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excessive insertion loss of homebrew BCB band stop filter

Discussion in 'Homebrew and Kit Projects' started by N2LKR, Jul 4, 2021.

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  1. VK2TIL

    VK2TIL Ham Member QRZ Page

    I didn't comment earlier because I could see no obvious mistakes but it would be a pity to abandon the project without learning something.

    I guessed from the toroid size, the number of turns and the light-grey colour that you used FT50-61 toroids; #61 ferrite makes good inductors up to about 7 MHz and I guess at a Q of at least 100 at 1 MHz.

    The white toroids appear to be T50-7; #7 is an uncommon material, #2 or #6 being more widely available, but #7 should work fine here.

    Polystyrene capacitors are an excellent choice for low-power filters; they are difficult to find these days so you are lucky to have a good assortment.

    Placing capacitors in parallel is fine; in fact, it places any stray inductances in parallel and so reduces them. (The same applies to resistors; for instance, two parallel 100R resistors have been shown to make a better calibration load for a VNA than does a single 50R device).

    The ground plane appears solidly-tied to the case, connectors etc so the double-sided board is not the problem.

    One minor point; the squares of PCB used as "floating" tie-off points will provide a small capacitance to ground but I don't think that's the fundamental problem. Cutting a gap across them and mounting them vertically would separate the tie-point from ground.

    Measurement with noise is not a very precise method; your noise source is far from "flat" but the filter response does seem to follow it, more-or-less.

    I take it that your spectrum analyser has no tracking generator or you would have used it.

    Measurement with a signal generator and 'scope, voltmeter or power meter is fine, just tedious; if you have the patience to take and plot a range of such measurements at, say, 100kHz intervals you will get a better picture of the filter performance.

    Sometimes, after much head-scratching, it's necessary to start again; it's easy, though, to repeat the mistake(s) made the first time.

    I try to measure every component that goes into something I'm building; the cheap RLC meters from ebay and elsewhere are very good for this, quick, simple and accuracy sufficient for this kind of measurement. Their only flaw is that they don't handle low values but the values in your design will be well within their capability.

    I have one of these;

    plus an apparently-identical one which has no case and uses a 9v battery but was less than 1/2 the price.

    I asked my girl Elsie to design a filter based (I hope) on your parameters; she provided different L & C values. I don't know much about bandstop filters but I guess there are several possible scenarios.

    I did note that there was little difference between Chebyshev & Butterworth responses but the Butterworth, of course does have minimal loss in the passband.


    Attached Files:

    VK4HAT likes this.
  2. N2LKR

    N2LKR Ham Member QRZ Page

    This is something I need to acquire, a cheap RLC meter! The culprit turned out to be a bad polystyrene capacitor. After removing all resonator sections yesterday and tuning them using my noise source and TinySA, I immediately discovered one resonator way below where it should have been. Since replacing it and tuning all sections, the filter works as it should!
    VK4HAT, PY2RAF, W1VT and 1 other person like this.
  3. KX4OM

    KX4OM Ham Member QRZ Page

    The LC-100A (or LC-200A, with a case) meter sold on eBay is a good choice, assuming that you get one from a vendor that allows refunds. Some recent ones have defective components, based on a long thread on the Antique Radio forums. Mine, which I bought several years ago on eBay tests very well. Chuck Adams, K7QO posted his LC-200A comparison with his AADE IIB and a few with his LC-100A equipment some years ago on his old Yahoo forum, qrp-tech. The files still exist on message #12641:

    Here are the results of tests I made with my LC-100A:

    Caps - w/clip leads - euro terminal -
    27pF 1% - 27.11 pF - 27.05 pF - 0805 ceramic with 1/8" leads soldered on
    .001 1% - 1016 pF - 1017 pF - Polypropylene #1
    .001 1% - 1017 pF - 1019 pF - Polypropylene #2
    .01 1% - 10.04 nF - 10.05 nF - TRW radial film cap

    2.2uH 5% - 2.253 uH - (no test) - SMD inductor with tiny leads
    10uH 5% - 9.717 uH - 9.672 uH small encapsulated conformal coated choke

    With regard to the other popular <$20 inexpensive instrument, the famous TransistorTestor, I don't think that device tests L and C ranges as low as the LC-100/200 units.

    Ted, KX4OM
    Last edited: Jul 6, 2021
  4. VK2TIL

    VK2TIL Ham Member QRZ Page

    It's Alive!!!

    It's Alive!! 2.jpg

    Your patience has been rewarded.

    Apropos the meters, KX4OM is correct; the LC-100A/200A (which, I suspect, is a clone of the AADE L/C meter, does measure small values whereas the others I mentioned do not.

    I also have an MK-328 which has the same limitation; it does not work for small values of L & C.

    The LCR-TC1 has the virtue of measuring R as well as L & C; this, with the ZIF socket, makes component-checking fast & simple compared to my previous method of using my AADE meter and a multimeter.

    These things are so cheap that we can have two and get the virtues of each.

  5. KI4IO

    KI4IO Ham Member QRZ Page

    You might want to go with a less complicated filter. For your stated need, a simple low-pass filter would fill the bill.
    I use on-line calculators like this: pi low pass.htm
    and I often fiddle with the cut-off frequency till it hit capacitors I have in my junk box

    Warrenton, VA

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