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RF transformer

Discussion in 'Electronic Design' started by Robert Baer, Jul 29, 2013.

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  1. Robert Baer

    Robert Baer Guest

    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)?
  2. Tim Williams

    Tim Williams Guest

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

  3. RobertMacy

    RobertMacy Guest

    use free femm 4.2 to design coil

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

    BOTH user groups, super helpful
  4. RobertMacy

    RobertMacy Guest

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

    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 [email protected]#$#[email protected]#$ 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'.
  5. RobertMacy

    RobertMacy Guest

    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 [email protected]#$#@!
    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
  6. Bill Martin

    Bill Martin Guest

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

    Glenn Guest


    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


    According to this site there is a 90 degree shift needed:
    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: de Luxe-7659700-1968-diagram.png
    (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

    Ratio Discriminator:
    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.

  8. Tauno Voipio

    Tauno Voipio Guest

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

    Robert Baer Guest

    * 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).

    * 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.
  10. Robert Baer

    Robert Baer Guest

    * I disagree; look at the phase diagrams.
  11. Robert Baer

    Robert Baer Guest

    Now digging into my old,dusty RCA Radiotron.
  12. Robert Baer

    Robert Baer Guest

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

    Robert Baer Guest

    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.
  14. Robert Baer

    Robert Baer Guest

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

    Fred Abse Guest

    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/
  16. Tim Williams

    Tim Williams Guest

    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. :)

  17. Guest

    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.
    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

    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
  18. Robert Baer

    Robert Baer Guest

    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
    Nineteen inch rack does not match wavelength..
  19. Tauno Voipio

    Tauno Voipio Guest

    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.
  20. Robert Baer

    Robert Baer Guest

    * So not as quacked-up as i thought..
    * That is good for FM, not AM; better would be the equivalent to an
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