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MC12079 internal structure

  1. Jul 14, 2015 #1
    Hello. I use a MC12079 as UHF divider by 256. Unfortunately this IC was oscillating at about 10 MHz. To correct this problem I insert a 47 k resistor between non inverting input and Gnd as Inverting input is used for signal to divide. It works, but I would like to know if this can have unwanted consequence on other parameters for example sensitivity. My question : has someone a precise idea of what is inside the input module ( in+ & in-) just in front of 1st flip-flop ( detail schematics) ?
     
  2. jcsd
  3. Jul 14, 2015 #2
    You put the resistor in parallel with the capacitor?

    Can you post schematic?
    What is the input freq and amplitude?

    What do you mean "oscillating at 10MHz"? Is the output 10MHz superimposed on the intended RF, or is it just outputting 10MHz?
    Is the 10MHz oscillation full power or at a small parasitic level.

    Sounds to me like you may have a layout issue, I would not feel comfortable applying the 47K fix not knowing the root cause of the problem.
     
  4. Jul 15, 2015 #3
    Hi. Oscillation occurs even with unconnected signal imput. I made several tentatives to join schematics pic but unsuccessfully ! Sorry
     
  5. Jul 15, 2015 #4

    tech99

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    Gold Member

    Seems to have an op amp input. The data sheet shows a 1.2k resistor load between output and ground. The non inverting input has a 50 ohm load resistor. Wondering if you have these in place, obviously with very short connections?
     
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