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MK484 single chip AM radio question

Discussion in 'Electronic Repair' started by Albert, Apr 4, 2005.

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  1. Albert

    Albert Guest

    I hope to use the MK484 single chip AM radio at 20 kilohertz for a
    very low power receiver. The spec sheet says it only goes down to 150
    kilohertz however.

    Has anyone used the MK484 chip below 150 kilohertz?

    I know it works well at lowfer runs circles around my
    ICOM receiver! But, I'm not sure about the minimum usable frequency.

    I have the spec sheet for it (and it's more modern variant, the
    MLF501), if anyone wants a copy, please speak up.

    Any comments appreciated.

  2. Highland Ham

    Highland Ham Guest

    Have you considered a mixer arrangement with a xtal osc.

    Frank GM0CSZ / KN6WH
  3. Dale Parfitt

    Dale Parfitt Guest

    Hi Albert,
    What is the upper freq limit on this chip? I am always looking for a good AM
    dtector. 9MHz is current interest.

    BTW, if you are the same fellow asking about wire gauge- a
    micrometer/calipers is by far the more accurate method. The gauges just do
    not work well for small diameters. And, you'll find lots of other uses for
    the calipers.

  4. I read in alt.binaries.schematics.electronic that Albert <[email protected]?.?.invalid>
    Is there any reason to suppose it contains a coupling capacitor or
    something that would limit the low-frequency response? It seems rather
    unlikely. But the definitive test invokes a little-used technique in
    these days of calculations and simulations. It's known as 'trying it'.

    It isn't as if testing at 20 kHz is rocket science, after all.
  5. Chuck Harris

    Chuck Harris Guest

    Since this is a superhet chip (IIRC), the problem is more likely one of LO
    bleed through into the IF. The LO frequency would have to be on the
    high side of the receive frequency, so in this case, with a 455Khz IF,
    the LO would be at 475Khz. The LO signal bleeding through the IF filter
    would probably be as strong, or stronger than the 20KHz signal you were
    intending on receiving.

  6. I read in alt.binaries.schematics.electronic that Chuck Harris
    No, it's a TRF.
  7. At some (very) low frequencies the 1/f noise will make the stated
    noise figure invalid :).

    Some microwave bipolar transistors will have a quite bad noise
    performance at HF (and some even VHF) frequencies due to the 1/f
    noise, but since the maximum frequency for that AM radio is not very
    high, I very much doubt this would be an issue at 20 kHz.

    After all, the environment noise is quite high at these frequencies,
    so unless the antenna is very lossy, the receiver noise figure is of
    very little interest anyway.

    Paul OH3LWR
  8. Chuck Harris

    Chuck Harris Guest

    Ahhh! Well, that changes everything. I took a look for the spec
    sheet, and found the sheet for the ZN414Z, which is apparently almost
    identical to the MK484. There are two things that I think would limit
    operation at lower frequencies:

    1) the gain appears to be purposefully rolled off at about 40KHz. That
    is to say, it approaches zero at that frequency.

    2) the detected audio stage has bandwidth out to 20KHz.

    Imagine what would happen if you had a signal with a 20KHz carrier
    frequency. The carrier frequency would pass right through the detector
    and into the audio channel. The AGC, which appears to be audio derived,
    would get seriously upset, and if the 20KHz signal was modulated, you
    create some very interesting hetrodynes.

    That being the case, it appears that the manufacturer tried to keep the
    operating frequency far enough away from the audio passband that a simple
    filter would suffice to prevent interferrence.

    It probably won't work well at all as a 20KHz receiver.

  9. I read in alt.binaries.schematics.electronic that Chuck Harris
    Yes, having found a 'data sheet' (handwritten), I see it has four
    coupling capacitors between the stages. Not good news.
    That may be controllable, but it's beside the point of there is no
    useful gain at 20 kHz. In that case, a 741 would be usable, but a 709
    would be better (small signals). (;-)
  10. Albert

    Albert Guest

    Thanks Frank,

    Yes, I have considered a mixer/superhet.

    Can you suggest any particular chip that runs on 1.5 volts and pulls
    less than 300 microamps (which is what the mk484 draws)??

    Op amps don't have the gain/bandwidth necessary without drawing
    millamps from the battery. There are some low noise chips that do a
    little better than others, but they all pull way to much supply

    I'm wide open to suggestions, suggest away.

  11. Two reasons for using an IF amp are to amplify a fixed freq even as the
    receiver is tunes, and to amplify a lower freq so that the bandwidth is
    narrower. But since neither of these is necessary, it makes less sense
    to upconvert to 455kHz, or to use 455kHz as the IF.

    So it seems more logical to reduce the IF amps to the receiving freq, by
    padding the IF tuned circuit caps. There is one barrier though: you
    can't pad a 455kHz ceramic filter.
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