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good radio, bad radio

Discussion in 'Electronic Design' started by RichD, Jun 12, 2013.

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  1. RichD

    RichD Guest

    I wear an armband scanning FM radio on my
    regular jog. (some good non-commercial stations
    nearby) It croaked, so I replaced it with
    another, same function - reset and scan buttons,
    2 AAA batteries.

    The new one is noticeably superior; better S/N,
    picks up more stations, and stays more solidly
    locked, when the signal gets funky. Both decode
    stereo, switching to mono when the signal weakens.

    FM receiver is a very mature technology, wouldn't
    the circuits be boilerplate, at this point? Price?
    The new one is 20% higher, that's in the noise,
    for a cheap consumer product

    Your comments, gentlemen -
  2. Syd Rumpo

    Syd Rumpo Guest

    SWMBO has a fairly old Roberts small portable (jogable?) which I've
    repaired several times - the soldering on the earphone socket fatigues.
    I've tried several others, but nothing comes close in terms of
    sensitivity and noise on FM.

    I had a Philips jogable, the FM was crap but the long wave was better
    than any other I've tried. I gave it to a cricket fan so he could
    listen to the Test Match at work on BBC R4 198kHz.

    But yes, you'd think they'd all be the same.

  3. Jamie

    Jamie Guest

    So I would assume that chip must have a rather good sample and hold
    circuit in it? I mean, what if you want to sit there for a while?

    Years ago I made a frequency to analog converter that would react in
    change per Sin. I used bimos op-amps, 4066 as the switch, edge trigger
    transistor circuit to activate the analog switch for the sample and hold
    refresh. The lowest drift I could hold was ~2mv/s.

    That circuit worked well for what it was designed for, and this was
    before the internist came to my place. All we had was college access
    main frame and dialup back then, 300 baud, 9600 on a good day :)

  4. Tim Williams

    Tim Williams Guest

    Don't need S&H, PLL holds it on target. Standard fare for FM, plus the
    PLL demodulates. Make it so the PLL filter cap (integrator) has a slight
    pull-up bias, and the PLL only has to 'tug' on it (open collector style)
    to keep it steady. Disable the PLL and frequency drifts up; enable and it
    captures the next one. Finally, a comparator with hysteresis detects when
    it's at the top of the band, resetting it to the bottom (holding SEEK
    reduces it to a relaxation oscillator).

  5. Uwe Hercksen

    Uwe Hercksen Guest


    a calculation error may exist, my HP 32S and mee found only 75 cm.

  6. sms

    sms Guest

    The better FM tuners use DSP for less adjacent-channel interference and
    for more noise suppression. But I wonder if an armband type radio would
    go to that expense.
  7. I'm prototyping a new 80m foxhunt receiver using the Si4735.
    The manufacturer has provided (under NDA) some SSB software for the DSP,
    which allows off-tuning to produce a whistle... nice. It would be nicer
    if the software wasn't encrypted and the DSP was documented.

    The QFN is a 3mmx3mm chip, quite small enough for an arm-band receiver.

    Clifford Heath.
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