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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. Guest


    I am trying to filter out a 13.56 MHz signal (and if possible I would
    want to filter some
    of its harmonics in succeeding circuits).

    I have tried a LC parallel resonance circuit put in series with the
    load. In theory
    the impedance of the LC parallel circuit becomes infinite at resonance
    frequency, i.e.
    the circuit becomes open.

    I used a fixed inductance L=10uH and a variable C, i.e. a trimmer to
    get the product
    L*C = 1 / (2*pi*13.56MHz)^2 right. C should be approx 14 pF, however,
    due to +/-20%
    tolerances in L, I use a trimmer.

    However, I can turn the trimmer (in the range from 10 to 20pF) as much
    as I want and
    I don't see ANY effect at all on my scope.

    Any hints ? What am I missing ?

    I have also looked at active notch filters, but this seems to be
    rather difficult at these
    high frequencies (see

    Thank you!!

  2. Well, 'infinite'.... it depends how much is your termination (the impedance
    you drive _from_ and drive _into_?

    ------------ ----------------
    Zin C Zout
    But Zin _and_ Zout need to be a lot lower then the high impedance of the parallel LC
    in resonance for anything significant to happen.

    Often something like this is easier:

    Zi --------------------------Zo

    Now Zi and Zo can be a few kOhm, and will be practially shorted at resonance.
  3. Tom Bruhns

    Tom Bruhns Guest

    What input impedance is your scope? In a very slightly more accurate
    theory, and a much more useful one, the impedance does NOT become
    infinite, but rather becomes Q times the reactance at resonance. The
    reactance in your case is about 850 ohms. The Q I have little idea
    about: it could be 10 (pretty easily), it could be 1000 (with quite a
    bit of difficulty). You may do much better if you put a lower load
    resistance on the output of the filter -- in the RF world, 50 ohms
    would be usual, but at least something much lower than a 1 megohm
    scope input (as I suspect you're using).

    There are better circuits for implementing RF notches. With only
    passive parts (and no superconductors), you can't get a infinite Q
    peak, but you can get an infinitely deep notch. Google 'bridged T
    notch circuit.' The key concept is that if you give the RF two paths
    to follow that cause phase shifts that are exactly 180 degrees out of
    phase at the output at one frequency, and you can adjust the
    amplitudes while summing them back together, you can get them to
    cancel perfectly. The bridged T notch should work quite well for you
    at 13MHz.

  4. Joel Kolstad

    Joel Kolstad Guest

    Say Tom,

    Any suggestions for building electronically adjustable notch filters where
    there's a ballpark of a watt flowing around (i.e., 30dBm in a 50 volt
    system -> 10V peak voltage)? Such high voltage seem to rule out using a
    varactor diode as the capacitor or using a DC bias current in an inductor to
    push it towards saturation. For UHF and above the obvious answer seems to be
    YIG filters, but how about for HF and VHF? A single octave of tunability
    would do wonders for me at times...

  5. Phil Hobbs

    Phil Hobbs Guest

    One approach would be to use a coaxial stub filter with a bunch of PIN
    diode shunt switches arrayed along its length. It would tune in steps,
    of course, but depending on the notch width you want, that might work.

    I often use a coax patch cord and a thumbtack for this sort of thing.


    Phil Hobbs
  6. Joel Kolstad

    Joel Kolstad Guest

    Hi Phil,

    Yeah, that's similar to what I typically do now -- albeit with lumped L-C's by
    the time I'm down at HF. By the time you get, e.g., 5% tuning steps though,
    that's 15 sections to make an octave. Not bad, I'm just hoping someone knows
    some magic that will work even better. :)

    I have seen some commercial notch filters that were electro-mechanical in
    nature: something like a motor-driven roller-inductor. That and a handful of
    capacitors gets you a very wide tuning range, tons of power, etc. with the
    only drawback being that the time to change frequencies is going to be
    measured in seconds. It almost seems like the most elegant approach

    Thanks for your input,

  7. In the '60s it was done with motor driven roller inductors and motor
    driven variable capacitors.

    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
  8. mpm

    mpm Guest

    You could reverse feed an isolator into a selective load....
    The beauty of this approach is low (even very low) insertion loss.
    The drawbacks or course are deep notches, and maybe only 20MHz BW at
    The latter being primarily a function of the isolator response.

    You would still have to make the load (cavity, line section, etc...)
    electronically adjustable if you truly wanted to meet your above
    criteria, but it should work.

  9. ehsjr

    ehsjr Guest

    Can you sweep it to determine where it is resonant?
    Sounds like it is either off frequency or has miserable
    Q or maybe both. 'Course as others have indicated
    the meaurements set up may be part of the problem.

  10. jasen

    jasen Guest

    an LC won't touch harmonics.
    depends on Q
    parasitic capacitance, parasitic inductance.....
    if you have room you could maybe try a stub filter.

  11. AF6AY

    AF6AY Guest

    From: on 26 Apr 2007 02:36:19 -0700
    Only if you have the theoretical zero-loss components.
    OK. At 13.56 MHz, the reactance of the inductor is ~ 852 Ohms. The
    magnitude of a parallel-tuned circuit at resonance is very close to Q
    * X where
    Q is the total quality factor of both inductance and capacitance and X
    is the
    resonance reactance of either L or C.

    Using a toroid inductor with a Q=150 will get you a resonance
    magnitude of
    about 127.8 KOhms. Using a solenoidal form inductor the Q is closer
    50 and the resonance magnitude would be about 42.6 KOhms.

    A series impedance between source and load, the load being a finite
    resistance of some sort, makes a simple voltage divider...the
    of the parallel-resonant circuit being resistive at resonance. If the
    load is
    on the order of 100 KOhms or more, the voltage drop will be small; if
    it is
    on the order of 100 Ohms, the voltage drop at resonance is great.
    with a low load resistance, there will be a decided loss of voltage on
    side of resonance due to finite impedance magnitude of the tuned
    That depends on just where you are observing. Putting a scope probe
    the load end will detune the L-C since the probe itself has a
    which is very close to what you've chosen. That can be calculated and
    proven but the impedance math gets more complicated. Note: The load
    end also has some capacitance at this frequency and that will detune
    the resonance as well.

    Someone else suggested a shunting trap of a series L-C rather than a
    parallel L-C. That wouldn't be much better since the off-resonance
    impedance of a series trap will affect the source end's impedance and
    thus its gain. Such an application needs to take into account the
    circuit's impedances including circuit capacitance of both source and
    as with the parallel L-C that needs to include pass frequency as well
    notch frequency..
    In general, the "trap" circuits used in the past (early TV receivers
    of 50
    years ago) were only partially-successful, primarily concerned with
    bandpass shaping without assuming anything close to high attenuation
    at a single frequency. They worked fine at the high source and load
    impedances for tubes but not at all optimum for solid-state active

    There are some bridge circuits that might work at a specific frequency
    attenuation, but those would need to be analyzed for their response
    you want to pass. A better bet for attenuating both a specific
    frequency - and -
    harmonics is to use a lowpass L-C. If your desired bandpass frequency
    only about a third of the "trap" desired, an Elliptic (aka Cauer)
    with one of its maximum attenuation frequencies at 13.56 MHz could do
    that and attenuate the higher harmonics. The Elliptic function
    filters have definite attenuation frequencies in their stopbands.

    I'd like to suggest an easy way out, but there really isn't
    any...without going to a more elaborate circuit than first realized.

    If you wish to pass a rather narrow band of frequencies but attenuate
    specific frequency well away from those, an ordinary tuned circuit
    be better. Depending on the frequency desired and Q of the L and C,
    impedance magnitude drop-off away from resonance might be enough to
    do whatever it is you want to do.

    73, Len AF6AY
  12. Drive from a low impedance with a series resistor.
    One big advantage of series LC to ground is that you can connect the trimmer cap
    to ground, so it does not detune when you stick a normal screwdriver in it :)
    Somebody may remember the 'tol-trimmer'.

    Else I agree with yet an other poster that the T filter is likely the way to go.
  13. oopere

    oopere Guest

    What is your load? If it is high impedance, then your parallel LC has to
    exhibit an even higher impedance at resonance, which is difficult for
    real inductors.
    To suggest a solution we should know more details on the source and the

  14. Tom Bruhns

    Tom Bruhns Guest

    Seconds? You should check out the old Collins 490T antenna tuner. As
    I recall, the spec was something like 5 seconds max to tune any new
    load within the specified range, any new frequency within range, but
    you knew that if it didn't tune in a second and a half, something was
    most likely broken. Motors don't have to be slow. The inductor went
    from one end to the other in I suppose under half a second. Seems
    like it was 8 or 10 turns. Obviously PIN diodes would be faster
    though. Also, I wouldn't count varactors out of the race, for at
    least part of the job. You might not use diodes rated specifically
    for varactor service, since you'd be biasing them with tens of volts
    most likely. But there's a long ways between these embryonic ideas
    and a working design, and I leave that to you. ;-)

  15. Tom Bruhns

    Tom Bruhns Guest

    This particular part is not true. A parallel-resonant trap placed in
    series is not detuned by load capacitance at the output. It's still
    parallel-resonant at the same frequency. A simulation shows this
    easily. The same is true of a series resonant shunt trap.

  16. Joel Kolstad

    Joel Kolstad Guest

    HI Tom,

    Yeah, I suppose that if you start back-biasing your diodes at 100V, suddenly a
    volt starts to look like a small signal again.

    Someone else mentioned that a straightforward means to drop the voltage swings
    is by dropping the "system" impedance. A 1:100 transformer takes 10V to a
    mere 100mV, although now the 50 ohm system is 5 ohms so Q has to be 10 times
    better to obtain the same notch depth. Still, probably worth pursuing.

    Speaking of Collins radios, here's a rather sad site:

  17. Phil Hobbs

    Phil Hobbs Guest

    Nope, you've got the square root in the wrong place: a 100:1
    transformer changes the impedance by 10,000:1, not 10:1. Try 5
    milliohms. You'd need as many varactors in parallel for that as you'd
    need in series-parallel for the other approach.


    Phil Hobbs
  18. Tom Bruhns () writes:

    Who's the idiot who suddenly decides to cross-post this to so many newsgroups:,,,

    You bozos think that just because it might be relevant to a newsgroup, it's
    acceptable to cross-post.

    The reality is that there are virtually no times when crossposting is
    necessary, and it's the mark of iditots too lazy to find the right newsgroup,
    or too clueless.

    ANd when you see fit to add in newsgroups, you're even greater idiots.

  19. Many years ago I made a digital system, it consisted of 4 coils
    10, 20, 40, and 80 uH, and 4 relais to put these in series as needed
    (this was a rather high power system).
    So the the tuning software switched the relais, giving 15 presets.
    That was accurate enough for antenne matching.
    And there were 12 of these in a rack....
  20. ehsjr

    ehsjr Guest

    Why are you asking me? I have no idea what the load
    is - I'm not the OP.

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