SM Inductors As BP Circuit
 

I'm seeking information about using 2 or more closely spaced SM
inductors (unshielded - not torid or shielded) in a bandpass circuit.

The application I'm thinking of would use two closely spaced SM
inductors and varicaps to tune both the center frequency and bandpass
and allow simple and repeatable construction without the need to
locate and wind torids or find specialty inductors.

Please - experiences and not pure speculation.

Thanks,
Bill - WB1GOT

SM Inductors As BP Circuit
 

The FAR circuits 9850 VFO board uses SM parts for the filter - elliptical
not bandpass, but it seems to work fairly well. The inductors aren't right
against each other, but they are pretty close and in the same orientation
(not my idea of a good plan). There is a picture of the arrangement at
http://www.qsl.net/wb8rcr/AD9850.html

...

"Bill Powell" wrote in message
...
I'm seeking information about using 2 or more closely spaced SM
inductors (unshielded - not torid or shielded) in a bandpass circuit.

The application I'm thinking of would use two closely spaced SM
inductors and varicaps to tune both the center frequency and bandpass
and allow simple and repeatable construction without the need to
locate and wind torids or find specialty inductors.

Please - experiences and not pure speculation.

Thanks,
Bill - WB1GOT

SM Inductors As BP Circuit
 

Bill Powell wrote:
I'm seeking information about using 2 or more closely spaced SM
inductors (unshielded - not torid or shielded) in a bandpass circuit.

The application I'm thinking of would use two closely spaced SM
inductors and varicaps to tune both the center frequency and bandpass
and allow simple and repeatable construction without the need to
locate and wind torids or find specialty inductors.

Please - experiences and not pure speculation.

Thanks,
Bill - WB1GOT

I have gotten 80 dB rejection from 80 MHz to 500 MHz (the limit of the
network analyzer) in a 23 MHz 7th order butterworth filter I built
using 4 surface mount inductors in a 20 mm line. I would give it a try.

--
Darrell Harmon
http://dlharmon.com

SM Inductors As BP Circuit
 

Yes, it can be done. Murata especially has a family of wire wound coils
size 1002 , unshielded, which are quite good at that. i designed
bandpass filters around 434 MHz with losses of about 1dB per dual
sections filters. point is that you can't calculate the coupling with
areasonable degree of accuracy, so you have to experiment with it. But,
once found, they are very repeatable. in this design the inductors were
22nH.
the long axes of the inductors are supposed to be parallel, since you
are interested in the magnetic coupling between them.
Saandy 4Z5KS

SM Inductors As BP Circuit
 

From: Bill Powell on Feb 5, 12:41 pm

I'm seeking information about using 2 or more closely spaced SM
inductors (unshielded - not torid or shielded) in a bandpass circuit.

The application I'm thinking of would use two closely spaced SM
inductors and varicaps to tune both the center frequency and bandpass
and allow simple and repeatable construction without the need to
locate and wind torids or find specialty inductors.

Please - experiences and not pure speculation.

Experience with filters here indicates that EVERY filter design
BEGINS with speculation. :-)

Bandpass filters can be designed for specific bandpass without
using inductive coupling. Those with percentage bandwidths
less than 5% can be tuned around the bandcenter without much
change in bandwidth. What bandcenter and bandwidth were you
considering?

From a scan of several different manufacturers of SMD inductors,
they don't specify any coupling so it would be experiment time
to find it out as one has already observed in here. I've found
that trying to determine coefficient of coupling by experiment
is more difficult than designing and tweaking a non-coupling
filter. Mileage varies.

If you want a small program that does passive (non-inductive
coupling) L-C filters (low, high, band, reject) synthesis and
analysis (with ability to set individual component values and
simulate Q), ask me in e-mail for LCie4. Ain't flashy but it
works. Freeware.

Small-quantity toroid forms are available at reasonable prices
quickly from www.kitsandparts.com. For home use, I like Neil
Hecht's (www.aade.com) little L-C Meter for much-easier
zeroing-in on inductance values when winding toroids along
with checking out fixed, small-value capacitors.

SM Inductors As BP Circuit
 

wrote in message
oups.com...

Bandpass filters can be designed for specific bandpass without
using inductive coupling.

hmmm ... I had lept to the conclusion that OP was concerned about undesired
coupling, rather than desired coupling.

I think SMD inductors are typically designed so as to not maximize their
susceptibility to objects in their vicinity, although not necessarily
minimize it.

Another issue of concern is self-resonance. Some SMD inductors have rather
surprisingly low self-resonant frequencies. Obviously, some designed for
cell phone service typically won't, but it is yet another parameter to fret
over.

...

SM Inductors As BP Circuit
 

A bandpass filter can be designed as two coupled L and C tuned
circuits.

The two circuits are coupled together via the mutual inductance
between the two spaced-apart coils. As in a 455 KHz interstage
transformer in a screening can.

The degree of coupling is approximately 1/Sqrt(Q1*Q2), where Q1 and Q2
are the tuned-circuit Q's which are determined by the filter's
terminating resistances.

Closer coupling increases the bandwidth.

For details see Terman.
----
Reg.

SM Inductors As BP Circuit
 

From: xpyttl on Feb 6, 12:19 pm

wrote in message

Bandpass filters can be designed for specific bandpass without
using inductive coupling.

hmmm ... I had lept to the conclusion that OP was concerned about undesired
coupling, rather than desired coupling.

To make an inductively-coupled BPF one IS concerned about DESIRED
coupling. That's how all those "IF cans" have been designed for
about the last 65 years (give or take). EXACT desired magnetic
field coupling.

Two-resonator bandpass filters (narrow bandwidth relative to
center frequency) can be done with "inductive" (magnetic field
coupling), capacitive coupling in two ways, or inductor coupling
(without magnetic field coupling) in two ways. Kind of old hat.
I prefer the non-mag-field approach since I LIKE using toroid
inductors and do NOT want any magnetic field coupling in there.
Makes the packaging more flexible; i.e., no sweat.

I think SMD inductors are typically designed so as to not maximize their
susceptibility to objects in their vicinity, although not necessarily
minimize it.

I've never done a mind-meld with any SMDs but I have looked in
a lot of databooks, seen a lot of datasheets. SMD inductors
are designed for inductance, Q, and current-carrying capacity.
Unless they are SPECIALS, shielded, or some other thing stated,
no one knows what their magnetic field is unless they try it
out. Or discuss it with the SMD makers.

If you want to find out the magnetic field coupling, the setups
have been described in the better texts for decades. It is a
lot of trouble to do but that time can be amortized nicely if
one is making thousands of identical IF cans (narrow percentage
bandwidth bandpass filters).

Another issue of concern is self-resonance.

NS? :-) That's a built-in concern every time one designs
filters of any kind. Try a highpass filter and see where
inductor (or capacitor) self-resonance messes up response.

A two-resonator type of narrow bandwidth bandpass filter
DEPENDS on resonance. Self-resonance implies a parallel
capacitance component equivalent on the inductor. That, in
turn, dictates the exact choice of fixed or trimmable
capacitor one adds across the inductor.

Some SMD inductors have rather
surprisingly low self-resonant frequencies.

Quantitatively define "surprisingly," sir. That is
a null statement. Show some examples of SMD inductors'
self-resonance versus other types of inductors not SMT.
Crib from existing data if you like, some does exist.

Obviously, some designed for
cell phone service typically won't, but it is yet another parameter to fret
over.

Are you designing cellular telephones? There is PLENTY of OTHER
applications at UHF where some small, two-resonator BPFs can be
used. Or at VHF. Or at HF. Even at MF and LF.

"Designed for cell phone service" implies SMT, obviously. Cell
phones DEMAND smallness in the market. HOW they eventually get
so small doesn't depend solely on physical package sizes or the
other factors you allude to. I like jumping but not doing so
on conclusions.

I've been sharp and nasty in this reply. I've also skim-read a
lot of inane and time-wasting blather on the "FAQs" cross-
posted to a bunch of newsgroups...that should not have been.
I'm tired of a bunch of self-righteous arguers busy, busy
arguing for the love of arguing. :-(

In HOMEBREW the replies ought to concern themselves with more
factual responses, not cut-and-paste generalities nor inability
to understand the original question. That's the best help for
anyone who asks questions.

SM Inductors As BP Circuit
 

I have done a 70 MHz LPF with SM inductors, though the physical spacing
wasnt close. Performance agreed roughly with theoretical, though I must
admit I did not test rigorously.
Closer coupling will widen bandwidth/lower Q.
You could use a small capacitor between parallel tuned sections as
coupling, and capacitive dividers as impedance matching
Richard

Reg Edwards wrote:
A bandpass filter can be designed as two coupled L and C tuned
circuits.

The two circuits are coupled together via the mutual inductance
between the two spaced-apart coils. As in a 455 KHz interstage
transformer in a screening can.

The degree of coupling is approximately 1/Sqrt(Q1*Q2), where Q1 and Q2
are the tuned-circuit Q's which are determined by the filter's
terminating resistances.

Closer coupling increases the bandwidth.

For details see Terman.
----
Reg.