Frequency up-converter with low IF and high LO

Discussion in 'The Projects Forum' started by katey, Sep 6, 2012.

  1. katey

    Thread Starter New Member

    Sep 6, 2012
    2
    0
    Hi,
    I've been working on an up-converter based on two gilbert cells operating in quadrature. The LO frequency is 2.5 GHz and the IF frequency is up to 250 kHz. Has anyone ever designed a converter like that? I can't make it produce a clean single tone frequency.
    Thanks for any help
    Katey
     
  2. vk6zgo

    Active Member

    Jul 21, 2012
    677
    85
    The problem is that your IF frequency is too low,compared to your LO frequency.
    You need an IF at VHF,or at the very least,high HF,around 30MHz.

    The resultant output frequency with your setup will include, at the least,two signals:

    at 2.49975 GHz

    &

    at 2.50025 GHz.

    A filter at those sort of frequencies won't be able to differentiate between them.
     
  3. katey

    Thread Starter New Member

    Sep 6, 2012
    2
    0
    Thanks for your reply. I'm using two mixers in quadrature and subtract the resulting signals to get my up-converted 2.50025 GHz, so I don't need a filter. The resulting frequency is fairly accurate but there is some amplitude modulation on the output signal, which I'd like to get rid of without using an additional filter.

    So you would say that this design is not possible due to the low frequency, as I've already suspected?

    Thanks.
     
  4. vk6zgo

    Active Member

    Jul 21, 2012
    677
    85
    The method you are using should cancel the IF frequency & the original 2.5GHz LO,but I don't think the quadrature subtraction will remove the resultant at 2.49975 GHz ,as that will be generated in both mixers.

    The only way to remove that is filtering,& filters around 2.5 GHz are not sharp enough.

    Google for "microwave up-converter" & you will find they always use a high IF because of this problem.
     
  5. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    656
    Can you show the math that results in the upper sideband only?
     
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