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Why are mixers noisy?

Discussion in 'Electronic Design' started by Clifford Heath, Jun 17, 2013.

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  1. I've often heard it repeated (by radio hams) that mixers are noisy, but
    never seen a good explanation of why. I'm not talking about injecting
    noise either directly or via jitter/phase-noise, I mean the mixer
    itself. Where does the noise come from? How can the noise be minimised?

    Anyone care to expound?
  2. Phil Allison

    Phil Allison Guest

    "Clifford Heath"
    ** Why do radio hams say shit like that ?

    Two reasons come to mind:

    1. Because they have heard other hams say it and it sounded clever to them.

    2. To impress fools and make themselves sound clever.

    YOU need to go ask them one simple question:

    What does that even mean and does it matter in a typical receiver ?

    Come back when you have a sensible answer.

    I won't be holding my breath.

    .... Phil
  3. Robert Macy

    Robert Macy Guest

    don't mixers automatically split the tones, dropping them 6dB? Noise
    stays put, thus effectively making it 'look' like noise was added.
  4. Guest

    noise cals in a rf system are based on nf ---

    depending on the system design, the mixer tends to be one of the
    first elements in the gain lineup. and as jim said, if there's a
    lossey element it adds to the nf.

    that's why some systems offer low noise pre-amps for certain apps.

    it's all about trade off's.
  5. Joerg

    Joerg Guest

    The reason mixers are noisy is the cheap brush type motor in them, the
    gears, and the large ice cubes people drop in there for their margarita.

    Oh wait ...
  6. A silly question. First is conversion loss the same as insertion
    loss? And if so why does insertion loss add to the noise? I'd figure
    that if I attenuate the signal by 3dB I'd also attenuate the noise by
    the same amount. I was thinking that mixers are noisy because they
    add signals from both the carrier and the image frequency... (I may be
    using those words wrong. The output sees the noise from both sides of
    the LO frequency.)

    George (not an RF guy) Herold
  7. miso

    miso Guest

    This implies if you use a Hilbert transformer and produced SSB, there is
    no mixer noise. Something to meditate on when your navel is no longer a
    sufficient target.

    It is interesting how the better radios are the ones with more mixers,
    i.e. triple conversion is praised while double conversion is lambasted
    for too many images. Of course there is more to the foo than just mixer
  8. Hi guys.. sorry for the repeat question.
    (I must have a pencil stuck from ear to ear, 'cause my brain ain't
    So let's stick with the 'plain' diode ring mixer. It's got 6dB
    (voltage goes down by 2) of conversion loss. No problem. But why is
    this a 6dB noise figure?
    Doesn't the noise at the input suffer the same conversion loss?
    Maybe I don't know what the noise figure is. I thought it was the S/N
    ratio from input to ouput. If the ratio stays the same isn't that

    George H.
  9. Jeroen

    Jeroen Guest

    Everything coming in from the antenna is signal, by definition. The
    noise is what originates from the receiver itself. Since most of that
    comes from the first IF, it doesn't get attenuated by the mixer. So
    since the mixer *does* reduce the signal by 6dB, that's a 6dB noise
    figure right from the start.

    The noise figure is S/N at the output over S/N at the input. In other
    words, it's a measure of how much the receiver deteriorates the
    original S/N.

    By the way, putting attenuators on a noise source does not necessarily
    reduce the noise level. I have one where it would actually *increase*
    the noise. I use it for amplifiers with noise figures well below 3dB.

    Jeroen Belleman
  10. You need one of those nice old Hamilton Beach blenders not a mixer.

  11. Ahh, I guess it depends on where the noise comes from.
    A resistor divider will attenuate both noise and signal.
    (As long as the resistors are low enough... Johnson noise of divider
    R's less than input noise.)
    Maybe I'm confused about what the 'noise' is.

    George H.
  12. ohh.. OK. I figured there must be some noise from the antenna too...
    I mean you still get something from the antenna even when the signal
    source is turned off.
    I think I've got it. The 'noise' is mostly after the mixer. I was
    thinking of just adding gain before the mixer. But I guess the high
    frequency gain 'costs more', noise-wise, than the mixer and lower
    frequency gain.
    Well OK if you've got big value resistors.
    (Just give me enough power and these 1% milli-ohm resistors and I'll
    have that excess noise down to almost nothing. :^)

    George H.
  13. Thanks, I sorta understand noise, but some of that is stil
    I guess if I knew how to measure the noise factor I'd understand it

    (Seems what we need is a way to terminate the signals without a
    resistor, suck 'em into an active device.)

    George H.
  14. Tim Williams

    Tim Williams Guest

    Ah, but the low-R divider shunts the (line impedance) source, so its noise
    doesn't go anywhere, anyway; the result is mostly noise from the divider
    itself, at whatever impedance it has. :)

  15. Mmmh. Attenuators are usually impedance matched. What you are
    actually doing by choosing very high or very low resistor
    values is to intentionally mis-match the corresponding
    noise sources, so that their available noise power is mostly
    reflected and doesn't end up in your signal. That's one of
    the keys to low-noise design.

    In my mind, all resistors have the same noise: P=kTB.

    Jeroen Belleman
  16. Tauno Voipio

    Tauno Voipio Guest

    Sorry to disappoint you, but but...

    As long as you have something with the ratio of voltage and current
    like a resistor, it will have at least the thermal noise of an
    equivalent resistor, due to the thermodynamics' law of entropy.

    For terminating a transmission line, you need a thing looking
    like a resistor of the characteristic impedance.
  17. Tauno Voipio

    Tauno Voipio Guest

    That's why you have to pay an arm and a leg for filters in the
    first and second IF. The same applies for oscillator and mixer
    shielding. A mixer is a very potent spewer of oscillator spurs.
  18. That isn't true! It's quite possible, using active devices, to
    make a resistive impedance with less noise than a resistor of
    that value. I've done it. It works.

    Jeroen Belleman
  19. Joerg

    Joerg Guest

  20. Thanks Jeroen, I guess I'm mostly living in the low frequency world, whereI can buy an opamp and lower the source impedance. (Given opamp noise andall the other caveat’s.)

    (Crap... Google won't let keep using the old groups and now I'm posting viathe new groops... which looks like $H!t. Sorry.)

    George H.

    That's one of the keys to low-noise design. In my mind, all resistors have the same noise: P=kTB. Jeroen Belleman
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