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RF receiver circuit

Discussion in 'Radio and Wireless' started by minh2211, Sep 29, 2015.

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  1. minh2211

    minh2211

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    Sep 29, 2015
    Hi, Im new to RF system design. I have a couple of questions regarding to it

    In RF receiver, why people put bandpass filter before LNA? Also if I have RF signal at 100 MHz, what should be the bandwidth of the bandpass filter?

    Why high order of filter is necessary in RF systems?
     
  2. AnalogKid

    AnalogKid

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    Jun 10, 2015
    An input bandpass filter reduces the chance of an input overload by a strong signal at another frequency.

    What is the bandwidth of the signal you want to recover and demodulate?

    High order filters have steep attenuation curves, which are necessary for selectivity.

    ak
     
  3. minh2211

    minh2211

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    Sep 29, 2015
    Thanks for your reply. I slowly understand things now. Ive been reading through a book. It says RF design requires impedance matching.
    Lets say my RF source at input has 50ohms and it goes through some blocks of circuit such as BPF, LNA, mixers, oscillator before giving output. How can i make sure the impedance matching at input and output properly?

    Do i have to match impedance at each block?
     
  4. GPG

    GPG

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    Sep 18, 2015
    What frequencies are you dealing with How about a bock diagram
     
  5. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    You have to choose a filter response before you can start to work on this. Then you can use specific filter tables and normalising techniques as one option.
    Adam
     
  6. AnalogKid

    AnalogKid

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    Jun 10, 2015
    Yes, although all of the impedances do not have to be equal. For example, your filter could have a 50 ohm input impedance to match the source but a higher output impedance into the next stage, as long as that stage is designed for a higher source impedanc.. In general, however, most designs have a constant impedance through all of the blocks. Impedance matching minimizes reflections and maximizes energy transfer from one block to the next.

    ak
     
  7. minh2211

    minh2211

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    Sep 29, 2015
    ok, the thing I find difficult is that at the first block, I can do matching but if I connect with next stage block, the input and output impedance need to be matched at that block will be different. Because the impedance at the first block affect the later stage.
     
  8. davenn

    davenn Moderator

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    Sep 5, 2009
    Interstage coupling/matching in both receivers and transmitters is commonly done using tuneable L/C circuits

    have a look at this VHF receive ( part) circuit I used to work on ....

    Comm2000 RX-TX Brdsm.gif

    It has filtering on the input consisting of C35-37 and L9 -L10
    then T1, LC matching to the FET's input impedance ( Q9 - The RF Amp)
    Then T2 , LC for matching between Q9 and Q10 FETs ( Q10 is the mixer)
    You will also notice that Q11 ( the local Osc0 is also LC matched to Q10
    Then there is a final LC match between the output of Q10 and the 10.7MHz crystal IF filters

    Having a tuneable LC circuit between stages allows matching ( therefore receive sensitivity) to be peaked


    Dave
     
  9. minh2211

    minh2211

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    Sep 29, 2015
    thanks, if anyone can give me detailed guide on how to design a LNA. that would be great
     
  10. davenn

    davenn Moderator

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    Sep 5, 2009
    --- what frequency ?
    --- what bandwidth ?
    --- what gain figure ?
    --- what noise figure is acceptable ?
    --- what IF frequency do you want ?


    Dave
     
  11. minh2211

    minh2211

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    Sep 29, 2015
    I want to design a LNA with RF at 100 MHz, IF at 10 MHz, gain about 15 dB. Im not sure how to decide on the NF. The LNA is after the bandpass filter
    It would be great if you can guide steps by steps. Im familiar with other amplifiers using bjt and mos. But what makes the design of lna different (i know it wuould be the noise level)?
     
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