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  1. #1
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    An applied RF system

    Hi all,

    Please look at the attached block diagram first.

    I'm generating a sinusoidal RF wave using ZX95-2536C-S+ with a ramp function applied to tuning input. Hence a sinusoidal wave having a sweeping frequency is supposed to be generated.

    Then I split the generated wave using ZX10-2-42-S+ splitter. The two splitted waves are then applied to a mixer (ZX05-43MH-S+). Then after LPF, I observe the output.

    I got a sinusoidal wave with a frequency of 5.52Hz and 550mV amplitude.

    Then I put a 3dB attenuator (VAT-3+) on one of the inputs of the mixer. Hence I'm attenuating one of the inputs of the mixer. And I expect a sinusoidal wave as the output of the mixer with again around 5.52Hz frequency with a decreased amplitude. However I got a sinusoidal with 884mV amplitude and 11.34Hz frequency. Why, do you have any idea?

    Cheers,

    •   Alt8th November 2013, 15:09

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  2. #2
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    Re: An applied RF system

    It sounds like the addition of your attenuator has changed the mixer diodes into full wave rectifier. Are you shorting out some DC path? - try putting a capacitor in series with it.

    Frank



    •   Alt8th November 2013, 17:47

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    Re: An applied RF system

    Thanks a lot.



    •   Alt21st November 2013, 14:43

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    Re: An applied RF system

    you are getting spurious outputs. since you are applying identical RF signals (same amplitude and phase) to two ports of the mixer you should be getting a constant DC output voltage (assuming the rf levels are high enough to drive the mixer diodes). Try putting a time delay in one of the paths, like 10 feet of coaxial cable, sweep the oscillator, and THEN tell us what u see.



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    Re: An applied RF system

    I do not know the purpose of your circuit. If you feed mixer LO and RF ports with equal signal frequency and power as shown, then a balanced mixer operates as a multiplier (others say a correlator). Then the IF output will contain the DC component proportional to input power, and harmonics of the input signal .You did not specify the LPF.
    With one input attenuated by 3 dB, the DC output will drop a bit. DC coupling at any port can cause mixer imbalance, so make sure you have capacitor coupling at mixer ports.

    - - - Updated - - -

    By replacing the 3 dB attenuator with a variable phase shifter, you can adjust 90 degrees phase shift, then the RF carrier will be rejected and at mixer IF output you will get the phase noise of the VCO nicely plotted over frequency.



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    Re: An applied RF system

    What is the frequency of your sweep function ? Since you are sweeping the freq, how do you get a fixed 5.52Ghz at the output ?
    I assume you mean 'GHz' everywhere.....

    Lastly, I observed that your mixer has a max isolation figure at the 2.76 - 2.92Ghz range



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    Re: An applied RF system

    Let me add a comment although the topic is one year old. Do you know how FMCW radars measure the range? When a (linearly modulated, i.e. with a ramp)FMCW waveform is mixed with its delayed version, a single frequency component occurs at the output according to the amount of delay. Think about your setup in terms of signal paths/delays. At the first experiment nothing look causing delay but there may still delay difference between the paths.
    When you add a attenuator, note that you also put delay into the path. The frequency at the mixer output depends on the frequency of the ramp function strictly. If you lower the frequency of your ramp function, the output frequency lowers.



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