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Dumb question 'bout signal routing (RF)

Discussion in 'Electronic Design' started by Paul Burridge, Oct 13, 2003.

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  1. Hi,

    Let's say I'm trouble-shooting a circuit board with several stages of
    RF amplification in addition to a primary source in the form of a
    VCXO. I'd like to be able to bypass certain parts of the signal path
    by the use of a jumper wire. This wire may need to be up to 4 inches
    in length to have sufficient reach between stages. Can anyone see a
    problem with this? I mean, rather than a single wire, should I use
    some sort of grounded outer lead like coax to carry the signal?

    at HF
    at VHF
    at UHF
     
  2. Roy Lewallen

    Roy Lewallen Guest

    The main problem with it is that the two stages being connected with the
    wire will no longer see the same impedances they see when operating.
    This can cause a multitude of changes in their operation, including
    modifying their frequency responses and gains. It can also cause
    instability that's not present when the intermediate stage is in place.
    The amount of the changes depends greatly on the nature of the cicuits
    being connected, as well as the frequency. The changes will almost
    certainly be severe at UHF and considerable at VHF. At HF, you might get
    by with it and you might not, depending on the circuits.

    It's good practice to design each stage for a standard input and output
    impedance, with 50 ohms being overwhelmingly the most common choice for
    RF circuitry. That is, each stage should be designed for correct
    operation when driven from a 50 ohm source and terminated with a 50 ohm
    load. The great benefit to this approach is that it allows independent
    testing and characterization of each stage with a standard set of test
    equipment. A secondary benefit is that any stage can be replaced with a
    jumper of 50 ohm coax without changing the impedances seen by the
    connected stages.

    Roy Lewallen, W7EL
     
  3. maxfoo

    maxfoo Guest

    Don't hack up the board just to troubleshoot...(if it is a pcb)
    make a rf sniffer instead. one method I use is a piece of coax
    connected to a spectrum analyzer with a dc block connector.
    the other end of the coax is stripped to expose the center conducter
    as a probe tip.
    course if this is a hobby project you probably don't have a spectrum
    analyzer...if that's the case...my heart bleeds...
     
  4. Active8

    Active8 Guest

    i don't usually have to bypass anything to shoot. you look for last
    known good, first known bad, with your RF probe/scope/sniffer/whatever
    and go from there. lift an input component leg, terminate an output,
    check signal, if good, plug it back in. find the stage that's bad and
    check transistor DC levels first.
    getting dangerous. 50 ohm systems and coax looking good
    same as above but remove "getting"

    you don't mention max allowed power out and freq. i know what your up
    to, but can't remember the rules.

    have you considered Mini-Circuits MMIC amps? 50 ohms in and out. if you
    need more power, just put them in parallel fed with a wilkinson divider
    and flip the divider around on the output for a combiner. alternately,
    if the divider legs are too long at your freq, use the coil version. you
    could probably get away with tearing open those 75 ohm CATV splitters
    and hacking them. you'll loose a little power, but it's doable. a quick
    solution.

    too much time on radios and not enough on bot killing weapory. quick
    solution for that... shotgun?

    HTH,
    mike
     
  5. Agreed. And that's what I'd do in practice.
    I'm just curious, however as to what effect jumping RF from point A in
    a circuit to point B with just a single wire has on the signal. IOW,
    will the integrity of the signal be preserved by this quick and simple
    expedient? The great and the good here seem to believe it shouldn't be
    a problem up into VHF.
    Mike, I do do *other* stuff apart from build killer robots! :-D
    Actually it's been the other way around lately. But the new
    disemboweler is quite impressive, though I do say so myself. I'll
    upload some pix of it when they're available.
     
  6. Active8

    Active8 Guest

    where is here? W7EL stated it might be. if it's just to jump a signal
    over, a wire or piece of RG-58 might be ok.
    why?!!! :)
    good name. i saw a beast in with the educational exhibits at the state
    fair recently. can't remember which competition it was in, Battle Bots
    or what. it had a thick piece of steel cut as a lawnmower blade on top.
    i'd love to see it up against a bot that just got under the blade and
    ran it through with a skewer, but it was some pretty thick plate.
    couldn't see much in the way of electronics, just a bunch of thick metal
    partitions with heavy wiring going to the hulking motors. it looked like
    a lot of other bots with the sloped sides to get under the enemy.

    brs,
    mike
     
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