Some questions on IF transformers
 

I graduated from college back in the 1994, and even then we were admonished to
avoid magnetics whenever possible. Of course, these days I know better, but
as a result my academic coverage of IF transformers was non-existant.

I'm now trying to make up for that transgression. :-)

I've done a fair amount of reading and have a good understanding on how IF
transformers work, how they should be modeled, how to build them, etc. (Most
of the books that address this in detail are from the 1970s or older, it
seems...) I still have a few questions, though, that I'm hoping a few of the
older reads could help me out on. They a

1) The really big 450kHz IF transformers you see in tube sets... why did they
wind the coils in the form of "pancakes" rather than "the usual way"
(single-layer coils)? Is it just a consequence of needing lots of turns (to
get enough magnetizing inductance) but, for the coupling coefficient desired,
finding that you'd end up with, e.g,. a foot-long tranformer if you only used
a single layer?
2) I can readily see why you'd want a center-tapped primary, or a primary
with, say, a tap 10% "up" as a small feedback winding, but why do you get such
things as an IF transformer with 103 and 50 turns on the primary (on either
side of the tap) and then 27 turns on the secondary? (E.g.,
http://www.mouser.com/catalog/specsheets/XC-600014.pdf ). None of my books
address this, and the only thing that looks close on the web is this article:
http://hem.passagen.se/communication/ifcan.html . Is his conclusion, "by
tapping the transformer the Q value increases" the main reason?
3) Sticking a parallel capacitor on the primary to resonate out the
magnetizing inductance makes sense to me. I'm a little less clear on parallel
capacitors on both the primary and secondary -- a double-tuned arrangement.
Hagen's "Radio Frequency Electronics" assigns leakage inductance to the
secondary and then converts the resonating capacitor in parallel with your
load resistance back into a series circuit and, voila!, you now have a series
RLC circuit so clearly bandpass behavior... but this approach implies that you
could just use a *series* resonating capacitor on the secondary instead. Is
that correct? (I am aware that there are a handful of commonly used
transformer equivalent circuit models, you can transform magnetizing or
leakage inductances and losses from primary to secondary or vice versa at
will, etc.)
4) Anyone have pointers to good books or articles that ideally discuss some
actual design examples of the more complicated cases (weird primary turns
ratios, double-tuned circuits, etc.)? -- The ones I've found so far as the
simpler single-tuned case, just center-tapped, etc.

Thanks a lot... I appreciate the help here!

---Joel

Some questions on IF transformers
 

In article ,
Joel Koltner wrote:

2) I can readily see why you'd want a center-tapped primary, or a primary
with, say, a tap 10% "up" as a small feedback winding, but why do you get such
things as an IF transformer with 103 and 50 turns on the primary (on either
side of the tap) and then 27 turns on the secondary? (E.g.,
http://www.mouser.com/catalog/specsheets/XC-600014.pdf ). None of my books
address this, and the only thing that looks close on the web is this article:
http://hem.passagen.se/communication/ifcan.html . Is his conclusion, "by
tapping the transformer the Q value increases" the main reason?

Tranformers from the 1960's vintage for bipolar transistors were
tapped in strange ways. I don't remember any special use of taps
on tube era stuff.

3) Sticking a parallel capacitor on the primary to resonate out the
magnetizing inductance makes sense to me. I'm a little less clear on parallel
capacitors on both the primary and secondary -- a double-tuned arrangement.
Hagen's "Radio Frequency Electronics" assigns leakage inductance to the
secondary and then converts the resonating capacitor in parallel with your
load resistance back into a series circuit and, voila!, you now have a series
RLC circuit so clearly bandpass behavior... but this approach implies that you
could just use a *series* resonating capacitor on the secondary instead. Is
that correct? (I am aware that there are a handful of commonly used
transformer equivalent circuit models, you can transform magnetizing or
leakage inductances and losses from primary to secondary or vice versa at
will, etc.)

There's a discussion older ARRL handbooks about getting broad bandwidth
with dual tuned over-coupled IF transformers.

4) Anyone have pointers to good books or articles that ideally discuss some
actual design examples of the more complicated cases (weird primary turns
ratios, double-tuned circuits, etc.)? -- The ones I've found so far as the
simpler single-tuned case, just center-tapped, etc.

Digging out an copy of Terman's _Radio Engineer's Handbook_ (that I
bought at a used bookstore and never got around to reading), it has
a discussion of point 3 in the section on "tuned amplifiers", (which
appears to be what the ARRL was cribbing from).

Mark Zenier
Googleproofaddress(account:mzenier provider:eskimo domain:com)

Some questions on IF transformers
 

"Joel Koltner" wrote in message
...
I graduated from college back in the 1994, and even then we were admonished
to avoid magnetics whenever possible. Of course, these days I know better,
but as a result my academic coverage of IF transformers was non-existant.

I'm now trying to make up for that transgression. :-)

I've done a fair amount of reading and have a good understanding on how IF
transformers work, how they should be modeled, how to build them, etc.
(Most of the books that address this in detail are from the 1970s or
older, it seems...) I still have a few questions, though, that I'm hoping
a few of the older reads could help me out on. They a


[snip]

2) I can readily see why you'd want a center-tapped primary, or a primary
with, say, a tap 10% "up" as a small feedback winding, but why do you get
such things as an IF transformer with 103 and 50 turns on the primary (on
either side of the tap) and then 27 turns on the secondary? (E.g.,
http://www.mouser.com/catalog/specsheets/XC-600014.pdf ). None of my
books address this, and the only thing that looks close on the web is this
article: http://hem.passagen.se/communication/ifcan.html . Is his
conclusion, "by tapping the transformer the Q value increases" the main
reason?

Short answer: yes.

Long answer:

That IF transformer is intended for driving a stage with an input impedance
of 5K. When they say the primary impedance is 20k, they mean between pins 2
and 3. 50:27 = ~ 2:1 turns ratio = ~ 4:1 impedance ratio. So you connect
pin 2 to the supply and pin 3 to the collector of your transistor and it
sees a load of ~ 20k, ignoring coil losses. The 180pF tuning capacitance is
connected between pins 1 and 3. You could try to resonate it by connecting
a much larger cap (1.7nF !) between 2 and 3 and not use the 1-2 section at
all; but the smaller cap has higher stability, tighter tolerance and lower
loss.

The 20k and 5k impedances are dictated by the nature of bipolar transistors.

The required bandwidth and IF centre frequency fix loaded Q.

Dynamic impedance and loaded Q in turn determine the reactance needed at
resonance.

You could design an untapped 4:1 impedance ratio transformer to do the same
job; but you would have difficulty achieving the required Q.

Some questions on IF transformers
 

Thanks Andrew, that helps a lot!

(...I do think in the example I linked to --
http://hem.passagen.se/communication/ifcan.html -- his circuit's Q as drawn
actually increases 1/(0.67^2) times rather than 1/(0.33^2) times, though; I've
e-mailed the guy to ask.)

---Joel

Some questions on IF transformers
 

On 02/23/2011 06:43 PM, Joel Koltner wrote:

1) The really big 450kHz IF transformers you see in tube sets... why did
they wind the coils in the form of "pancakes" rather than "the usual
way" (single-layer coils)? Is it just a consequence of needing lots of
turns (to get enough magnetizing inductance) but, for the coupling
coefficient desired, finding that you'd end up with, e.g,. a foot-long
tranformer if you only used a single layer?

They do this to try to keep the self-resonance of the winding up above
the operating frequency. By spreading the winding out into series
connected pancakes the stray capacitances have a harder time shunting
large inductances. You see this kind of construction on RF chokes too.

Paul Probert

Some questions on IF transformers
 

"Joel Koltner" wrote


1) The really big 450kHz IF transformers you see in tube sets... why did
they wind the coils in the form of "pancakes" rather than "the usual way"
(single-layer coils)? Is it just a consequence of needing lots of turns
(to get enough magnetizing inductance) but, for the coupling coefficient
desired, finding that you'd end up with, e.g,. a foot-long tranformer if
you only used a single layer?


See

http://www.crystal-radio.eu/enkoppelfactor.htm

Some questions on IF transformers
 

"Paul Probert" wrote in message
...
On 02/23/2011 06:43 PM, Joel Koltner wrote:

1) The really big 450kHz IF transformers you see in tube sets... why did
they wind the coils in the form of "pancakes" rather than "the usual
way"?
They do this to try to keep the self-resonance of the winding up above the
operating frequency. By spreading the winding out into series connected
pancakes the stray capacitances have a harder time shunting large
inductances. You see this kind of construction on RF chokes too.

Ah, clever!

One I was looking at the other day has some fancy weaving involved as well; it
must have been quite an interesting machine that made them.

Poking around You Tube some I see that winding single-layer (no fancy waving)
coils is quite doable at home (the Tesla coil guys are quite into it). It's
pretty slick: http://www.youtube.com/watch?v=N3N-tw6OEXw .

Thanks for the help,
---Joel

Some questions on IF transformers
 

"John J" wrote in message
...
See
http://www.crystal-radio.eu/enkoppelfactor.htm

Thanks John.

What stymies me about the double-tuned transformer is this: If you look at
Hagen's exaplanation for how they can be modeled -- I've stuck a scan he
http://koltner.com/Hagen.png -- he's modeling it as the primary's parallel
capacitor is resonating with the magnetizing inductance of the IFT and the
secondary's parallel capacitor is resonating with the leakage inductance...
and this is then made more obvious if you use the high-Q approximation and
transform a parallel RC circuit into a series RC circuit. But if that's the
case... doesn't it seem as though the most straightforward way to use an IFT
would be to have a parallel capacitor on the primary and a *series* capacitor
on the secondary?

I realize that you can move impedances from one side of an IFT to another and
change the equivalent circuit model and so on and present numerous different
"views" that all end up with the same correct mathematical behavior to model
what are really just two coupled inductors, but still... does anyone using a
series resonating capacitor on their secondaries?

---Joel

Some questions on IF transformers
 

Answering myself he

"Joel Koltner" wrote in message
...
does anyone using a series resonating capacitor on their secondaries?

Googol books provides some pages from the "Electromagnetic Compatibility
Handbook" by Kenneth Kaiser
(http://books.google.com/books?id=nZz...9&lpg=RA1-PA19) and
states:

"There are four different configurations for the double-tuned transformer
based on the positions of the capacitors on the primary and secondary sides.
For example, one capacitor can be in series with the primary winding, whereas
another capacitor can be in parallel with the secondary winding. A few
reasons why one configuration is selected over another will be discussed after
a few circuits are analyzed."

....and so on...

This is Good To Know.

---Joel

Some questions on IF transformers
 

"Joel Koltner" wrote

What stymies me about the double-tuned transformer is this: If you look at
Hagen's exaplanation for how they can be modeled -- I've stuck a scan
he http://koltner.com/Hagen.png -- he's modeling it as the primary's
parallel capacitor is resonating with the magnetizing inductance of the
IFT and the secondary's parallel capacitor is resonating with the leakage
inductance... and this is then made more obvious if you use the high-Q
approximation and transform a parallel RC circuit into a series RC
circuit. But if that's the case... doesn't it seem as though the most
straightforward way to use an IFT would be to have a parallel capacitor on
the primary and a *series* capacitor on the secondary?

I realize that you can move impedances from one side of an IFT to another
and change the equivalent circuit model and so on and present numerous
different "views" that all end up with the same correct mathematical
behavior to model what are really just two coupled inductors, but still...
does anyone using a series resonating capacitor on their secondaries?

If you use a capacitor in series with the winding to create series resonant
circuit, then the impedance at resonance will be low.

For a tube IF, you really want high impedances at resonance.

In the plate circuit this is to minimise current draw.

The grid input of the preceeding stage is high impedance and is voltage
driven, therefore a parallel tuned circuit is ideal.

So hence the use of dual parallel tuned circuits.

Some questions on IF transformers
 

"John J" wrote

What stymies me about the double-tuned transformer is this: If you look
at Hagen's exaplanation for how they can be modeled -- I've stuck a scan
he http://koltner.com/Hagen.png -- he's modeling it as the primary's
parallel capacitor is resonating with the magnetizing inductance of the
IFT and the secondary's parallel capacitor is resonating with the leakage
inductance... and this is then made more obvious if you use the high-Q
approximation and transform a parallel RC circuit into a series RC
circuit. But if that's the case... doesn't it seem as though the most
straightforward way to use an IFT would be to have a parallel capacitor
on the primary and a *series* capacitor on the secondary?

I realize that you can move impedances from one side of an IFT to another
and change the equivalent circuit model and so on and present numerous
different "views" that all end up with the same correct mathematical
behavior to model what are really just two coupled inductors, but
still... does anyone using a series resonating capacitor on their
secondaries?

If you use a capacitor in series with the winding to create series
resonant circuit, then the impedance at resonance will be low.

For a tube IF, you really want high impedances at resonance.

In the plate circuit this is to minimise current draw.

The grid input of the preceeding stage is high impedance and is voltage
driven, therefore a parallel tuned circuit is ideal.

So hence the use of dual parallel tuned circuits.

I should have added

If you want to see an application for series tuned curcuits, although not in
a IF application, have a look at the base circuit of almost any VHF
single ended RF power amplifier using a bipolar transistor. The input
impedance of such amps is low hence the appropriate use of a series tuned
circuit.

Some questions on IF transformers
 

On Wed, 23 Feb 2011 16:43:10 -0800, Joel Koltner wrote:

I graduated from college back in the 1994, and even then we were
admonished to avoid magnetics whenever possible. Of course, these days
I know better, but as a result my academic coverage of IF transformers
was non-existant.

I'm now trying to make up for that transgression. :-)

I've done a fair amount of reading and have a good understanding on how
IF transformers work, how they should be modeled, how to build them,
etc. (Most of the books that address this in detail are from the 1970s
or older, it seems...) I still have a few questions, though, that I'm
hoping a few of the older reads could help me out on. They a

1) The really big 450kHz IF transformers you see in tube sets... why did
they wind the coils in the form of "pancakes" rather than "the usual
way" (single-layer coils)? Is it just a consequence of needing lots of
turns (to get enough magnetizing inductance) but, for the coupling
coefficient desired, finding that you'd end up with, e.g,. a foot-long
tranformer if you only used a single layer?

Googling "Pi-wound coil" may help. It was to reduce shunt capacitance,
as mentioned.

2) I can readily see why you'd want a center-tapped primary, or a
primary with, say, a tap 10% "up" as a small feedback winding, but why
do you get such things as an IF transformer with 103 and 50 turns on the
primary (on either side of the tap) and then 27 turns on the secondary?
(E.g., http://www.mouser.com/catalog/specsheets/XC-600014.pdf ). None
of my books address this, and the only thing that looks close on the web
is this article: http://hem.passagen.se/communication/ifcan.html . Is
his conclusion, "by tapping the transformer the Q value increases" the
main reason?

3) Sticking a parallel capacitor on the primary to resonate
out the magnetizing inductance makes sense to me. I'm a little less
clear on parallel capacitors on both the primary and secondary -- a
double-tuned arrangement. Hagen's "Radio Frequency Electronics" assigns
leakage inductance to the secondary and then converts the resonating
capacitor in parallel with your load resistance back into a series
circuit and, voila!, you now have a series RLC circuit so clearly
bandpass behavior... but this approach implies that you could just use a
*series* resonating capacitor on the secondary instead. Is that
correct? (I am aware that there are a handful of commonly used
transformer equivalent circuit models, you can transform magnetizing or
leakage inductances and losses from primary to secondary or vice versa
at will, etc.)

Are you talking transistor IF transformers, with one slug each, or tube-
type, with two? Two-slug IF transformers, with lightly coupled and
independently tuned coils, gave you two filter sections in one can.

4) Anyone have pointers to good books or articles that ideally discuss
some actual design examples of the more complicated cases (weird primary
turns ratios, double-tuned circuits, etc.)? -- The ones I've found so
far as the simpler single-tuned case, just center-tapped, etc.

Old radio texts. If you still get up to Portland from time to time, dig
through Powell's Technical books.

What are you trying to do? There's good reason for not wanting to have a
circuit with a bazillion tweaks that all have to be right for the thing
to be in tune.

--
http://www.wescottdesign.com

Some questions on IF transformers
 

Hi Tim,

"Tim Wescott" wrote in message
...
Are you talking transistor IF transformers, with one slug each, or tube-
type, with two?

One slug each. I hadn't remembered two two-slug types, but now that I think
about it, I think I have seen them.

Old radio texts. If you still get up to Portland from time to time, dig
through Powell's Technical books.

Good idea, will do.

What are you trying to do? There's good reason for not wanting to have a
circuit with a bazillion tweaks that all have to be right for the thing
to be in tune.

Yeah, although it's kinda a sad commentary that many an "FM receiver" today
doesn't have a tuned front end and, as such, actually performs rather wose
than radios from 30 years ago now, you know?

As for what I'm trying to do... mostly just fully understand how the cheap
little transistor radios they made up until a decade or so ago operated; I
sure couldn't have designed one at the point I graduated from college, and
these days I finally feel as though I'd have a decent shot at it. (On the
other hand, I did feel I could have designed a 6502 when I graduated from
college, and then some fancier superscalar processor by the time I finished
grad school -- so it's not like I didn't get anything out of it.)

(On the other hand, I *have* designed various radio transmitters and receivers
that work well, they were just done more at the MMIC/IC/MiniCircuits-parts
level rather than the "discrete transistor/coupled coil" level -- you know,
the kind of designs industry will actually *pay* you to do. :-) You probably
don't do many PID loops with op-amps anymore I expect?)

Thanks for the help,
---Joel

Some questions on IF transformers
 

On Mon, 28 Feb 2011 11:41:15 -0800, Joel Koltner wrote:

Hi Tim,

"Tim Wescott" wrote in message
...
Are you talking transistor IF transformers, with one slug each, or
tube- type, with two?

One slug each. I hadn't remembered two two-slug types, but now that I
think about it, I think I have seen them.

Old radio texts. If you still get up to Portland from time to time,
dig through Powell's Technical books.

Good idea, will do.

What are you trying to do? There's good reason for not wanting to have
a circuit with a bazillion tweaks that all have to be right for the
thing to be in tune.

Yeah, although it's kinda a sad commentary that many an "FM receiver"
today doesn't have a tuned front end and, as such, actually performs
rather wose than radios from 30 years ago now, you know?

As for what I'm trying to do... mostly just fully understand how the
cheap little transistor radios they made up until a decade or so ago
operated; I sure couldn't have designed one at the point I graduated
from college, and these days I finally feel as though I'd have a decent
shot at it. (On the other hand, I did feel I could have designed a 6502
when I graduated from college, and then some fancier superscalar
processor by the time I finished grad school -- so it's not like I
didn't get anything out of it.)

(On the other hand, I *have* designed various radio transmitters and
receivers that work well, they were just done more at the
MMIC/IC/MiniCircuits-parts level rather than the "discrete
transistor/coupled coil" level -- you know, the kind of designs industry
will actually *pay* you to do. :-) You probably don't do many PID loops
with op-amps anymore I expect?)

Does Mouser still sell them?

The point of the IF transformer was twofold: to provide selectivity, and
to give a good impedance match between stages. I think there must have
been a standard receiver design, as there just seemed to be one choice
for each coil -- this in spite of the fact that as soon as you start
juggling feedback and/or standing currents, you change the impedances,
and therefor the required transformer.

I think if I were going to design a broadcast-band receiver, I wouldn't
just re-do the old schematic from 1960 -- I'd start from a clean sheet of
paper, and see where I could go from there. (Actually, I think the first
thing that'd go onto that clean sheet of paper would be an ADC -- I'm a
luddite in a lot of ways, but not in how I'd like to see a receiver laid
out).

--
http://www.wescottdesign.com

Some questions on IF transformers
 

"Tim Wescott" wrote in message
...
Does Mouser still sell them?

Somewhat surprisingly, yes, they do. (Although of the many hundreds they have
listed in their system, it's only something like a couple dozen they actually
have stock of.)

The point of the IF transformer was twofold: to provide selectivity, and
to give a good impedance match between stages. I think there must have
been a standard receiver design, as there just seemed to be one choice
for each coil -- this in spite of the fact that as soon as you start
juggling feedback and/or standing currents, you change the impedances,
and therefor the required transformer.

Agreed -- and I additionally suspect that many people who made those receivers
didn't necessarily understand the design itself that well. I.e., if it
basically worked and wasn't clearly deaf, it became a product -- few if any
AM/FM receivers found on the shelves of, e.g., Sears listed things like their
sensitivity, adjacent channel rejection, etc. (From the flip side, though, I
suppose that's one of the things the FCC did: Coordinated frequencies and
power levels such that just about any radio would work "reasonably" well; it's
a very different market from, say, amateur radio where there's little or no
coordination of these parameters and the customer may very well want to try
to listen to some QRP station on 14.15MHz while there's some big gun blasting
away on 14.14MHz...)

I think if I were going to design a broadcast-band receiver, I wouldn't
just re-do the old schematic from 1960 -- I'd start from a clean sheet of
paper, and see where I could go from there.

I agree insofar as the actual design goes, but I like to study these older
technologies because I think it's all too easy to not realize just how good
the performance of some of the old designs were (for your new design you'd
like to start with specs that are hopefully some improvement or at least as
good as the old ones...), and also because a lot of the same *techniques* can
be applied to modern designs just as well as they could to old ones (e.g.,
varactor tuning is just an evolution of mechanical tuning, neutralization
applies just as much to BJTs as it does to tubes, etc.).

(Actually, I think the first
thing that'd go onto that clean sheet of paper would be an ADC -- I'm a
luddite in a lot of ways, but not in how I'd like to see a receiver laid
out).

That works, but consider that if you digitize the entire broadcast FM band at
once (all 20MHz of it), compared to a $20 superhet receiver:

-- Your weak signal sensitivity may be worse, since your dynamic range is
spread across the entire band rather than just what'll fit through an IF
filter.
-- You'll likely suck rather more power from a battery.
-- It'll probably cost more.
-- For the channels you can receive well, you'll have infinitely more options
on being able to reduce noise, change your audio bandwidth, recovery stereo,
decode RDS, an so on. :-)

---Joel

Some questions on IF transformers
 

On Thu, 03 Mar 2011 10:15:42 -0800, Joel Koltner wrote:

"Tim Wescott" wrote in message
...
Does Mouser still sell them?

Somewhat surprisingly, yes, they do. (Although of the many hundreds
they have listed in their system, it's only something like a couple
dozen they actually have stock of.)

The point of the IF transformer was twofold: to provide selectivity,
and to give a good impedance match between stages. I think there must
have been a standard receiver design, as there just seemed to be one
choice for each coil -- this in spite of the fact that as soon as you
start juggling feedback and/or standing currents, you change the
impedances, and therefor the required transformer.

Agreed -- and I additionally suspect that many people who made those
receivers didn't necessarily understand the design itself that well.
I.e., if it basically worked and wasn't clearly deaf, it became a
product -- few if any AM/FM receivers found on the shelves of, e.g.,
Sears listed things like their sensitivity, adjacent channel rejection,
etc. (From the flip side, though, I suppose that's one of the things
the FCC did: Coordinated frequencies and power levels such that just
about any radio would work "reasonably" well;

In fact, they did: if you pay attention to the analog TV channels and the
radio channels in any one market, you'll find that they're spaced apart
by at least one 'dead' channel, while those 'dead' channels are 'live' in
adjoining broadcast areas.

For instance, in Portland, Oregon the TV channels were 2, 6, 8, 10 and
12, while Salem got the odd-numbers.

it's a very different
market from, say, amateur radio where there's little or no coordination
of these parameters and the customer may very well want to try to
listen to some QRP station on 14.15MHz while there's some big gun
blasting away on 14.14MHz...)

A different problem, too: broadcast implies a few transmitters and lots
of receivers, so you hold down system costs by skewing the requirements
very heavily toward cheap receivers, even at the cost of efficient
bandwidth usage.

Contrast that with pre-broadcast satellite TV, where you save some bucks
in the satellites, at the cost of needing big dishes and sensitive
receivers. That's OK, because the dishes are few (before they started
getting into the hands of the consumers). Then look at the direct-to-
consumer satellite systems, where the dish is just two feet across, and
you have to assume that the satellite is way more expensive.

I think if I were going to design a broadcast-band receiver, I wouldn't
just re-do the old schematic from 1960 -- I'd start from a clean sheet
of paper, and see where I could go from there.

I agree insofar as the actual design goes, but I like to study these
older technologies because I think it's all too easy to not realize just
how good the performance of some of the old designs were (for your new
design you'd like to start with specs that are hopefully some
improvement or at least as good as the old ones...), and also because a
lot of the same *techniques* can be applied to modern designs just as
well as they could to old ones (e.g., varactor tuning is just an
evolution of mechanical tuning, neutralization applies just as much to
BJTs as it does to tubes, etc.).

True, but you need to know where the old techniques aren't going to serve
you well (e.g. the passives involved in neutralizing a transistor stage
to get a few more dB gain cost more than another stage).

(Actually, I think the first
thing that'd go onto that clean sheet of paper would be an ADC -- I'm a
luddite in a lot of ways, but not in how I'd like to see a receiver
laid out).

That works, but consider that if you digitize the entire broadcast FM
band at once (all 20MHz of it), compared to a $20 superhet receiver:

-- Your weak signal sensitivity may be worse, since your dynamic range
is spread across the entire band rather than just what'll fit through an
IF filter.
-- You'll likely suck rather more power from a battery. -- It'll
probably cost more.
-- For the channels you can receive well, you'll have infinitely more
options on being able to reduce noise, change your audio bandwidth,
recovery stereo, decode RDS, an so on. :-)

Good point. But getting things through an IF filter, _then_ going into
an ADC isn't a bad thing, unless you're trying for the absolute minimum
of power and circuit cost.

--
http://www.wescottdesign.com

Some questions on IF transformers
 

On Feb 25, 1:44*pm, "Joel Koltner"
wrote:
"Paul Probert" wrote in message

...

On 02/23/2011 06:43 PM, Joel Koltner wrote:

1) The really big 450kHz IF transformers you see in tube sets... why did
they wind the coils in the form of "pancakes" rather than "the usual
way"?
They do this to try to keep the self-resonance of the winding up above the
operating frequency. By spreading the winding out into series connected
pancakes the stray capacitances have a harder time shunting large
inductances. You see this kind of construction on RF chokes too.

Ah, clever!

One I was looking at the other day has some fancy weaving involved as well; it
must have been quite an interesting machine that made them.

Poking around You Tube some I see that winding single-layer (no fancy waving)
coils is quite doable at home (the Tesla coil guys are quite into it). * It's
pretty slick:http://www.youtube.com/watch?v=N3N-tw6OEXw.

Thanks for the help,
---Joel

Back as far as the 70's hand operated machines could be purchase for
winding coils. I think I remember seeing them in my old Allied
Electronics catalog.

Jimmie