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Discussion in 'Electronic Design' started by RealInfo, Feb 1, 2013.

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  1. RealInfo

    RealInfo Guest

    Hi all
    What is the "right" total amplification factor of an IF amplifiers chain in a radio receiver ?

    For example a receiver employs 4 IF amps cascade , what is the proper total
    voltage amplification of that IF cascade ?

    Thanks in advance

  2. AndyS

    AndyS Guest

    I liked putting the total gain of the RF, mixers, and IFs so that the
    system would start AGC on noise. Then I could use the RF gain
    or If gain control to back off on gain for specific applications.
    I also liked to put a primary bandwidth filter as close to the
    antenna as possible, usually at the IF input, in order to eliminate
    adjacent channel signals, AND, if economics allowed, putting another
    primary bandwidth filter at the IF output, to set the system noise
    bandwidth.... If good Xtal filters are used, this is nearly always
    too expensive for commercial production, so there is usual
    a compromise position determined, depending on the system

    So my answer to your question is that the IF gain should be
    combined with the other gains/losses to develop an overall
  3. brent

    brent Guest

    As Jan said,

    It can be next to nothing or a lot. It all depends on architecture.
    If you are digitizing the signal you likely do not need a whole lot.

    I was taught to build a table which tracked noise power through the
    channel on one end and how close to compression the highest expected
    signals got on the other end. Build a table which shows the
    increases /decreases in noise power and also for the strongest
    expected signal coming in. This table should get you started
  4. brent

    brent Guest

    I forgot to add, you need to also track the weakest expected signal
    and keep track of S/N ratio degradation throughout.
  5. Fred Abse

    Fred Abse Guest

    Depends on the required level at the demodulator, the gain (if any) at the
    mixer, any pre-mixer (RF amplifier) gain, the available signal voltage. If
    it's AM, AGC is usually used. FM receivers are usually designed to hard
    limit (clip).
  6. Guest

    What these guys are talking about is something called “gain line-up”.

    It’s the system architecture design that balances the “large signal” distortion effects vs small signal s/n requirements.

    As they've said, final “baseband” requirements will dictate overall specs.

    I'd tell you to do a search, but you most likely already have; it's not a trivial exercise if you don't have any rf experience.

    good luck
  7. Guest

    Here's a link to hp's old appCad.
  8. Bill Bowden

    Bill Bowden Guest

    So, if the smallest signal input is 1 microvolt, and you want 1 volt
    out, the gain is 120db? And that includes audio gain of maybe 20db?

  9. Guest

    But, but, but dB only refers to power, right? ;-)
  10. Guest

    it's just a conversion, note the reference in the following;


    also note rf systems generally are 50 ohms, cable 75 ohm.

    take that into account when doing the conversion.
  11. Jeroen

    Jeroen Guest

    Decibels are fertile ground for confusion, much like percentages are
    for bankers. The basic definition is 10*log10(P1/P2). _If_ both are
    electrical signals working into the same impedance, this is the
    same as 20*log10(V1/V2). The trouble is that not everyone is equally
    careful with this.

    For example, control system engineers employ 20*log10(V1/V2) and
    totally ignore impedance. They even use dB to express ratios between
    different units, e.g., as the ratio between say, voltage and pressure
    of a transducer. (I once got into an argument over that with Dr.
    Middlebrook.) RF engineers will usually, but not systematically,
    take the impedance into account. For example again, my Rohde &
    Schwartz 50<->75 matching pads are marked as having the same
    attenuation in both directions, while my Radiall matching pads
    have two different values, despite doing exactly the same thing in
    the same way.

    Then there is the jungle of reference levels, with an abundant number
    of definitions, sometimes conflicting, often illogical. For example,
    dBm is dB referred to 1 mW, but dBu is referred to 1uV. I wouldn't
    be at all surprised if with some digging, we could come up with over
    a hundred different definitions of reference levels that are in
    common use in one corner or another.

    Jeroen Belleman
  12. Guest

    it's pretty straight forward if people are clear when they use the terms.

    gain = dB.

    input and output ports should always have an absolute value referenced to them.

    keep the impedance straight and it becomes simple arithmetic.

    btw, it's virtually impossible to do a gain line-up correctly if you don't keep this stuff straight when using mixed impedances and equiv nf and IM specs.
  13. Bill Bowden

    Bill Bowden Guest

    The original question was about voltage gain. 20dB is a factor of 10
    times the voltage. 20dB=10 , 120dB= 1 million.

  14. Guest

    I understand that it's late. Go back to bed.
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