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Noisy Fluke 6060A/AN signal generator

Discussion in 'Electronic Repair' started by [email protected], Mar 11, 2007.

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  1. Guest

    I own a Fluke 6060A/AM (10 kHz to 520 MHz) synthesized signal
    generator that appears to be a source of noise. For example, if I set
    it to 1 Mhz and tune an AM radio to this frequency, I hear a light
    crashing noise and an occasional pop/click. If I put a scope across
    the radio's speaker, I can see the pops/clicks. Some other facts:

    (1) I've poked around the power supply with my scope and find nothing
    unusual.
    (2) The magnitude of this noise is independent of the generator's RF
    output level.
    (3) When I switch-off the generator's output, but keep it powered and
    running, the noise and clicks goes away.
    (4) The RF output of the generator is clean. No noise or pops/clicks
    are "floating" on the signal.
    (5) The noise and pops/clicks only appears after, say, a 10-minute
    warm-up.

    Other than this, the signal generator work fine. I still suspect the
    power supply--bad capacitors? Or perhaps some other noise source in
    the house is getting a free ride at the generator's expense.

    Any suggestions?

    -Dave
     
  2. The AM broadcast band is noisy. Have you tried running the amplitude of the
    generator all the way up and loop coupling to the AM receiver? This would
    tend to swamp out external sources of noise.

    If it's still noisy, then the generator is indeed acting up. A phase-locked
    loop is not the easiest thing to troubleshoot.
     
  3. Jamie

    Jamie Guest

    I would start monitoring the AC line it's plugged into.
    it's noise that is just getting a free ride..
    also, you should check the PS on the AC side of the
    Diodes/rectifiers. Bridges/Rectifilers have a happen
    of causing intermitting noise like this one they warm
    up and are getting ready to go bad.
    Also, even though you may not see any signal pops on the
    PS cap leads, that does not mean the cap is good. I've seen
    caps arc internally due to bad dielectric but not short.
    They would at some time how ever. Bad contact switches will
    also do this.
    What i do to find problems like this is to use a R.F. sniffer
    with a small loop so that you can tightly close in the area of
    detection.
    I have a grid meter i use for things like this. it works like a
    charm.. I have found a good many bad components that exhibit this
    behavior.
     
  4. Yipes. The Fluke 60xx generators are very complex and touchy. There
    are very complex delay-line phase-noise-reduction loops, with about a
    dozen delicate adjustments, plus a handful that are "factory adjust
    only". Plus a few adjustments that are burned into a EPROM.

    I'd take it to the top of a tall building and play "how loud is the
    crash".
     
  5. Guest

    I'm fairly sure phase noise is not significantly contributing to what
    I'm hearing.

    Using my amateur radio transceiver interfaced to my home computer
    running a spectral analysis program, I've looked at the unmodulated
    signals from the Fluke signal generator and found them to be narrower
    than the carriers of the local AM broadcast stations. These AM
    stations provide "full quieting," having no noticeable noise similar
    in character to what I hear on the signal generator.

    Even if the phase-noise reduction loop is the source of the problem,
    it's no going to matter much because I principally use the generator
    for receiver alignment. It's the pops and click that I'm looking to
    eliminate.

    -Dave
     
  6. Dave ... since the noise starts after 10 minutes, it might be temperature
    related. Have you tried cold spray?
     
  7. Guest

    Yes, I did.

    I sprayed the circuit boards containing the output amp/modulator,
    controller, attenuator and power supply--no change. I made sure I
    sprayed the master oscillator--no change. I also made sure I sprayed
    the power supply capacitors--also no change. However, they are large
    and covered in plastic, so I doubt the spray had much internal effect
    on these insulated thermal masses. The only thing I don't recall
    spraying is the display circuit board.

    In any case, I plan to replace the power supply capacitors anyway.
    They're probably 25 years old and one of them is "bulging."

    -Dave
     
  8. Ah bulging! Yes, change those for sure.

    Good luck. By the way, I have worked on several RF generators and have been
    able to fix them. It is worth the effort, since the good ones are costly to
    replace.
     
  9. Yeah, that sounds like a robust, field proof design.
     
  10. Guest

    I finally fixed it, taking more time to find the problem than I'll
    ever care to admit.

    The problem was a surface mount capacitor that's in parallel with the
    varactor diodes in the voltage controlled oscillator (VCO).
    Specifically, it was capacitor C3, a 1.5 picofarad (pF) capacitor
    that's in series with R1, a 180 Ohm resistor. I removed this
    capacitor and in its place I've temporarily soldered in a 1 pF ceramic
    disc, zero-temperature coefficient capacitor.

    Incidentally, I took Charles Schuler's advice and replaced the power
    supply capacitors, both electrolytic and tantalum. Although none of
    these proved to be the problem, the supply voltages are now cleaner.

    -Dave, K3WQ
     
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