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That Field-Strengh Meter again...

Discussion in 'Electronic Repair' started by Paul Burridge, Jan 14, 2004.

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  1. HI all,

    I refer you to Analogue's design which can be seen on this page:

    I have now built the circuit, but it has an immediate problem on
    switch-on: the meter (represented by "Lmeter" in the diagram shoots
    over to nearly full scale in the absence of any input signal. I've
    tried adjusting the zero setting for it but to no avail. I then set
    about carrying out the usual DC voltage checks and found that by
    placing a probe tip on nodes 5 or 6 I was able to get the needle to
    back down to nil reading as it should. But this effect only lasts as
    long as I'm holding any metal object against nodes 5 or 6. If I
    release them, then the needle swings over towards FSD again. Any idea
    what might cause this? Initially I thought is was maybe the DVM
    loading down a hi impdence point in the circuit, but it turns out the
    probe you apply doesn't even have to have a ground connection; single
    hand contact alone by a metal screwdriver is sufficient. It's not a
    dry joint and I've checked the wiring etc. Any ideas? Could it be
    self-oscillating for some reason? If so what can be done about it? I'm
    applying no input signal whatever at this stage, BTW. And ignore the
    power supply arrangement as I'm using 2X3V dry cells at this stage so
    it's nothing to do with the zeners jittering eitiher.


  2. Jim Thompson

    Jim Thompson Guest

    Gee, Paul, I thought you knew it all. And watch out for those "zeners
    jittering" ;-)


    ...Jim Thompson
  3. John Smith

    John Smith Guest

    I'd bet it's oscillating and your probe is detuning it enough to stabilize
    it. Try shorting node 2 or node 3 to ground and see if you get the same

    -Assuming you've done what I did (build it on a piece of copper-clad FR4),
    you can solder little walls of thin brass sheet between the stages.

    -You should lay the whole thing out in a long, thin, line--about like the
    schematic--to keep the input away from the output.

    -Make sure the axis of L1 is perpendicular to L2. Detune L2 a little. Or
    change C2 temporarily to see if it stops. In feedback situations like this,
    resonance is not necessarily your friend.

    -Remove any antenna temporarily.

    Just some things to try.

  4. mike

    mike Guest

    What does your oscilloscope say?
    If you don't have one available, put down you simulator and
    go find one.

    How are you shielding between stages and the whole thing
    to keep the amplified signal out of the antenna?
    How are you keeping L1 from talking to L2?

    I know I've said this more than once before: "Design first, Simulate
    second." That way you can simulate the strays in your DESIGN.
    And not waste your/my time simulating stuff with fundamental flaws.

    I've also said this before: "This is a funky circuit that depends
    on extreme impedance transformations done in a very component sensitive
    manner to do it's job." Yep, on paper
    and in the simulator, it may look ok. Just try to build one. It has
    multiple oscillators just waiting to happen. See if
    it works in the real world.

    I really enjoy these threads. I don't have to subscribe to the
    newspaper to read the funnies. Keep 'em comin'. I'm saving 3 bucks a week.

    Return address is VALID.
    Bunch of stuff For Sale and Wanted at the link below.
    Toshiba & Compaq LiIon Batteries, Test Equipment
    Honda CB-125S $800 in PDX
    TEK Sampling Sweep Plugin and RM564
    Tek 2465 $800, ham radio, 30pS pulser
    Tektronix Concept Books, spot welding head...
  5. Active8

    Active8 Guest

    London/S.E.D. bridge is

    | /|

    burn-ing down, burn-ing down...

    I didn't see any littering beaners last time I was in Houston. Do
    they have a problem with that in the UK?
  6. Leon Heller

    Leon Heller Guest

    I'd scrap that circuit and use cascaded broadband amplifiers with 50 ohm
    input/output impedances

  7. It's probably a lay-out/shielding problem as some people here have
    suggested. I'll get it working eventually with or without assistance;
    it'll just take longer without. Anyway, *some* people here have given
    helpful pointers which I shall investigate. I shall stick with it.
    Everyone said what a great design it was when Analogue first posted it
    so I see no need to "scrap" it at this initial stage!
  8. You mean you've never heard of "zener jitter"??
  9. Leon Heller

    Leon Heller Guest

    This is the NG - note the word "design"! I was
    just suggesting a better, easier and quicker way to go about it.

  10. I don't know what it is.
  11. Active8

    Active8 Guest

    A new dance or an ancient one?
  12. Active8

    Active8 Guest

    This is the NG - note the word "design"! I was
    just suggesting a better, easier and quicker way to go about it.

    Like those affordable broadband MMICs. This *was* supposed to be a
    relative FSM, not a SLM, so if a broadband FSM is reading signal
    with the TX unkeyed, and more signal with it keyed, you have a
    "relative" measurement.

    It's a good enough project, though. More learning involved than a
    poor-mans spectrum analyzer.
  13. Leon Heller

    Leon Heller Guest

    Like those affordable broadband MMICs. This *was* supposed to be a
    relative FSM, not a SLM, so if a broadband FSM is reading signal
    with the TX unkeyed, and more signal with it keyed, you have a
    "relative" measurement.[/QUOTE]

    I was thinking of using discrete transistors like the 2N5179 with 4:1
    transmission line transformers.
  14. Active8

    Active8 Guest

    I was thinking of using discrete transistors like the 2N5179 with 4:1
    transmission line transformers.[/QUOTE]

    That'll work.
  15. Yeah, I've got one but it seemed a bit of a waste of time trying to
    make measurments with it since the fault appeared to disappear with
    the application of any prod/probe or whatever to that part of the
    circuit. *However* I've made some checks nonetheless and the situation
    ain't good. There are multiple parasitic oscillations of all sorts,
    shapes and frequencies at just about every blasted node on the circuit
    board. The whole thing is about as unstable as you could possibly
    imagine and I'm at a complete loss as to know where to begin, quite
    frankly. :-( And this is before it's even subjected to any input
    signal stimulus!
    There's no antenna yet! There's no shielding either. I'm relying on
    trying to keep the components reasoably well spaced-apart. I'm going
    to post a photo of it later in case there's some obvious construction
    error I've made with the layout on the PCB.
    I've tried interposing a grounded copper plate between them but it
    made no difference. Pity as it was a good suggestion.
    This isn't my design, remember. It was posted here by someone who
    knows what they're talking about, was widely applauded by others who
    presumably know the same, and built in a modified form by John Smith
    (only one RF stage instead of two) and reported to be working fine.
    "Multiple oscillations just waiting to happen" Seems you're right! How
    could you tell that from just inspection of the schematic?
  16. In that case I want it named after me. Miller and Early shove over.
  17. For the sake of completeness, I've shorted nodes 2,3,4 & 5 to GND
    sequentially and noted the following effect on the meter needle:

    N2: Slightly worse (higher meter reading)
    N3: Big reduction to about 5% of FSD
    N4: Total zeroing to natural resting position
    N5: Reduction to about 10% of FSD.

    Shorting out N4 therefore produced the most dramatic reaction but
    maybe the whole thing gets killed by doing that; I don't know...
  18. Okay, but I still don't know what it is.
  19. Fred Bartoli

    Fred Bartoli Guest

    Guess what ?
    Neither Paul, nor every body here.

    There's simply nothing like that.

  20. No, no, no. I'm gonna get me a Nobel Prize for this one. I just don't
    want it (this thread's deviating) upsetting the current question and
    thereby robbing me of a solution to the FSM problem.
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