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  1. #1
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    need clarification on IF frequencies.

    hi please clarify me .
    how to select the IF frequency.based on what criteria we have to select particular freq.
    for example in our project RF FREQ is down converted to 240 MHZ IF freq...
    based on what criteria it is decided.
    if you have two IF frequencies 240 MHZ and 500MHZ.....
    Which one will you select....



    regards
    ram.....

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  2. #2
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    Re: need clarification on IF frequencies.

    Without considering receiver frequency range and bandwidth, the discussion is pointless.



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  3. #3
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    Re: need clarification on IF frequencies.

    hi we are using 6 MHZ SAW filters in our down converter.
    in our project(4.4GHZ TO 5 GHZ) transmitter IF is 140 MHZ and receiver IF is 240 MHZ..
    why two different frequencies are used and how to select.
    and does it depends on ADC OR DAC .



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  4. #4
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    Re: need clarification on IF frequencies.

    Usually, the first IF in a double heterodyne receiver is selected according to image rejection requirements and available input filter specifications. ADC parameters may matter in a digital receiver.

    All in all, there's too little information.



  5. #5
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    Re: need clarification on IF frequencies.

    please can you eloborate the explanation more clearly.how adc /dac parameters decide IF frequencies..
    i had a telephonic interview .in that they asked me regarding IF.
    THEY SIMPLY GAVE ME TWO if FREQUENCIES 240 mhz AND 500 mhz.which one will you select and on what basis.
    regards
    ram



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  6. #6
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    Re: need clarification on IF frequencies.

    They may just want to hear your reasoning. There are a number of factors, some are trade offs.

    Generally the high end limit of an I.F. is 1/5 to 1/7 th of RF frequency. Image response and RF filter is one of the first criteria. Too low an I.F. may put an unrealistic requirement on RF filter image rejection. Able-Baker spurious responses are another, basically harmonics of L.O. and RF frequencies. Able-Baker spurs will get you if I.F. is too high. All mixers create harmonics of their inputs. Sometimes A-B spurs are referred to as crossover spurs.

    If there are other clocks or oscillators in the circuit, from transmit side oscillators to microprocessor oscillators and bus frequencies, should be considered as possible internally generated interferers. For example if processor memory has to operate at 140 MHz then it would not be a good idea to use receiver I.F. of 140 MHz.

    If transceiver is full duplex (Tx and Rx operating at same time) then you would want to avoid using same I.F. freq for both to avoid cross interference.

    Where spurious responses show up maybe of importance. For example, it might not be wise to put the image in the UHF high power transmitter T.V. band.



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