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AM radio receiver - design

Discussion in 'Electronic Design' started by Joe G \(Home\), Feb 6, 2009.

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  1. Hi All,

    I am looking for a super sensitive / selective AM radio design.

    Where I am I get lots of interference ....

    Have you seen any good designs on the internet you can suggest.

    AM broadcast band 500K - 1.6MHz

    PS... I will be able to manually tune to correct frequency.

  2. Rich Grise

    Rich Grise Guest

    For sensitivity, more antenna, preferably vertically polarized;
    possibly an antenna tuner:"antenna+tuner"
    For selectivity, a Q multiplier:"Q+multiplier"+circuit

    Hope This Helps!
  3. Charles

    Charles Guest

    One then adds a noise canceling (phasing) circuit (with two antennas feeding
    it) in front of the above radio (or its equivalent).

    And one then regrets the whole experiment.

    Conclusion: it can be done but it is a major PITA and the results are iffy.
  4. Guest

    The antenna is where you control noise. Google magnetic loop antenna.
    There are commercial versions, such as made by Wellbrook. There are
    also home brew designs, though generally tuned rather than broadband
    like the Wellbrook.
  5. MooseFET

    MooseFET Guest

    If this is for voice, you can get some improvement by band limiting
    the audio.

    If you make the AM radio a phase lock loop, you can get some
    improvement in the interference rejection. The simplest way to do
    this is to build a normal AM radio up to the detector stage. At the
    detector, use something like a HC4046 to phase lock onto the 455KHz
    signals carrier. The filter in the PLL needs to be quite slow and you
    have to use the XOR phase detector.

    The VCO of the PLL needs to run at at least 2x the IF. You use some
    flip flops to make a 90 degree version of the IF for the XOR and a in
    part to drive a HC4053.

    The 4053 sync demodulates the IF giving you the audio. Noise energy
    that happens to land at 90 degrees to the long term average of the
    carrier is rejected.
  6. Guest

    What makes you think that?

    A PLL detector can give THRESHOLD reduction but that is not the same
    as interference rejection.

    For broadcast AM reception, a directional magnetic loop antenna is a
    good start as others have said. Where/ what is causing the
    interference? Get rid of the lamp dimmers in your house.

  7. JosephKK

    JosephKK Guest

    If they are that old, they are probably still good. They still built
    forever back then. Besides, what would the failure mechanism be?
  8. MooseFET

    MooseFET Guest

    The PLL locks onto the carrier and has a filter with a low bandwidth.
    This means that the phase of the VCO won't change due to noise that is
    far from the carrier. For purposes of understanding why it works,
    assume that the VCO is running exactly in step with the carrier and
    that in effect the PLL has zero band width.

    Now consider the side bands that get demodulated. A noise component
    that can be thought of as resulting in a signal like:

    Y = f(t) * cos(wt) + sin(wt)

    Y = the signal
    f(t) = some random (noise) function with no DC
    sin(wt) = the carrier
    cos(wt) = 90 degrees to the carrier

    is the important one for the argument. This signal will result in
    noise from a standard AM demodulation but no noise from the PLL and
    sync demodulation one.

    Now consider the intelligence on the AM signal. It is a function

    Y = f(t) * sin(wt) + sin(wt)

    The two methods of doing the AM demodulation give equal values for
    this input.

    The result is that the PLL based method makes the same signal and less
    noise on its output.
  9. Guest

    Maybe in theory you are right. But anyone with a radio that has
    synchronous demod will testify that synch demod doesn't help with a
    noise source. It is good for rejecting noise from the adjacent channel
    if you passband shift.

    The magnetic loop really does the job. The designs that float the loop
    above ground are more effective. [The Wellbrook does that.]
  10. TheM

    TheM Guest

    TCA440 was a popular choice as well.

    Anyone know of a modern variant IC that does AM (linear AGC IF)
    decently. All I can see now are FM IF receivers and some completely
    integrated radios (FM+AM+PLL etc) that are too complex and
    draw lots of current (>100mA).

  11. MooseFET

    MooseFET Guest

    3dB of improvement may be hard for people to notice but it is none the
    less real.
    It prevents the carrier on the adjacent channel from multiplying with
    anything so it keeps all of its side bands at high frequencies so yes
    it does a good job of that.

  12. amdx

    amdx Guest

    As you have seen the problem is generally not the receiver, it's the noise
    on the band.
    Check out this site, Dallas Lankford has been working at this problem for
    many years.
    Reviewing his many projects will give you an education on the source of
    problems and
    some ways to minimize them.
    I built one of his phasing unit about 13 years ago, had a lot of fun with
    it. Several times
    I had a situation where I nulled a station, and then by flipping a switch I
    could hear another
    station on the same frequency. I think he is now 2 maybe 3 generations past
    the one I built.

  13. Guest

    Some loops are built over a ground plane, other use a transformer and
    float above the ground.

    I'm pretty sure I'd hear a 3db reduction in noise with synchro. I'm
    not sure why you think the snr would be 3db better. If it is because
    you are listening to just one side of the AM signal, you need to
    remember that you are getting half the energy as well. Thus the narrow
    bandwidth doesn't buy you much unless you managed to avoid the QRM/N.
    Synchro helps a bit with fading since that phenomena is due to the
    mixing of signals (direct and reflected). [Narrow the BW, less room
    for constructive and destructive interference.]

    It's really hard to design a decent synchronous AM receiver. Most of
    the built-in synch demods are real crap. Even Drake has to rev their
    synch a few times. Sherwood sells an external synchro box because
    everyone else does a piss poor job. Probably the R8b and AR7030 are
    the only receivers out there with passable synchronous demod.
  14. Guest

    I've seen Lankford's work. A lot of it is dubious at best. I recall he
    had a "paper" where adding a lowpass filter in the audio path would
    reduce fading.

    The phasing schemes work if the QRM/N is local. I have a ANC-4 for
    that purpose. MFJ makes one as well. Still, nothing is a good as using
    a Wellbrook. There is something about the "large aperture" of the
    Wellbrook that reduces the local noise.
    Having hacked with the older ALA100 (there are a few versions now), I
    find a 4x4 loop of copper pipe (easy to build) does the job for AM
    BCB. If you want a deep null, you need to give the antenna space. I've
    done handheld direction finding with the ALA100 and a 2x2 loop. For
    most urban areas, about 4x6 is the practical limit. I find the
    attenuator turning on for some local stations with that much aperture.
    In the boonies, anything goes I suppose. I have use the ALA100 with an
    8ft on a side loop. I could pull in NDBs from 1000 miles away in the
    daylight, but only in remote areas.
  15. MooseFET

    MooseFET Guest

    You'd have to do an A/B test quickly. 3dB isn't huge.
    Go back and re-read what I said. Look carefully at the noise it
    removes. It takes out 1/2 the noise. Since we are talking about
    random stuff the difference is 3dB.

    No, it is because the sync demod rejects half the noise in the
    combined sidebands. The is double sideband with a carrier. It is
    standard AM radio not SSB. The 3dB improvement happens only if you
    have both side bands and the carrier. Only if you have both sidebands
    can you have a signal like:

    Y = f(t) * cos(wt) + sin(wt)

    That equation can't exist in a SSB world.
  16. amdx

    amdx Guest

    Please look through the papers and on the URL I postedm and post any
    material you find. Inquiring minds want to know!

    Phasing schemes are also helpful to null out unwanted stations on the same
  17. Guest

    That silly paper about the elliptic filter reducing fading comes to
    mind. As far as I know, Lankford writes those papers in the format of
    a technical journal, but they are neither published or peer reviewed.
  18. Guest

    First, I need to dig up a communications book since I don't know if
    you noise model is legit. But look at this this way. You are
    receiving one side band. Half the signal, half the noise. Where did
    you gain 3db?

    y(t) = (1+f(t))*cos(wt)) + n(t)
    where n(t) is the noise seems like a more reasonably model.
  19. MooseFET

    MooseFET Guest

    Trust me it is. With noise, you can say that any function is part of
    it and then say that the rest of it is everything but that function.
    This is a nice thing about talking about random stuff.

    I am talking about a normal AM signal with two side bands and a
    carrier. You are changing to talking about something else.

  20. MooseFET

    MooseFET Guest

    The OP is talking about AM band radio. This is not single side band
    stuff. There is a huge difference between the two in terms of sync
    demodulation's effect.

    In single sideband, the phase of the carrier is modulated.
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