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  • Stopping HF Tx/Rx SMPSU QRM

    From I0OJJ@21:5/101 to TECH on Sat Jun 29 05:32:50 2019
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    From i0ojj@i0ojj.ita.eu Sat Jun 29 10:25:05 2019
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    From: G8MNY@GB7CIP.#32.GBR.EURO
    To : TECH@WW

    By G8MNY (Updated Jul 06)
    (8 Bit ASCII graphics use code page 437 or 850, Terminal Font)

    Where a 13.8V SMPSU is used directly to power an HF Tx/Rx the normal mains & DC filtering is often inadequate to remove all the switch mode spogies that a sensitive Rx & nearby aerial can pick up.

    10mV─┤~~`─-..__ │_S9+40dB
    1mV─┤ ~`-._ │_S9+20dB
    100uV─┤ ~`-. │_S9
    10uV─┤ `\ │_S5
    1uV─┤ `\._ │_S1
    100nV─┤ ~`─-._ │-S0
    ┼──┬──┬──┬──┬──┬──┬──┬──┬──┬──┬──┬──┬──┬──┬──┤
    0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 MHz

    Typical QRM as measured on a commercial "RADIO HAM SMPSU" using a 50kHz wide spectrum analyser. The 'S' meter readings depends on your Rig's QRM imunity to power rail QRM as well as the SMPSU radiation & pickup to your aerial.

    SMPSU often pulse on & off on no load, this is good indication that the following Rx QRM elimination method will work 100%. But before you under take a SMPSU modification, test the principle works OK with a variable bench PSU first. If the test is OK & not too high a voltage is needed (watch those dial lamps if over 15V!), then look for room inside the SMPSU or add a bolt on box to the SMPSU unit if you like (case as heatsink).

    The strategy is to stop the SMPSU from running while on Rx, by using a small linear PSU for Rx instead. Stopping the SMPSU running is normally possible on all SMPSU if the output goes slightly higher than the nominal 13.8V by only a few 100mV. Note that too much overvoltage may trigger a crowbar overvoltage circuit if that sort of protection is built in (I recommend U add one if not).

    On a good insulated Heatsink
    in┌───────┐out
    ┌───┬──────┬──┤ 7812 ├──┬──┤>├──────────────┬────────> +Tx/Rx
    _│_ _│_ │ └───┬───┘ │ 1N5401 │
    MAINS /_\ /_\ │ │ │ --------│---
    ──┐ ┌──┤ │ │ ├─2k2──┤ | ─┤>├─┤ | 14V Rx < 1A
    )││( │ │ 4700u│+ │ │ ──| TX │+ | 13.8V Tx > 1A
    )││( │ │ 30V=== Pot │ MAINS | SMPSU === |
    )││ └──)───┤ │ ┌─>1k === ──| 25A │ |
    ──┘ 18V _│_ _│_ │ └──┤ u1 │ --------│---
    1.5A /_\ /_\ │ │ │ │
    └───┴──────┴──────┴──────┴───────────────────┴─────────> -ve
    Linear Rx PSU Noisy PSU

    diodes 1N4001 or better.

    The rating of the current limiting linear PSU needs to be just enough to run the Rx at normal volume etc. Normally 1A will do, but 2 x 1A regs could be used in parallel (with an 0.1Ω Rs in each output) if 1A is not enough, or use some other regulator.

    Other than eliminating the SMPSU QRM there is 1A more current available on Tx.

    The regulator(s) must be the current limiting type & well heatsinked, as it will be working very hard all the time!

    See my bul "Reducing Electronic RF QRM" for more tips.


    Why don't U send an interesting bul?

    73 De John, G8MNY @ GB7CIP


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