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MK484 single chip AM radio question

Discussion in 'Electronic Components' 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. Joerg

    Joerg Guest

    Hello Albert,
    I haven't but isn't this chip discontinued as well? Just FWIW, I would
    build LF stuff in discretes and plain jelly bean parts. After all, at
    these low frequencies any old opamp works well. If it is for series
    production that will make the purchasers much happier. If it is for a
    prototype you can still get parts a few years from now if something breaks.

    Regards, Joerg
  3. Albert

    Albert Guest

    Yes, any old op amp will do....but, they all draw way too much power
    for my receiver. The MK484 only draws 300 microamps.

    The TLl081 draws 2 ma .

    I am thinking about using plain ole transistors, the receiver I am
    building doesn't need to be extremely sensitive.

    I have a source for the MK484 at present, and I might just order some
    and try it out myself.



    PS, did the vlf schematic show up on abse? If not, I'll gladly email
    it to you.

    Just as an update, I did some more work on the vlf transmitter. I
    disconnected the coils, whcih allowed me to probe the ASIC output
    feeding each audio amp chip.

    The very wide 17 Khz output peaks at 18.8 khz, but there are many many
    60 hz peaks with 15 hz peaks on each side.

    So, the 60 hz signal that many said must be power supply hum is
    clearly intentional, and is not a product of power supply filtering.
  4. Joerg

    Joerg Guest

    Hello Albert,
    Not all of them. The LPV324 is under 10uA per amp. GBW is just 150kHz
    but for this app it should suffice. At 30-40 Cents a quad not a bad
    deal. Large gains I'd do with transistors anyway.
    No, I couldn't. If you have a web site just post it there and a link
    here so all can see it. Else, yes, just email me.
    Beats me why they chose 60Hz. Are you sure it isn't ripple from an
    unregulated power supply? Maybe intended ripple?

    Regards, Joerg
  5. Albert

    Albert Guest

    Ok Joerg, I'll email it.

    Be on the lookout for it.

    I am positive there is no intentional leakage or injection of 60 hz.
    It should be noted that rectified and filtered residual ripple would
    be 120 hertz. I put spectrum lab on the dc at the filter cap, and
    there was no sign of 120 hz or 60 hz.

    There is no connection to any part of the ASIC to anything except dc
    power. I can't get info on the ASIC, but they were nice enough to
    include labels on the pins, so it is quite easy to figure out what's
    going on.

    FYI, there are 4 coils, each large and air wound. The dc resistance of
    each coil is 4.5 ohms.


  6. Joerg

    Joerg Guest

    Hello Albert,
    Thanks, got it. I just hope others who read here can see your post on
    They do feed non-regulated DC directly into the supplies of the audio
    chips. But considering their good PSRR you are right, this would not
    explain the rather high spectral level of 60Hz.
    This must be a pretty nifty concept. I remember when we tried 3D
    localizing with such fields back at the university. It wasn't easy. The
    minute something metallic entered the area it distorted the field.
    Probably the receiver has a 3D coil arrangement as well. Maybe it
    watches the field ratios to determine the distance from the transmitter.
    But that would still not explain the sidebands.

    The only plausible explanation for desired sidebands I can think of is
    the creation of a unique yet easily detectable transmission "footprint"
    in order to dodge interference from man-made noise sources. Such as TV
    flyback transformers, PC power supplies, energy saver fluorescents and
    so on. They are usually all in the 15-30kHz range.

    Regards, Joerg
  7. Albert

    Albert Guest

    Joerg, I have photos of the inside and out, plus all the FCC data (FCC
    ID = KE3IF100). You can search the FCC data base and download all the
    documents OR I can send them to you. The files are big though, be

    I'll send them or will give you the URL for them.

    Notice that the 5 volt supply for the ASIC has it's own filter and is
    derived from the 16 volt dc supply. I checked the 16 volt supply with
    spectrum lab, so it's pretty unlikely that the ASIC supply has large
    ripple on it.

    It appears to me that hopping the thing up would be easy...either
    change R26, R24 and R18 to give the amps more voltage gain, or perhaps
    changing R30 to a lower value Which is in series with the power output
    pot (R33).

    To get the range I want (300 feet) requires much more transmitter
    power, so I'd probably need a larger wall wart and larger heat sinks
    for the amp IC's.

    I think this is a wide band FM signal, centered around 18.7 kilohertz.
    Which probably explains WHY the range is so short:>:


  8. Albert

    Albert Guest

    Oh, forgot to say in the last message.....

    I think they use FM because it rejects amplitude modulated noise,
    hence it provides good rejection for most of the beeps and whistles
    down there.

    The wide band FM also restricts the range pretty severely as well, but
    it's simple, quick and dirty:>:


  9. Joerg

    Joerg Guest

    Hello Albert,
    A URL would be better. Then others can participate, too.
    I think the amps are rated 20W. But there will also be FCC limits.
    Yes, and sometimes that is necessary to stay within FCC limits. Like the
    dithering that is done to disperse the nasty peaks from processor clocks.

    One issue with transmitters inside houses is the insulation. Ours has
    aluminum foil backed fiber in the walls, often one roll stapled to the
    next. That muffles RF pretty good.

    Regards, Joerg
  10. "Any old opamp" means a 741 and similar such as the 1458, and they can't
    even do well above 10kHz. So use something a lot better.
  11. Albert

    Albert Guest

    The product is an Instafence (also called Instant Fence or wireless
    fence). The manual for the thing is available on, look for
    the PIF-300 product (which also includes the collar). It transmits a
    short range signal at 18.7 kilohertz using large air coils as
    antennas. It is designed to contain a dog within a 90 foot radius and
    to issue mild shocks if/when the pet attempts to leave the area. It
    does not work with a buried wire loop, it uses the transmitter to
    define the dogs play area.

    For anyone interested in purchasing one, they are horribly expensive
    (list price $280) and they have a special battery for the collar,
    which is only used by petsafe, available at 6 dollars each. The
    batteries don't last long. The range is only 90 feet, so it doesn't
    allow the pet to move very much.

    It appears to be grossly over-engineered as well, an ASIC feeds one
    each amplifier separately and the amps appear to run at very low
    power. No doubt a much cheaper transmitter could be made, this one is
    also physically large as well!

    For anyone interested in the petsafe IF100 vlf transmitter, you can
    look on for the manufacturers info-

    Or, check out pictures, schematic and field strength documentation at:

    Here are some notes based on my personal observations (before I
    returned the device).

    SW2 is hardwired, and is not adjustable as shown in the schematic. For
    some reason, this switch was called 'mode' back in 1998 when petsafe
    submitted the documentation to the FCC. The schematic shows it as a
    switch though. When looking at the schematic, the position of the
    switch in the schematic is shown correctly.

    Switching S3 to ground selects low power transmit.

    The pot, R33 is the power control.

    The ASIC is marked as follows:

    It is impossible to tell how many turns are on each loop (transmit
    antenna). But, each of the 4 loops has the same size wire and has a dc
    resistance of 4.5 ohms.
    Vdd for the ASIC is 4.85 volts.

    I have spectral output for each of the output lines (X, Y, Z). The
    spectral display shows the same signal on each output although the
    same output signal on all 3 output lines, although there are different
    peak amplitudes (some of the amp stages are driven harder than others,
    but all have the same basic input signals).

    There are 3 audio amp chips, they are each the LM 1875T. They can run
    20 watts output, but the output power is much lower than 20 watts for
    each stage. This is based on the V+ being very low (16.8v) and the
    observation that the 1.8A wall wart (at 14v ac) runs very cool. Also,
    although the heat sinks for each stage are very small, they are barely
    warm to the touch.

    If anyone wants to see the spectral output of the ASIC that feeds the
    amps, send me an emmail to:

    K Y 1 K at pivot dott nett.


  12. Albert

    Albert Guest

    Just noticed an error in my previous post. The amanfacturers website
    is, and is NOT
  13. Joerg

    Joerg Guest

    Hello Watson,
    Actually I did build my first DCF77 receiver with several 741. It's the
    output that peters out above 10kHz but only if you need a few volts of
    swing. In a receiver application these amps can go well above 100kHz,
    provided that you stay under 15dB or so per stage with your closed loop
    gain and signals are in the uV or mV range. It worked nicely. I did like
    the 709 more though but it was kind of expensive in the 70's.

    Then I made another receiver with transistors only. With the good old
    hot rods like the AF126. There were two in each scrapped TV tuner, but
    very hard to pry out without damage. This receiver was better and much
    smaller. It also ran on a couple of battery cells with ease, something
    none of the old opamps could do.

    Regards, Joerg
  14. Joerg

    Joerg Guest

    Hello Albert,
    Somehow I still think training is worth more. Worked for our dogs, most
    of the time... And if they are after something that is really important
    to them (cat) no beep or shock is going to stop them.
    If it is a 6V or 12V stick sometimes you can even buy them at
    Raleys/BelAir and at Longs Drugs. 90ft is not a lot, it would not cover
    most of the yards around here.
    Judging from the 32kHz crystal hanging off of it, the ASIC could also be
    a uC. But likely not if it says Supertex on there. I believe the concept
    could be realized with a sub-$1 uC.

    It doesn't seem to be too over-engineered. The LM1875 is quite cheap but
    I would probably not base a design on an audio chip. They can quickly
    become obsolete when the consumer industry migrates to some other chip.
    Probably a whole lot of turns.

    Another concern with a shock collar is water. I have seen reports about
    badly burned necks when a collar went into continuous zap. It wasn't
    from this system but for our dogs I wouldn't leave it on unattended.

    Regards, Joerg
  15. Yeah, like 15 dB is a V gain of 5 or 6. So it takes three 741s to get
    the gain of a single transistor(!)

    Sometime back in the '70s, in some amps, I replaced a number of
    Germanium transistors with Si ones, and rebiased the stages. Worked
    okay for me.
  16. Joerg

    Joerg Guest

    Hello Watson,
    If you needed a really hi-Z input like for the tuned loop antenna you
    had no choice. Germaniums in those days just wouldn't cut it and a JFET
    was only affordable to high society.

    Then the discerning and financially strapped hobbyist of those days had
    to live with what's there. Scavenging AF126 from a tuner that was
    soldered shut took almost an hour and several items from the first-aid
    kit. Buying them would have cost me several weeks of allowance per
    piece. I lucked out, or at least I thought so, when a local electronics
    store had a special on uA741. Re-labeled, so you had to live with some
    discrepancies with respect to the data sheet.
    I rarely did that because ye olde BC107 cost more ($1+) than the
    transistors scavenged from TV sets and radios.

    Regards, Joerg
  17. Well, that's what a tap on the inductor is for. And a couple megs at
    100s of kHz is easy to do with a Germanium bootstrapped emitter
    follower. And you include the shield by connecting it to the emitter,
    so much of the shield's capacitance is nulled out.
    That sounds familiar. Some of those old chips were not even marked
    because they were so far out of specs. Then these 'seconds' were sold
    to hobbyists at a high markup.
    Well, I did my fair share of scavenging, too. Trouble was that the
    transistors were often from TV sets with house part numbers, like
    Zenith. So it was a wild guess for me of what I was scrounging. I had
    enough sense to know that a transistor from the IF was a lot better for
    Rf work than one from the audio amp. ;-)
  18. Joerg

    Joerg Guest

    Hello Watson,
    Problem is that Germanium RF transistors in the 60's had no beta to
    speak of. So for a bootstrap scheme you needed several. Considering that
    it was an hour's job with cuts and bruises to pry two AF126 out of a
    tuner these things became just too precious for that. Often they were
    literally jammed into divider walls that were more than a millimeter thick.

    When I was a kid I tried to buy one. They wanted the equivalent of two
    hours of hard work in a meat factory. No way.
    In Germany they used mostly regular issue transistors. In cases where
    they didn't I placed them in bags and wrote down exactly which part of
    the TV they came from.

    Regards, Joerg
  19. I remember when I was in the army, stationed in Wuerzburg in '68. Guys
    would give me broken radios and cheap cassette tape recorders, etc. I
    would scrounge parts out of them. But I didn't know squat about Euro
    parts; up to then everything I had done was with JEDEC "1N" and "2N"
    parts. So when I came across those AC or OC or whatever 'Euro' parts, I
    didn't have a clue what they were. It wasn't until the Web came into
    being that I realized how common they were in other parts of the world.

    Ooh! Toobs! Check this out. Proves that wirewounds take a licking and
    keep on ticking! I'd like to see the Klystron that goes with that PS!
  20. Joerg

    Joerg Guest

    Hello Watson,
    I bought a "Transistoren- und Kristalldiodentaschentabelle". They can
    really make long words over there. This book helped me to figure out
    what the AC and AF types could do. OC was short-lived but I still have
    some, including "micro transistors" for hearing aids.

    When I was in the army we had one common locker in the quarters. This
    contained a tube radio that was never to be seen by the drill sergeant,
    unless we were sure he'd know nothing about electricity. Bare chassis,
    no housing, no fuse, stuff hanging out of it. But it played.
    Wow. My last 1kW power resistor was made out of about 300 single carbon
    resistors in a large metal honey vat. Tractor oil cooled it. Until that
    stuff leaked...

    Regards, Joerg
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