Switching Power Supply Hash - Questions

Discussion in 'General Technical Questions and Answers' started by KB3MOW, Sep 19, 2019.

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  1. KB3MOW

    KB3MOW Ham Member QRZ Page

    I've heard horror stories about the noise generated by switching power supplies, but now that I actually have one, I have a couple of questions.

    Where does the noise show up? Is each supply unique or do they all show up on one frequency more or less?

    Does it 'migrate' or change frequency?

    What does it sound like on the air?

    Can I tell by looking at one of the digital waterfalls?

    Called a guy on CW last night on 3.548. Frequency was clear for about five minutes when I called 'QRL' then called him. He replied, and in about 30 seconds, I started getting a +20 noise I couldn't identify. Wasn't a digital mode for sure, and sounded like some type of QRN, but it was isolated to a small area - about 1kHz. Today the same frequency is clear, so I doubt it was my PS.

    I went through 160 thru 10 this morning and didn't find any 'hash' that appeared in sort of pattern, but would appreciate if someone would tell me what I'm looking for.

    Tnx es 73
    Dave KB3MOW
     
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    This varies by model, but generally the RFI ("hash") is based on the switch frequency inside the power supply, which is not crystal controlled or synchronized to any stable source, so it can and will drift.

    Usually the hash is harmonics of the switch frequency, and how strongly it is received is based on lots of variables including power supply cable length, your antennas, antenna proximity, and whether it's radiated by the supply and/or its DC cable, or line-conducted and then radiated by power lines.

    But don't expect it, if you have this problem, to always be on the same frequency...it won't be.
     
  3. SM0GLD

    SM0GLD Ham Member QRZ Page

    It is very easy to check the PS for RFI noise with the help of a current probe.
    I use a snap-on-ferrite with 1 turn cable thru the ferrite soldered to a BNC connector so I can easily connect it to my antenna input.
    Then I turn on the PS and clamp the ferrite to the AC or DC cable and listen on all bands for noise.
    If you have a high level of noise you may need an attenuator between the current probe and RX input.

    Make sure not to touch the PTT :D
     
    KT5WB likes this.
  4. KM3F

    KM3F Ham Member QRZ Page

    I have two different switching power supplies that both generate hash.
    One begins on 160 and in a harmonic relationship appears on 80, 40 20, 15 and 10m.
    For my operating it never is a problem but I use an ANC 4 Noise canceller to totally clear the interference on any of the bands when needed. The ANC 4 adjustments can be very precision so needs to be done paying close attention to the result. Once nulled out, you can't detect interference in the audio. Be a where the ANC4 will have about a 6db loss through it's system that is normal when switched in. Otherwise you would not even hear your desired signal for the RFI present. ANC 4 is effective up to 6 meter band.
    As the supply temperature rises the hash drifts usually downward and can vary around depending on the load the radio provides that keep changing the operating temperature in the supply.
    I liken the noise to a wild animal scream.
    Problem as I see it is the supply case is not RF tight for direct radiation nor is the AC and DC power filtered and by-passed sufficiently, plus your radio is not RF tight to outside signals except from the antenna connected.
    Basically too much plastic used in the cabinets of both plus proximity to each other.
    Displays and meter faces on equipment let interference out and in those areas making full shielding impossible. Only way for practical metering is by remote shielded cable and jack by-passed and filtered.
    On the other hand I have a 220 transmitter built from English equipment that uses a SS sps fully double shielded and by passed. It has no noise output whatever proving the issue can be solved if enough investment is done and the customer is willing to pay the extra costs.
     
    Last edited: Sep 19, 2019
  5. KB3MOW

    KB3MOW Ham Member QRZ Page

    Thanks for the replies! I now have a starting point!

    I live in a rural area, so no neon signs, no mercury vapor lights, and no close neighbors, so noise like this shows up without having to search too hard! I gathered some empirical data today and it was interesting.

    The 'hash' does seem to be coming from the switching power supply, but it at least isn't as bad as I feared. It covers all of the 80M band, and, to a lesser degree, the 160M band. I 'measured' the instances at about 56 Hz separation (close enough to 60 for me). As soon as I keyed up the transmitter, the signal started drifting. I could find very faint images on 40M and up, but barely heard and wouldn't interfere with even the faintest DX signal!

    The solution seems pretty straightforward - Shorten my DC power leads first. They are about 6 feet long (Hate cutting them, knowing I'll need a wire-stretcher in a week or so!) and possibly adding ferrite cores to the power cords if there is still a problem.

    Again, thanks for the info! It helps a lot!

    73
    Dave - KB3MOW
     
  6. KM3F

    KM3F Ham Member QRZ Page

    There are two or more basic ways a switching power supply is designed.
    One is the AC line frequency is chopped from either the 60 or 120 hertz rate and shaped to drive a transformer.
    The other is a separate Oscillator and a control integrated circuit to do the same function.
    In some supplies the Oscillator can be varied with a supplied external control to shift the interfering frequency but not get rid of it.
    It pays to look at the schematic of the specific unit to see these things.
    Good luck.
     
  7. N8FVJ

    N8FVJ Ham Member QRZ Page

    Some switchers seem to be quiet and others not. I do not take any chances and pay a little more for linear type power supply by Astron. Linear type never makes any noise on the ham bands.
     
  8. SM0GLD

    SM0GLD Ham Member QRZ Page

    DonĀ“t cut the DC cable. You may need to add a common mode filter on the DC output.
    On the AC side.....I have a AC line filter in a box with extension cords that I can plug in for fault finding.
    Many times the AC line filter in the PS isn't enough or at least not enough for a ham.

    What is the type and brand of your PS ?
     
  9. KB3MOW

    KB3MOW Ham Member QRZ Page

    Thanks Mike! Good call! The PS is the MegaWatt S-400-12. This isn't the cheap "clone", and I did research before I bought it. I wanted a PS that would deliver the 22A I needed and allow some headroom. Cost was also a factor! There were a few reviews that mentioned hash, but the majority of them said it was clean. This one is mostly clean, with 160 and 80 the only bands it affects.

    I have the PS hooked up to a power strip (15A) for the on/off switch, but no filter on AC. The strip is plugged into a 4-foot heavy duty appliance extension cord to reach the outlet. This outlet is on its own circuit. I might be able to eliminate the extension cord, but that could be tricky.

    What AC filter are you using, and does it eliminate the hash??

    Tnx es 73
    Dave - KB3MOW
     
  10. KA5IPF

    KA5IPF Ham Member QRZ Page

    Use your 'scope and measure the garbage on the DC output. Then do it again under a load.
     

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