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UHF Inductor

Discussion in 'Electronic Design' started by Michael Drainer, Jul 16, 2003.

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  1. I am building a RF Field Strength Meter and need to tune it to the UHF Band.
    What size/kind of inductor and transistor should I use for this. All the
    plans I have found are for the VHF band.

    Thanks
     
  2. Rob Judd

    Rob Judd Guest

    1. A smaller one
    2. A faster one

    Rob
     
  3. Of course, the speed of the transistor depends on what it's being used
    for. There have been plenty of transistorized signal strength meters
    over the years where the transistor was merely a DC amplifier, coming
    after the diode detector. In those cases, changing it will do nothing,
    and there is no reason to change it.

    If the transistor is ahead of the detector, ie an actual RF amplifier,
    then it matters.

    Michael
     
  4. All sounds good...thanks
     
  5. Tom Bruhns

    Tom Bruhns Guest

    You can scale things if you make the inductance and its tuning
    capacitance smaller by the ratio of the frequencies. In fact, if you
    scale ALL dimensions of an inductor, you should end up with one that
    behaves just the same at the scaled frequency. (Smaller coils work at
    higher frequencies.) If the design's coil is tapped, you should be
    able to keep the tap at the same number of turns on the new smaller
    coil.

    As others pointed out, if the transistor is just a DC amplifier, you
    don't need to worry about it. The detector diode (assuming it uses a
    diode detector!) likely will work OK, but if you try to go TOO high in
    freq, you'll perhaps need a better detector diode. The
    dimension-scaling thing works for parasitics too: there is always
    extra capacitance and inductance in the wiring. So you'll want to
    build things smaller and smaller as you go to higher and higher
    frequencies. You eventually hit a practical limit where you will want
    to change your whole way of thinking from lumped capacitors and
    inductors and resistors to distributed things like cavities and
    waveguides and resonators, but with modern components and low power,
    you shouldn't hit such limits below a gigahertz, and people work
    successfully well above that with lumped components (with care).

    As an example, program "Coil" from Saratoga Software tells me that
    five turns of 28AWG wire wound on a 4-40 (US thread) machine screw
    (then remove the screw) will give you a 35nH coil with a
    self-resonance around 2.6GHz and an unloaded Q of about 250 at 500MHz.
    That should be useful to over 1GHz.

    Cheers,
    Tom
     
  6. Tom you information has been extremly useful. Thank You.

    The purpose if this device is to measure dBm at 663.25Mhz TV chanel 46. My
    theory behind this circuit is to capture the UHF band then convert it to DC
    and mix it with a fixed osocolator to create a resonating frequency for
    which I can measure the voltage. I have just started and the first problem
    is capturing the UHF band.

    I would rather tune the circuit to look at just 663.25 - 669.25 (6 Mhz
    bandwidth) convert the signal to DC and measure the voltage.

    If you have any other thoughts please let me know.

    Thanks

    Michael Drainer
     
  7. Siol

    Siol Guest

    There is, at least I think a maxim chip, which does this. Its like a wideband detector.
    I guess in your case you'd have to add a band pass at the input to remove
    any other harmonic content.

    I'll dig out more details, if you're interested.

    SIol
     
  8. Absolutly I'm interested...I would greatly appreciate the help.

    Thanks
     
  9. I think I found what your talking about.

    Its the MAX4003 - 100MHz to 2.5GHz 45dB RF Detector.

    Thanks
     
  10. Tom Bruhns

    Tom Bruhns Guest

    Ah, well, I wonder how accurately you wish to measure the voltage.
    Since it's a standard TV channel, why not use a television tuner to
    convert it down to baseband where you could measure it with a scope or
    similar? You might need some way to disable the AGC or control it,
    and to calibrate the gain. Since it's a signal that occupies some
    bandwidth, there are several different ways you could define the
    voltage. Do you want the (video) carrier level? The total signal
    power (as measured by an RMS voltage)? The peak voltage? Your
    mention of dBm suggests you want the total power available in that
    channel.

    We build equipment that would make the job very easy, no matter which
    sort of voltage you want to measure. Doing it yourself is usually not
    so easy! How large a voltage do you think you have? If it's large
    enough, you can measure it thermally or with a diode detector, and you
    just have to be sure you're not measuring other signals too. If it's
    microvolts, you'll probably want to convert it down to a lower
    frequency and amplify it and feed it through an appropriate filter. A
    simple diode detector could do the job if the signal is larger than
    perhaps a millivolt RMS, and you use a sensitive voltmeter to measure
    its output. You might be able to calibrate your detector with a
    known-good signal generator with metered output. But do you have a
    way to know you have good enough selectivity at the input so you're
    not measuring adjacent channels?

    Cheers,
    Tom
     
  11. Tom Bruhns

    Tom Bruhns Guest

    Thanks to you and SIol for the reminder about Maxim and Analog Devices
    RF detectors. Linear Technology also make them. If power in the
    channel (RMS voltage) is what you want, and the level is high enough,
    that should do fine for you. You could put an MMIC amplifier in front
    of it if you needed just a bit lower input power range.

    You'll probably want a pretty good bandpass filter, though, unless you
    can guarantee that there aren't many interferring signals strong
    enough to worry about and close enough in frequency. Designing,
    constructing and testing such a narrow-band filter isn't trivial, and
    that's why your idea of converting down to a lower (IF) frequency is
    basically sound. Practically all general-purpose spectrum analyzers
    do just that. Again, a TV tuner might be the way to do that easily,
    especially if you can fix the AGC voltage so the gain stays constant,
    and you have a way to calibrate the gain.

    Cheers,
    Tom
     
  12. UHF is a large spread of freqs, 300 MHz to 3 GHz. At these freqs,
    lumped constant components get to be very problematic, they are low
    values and their tolerances can't be easily controlled. The inductors
    are often made from PC board traces and a simple U or hairpin of wire
    between two pads has more than enough inductance to resonate at UHFs.
    Also it was common to silver plate the wire and conductors to get
    higher Q.

    The inductance of resistor leads can be sufficient to give a resonance
    to the circuit. It just depends on the freqs you have in mind. Get
    one of the ham manuals on this, to see examples of some homebrew
    equipment. Rec.radio.amateur.homebrew is another source of info.


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  13. I just plugged some values into my calculator and came up with 5 pF
    and 11 nanohenrys, both of which are really quite small values.
    The impedance is about 48 ohms.

    [snip]
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    My email address is whitelisted. *All* email sent to it
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    Subject: line with other stuff. alondra101 <at> hotmail.com
    Don't be ripped off by the big book dealers. Go to the URL
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    Just when you thought you had all this figured out, the gov't
    changed it: http://physics.nist.gov/cuu/Units/binary.html
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