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  1. #1
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    HUM in transmitter PSU

    Hi on my Classic 40 http://qrp.gr/classic40 I am using a SMPSU from a laptop. There is a bit of hum in the received audio on a nearby radio, so I thought this was because of the SMPSU.
    So I used a 24v transformer (ubloaded voltage 28v) and a bridge rectifier and a 4700uF after it to power the transmitter. The hum was much more!

    Why is that? Can it be a transmitter problem or a PSU related?

    Can it be cured?
    Professional engineering is the top, but amateur engineering is more fun.
    It is when you cross the barrier between these two, that things become really fascinating!

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    Re: HUM in transmitter PSU

    Aside from the fact that it looks like your transmitter is having a seizure (what was the point of that??), it also looks like you're using a protoboard with wires flying all over the place. That is a sure recipe for hum, noise, etc. Try grounding your audio input and see what happens.


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    Re: HUM in transmitter PSU

    In addition to proper audio grounding as suggested ,
    add a electrolytic filter cap at the input pin of lm317 at
    the input and ground pins of IC where DC is applied
    in your circuit.



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    Re: HUM in transmitter PSU

    laptop psu's do make a lot of CM noise ...



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    Re: HUM in transmitter PSU

    Quote Originally Posted by barry View Post
    Try grounding your audio input and see what happens.
    I did so, the hum was increased!
    Professional engineering is the top, but amateur engineering is more fun.
    It is when you cross the barrier between these two, that things become really fascinating!



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    Re: HUM in transmitter PSU

    Sounds like a ground hum loop to me then. There is a difference in voltage between your audio input ground and the LM317 ground. You should have a capacitor (at least 10uF) from the input pin of the LM317 to the emitter of the 2N2222 and the audio ground should also be connected directly to the emitter. Use a star connection, everything audio returned to the same physical point so they must be at the same potential.

    Another thing to watch for is the RF output side. If you have an antenna you may (should) also have a ground connection but it may not be at the same potential as the mains cord Earth. If it isn't, there will be an AC voltage developed between the two and as a consequence, a current flowing along the 0V connections.


    Brian.
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    Re: HUM in transmitter PSU

    Quote Originally Posted by betwixt View Post
    Sounds like a ground hum loop to me then. There is a difference in voltage between your audio input ground and the LM317 ground. You should have a capacitor (at least 10uF) from the input pin of the LM317 to the emitter of the 2N2222 and the audio ground should also be connected directly to the emitter. Use a star connection, everything audio returned to the same physical point so they must be at the same potential.
    Brian.
    Weird, changing the ground point a few CM away in the PCB changes the TX carrier level and hence the modulation.
    Professional engineering is the top, but amateur engineering is more fun.
    It is when you cross the barrier between these two, that things become really fascinating!



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    Re: HUM in transmitter PSU

    Hi,

    Weird, changing the ground point a few CM away in the PCB changes the TX carrier level and hence the modulation.
    Weird? I´m not sure.
    This is why a solid GND plane is that important (Like I very often write in my posts).

    And then one can see that there is a big difference between copper filled layer and a true GND plane layer.

    If you need some support with GND plane: just show us your PCB layout.

    Klaus
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    Re: HUM in transmitter PSU

    Quote Originally Posted by KlausST View Post
    Hi,


    Weird? I´m not sure.
    This is why a solid GND plane is that important (Like I very often write in my posts).

    And then one can see that there is a big difference between copper filled layer and a true GND plane layer.

    If you need some support with GND plane: just show us your PCB layout.

    Klaus
    THIS... is my PCB layout... http://qrp.gr/classic40/
    Professional engineering is the top, but amateur engineering is more fun.
    It is when you cross the barrier between these two, that things become really fascinating!



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    Re: HUM in transmitter PSU

    Hi,

    not that bad.
    Such copper arts usually are much better than any breadboard...or single layer PCB.

    Klaus
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    Re: HUM in transmitter PSU

    Quote Originally Posted by KlausST View Post
    Hi,

    not that bad.
    Such copper arts usually are much better than any breadboard...or single layer PCB.

    Klaus
    Yes, but there's also that problematic white protoboard, and another board in the background; are those part of the system?



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    Re: HUM in transmitter PSU

    Quote Originally Posted by betwixt View Post
    Sounds like a ground hum loop to me then. There is a difference in voltage between your audio input ground and the LM317 ground. You should have a capacitor (at least 10uF) from the input pin of the LM317 to the emitter of the 2N2222 and the audio ground should also be connected directly to the emitter. Use a star connection, everything audio returned to the same physical point so they must be at the same potential.
    Brian.
    It did not make a difference.
    I notice this hum only when switching the RX to AM. On CW no hum is heard. Does this tell you something?
    Professional engineering is the top, but amateur engineering is more fun.
    It is when you cross the barrier between these two, that things become really fascinating!



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    Re: HUM in transmitter PSU

    I notice this hum only when switching the RX to AM. On CW no hum is heard. Does this tell you something?
    Only that there is no frequency modulation on the carrier. When receiving in CW mode you probably wouldn't hear very low frequencies because of the receiver audio filtering.

    What happens if you short out the audio input, with and without the audio cables connected?

    Brian.
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  14. #14
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    Re: HUM in transmitter PSU

    Quote Originally Posted by betwixt View Post
    What happens if you short out the audio input, with and without the audio cables connected?

    Brian.
    The hum gets louder. But I am touching the shorting metal with my hand...
    Professional engineering is the top, but amateur engineering is more fun.
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    Re: HUM in transmitter PSU

    That suggests you are becoming part of the signal path.
    Can you check that the PSU you are using really has a ground connection. There should be one but I'm thinking if you are using a laptop supply it might have a two pin mains connector and if I'm right, there would be no ground at all on your gadget except through the audio cable.

    Brian.
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    Re: HUM in transmitter PSU

    Quote Originally Posted by betwixt View Post
    That suggests you are becoming part of the signal path.
    Can you check that the PSU you are using really has a ground connection. There should be one but I'm thinking if you are using a laptop supply it might have a two pin mains connector and if I'm right, there would be no ground at all on your gadget except through the audio cable.

    Brian.
    Hm... But I have tested this with the simple linear PSU and also with the LAB psu, which has a ground (the mains ground)
    Professional engineering is the top, but amateur engineering is more fun.
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    Re: HUM in transmitter PSU

    Quote Originally Posted by neazoi View Post
    Hm... But I have tested this with the simple linear PSU and also with the LAB psu, which has a ground (the mains ground)
    Don’t confuse mains ground with the negative supply terminal; they’re generally isolated.it’s the negative terminal(common) that needs attention.



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    Re: HUM in transmitter PSU

    Quote Originally Posted by barry View Post
    Don’t confuse mains ground with the negative supply terminal; they’re generally isolated.it’s the negative terminal(common) that needs attention.
    Tha LAB PSU has a ground terminal which is bridged with the negative terminal of the PSU. So you can optionally disconnect the GND of the house from the minus pole. I have it connected.
    Professional engineering is the top, but amateur engineering is more fun.
    It is when you cross the barrier between these two, that things become really fascinating!



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