you.

I was told 40Mhz is "practically DC" too. I guess it depends on where
you're coming from.
Actually I've dumped the factory inductor as suggested by Tom and
wound-up a large, air core job on 15mm plastic water pipe. It's made
a big difference. I'm happy to report I've now got the 5th! Could be a
little cleaner but who cares? Tom made a big deal out of the
importance of high-Q so it was the obvious thing to try. Fortunately,
it's worked. God knows how I'm going to squeeze this monster coil onto
the board, though! :-|
Can I infer from this experience that SMD inductors of over a few uH
are a waste of time?
--

The BBC: Licensed at public expense to spread lies.

On Sat, 20 Mar 2004 10:13:22 -0500, "Tam/WB2TT"
wrote:

Paul,
If you are trying to keep things small, have you considered one of the high
speed versions of the 4046 PLL/OSC? Some manufacturers spec these up to 20
MHz. No coils. If you go with the LC, and you have any fixed capacitors in
parallel with the inductor, use decent capacitors, like mica or RF approved
ceramic. I have seen cheap ceramic caps meant for bypassing just not work in
applications like yours.

That's a point well worth remembering, Tam: it's not just coils that
exhibit a Q-factor and just as much harm can be caused by using lossy
capactors in tuing applications, too.
The PLL suggestion is a good one, but I'm trying to keep things simple
for this fairly straightforward project. It's nice to be able to build
everything just from what's in the junkbox.
--

The BBC: Licensed at public expense to spread lies.

On Sat, 20 Mar 2004 10:13:22 -0500, "Tam/WB2TT"
wrote:

Paul,
If you are trying to keep things small, have you considered one of the high
speed versions of the 4046 PLL/OSC? Some manufacturers spec these up to 20
MHz. No coils. If you go with the LC, and you have any fixed capacitors in
parallel with the inductor, use decent capacitors, like mica or RF approved
ceramic. I have seen cheap ceramic caps meant for bypassing just not work in
applications like yours.

That's a point well worth remembering, Tam: it's not just coils that
exhibit a Q-factor and just as much harm can be caused by using lossy
capactors in tuing applications, too.
The PLL suggestion is a good one, but I'm trying to keep things simple
for this fairly straightforward project. It's nice to be able to build
everything just from what's in the junkbox.
--

The BBC: Licensed at public expense to spread lies.

On Fri, 19 Mar 2004 19:40:01 +0000, Paul Burridge
posted this:

Can I infer from this experience that SMD inductors of over a few uH
are a waste of time?

ANY part used to perform a function which it is not suited for is a
waste of time. It would be interesting to know which SMD inductors you used
that seemed to eat up the 40 MHz so effectively. I'd be willing to bet that a
quick look at the spec sheet would show why that particular part was a bad
choice for that particular job.

Jim

On Fri, 19 Mar 2004 19:40:01 +0000, Paul Burridge
posted this:

Can I infer from this experience that SMD inductors of over a few uH
are a waste of time?

ANY part used to perform a function which it is not suited for is a
waste of time. It would be interesting to know which SMD inductors you used
that seemed to eat up the 40 MHz so effectively. I'd be willing to bet that a
quick look at the spec sheet would show why that particular part was a bad
choice for that particular job.

Jim

On Sun, 21 Mar 2004 19:17:38 GMT, James Meyer
wrote:

On Fri, 19 Mar 2004 19:40:01 +0000, Paul Burridge
posted this:

Can I infer from this experience that SMD inductors of over a few uH
are a waste of time?

ANY part used to perform a function which it is not suited for is a
waste of time. It would be interesting to know which SMD inductors you used
that seemed to eat up the 40 MHz so effectively.

Eh? I've *never* used SMD inductors!
--

The BBC: Licensed at public expense to spread lies.

On Sun, 21 Mar 2004 19:17:38 GMT, James Meyer
wrote:

On Fri, 19 Mar 2004 19:40:01 +0000, Paul Burridge
posted this:

Can I infer from this experience that SMD inductors of over a few uH
are a waste of time?

ANY part used to perform a function which it is not suited for is a
waste of time. It would be interesting to know which SMD inductors you used
that seemed to eat up the 40 MHz so effectively.

Eh? I've *never* used SMD inductors!
--

The BBC: Licensed at public expense to spread lies.

Paul,

I probably should have included more details. A fellow I was working with
was trying to extract a clock signal from a synchronous data stream, and was
getting nowhere. We swept the frequency back and forth to be sure he was
tuned to resonance - he was. Changed the ceramic cap to mica, and everything
worked like a charm. We never analyzed why the ceramic did not work, but I
suspect it was because of the capacitance vs. applied voltage dependence. If
the cap had 6VDC on it, and he had a few mv of RF, I expect it would have
worked. Instead, he had 0 bias, and a couple of Volts p-p signal.

BTW, somebody mentioned powdered iron toroids. Sounds like a good idea.

Tam

Paul,

I probably should have included more details. A fellow I was working with
was trying to extract a clock signal from a synchronous data stream, and was
getting nowhere. We swept the frequency back and forth to be sure he was
tuned to resonance - he was. Changed the ceramic cap to mica, and everything
worked like a charm. We never analyzed why the ceramic did not work, but I
suspect it was because of the capacitance vs. applied voltage dependence. If
the cap had 6VDC on it, and he had a few mv of RF, I expect it would have
worked. Instead, he had 0 bias, and a couple of Volts p-p signal.

BTW, somebody mentioned powdered iron toroids. Sounds like a good idea.

Tam

On Sun, 21 Mar 2004 22:43:17 +0000, Paul Burridge
posted this:

On Sun, 21 Mar 2004 19:17:38 GMT, James Meyer
wrote:

On Fri, 19 Mar 2004 19:40:01 +0000, Paul Burridge
posted this:

Can I infer from this experience that SMD inductors of over a few uH
are a waste of time?

ANY part used to perform a function which it is not suited for is a
waste of time. It would be interesting to know which SMD inductors you used
that seemed to eat up the 40 MHz so effectively.

Eh? I've *never* used SMD inductors!

Sorry. When you said "this experience" I thought you meant "my
experience".

Carry on.

Jim