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LC notch filter not working!

Discussion in 'Electronic Design' started by [email protected], Apr 26, 2007.

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  1. Joel Kolstad

    Joel Kolstad Guest

    Oops, thanks for catching that Phil! Sheesh...
  2. AF6AY

    AF6AY Guest

    Yes, you are correct...caught me with a low level of caffeine on
    Thursday. :)
    Yes on the parallel L-C for the trap frequency. But, under low source
    impedance and high load impedance, with the approximate L and C given,
    there is a voltage increase at a frequency below the trap frequency.
    are four combinations of 3 components for L and C circuits, each with
    peak versus dip impedance response, me has to keep reviewing those to
    avoid confusion] To explain, my (later) analysis model was as

    One-Ohm impedance current source. Parallel L-C in series with load,
    L1 = 10 uHy with Q of 150, C1 = 14 pFd. Load is 1 MOhm in parallel
    C2, C2 varying 10, 20, 30 pFd. Capacitors were assumed essentially
    lossless since their typical Q at these frequencies can be 1000 or
    Minimum voltage response was at a nearly constant frequency regardless
    of C2 value. Maximum voltage response frequency varied considerably.
    Using 1.0 V RMS reference for 0 db, the response v. C2 value was:

    C2 = 30 pFd, Vout peak +22 db at 7.8 MHz, Vout minimum -35 db.
    C2 = 20 pFd, Vout peak +26 db at 8.25 MHz, Vout minimum -32 db
    C2 = 10 pFd, Vout peak +21 db at 10.4 MHz, Vout minimum -26 db

    I could have done the above with L1 Q of 50 but that would simply
    decrease the lower frequency peak voltage, show a lesser voltage
    minimum at the upper trap frequency, the rest about the same.

    * At this point someone will get hot about "ya can't have voltage
    * gain with no amplifier!" or equivalent. :) Yes, one can since
    * a voltage increase only means a current decrease at one
    * frequency...the only power loss is in the Qs of the components.
    Yes, but only for the series resonance frequency. There's a variation
    in the overall voltage response depending on the load resistance and
    its parallel load (and probe) capacity. For sure, a series-resonant
    circuit across the source is going to affect the gain of the driving
    source from its frequency variation of impedance.

    This is one of those seemingly-inocuous circuit applications which can
    get very tricky to apply with any repeatability. Especially so when
    source and load were unspecified. It's safe to say that EVERYTHING
    interacts over frequency and one cannot just assume anything. That
    includes scope probes which far too many apply thinking just of their
    10 Meg input resistance and forgetting they all have capacity to
    in parallel. :-(

    Thanks for reminding me to go back to earlier basics, Tom. A number
    of years ago I worked the math on impedance of the four basic 3-
    component combinations and wrote it up for a work application (that
    would have been a high production failure situation if used as-is) and
    thought memory "would always be there." Actually it was but my mind
    gets cluttered with other stuff on a disorganized basis. :)

    BTW, I used my own LINEA (DOS-only) analysis program and LTSpice
    (free Windows compatible full package from Linear Technology) to run
    this simple circuit model. Results agreed.

    73, Len AF6AY
  3. AF6AY

    AF6AY Guest

    Other than a pre-tuned Collins commercial transmitter at an Army
    station in the early 1950s, the first time I recall seeing an
    antenna tuner was in the T-195 transmitter built by Collins for a USMC
    contract (forget the AN/ number, its companion receiver was the
    R-392, the 28 V counterpart to the R-390 and R-391). On a quickie
    demo in 1955, the officer doing the demo disconnected one of the
    Jeep's whip antenna sections. The T-195 retuned its antenna is
    a few seconds, indicated by a little lamp on the front panel. Most
    amazing to me at the time, used to the huge built-to-last-forever
    HF monsters that were always most fussy on manual tuning. :)

    Much later I got a PDF of that T-195 TM and believe that this set
    might have been the first military radio to incorporate the Bruene
    voltage-current detector necessary for the automatic antenna
    tuning servos. Any delays in operation might have been just from
    the detector-sensor output time-constants in addition to motor
    speeds. The "Bruene Bridge" as it is sometimes called, is
    the basic form for nearly every other automatic antenna tuner built
    since then.
    Yes, the all-important "gestation stage." Ask any mother. :)

    73, Len AF6AY
  4. Tom Bruhns

    Tom Bruhns Guest

    I've trimmed off a couple of the groups since someone seems to have
    gotten his knickers all twisted up over the posting in, wow, four
    pissibly relevant newsgroups. Wish he'd take his venom out on the
    idiots that cross-posted from the wierd alt.* groups a couple weeks

    Anyway, Joel, WHY would you think that the Q needs to be any
    different?? You'd scale the impedance, and as Phil noted you got the
    impedance ratio vs turns ratio backwards, but you'd want the same Q at
    that frequency. It might be practical to scale by a 3:1 turns ratio
    or possibly even 4:1 at these frequencies, but I'd be wary of going
    beyond that.

  5. Tom Bruhns

    Tom Bruhns Guest


    Trimmed off a couple of the groups and much of the message, though all
    was noted. Thanks for the additional info; I trust the OP will finde
    it useful, if he's still around. (Pet peeve: posters who don't
    bother to get back to say "Hey, that helped," or "Huh?" or give some
    indication they are still lurking.)

    Yes, to be sure the response depends on the load. In fact, even at
    the notch frequency, if you start with a high load and add
    capacitance, you significantly affect the depth of the notch with the
    added capacitance.

    It can be quite useful to add another capacitor (or inductor) to a
    series or shunt trap, to get the response at a frequency you
    specifically want to pass to be high. You can do the same thing with
    transmission line stubs, which becomes practical at higher
    frequencies. For example, you can put a shorted stub across a line,
    where the stub length is 1/2 wave on the frequency you want to
    "kill." But then the response at nearby frequencies will also be
    attenuated. You can then view that first stub as a reactance at the
    frequency you want to pass, and add another stub of the same reactance
    magnitude but opposite polarity. You'll find, of course, that the two
    stubs total a wavelength, assuming both are shorted at the ends away
    from the point the join the through line. With low loss line, this
    can be a very effective way to get rid of a large signal in a fixed-
    frequency receiver system. The capacitor-or-inductor-added-to-the-
    trap is a lumped equivalent of this idea.

    I suppose in an absolutely accurate analysis, the impedance versus
    frequency charaterisitic of a load that includes capacitance may be
    such that the frequency of the maximum attenuation of a finite-Q notch
    is shifted ever so slightly, but for sure it won't be shifted enough
    to notice; the proximity of the metal in the probe to the coil is
    likely to affect the resonant frequency more.

  6. Joel Kolstad

    Joel Kolstad Guest

    Hi Tom,

    I was probably unclear in that I meant Q of the inductor (and am assuming Q
    of the capacitor is high enough to be ignore); *not* Q of the system.

    Say you're in a 50 ohm system. If, at resonance, your series shunt L-C
    exhibits a resistance of 0.5 ohms, that's about a 40dB notch. However, in a
    5 ohm system, it's only a 10dB notch. You need to get the resistance down
    to 0.05 -- implying a Q ten times large than what you started with -- to
    maintain the same notch depth.

    Do you buy this? :)

  7. AF6AY

    AF6AY Guest

    Agreed. However, the coax cable stub idea might be a tad
    impractical considering that 13.56 MHz is lower in wavelength
    than the 20m band. Stubs could get as long as around 15 feet
    at that frequency. :)

    I once had to "rotate" the impedance of some SAW filters (8 of
    them) for the 60 to 70 MHz region and couldn't get any more
    space for the matching other than some slots in a machined-out
    chassis. I couldn't have done it without skinny lil 1/8" OD coax
    held in place by some RTV. [roughly 3/8 of a rotation on the
    Smith Chart] I hate to think about doing that at 13 MHz.

    73, Len AF6AY
  8. Joel Kolstad

    Joel Kolstad Guest

    This should, of course, be 20dB. I know I was thinking 20dB, but clearly I
    typed 10dB. Oops.
  9. Guest

    Hi Tom,
    The input impedance of the scope is 1 MegOhms for the passive probes
    I use, but i can change the coupling to 50 Ohms as well.

    Do you know where the Q in an LC notch filter comes from ? Is this
    the Q of the inductor defined as (2*pi*f * L) / R ?

    What kind of inductor and capacitor is best suited for a notch filter
    at 13.56 MHz (RF frequencies) ?
    I use a ceramic trimmer right now, but I am not sure what kind of
    is best suited for RF circuits. It seems that the value of an inductor
    (even the L) is
    very frequency-dependent.

    When you say a low load is much better, do you mean for a parallel
    LC notch filter, or for a series LC notch filter ?

    Thanks for the great help!
  10. What are the circuit impedances?

    Consider that your circuit more or less looks something like this,

    input signal >-----+----/\/\/\/\/----+-----> output
    | |
    / /
    \ \
    Rin / / Rout
    \ \
    / /
    \ \
    | |
    | |
    ----- -----
    --- ---
    - -

    Hmmm, looks just like a plain old RC attenuation pad! Except the
    Rtrap is actually a parallel tuned circuit that is a high impedance
    at one frequency and lower impedances at other frequencies. So lets
    pick any handy set of values for Rtrap, as an example. Maybe your
    LC circuit is more, or maybe less... the effect is what you want to
    understand. Lets assume the value for Rtrap approaches 100 Ohms for
    non-resonant frequencies, and say 10,000 Ohms at the resonate frequency.

    So, if Rin happens to be high, say 100,000 Ohms or more we can just
    ignore it. (Which is practical, as all it does is provide a constant
    load for your source, and we'll assume it is sturdy and can handle
    anything from 0 to 1000 megs!)

    That means you have two circuits, one at the resonate frequency and
    one at all others, which both look like this,
    \ 100 Ohms, or
    \ 10,000 Ohms
    +------> out
    / Rout

    It's just a plain old resistance divider. If Rout is 100 Ohms the
    output will be 1/2 the input at non-resonate frequencies (insertion
    loss), and at the resonate frequency it will be 1/100th of the input.

    Obviously if the Rout value is 100,000 Ohms your divider is going to
    have virtually no effect at all! And if it is 10 Ohms the effect will
    be even greater than it was at 100 Ohms.
  11. Can that circuit ever produce any depth of notch? If Rtrap is
    a parallel tuned circuit then it has in parallel with it an
    effective resistance of Rin+Rout, and to get any reasonable
    selectivity Rin+Rout must be high compared to L/C.R, the
    dynamic impedance of the tuned circuit at resonance.

    If that is so, are there actually any values for Rin and Rout
    that could produce a reasonable selectivity?
  12. The first thing you must do is determine the self-resonance of the coil.
    That can be done with a "grid dipper". If that frequency is below the
    frequency you want to filter, it won't work. The next ting to do is
    determine the resonsnce frequency this time installed in the circuit
    witout a trimmer. Again, if that is below the frequency to be filtered,
    it won't work. Only when that resonance is above, 13 MHz in this case,
    can a trimmer be applied to tune it.
    The stray capacitance, possibly multiplied by the chip gain, may rule
    out operation at 13 MHz.

    Angelo Campanella
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