RF transformer

[Robert Baer]

I understand, that with the proper winding configuration, that a 90
degree phase shift can be achieved between primary and resonant secondary.
That is done with a tapped secondary on a FM discriminator transformer.
What are the guidelines and requirements needed to construct such a
transformer at a given frequency OTHER than the standard 10.7Mhz?
Say, on a form in the one inch diameter region, and frequency below 1Mhz?
Ideal coil diameter, length based on chosen frequency?
Turns / inductance (no resonating cap)?

 

[Tim Williams]

I understand, that with the proper winding configuration, that a 90
degree phase shift can be achieved between primary and resonant
secondary.
That is done with a tapped secondary on a FM discriminator
transformer.
What are the guidelines and requirements needed to construct such a
transformer at a given frequency OTHER than the standard 10.7Mhz?
Say, on a form in the one inch diameter region, and frequency below
1Mhz?

You need LL high enough (k low enough) that the resonant tank, at whatever
Q it ends up at, doesn't draw an excessive amount of current (excessively
double-humping that stage's IF response), without being so high that the
signal is too small. Just basic IF transformer and bandpass filter stuff.

Ideal coil diameter, length based on chosen frequency?

Well, you generally get best results with a coil 0.5 to 2" long, pitch
twice the wire diameter. Assuming that the form is 1" as stated. That's
true of coils in general; were you expecting something specific to this
application?

Turns / inductance

Depends on circuit impedance, but easily figured from the nature of the
network.

(no resonating cap)?

As in, self resonant?

At <1MHz? You're going to need more than 1" diameter to get a resonator
down there. Especially with any useful Q.

Tim

 

[RobertMacy]

I understand, that with the proper winding configuration, that a 90
degree phase shift can be achieved between primary and resonant
secondary.
That is done with a tapped secondary on a FM discriminator
transformer.
What are the guidelines and requirements needed to construct such a
transformer at a given frequency OTHER than the standard 10.7Mhz?
Say, on a form in the one inch diameter region, and frequency below
1Mhz?
Ideal coil diameter, length based on chosen frequency?
Turns / inductance (no resonating cap)?

use free femm 4.2 to design coil

use LTspice to design 'circuit' based upon parameters determined from femm.

BOTH user groups, super helpful

 

[RobertMacy]

On a sunny day (Mon, 29 Jul 2013 05:56:05 -0700) it happened RobertMacy

It is actually really simple:
..excellent construction directives snipped...
So need more info.
In the same time you look that up you can try it out.

I like your, "JUST DO IT!" approach. Tens to result in working breadboards.

LTspice is only for self-justification and to show customers ;-)

When LTspice first started out, seemed a bit like that. But now, LTspice
has mutated into a generic super tool and has essentially scooped the
SPICE simulation market for ease of use, accuracy, access to support,
access to models [thanks to an intrepid individual in Russia, Alex
Bordodynov]

I use LTspice extensively for modeling EVERYTHING, from electromechanics
to cabling to EMC/shielding to ESD protection, even antenna designs, etc.
which contain NO Linear components.

And now with Mike's new updated version... On a particularly mind boggling
topology that I've been working with since 1989! LTspice predicted at
100Hz there would be 35nV/rtHz noise and the breadboard had an actual
MEASURED noise of 36nV/rtHz!!! Considering that S/N is the next frontier
for improvement in my circuitry, LTspice is USEFUL! I used to be reduced
to soldering/resoldering, head scratching until hair was gone, and then
repair repair repair the !@#$#!@#$ overworked breadboard; now, can sit and
explore all kinds of 'improvements' to the topology ...using LTspice. and
even better, keep a record of the attempts. Breadboards tend to get
lost/destroyed along the way.

If you're goinng to pick on simulation tools that are self serving, you
should go after National and their online tool for SMPS design, with so
many 'unknown to you' approximations you're likely;, as happened to me, to
get wrong results. And, Analog Devices who likes to make horrific,
'unstable' models, or TI who likes to make TINA-only models. I'll stick
with any company that has the courage to be 'opensource'.

 

[RobertMacy]

Oh, I use LTspice, usually for small sub-circuits,
but hey, human mind has a trillion ? of neurons (or it should be ;-)),
and more than 60 years E design experience in my case.

I know. I saw the 'smiley' I just used the opportunity to vent my spleen
on three entities that should know better.

Using my own brain? I pride myself on looking at a schematic and actually
view it 'moving' Instantly see what it does, where it's weak, etc. But,
this topology has driven me NUTS! Using my intuition to increase
bandwidth, the change reduces bandwidth. Increase gain, nothing happens -
constant gain! I sat with a tablet of equations for two days solid to no
avail, even with super simplifications. Now even with LTspice this !@#$#@!
circuit doesn't 'act' right! where the circuit should null and be the same
voltage, it's not! I'm starting to simply take the attitude, it works,
because it works.

I wind 100 faster than I can write this.
And 100 you know, as the grocer says when weighting the cheese:
Can it be a little more?
It is easy to remove a few turns... if you end up too high.

Now, now it works, you can ask the online calecujilator what L is for 1
MHz,
and put the in LTspice, and show it to customer,
he (the custardtomer) now feels much more secure after seeing
'mathematical proof'.

I carry 'data points too, especially the 'stacking factor' and coil cross
section size.

Hey I just had a discussion at CNN.com. and found out how to get rid of
the US deficit:
Advertising on dollar bills, on banknotes,
how much does google make ... FED can do it too.
Made available to all for free.
(And in that discussion I invented gambling on banknotes too).
So there you are, I like LTspice, in runs in wine (that is a windows
emulator for the microsof users),
so I can actually use it.
It is the best I have seen (LTspice, not wine).

Great ideas!! If Facebook can turn a profit of $480M in this economy then
the govt should be able to do the same thing!

Uh, who paid that $480M? and, did THEY make a profit? Who's spending this
money?

 

[Bill Martin]

That's already being done, sorta:
<http://www.tosscards.com/million.htm>
About $0.05/ea. One of the local Pizza houses was passing them out
recently.

However, it might be possible to do what the various baseball and
football stadiums have done for many years. Like renaming the stadium
after major sponsors, we should rename government buildings, programs,
and monuments after corporate sponsors. Instead of the white house,
it should be the Verizon house, or something equally disgusting.
Instead of naming airports and freeways after famous politicians, they
should be named after corporate sponsors. For example, the Starbucks
freeway.

"The United States of Walmart" has sort of a ring to it...

 

[Glenn]

That would be a Foster Seeley Discriminator, except there is no 90
phase shift involved:
<http://www.tpub.com/neets/book12/51c.htm>
A 90 degree phase shift demodulator would be a quadrature demodulator.

....

Hi Jeff & Robert

I hope I some day will design a Foster Seeley Discriminator or better
yet a Ratio detector. The Ratio detector because it has built-in limiter
functionality.

-

According to this site there is a 90 degree shift needed:
http://www.radio-electronics.com/in...n/foster-seeley-fm-detector-discriminator.php
Quote: "...
To obtain the different phased signals a connection is made to the
primary side of the transformer using a capacitor, and this is taken to
the centre tap of the transformer. This gives a signal that is 90
degrees out of phase.
...."

-

By chance I found a schematic some months ago, with a Ratio
Discriminator with AFC functionality. I did not imagine that it could,
because I read some decades ago that only a Foster Seeley was associated
with AFC functionality.

Here is the schematic with the Ratio Discriminator with AFC functionality:
http://www.radiohistoriskforeningvest.dk/Diagrammer/Blaupunkt-Derby de Luxe-7659700-1968-diagram.png
Main:
http://www.radiohistoriskforeningvest.dk/Diagrammer/Diagramsiden.php
(look for: Blaupunkt-Derby de Luxe-7659700-1968-diagram.png )

AFC goes through R931 (top-right) to left to R784 (AFC limited by V40C2
two diodes) through R782, R755 (top mid) and finally to BA124 - used as
varactor.


Ratio Discriminator:
http://www.radio-electronics.com/in...reception/ratio-fm-detector-discriminator.php
Quote: "...
When circuits employing discrete components were more widely used, the
Ratio and Foster-Seeley detectors were widely used. Of these the ratio
detector was the most popular as it offers a better level of amplitude
modulation rejection of amplitude modulation. This enables it to provide
a greater level of noise immunity as most noise is amplitude noise, and
it also enables the circuit to operate satisfactorily with lower levels
of limiting in the preceding IF stages of the receiver.
...."

/Glenn

 

[Tauno Voipio]

...

Hi Jeff & Robert

I hope I some day will design a Foster Seeley Discriminator or better
yet a Ratio detector. The Ratio detector because it has built-in limiter
functionality.

-

According to this site there is a 90 degree shift needed:
http://www.radio-electronics.com/in...n/foster-seeley-fm-detector-discriminator.php

Quote: "...
To obtain the different phased signals a connection is made to the
primary side of the transformer using a capacitor, and this is taken to
the centre tap of the transformer. This gives a signal that is 90
degrees out of phase.
..."

-

By chance I found a schematic some months ago, with a Ratio
Discriminator with AFC functionality. I did not imagine that it could,
because I read some decades ago that only a Foster Seeley was associated
with AFC functionality.

Here is the schematic with the Ratio Discriminator with AFC functionality:
http://www.radiohistoriskforeningvest.dk/Diagrammer/Blaupunkt-Derby de Luxe-7659700-1968-diagram.png

Main:
http://www.radiohistoriskforeningvest.dk/Diagrammer/Diagramsiden.php
(look for: Blaupunkt-Derby de Luxe-7659700-1968-diagram.png )

AFC goes through R931 (top-right) to left to R784 (AFC limited by V40C2
two diodes) through R782, R755 (top mid) and finally to BA124 - used as
varactor.


Ratio Discriminator:
http://www.radio-electronics.com/in...reception/ratio-fm-detector-discriminator.php

Quote: "...
When circuits employing discrete components were more widely used, the
Ratio and Foster-Seeley detectors were widely used. Of these the ratio
detector was the most popular as it offers a better level of amplitude
modulation rejection of amplitude modulation. This enables it to provide
a greater level of noise immunity as most noise is amplitude noise, and
it also enables the circuit to operate satisfactorily with lower levels
of limiting in the preceding IF stages of the receiver.
..."

/Glenn

The circuit (while quite correct) in not too carefully drawn:
the signal from the diodes charges the electrolytic capacitor
in reverse polarity.

 

[Robert Baer]

You need LL high enough (k low enough) that the resonant tank, at whatever
Q it ends up at, doesn't draw an excessive amount of current (excessively
double-humping that stage's IF response), without being so high that the
signal is too small. Just basic IF transformer and bandpass filter stuff.


Well, you generally get best results with a coil 0.5 to 2" long, pitch
twice the wire diameter. Assuming that the form is 1" as stated. That's
true of coils in general; were you expecting something specific to this
application?

* OK, i confess my madness here. 455KC FM discriminator transformer,
reasonably optimized for linear operation and max AM rejection.
That is my initial target (for further madness).

Depends on circuit impedance, but easily figured from the nature of the
network.


As in, self resonant?

* No; what should the target inductance be (of the coil without a
resonating cap) WRT frequency? That way i could calculate the needed
resonating cap.

 

[Robert Baer]

That would be a Foster Seeley Discriminator, except there is no 90
phase shift involved:
<http://www.tpub.com/neets/book12/51c.htm>

* I disagree; look at the phase diagrams.

 

[Robert Baer]

use free femm 4.2 to design coil

use LTspice to design 'circuit' based upon parameters determined from femm.

BOTH user groups, super helpful

Thanks.
Now digging into my old,dusty RCA Radiotron.

 

[Robert Baer]

It is actually really simple:
Find a nice C value, 220 pF is my favorite.
Use online calculator to find L for resonance, or estimate from Xperience.
Use standard coil formers (8 mm diameter, or 6 mm diameter, whatever),
with adjustable cores.
Add some turns (how about 100? for 1MHz),
apply your F from source or generator.
Tune for maximum.

Now make 2, put them, 1 cm next to each other,

* RCA Radiotron indicated tight coupling to achieve a double-humpped
response for better linearty and AM rejection.

 

[Robert Baer]

There's a detailed explanation of how to design an FM discriminator
for AFC in the Radiotron Designers Handbook 4th edition (1953) Pg
1152-1156.
<http://www.paleoelectronics.com/RDH4/CHAPTR29.PDF>
Note the concern for controlling the Q of both the primary and
secondary.

Ineresting; have recently been looking at my old dusty Radiotron pgs
1090-1097 (same version).
Q control _and_ coupling control.
A bit messy for one that has not wound any transformers for 25 years,
and those were power xfmrs.

 

[Robert Baer]

I know. I saw the 'smiley' I just used the opportunity to vent my spleen
on three entities that should know better.

Using my own brain? I pride myself on looking at a schematic and
actually view it 'moving' Instantly see what it does, where it's weak,
etc. But, this topology has driven me NUTS! Using my intuition to
increase bandwidth, the change reduces bandwidth. Increase gain, nothing
happens - constant gain! I sat with a tablet of equations for two days
solid to no avail, even with super simplifications. Now even with
LTspice this !@#$#@! circuit doesn't 'act' right! where the circuit
should null and be the same voltage, it's not! I'm starting to simply
take the attitude, it works, because it works.


I carry 'data points too, especially the 'stacking factor' and coil
cross section size.


Great ideas!! If Facebook can turn a profit of $480M in this economy
then the govt should be able to do the same thing!

Uh, who paid that $480M? and, did THEY make a profit? Who's spending
this money?

....and $480M is a drop in the $20+ trillion deficit.

 

[Fred Abse]

TI who likes to make TINA-only models.

They provide Pspice models, too.

Ones that actually work something like the actual devices.

I downloaded a Burr-Brown rail to rail instrumentation amp from TI today.
Remarkably like the datasheet says it should perform/

 

[Tim Williams]

The circuit (while quite correct) in not too carefully drawn:
the signal from the diodes charges the electrolytic capacitor
in reverse polarity.

Probably a careless reprint of a circuit from the Navy's electronic manual
(I don't remember the website, but it turns up often on these subjects).
They, of course, used *electron current flow*, just to be different.

Often, the diodes in those old schematics (when electron and conventional
current were still open for debate) would specify with a + and -. So
you'd see a diode drawn anode-to-cathode with a plus and minus
respectively (i.e., the rectifier makes the minus side negative, because
the symbol is pointing *electrons* in that direction).

Don't forget those old circuits with germanium PNP BJTs, with a negative
supply on the top and backwards-pointing emitters on bottom.

Tim

 

[[email protected]]

Probably a careless reprint of a circuit from the Navy's electronic
manual (I don't remember the website, but it turns up often on these
subjects).

I have one on genuine dead tree slices. It has a Bohr atom, boats, and
a nicely drawn tube on the front. It also has a TO-3-ish transistor
hastily drawn in above the tube. The cover says it is NAVPERS 10087-C
and the inside front page gives a Stock Ordering No. of 0500-031-0110.

Often, the diodes in those old schematics (when electron and
conventional current were still open for debate) would specify with
a + and -.

Most of these schematics don't have the + and - on the diodes, but
Chapter 3 has a diode symbol with a separate "forward current" arrow
pointing against the diode.

The ratio detectors are in chapter 26. Figure 26-16 shows that the
"bottom" plate of the cap on the far right is positive, backwards to
what the above link shows. That same figure also has little arrows
drawn on it (in the original) to show the charging path for that cap,
and they run opposite to the way the diodes point.

Figure 26-18 sort of matches the above link, but the cap on the far
right isn't specifically called out as electrolytic - it just has the
flat plate on top and curved plate on the bottom, no plus sign by one
plate.

For some reason, the Navy liked to have a capacitor from the top
end of the primary winding to the middle of the secondary winding.

The appendices tell me that I need an AN/ABC-1 (or possibly an
AN/DBC-1, depending on the definition of "pilot") to implement RFC 1149.

Matt Roberds

 

[Robert Baer]

I had good AM rejection in the TV demod 5.5 MHz, 8 mm diameter formers,
at 1 cm spacing those almost touch.
You propbably want a good limiting amplifier.
But please tell us:
Is it narrow band FM (at 1 MHz????)
It should be... other methods may be better (PLL, quadrature).

I had some nice formulas for calulating bandfilters, no idea where that is.

LC coupling LC
and
LC -C- LC
etc
Once had to do a whole lot a 19 inch rack full ...

Find my confession to crazyness.
IF frequency: 455KC, bandwidth: standard AM of 10KC max; trial use on
SSB signals.
Moused up version of FM detector to enhance AM detection and suppress
"duck talk" SSB - which is backwards and upside-down use of standard FM
detector.
Nineteen inch rack does not match wavelength..

 

[Tauno Voipio]

Find my confession to crazyness.
IF frequency: 455KC, bandwidth: standard AM of 10KC max; trial use on
SSB signals.
Moused up version of FM detector to enhance AM detection and suppress
"duck talk" SSB - which is backwards and upside-down use of standard FM
detector.
Nineteen inch rack does not match wavelength..

You have ordered a disappointment - SSB comes out as Donald Duck
even from a FM detector.

For the 455 kHz IF bandwidths, FM must be narrow-band (not much energy
outside the first pair of sidebands). IIRC, a hard-limiting IF strip
and a discriminator is the way to go, instead of a ratio detector.

 

[Robert Baer]

You have ordered a disappointment - SSB comes out as Donald Duck
even from a FM detector.

* So not as quacked-up as i thought..

For the 455 kHz IF bandwidths, FM must be narrow-band (not much energy
outside the first pair of sidebands). IIRC, a hard-limiting IF strip
and a discriminator is the way to go, instead of a ratio detector.

* That is good for FM, not AM; better would be the equivalent to an
un-limiter..