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  1. #1
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    All HF bands modest BPF?

    Hi, I have noticed that high L/C ratios in a series LC can make a usable BPF in a single stage (pole) which can be converted to all bands by tuning the C and a few taps on the L.
    Here is a simulation at 40m.

    I only use 3 inductor values and a small 6-50pF trimmer to cover the whole HF.
    The 100pFs are there just to improve the response on higher bands and have no effect on lower.
    Components values are practical and the passband attenuation is very low, whereas the stopband can be very high at low C values.
    I have also played around with the I/O impedances and the shape does not change too much, but most importantly the passband peak frequency does not change too much.

    Can it really be that simple?
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    Re: All HF bands modest BPF?

    Your plot shows a central frequency of 7.018MHz, when for the filter values that you posted the central frequency should be 3.6MHz.
    In the simulation you don't use a physical model for the capacitors. Add a value of about 150 for the Q of the series capacitor, and you will find that things are not so good.

    However, band pass filters like this are used in almost all the commercial HF receivers/transceivers. Sometime with few more poles, to get better out of band rejection.


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    Re: All HF bands modest BPF?

    Quote Originally Posted by vfone View Post
    Your plot shows a central frequency of 7.018MHz, when for the filter values that you posted the central frequency should be 3.6MHz.
    In the simulation you don't use a physical model for the capacitors. Add a value of about 150 for the Q of the series capacitor, and you will find that things are not so good.

    However, band pass filters like this are used in almost all the commercial HF receivers/transceivers. Sometime with few more poles, to get better out of band rejection.
    I am sorry you are right. It should be 13pF. I simulated it with 13pF and then I changed the value to 40 to do another simulation.

    What is this thing about the Q you told me, this program does not have this parameter I think. Here are some parameters for the cap.

    I am thinking of T106-2 amidon core with taps and ceramic or better air spaced trimmer, won't the Q improve?
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    Re: All HF bands modest BPF?

    Double click on the series capacitor, check "Use Physical Model", and set the Q value.


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    Re: All HF bands modest BPF?

    Quote Originally Posted by vfone View Post
    Double click on the series capacitor, check "Use Physical Model", and set the Q value.
    Great thanks!

    Starting values to put in this window? roughly.
    I put a Q of 150 at 30MHz and no other settings changes and indeed the loss increased to almost 4db from when the Q was 100 at 100MHz!

    (using air variable trimmers and the *-2 amidon core material)
    Last edited by neazoi; 6th December 2018 at 22:27.
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    Re: All HF bands modest BPF?

    Now make sense. This is the curse when use real life component models.
    Generally Q increase with frequency, so Q mentioned at 30MHz should be fine.


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    Re: All HF bands modest BPF?

    Quote Originally Posted by vfone View Post
    Now make sense. This is the curse when use real life component models.
    Generally Q increase with frequency, so Q mentioned at 30MHz should be fine.
    I wonder if one could use a preamplifier to compensate for the loss of the filter with real world components.
    Then it could be a nice preselector (since it is easy to build) and the actual LC loss will be compensated with the preamp.
    How does it sound?
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    Re: All HF bands modest BPF?

    Look at this one
    http://openhpsdr.org/wiki/index.php?title=ANICETUS
    How does it seem to you? It seems he has made it working by lowering down the impedance prior to filtering
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  9. #9
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    Re: All HF bands modest BPF?

    This is about port impedance matching, which should be done in any filter design. The posted link use transformers to do this impedance matching.
    What I don't like in the mentioned link, is that the designer is placing components (relays, and ferrite core inductors) inside of the big toroid. There are currents that affect the field inside of the toroid.
    https://upload.wikimedia.org/wikiped...ing_Vector.jpg


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