In a coupling circuit such as:

C1|| C3||
IN----||----o----o----||--OUT
|| | | ||
--- C
--- C L1
C2| C
| |
'----'
|
===
GND

(see, eg, the "38 Special" NorCal 30m QRP tranceiver sch connecting
the NE602 single-ended output to an amplification stage:
http://www.amqrp.org/kits/38spcl/ )

What is the purpose maximizing the Q of L1 by using, say, a hand-
wound toroid vs a molded inductor? Rs of a 1.8uH moulded inductor
might be 1.5 ohms vs .1 ohm for 5 inches of 24ga wire. However the
expected effect in tank bandwidth doesn't seem to matter compared to
the large effect of varying C1 (very narrow for small C1, 5p in the
example).

I can see why a homebrewer would prefer to keep a bag of T37-2 and
some enamelled wire around, but in a kit such as that, wouldn't a
moulded inductor do just as well?

--
Ben Jackson AD7GD

http://www.ben.com/

The much lower Q of the molded inductor can result in significantly
worse rejection of spurious outputs from the amplifier. I don't know
whether that's important for this particular design or not. But a toroid
also has other advantages over a solenoidal inductor. A toroid has a
much smaller external field, so it can be mounted close to other
components including other inductors with minimal mutual coupling. For
the same reason, a solenoid's Q can be degraded substantially by
proximity to other components or conductors, while a toroid is
relatively immune to this problem.

Roy Lewallen, W7EL

Ben Jackson wrote:
In a coupling circuit such as:

C1|| C3||
IN----||----o----o----||--OUT
|| | | ||
--- C
--- C L1
C2| C
| |
'----'
|
===
GND

(see, eg, the "38 Special" NorCal 30m QRP tranceiver sch connecting
the NE602 single-ended output to an amplification stage:
http://www.amqrp.org/kits/38spcl/ )

What is the purpose maximizing the Q of L1 by using, say, a hand-
wound toroid vs a molded inductor? Rs of a 1.8uH moulded inductor
might be 1.5 ohms vs .1 ohm for 5 inches of 24ga wire. However the
expected effect in tank bandwidth doesn't seem to matter compared to
the large effect of varying C1 (very narrow for small C1, 5p in the
example).

I can see why a homebrewer would prefer to keep a bag of T37-2 and
some enamelled wire around, but in a kit such as that, wouldn't a
moulded inductor do just as well?

Ben,

Don't confuse the 'unloaded' Q of the part with the 'loaded Q' of the
circuit.

The unloaded Q of the part is a measure of internal resistance in the part
and will mainly affect the loss of the network, not the bandwidth. The
loaded Q of the network is a measure of the external network resistance that
the part is embedded in and will mainly determine the bandwidth.
The ratio of unloaded Q to loaded Q will determine the losses of the network
(higher unloaded Q is better).

In general, high unloaded component Q is 'goodness'.

Joe
W3JDR


"Ben Jackson" wrote in message
...
In a coupling circuit such as:

C1|| C3||
IN----||----o----o----||--OUT
|| | | ||
--- C
--- C L1
C2| C
| |
'----'
|
===
GND

(see, eg, the "38 Special" NorCal 30m QRP tranceiver sch connecting
the NE602 single-ended output to an amplification stage:
http://www.amqrp.org/kits/38spcl/ )

What is the purpose maximizing the Q of L1 by using, say, a hand-
wound toroid vs a molded inductor? Rs of a 1.8uH moulded inductor
might be 1.5 ohms vs .1 ohm for 5 inches of 24ga wire. However the
expected effect in tank bandwidth doesn't seem to matter compared to
the large effect of varying C1 (very narrow for small C1, 5p in the
example).

I can see why a homebrewer would prefer to keep a bag of T37-2 and
some enamelled wire around, but in a kit such as that, wouldn't a
moulded inductor do just as well?

--
Ben Jackson AD7GD

http://www.ben.com/

For a quick graphic demonstration of what the other posters are
pointing out without having to build and measure a physical circuit,
try putting this circuit into "RFSim99" (a free program), and select
"Use physical model" in the inductor value dialog box. Then change the
unloaded Q of the inductor there to see the effects on performance,
both loss and filter sharpness (-- rejection of unwanted signals).
You'll need to connect "IN" and "OUT" to measurement ports (one of the
available components in RFSim99) that you set to appropriate
impedances.

Such a simulation doesn't address the effects that Roy mentioned of
coupling to external fields and lowering of Q by coupling to external
material. On the other hand, though toroid coils have lower coupling
to externals, their coupling isn't zero, especially when you use a
low-mu core---so be careful how you mount the toroids, too. In a
couple minutes, the simulation can give you some insights to get you
started and save a lot of time soldering, but expect to ultimately
learn even more by actually building and measuring the circuit.

Cheers,
Tom


Ben Jackson wrote:
In a coupling circuit such as:

C1|| C3||
IN----||----o----o----||--OUT
|| | | ||
--- C
--- C L1
C2| C
| |
'----'
|
===
GND

(see, eg, the "38 Special" NorCal 30m QRP tranceiver sch connecting
the NE602 single-ended output to an amplification stage:
http://www.amqrp.org/kits/38spcl/ )

What is the purpose maximizing the Q of L1 by using, say, a hand-
wound toroid vs a molded inductor? Rs of a 1.8uH moulded inductor
might be 1.5 ohms vs .1 ohm for 5 inches of 24ga wire. However the
expected effect in tank bandwidth doesn't seem to matter compared to
the large effect of varying C1 (very narrow for small C1, 5p in the
example).

I can see why a homebrewer would prefer to keep a bag of T37-2 and
some enamelled wire around, but in a kit such as that, wouldn't a
moulded inductor do just as well?

--
Ben Jackson AD7GD

http://www.ben.com/

On 2006-07-10, K7ITM wrote:
try putting this circuit into "RFSim99" (a free program)

Thanks, I'd been looking for something like that.

unloaded Q of the inductor there to see the effects on performance,
both loss and filter sharpness (-- rejection of unwanted signals).

I see. So the reason that the Q matters so much is not that it affects
the filter shape much (in this topology), but that it has a big effect
on the amplitude of the (tiny) passband which provides additional
separation from the unwanted signals. In this case it looks like
raising from Q=50 to 250 gives about 4dB of "wanted" signal without
really affecting anything else.

couple minutes, the simulation can give you some insights to get you
started and save a lot of time soldering, but expect to ultimately
learn even more by actually building and measuring the circuit.

I actually have such a circuit built, and I took some measurements
today. I'll wind a 1.8uH inductor on a T37-2 tonight and compare.

Thanks to all for the responses.

--
Ben Jackson AD7GD

http://www.ben.com/

On 2006-07-10, Ben Jackson wrote:

I actually have such a circuit built, and I took some measurements
today. I'll wind a 1.8uH inductor on a T37-2 tonight and compare.

Ok, I wound 20 turns of about 30ga wire on a T30-2 and compared before/
after (each with the trim cap set for peak) and it's about 2.75V p-p
vs 2.0V p-p, or ~3dB. I'll put it on the spectrum analyzer at work
again to make sure the unwanted products are still in the same place,
but it looks like a win for high Q.

Thanks to everyone who replied, it was very educational!

Anyone have any tips for securing fine-wire toroids in
Ugly/Manhattan construction?

--
Ben Jackson AD7GD

http://www.ben.com/

Ben Jackson wrote:
. . .
Anyone have any tips for securing fine-wire toroids in
Ugly/Manhattan construction?

A dab of hot melt glue or RTV. Or a Nylon screw through the middle. Or a
couple of holes in the board and a cable tie. The only thing to avoid
is laying it down flat on a solid copper plane.

Roy Lewallen, W7EL

On Tue, 11 Jul 2006 08:29:05 +0100, Roy Lewallen wrote:

Ben Jackson wrote:
. . .
Anyone have any tips for securing fine-wire toroids in
Ugly/Manhattan construction?

A dab of hot melt glue or RTV. Or a Nylon screw through the middle. Or a
couple of holes in the board and a cable tie. The only thing to avoid
is laying it down flat on a solid copper plane.

Roy Lewallen, W7EL

Roy

Excuse my ignorance, but what is the problem with laying a toroid down on
a solid copper plane? I thought the magnetic field was contained within
the toroid and thus minimised external effects. I have completed a couple
of projects recently with some of the inductors like that and didn't
notice any real problem. Mind you, I guess they might have performed
better if not mounted that way )

Best 73

John, G4GOY

--
Using Opera's revolutionary e-mail client: http://www.opera.com/mail/

On Tue, 11 Jul 2006 13:02:48 +0100, "John Hague"
wrote:

On Tue, 11 Jul 2006 08:29:05 +0100, Roy Lewallen wrote:

Ben Jackson wrote:
. . .
Anyone have any tips for securing fine-wire toroids in
Ugly/Manhattan construction?

A dab of hot melt glue or RTV. Or a Nylon screw through the middle. Or a
couple of holes in the board and a cable tie. The only thing to avoid
is laying it down flat on a solid copper plane.

Roy Lewallen, W7EL

Roy

Excuse my ignorance, but what is the problem with laying a toroid down on
a solid copper plane? I thought the magnetic field was contained within
the toroid and thus minimised external effects. I have completed a couple
of projects recently with some of the inductors like that and didn't
notice any real problem. Mind you, I guess they might have performed
better if not mounted that way )

Twofold. One is added capacitance across the windings and secondary
is the proximitry of a conductor to the small field around the wire(s)
that are not in direct contact with the toroid.

Allison



Best 73

John, G4GOY

John Hague wrote:
On Tue, 11 Jul 2006 08:29:05 +0100, Roy Lewallen wrote:

Ben Jackson wrote:
. . .
Anyone have any tips for securing fine-wire toroids in
Ugly/Manhattan construction?

A dab of hot melt glue or RTV. Or a Nylon screw through the middle. Or
a couple of holes in the board and a cable tie. The only thing to
avoid is laying it down flat on a solid copper plane.

Roy Lewallen, W7EL

Roy

Excuse my ignorance, but what is the problem with laying a toroid down
on a solid copper plane? I thought the magnetic field was contained
within the toroid and thus minimised external effects. I have completed
a couple of projects recently with some of the inductors like that and
didn't notice any real problem. Mind you, I guess they might have
performed better if not mounted that way )

Two potential problems. One is that the field isn't completely
contained. Leakage is greater with more sparsely wound toroids and ones
with lower permeability cores. The second is the "one turn effect" -
There's a net field equivalent to that of a single turn running
circumferentially around the core. A solid plane parallel to this would
act as a shorted turn. Both effects would act to lower the Q, and might
be the cause of some drift or microphonics if the inductor was in an
oscillator tank.

But to be honest, I've never run any experiments to see just how much of
a problem it might cause -- it's quite possible you could get away with
it in some or even most applications. I'll put it on my list of things
to do when time permits -- unless somebody else is willing to take on
the job.

Roy Lewallen, W7EL