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Radio Intermediate Frequencies

Discussion in 'Electronic Design' started by James Douglas, Feb 15, 2006.

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  1. I am looking for a schematic that I can build that will teach me about
    intermediate frequencies and radio reception. I want to be able to
    simulate the circuit in CircuitMaker then build it and test it.

    I am building/learning about radio reception and am not going to
    continue building the radio without a through understanding of how
    these intermediate frequencies work.

    Thanks!
     
  2. Tim Wescott

    Tim Wescott Guest

    I'm not sure that any general-purpose circuit simulator will be up to
    helping you much with superheterodyne radio design. AFAIK you pretty
    much have to use mathematics, red in tooth and claw, to analyze the
    performance if the thing. Fortunately the math doesn't have to be that
    hard.

    I suggest you look up the Amateur Radio Relay League (ARRL). Their
    "Handbook" has many many basics including (probably) a superhet receiver
    if not two or three. Also look at "Experimental Methods in RF Design"
    by Hayward et all, and "Introduction to Radio Frequency Design" by
    Hayward. I have the latter, and I've seen good recommendations for the
    former.

    On to your problem. Since it isn't clear about what you don't
    understand it's hard to recommend a circuit. Let me give you this
    thumbnail explanation, and you tell me where things are unclear:

    The principal of the superheterodyne radio (i.e. one that uses an
    intermediate frequency) is this: it is relatively easy to build a good
    radio as long as all of the finely tuned circuits have fixed tuning, and
    it is relatively easy to build a gizmo that takes a radio frequency
    signal and shifts all of it's content by a fixed frequency offset. So
    you build a really good fixed frequency radio and a really good
    frequency shifter, stick them together, and voila! a good radio.

    The fixed-frequency radio part is called the 'IF strip', and the
    frequency at which it operates is the 'intermediate frequency (IF)'.

    The frequency-shifting part is called the 'front end*' or the 'mixer' or
    (in older material) the 'first detector'. It's frequency is set by the
    variable frequency oscillator (VFO) if the radio is tunable.

    * Purists: Yes, the mixer is just part of the front end, at least in
    many radios. This is a basic, educational post. Go away.

    --

    Tim Wescott
    Wescott Design Services
    http://www.wescottdesign.com

    Posting from Google? See http://cfaj.freeshell.org/google/
     
  3. This stuff has been beaten to death lately, but here goes. ;-) Since
    all RF circuits are, by nature, sensitive to variations in frequency,
    radios tend to use an IF (intermediate frequency) for achieving good
    sensitivity, linearity and selectivity. The IF is usually chosen to be
    the "difference" between the desired incoming signal and the LO (local
    oscillator).

    In most portable AM type receivers, the IF is chosen to be 455kHz. That
    means in order to receive 545kHz thru 1650kHz, the LO must be able to
    tune a range of 1000kHz thru 2105kHz. The LO is "mixed" (technically
    this is a multiplication process even though it doesn't seem that way)
    with the incoming signal. This process produces four output frequencies
    (the two originals, the sum and the difference).

    For example if we're trying to receive a station on 1000kHz, the LO
    would be tuned to 1455kHz. This would produce a mixer output containing
    signals on 455kHz (the difference), 2455kHz (the sum),1000kHz and
    1455kHz. At this point the signal is passed thru a low pass filter to
    remove everything above about 500kHz, a relatively simple process at
    these frequencies. Then the signal is fed to an amplifier stage(s) that
    are designed to work best at exactly 455kHz. Then this signal is passed
    thru a diode (detector) and another low pass stage to remove the RF and
    leave only the audio signal.

    This is a simple description that only scratches the surface of the
    theory behind all of it. There are technical reasons to pick specific
    IF frequencies and to even have multiple IFs in a radio. Still other
    times you may want the ultimate in simplicity and choose not to have an
    IF at all. Google must surely be filled with pointers to information on
    this.
     
  4. Geez, and you didn't even mention the preselector. (;-)
     
  5. CWatters

    CWatters Guest

    Building and simulating may not be the best way to learn about this. Google
    for articles on the Superhet principle.

    Figure out why this equation is important..

    sin(x) sin(y) = 0.5 cos(x ? y) ? 0.5 cos(x + y),.
     
  6. The hell you say?

    Jim


    At this point the signal is passed thru a low pass filter to
     
  7. Guest

    I want to light a torch bulb from a battery source.
    The torch bulb specification is it is the smallest one found in the
    market.A MES is good enough.

    I do not know of any other power sources so I am thinking of a pencil
    battery. But I would prefer any LIGHT WIEGHT power source. Cost is not
    any factor but the whoile system should be light weight. I would like
    to build a car that have head lights and so is all this. My car is 7
    inch in length. and should be very light wieght again!

    Thanks.

    Masroor.
     
  8. Guest

    Guest Guest

    : I am looking for a schematic that I can build that will teach me about
    : intermediate frequencies and radio reception. I want to be able to
    : simulate the circuit in CircuitMaker then build it and test it.

    : I am building/learning about radio reception and am not going to
    : continue building the radio without a through understanding of how
    : these intermediate frequencies work.

    James,

    Find a diagram of a superheterodyne receiver. I'm sure you might
    find one by just doing a google search for "superheterodyne receiver." It
    will help you follow along with what I am saying.

    I don't have time to post a detailed explanation, but in a
    nutshell, the IF is a free parameter in a (super)heterodyne receiver that
    is chosen to trade off the first stage filter "Q" or bandwidth/sharpness
    with second stage filter/amplifier bandwidth/complexity.

    The higher the IF, the the lower the Q of the first stage filter.
    Choosing a higher IF may allow for the use of less expensive first stage
    (RF) filters (Lower Q generally = Lower cost,) although these days, it's
    more common to see lower or even 0 IF (also called direct conversion)
    receivers.

    The lower the IF, the lower the band of interest of the components
    of the second stage (filter, amplifier, mixer.) Choosing a low IF may
    allow for the integration of the IF filter in an IC receiver (which is
    what I am most familiar with) or using a cheaper external filter (crystal
    vs. SAW, etc)

    So, in a general sense, you can think of the choice of IF as a way
    of trading off first vs. second stage complexity. For lots of types of
    receivers (FM, TV, etc) there exists a "customary" IF around which lots of
    ICs (which may implement an entire second stage of an FM receiver, for
    instance) have already been designed. Therefore, you don't see the IF
    being changed in those types of receivers, although it can be.

    That's the general idea...

    Joe
     
  9. Guest

    It generates double side band, local to receiver.
    By the way, why is it important mixing to get intermediate freq instead
    of detecting audio directly from RF?
     
  10. Thanks for all the great information! I will be reviewing everything
    today as I'm sitting home with a sick kid.
    I was hoping that I could simulate a circuit with two input frequencies,
    for example 1Khz and 5Khz and somehow view/measure the output to be
    XKhz? I will continue to research the superhet type devices. I do have
    that AARL book around here somewhere.
     
  11. Well? I'm guessing that I've made some horrible error. Perhaps it's a
    band-pass filter?
     
  12. Better yet, go for broke. Pretend that the whole world is made of
    nothing but complex numbers. In other words, pretend that complex
    numbers are not a special case of regular numbers that we learned in
    grade school, but the end-all in general of quantities, and that it is
    we, the humans, who have been operating in a mode of deficiency since
    the very first time we learned to count.

    Then you can assert that all functions are complex, where every part
    that make them up is potentially complex. Then, for a wide variety of
    functions, it is true that those functions can be represented as sums
    of complex exponentials on t:

    x(t) =Sum {-infinity, +infinity} (complex coefficient)*e-to-j-omega-t.

    It would do you great benefit to take random "grade-school" functional
    patterns of t that you make up yourself (sines, cosines, ramps, boxes),
    and see if you an represent the functions as a sum of clumps where each
    clump is a complex co-factor applied to e raised to j omega t. Keep
    figeting with the per-omega clumps to get the signal to look right in
    the time domain. This is most likely what Fourier did before he
    arrived at his convictions.

    If you view the world this way, as if all numbers were complex,
    including the number of pieces of fruit that you last bought at the
    supermarket, you will feel a lot better about all of this, because
    there will be no more special cases, as everything will be complex, and
    the vast majority of quantities that we experience each day, the
    complex part just happens to be zero.

    Then take the two pure sinusoids that you plan to mix, use Euler's
    Theorem to treat them as two complex functions as above.

    Multiplying them together (heterodyning) will quickly reveal, by
    definition of multiplication of *any* two exponential functions (add
    the exponents), that the frequencies will add in the resulting signal.

    Then if you take a x1 to be sum of two sinusoids, and x2 to also be sum
    of two sinusoids, and multiply them, you can see the blobs that they
    make in the frequency domain (again by adding).

    If you keep adding sinusoids to x1 and x2 so that they become "rich" in
    time (and therefore spectral pattern becomes less spike-like), you will
    see that the multiplication in time domain results in convolution in
    frequency domain.

    -Le Chaud Lapin-
     
  13. CWatters

    CWatters Guest

    oops where did all the ? marks come from.

    Well I guess you are all clever enough to know what I meant.
     
  14. The normal coupling out of a mixer is with an RF transformer tuned to the IF
    frequency. Let's see why a lowpass filter isn't of much use using your
    example of a 1000kHz signal, a 1455kHz local oscillator, and 455kHz. IF
    frequency. Let's go still further and postulate a 2-pole LPF which can be
    made about the same size as that IF transformer. Put the cutoff right at
    500 kHz. so that we don't lose a lot of the 455kHz. energy.

    What's the attenuation at the signal frequency? Well, the slope of a
    2-section RC filter is 12 dB per octave, and that is exactly an octave, so
    you reject 12 dB of the unwanted signal frequency. 1455 isn't much above
    that, so you lose perhaps another couple of dB. Big deal. When dealing in
    an environment that has to handle microvolts to millivolts, 15 dB or so is a
    drop in the bucket.

    However, with an IF transformer, you will be down something on the order of
    50 to 60 dB at the signal and about the same at the LO. Now we're talking
    some decent attenuation.

    Jim
     
  15. Tim Wescott

    Tim Wescott Guest

    The basic process of detecting audio (or data) from RF involves
    filtering the RF, then detecting it as if the signal of interest were
    the only thing there. That filtering step gets extremely complicated if
    you try to make it tunable to any old RF frequency. By translating the
    signal to match a fixed frequency filter-and-detect strip (the IF strip)
    you simplify the radio design.

    --

    Tim Wescott
    Wescott Design Services
    http://www.wescottdesign.com

    Posting from Google? See http://cfaj.freeshell.org/google/
     
  16. Zak

    Zak Guest

    You can: you will see an output consisting of 4 and 6 KHz tones.

    Mix, say, 4 and 5 KHz and you will get 1 and 9. Filter that and you are
    left with 1 KHz. Then, vary one of the frequencies and see what happens.
    Or apply 4 and 4.5 on one input and5 on the other of the mixer, and
    check what happens.
    Excellent books. You need to get a feel for how this works - once you
    graps it all kinds of things become possible.


    Thomas
     
  17. Don Bowey

    Don Bowey Guest

    I believe a Direct Conversion design, which eliminates the need for an IF,
    makes separating the modulation from the RF signal quite simple. Direct
    Conversion uses demodulation rather than envelope detection, so the signal
    is baseband audio straight out of the mixer. A simple low pass filter (RC)
    will prevent RF from affecting the following audio stage(s). If one wishes,
    wideband and narrowband audio filters can be switched in/out for "music" and
    CW.

    Don
     
  18. Damm, you guy's are either really smart or good bullshitters, I am
    thinking smart. I appreciate all the help and now am armed with new
    information to continue my experimentation.
     
  19. <snip>

    They are not necessarily mutually exclusive characteristics.


    Best regards,
    Spehro Pefhany
     
  20. Rich Grise

    Rich Grise Guest

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