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High Frequency VCO

Discussion in 'Electronic Design' started by Marco, Oct 6, 2004.

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  1. Marco

    Marco Guest

    Hi to all,
    in my research activities I need a suitable VCO working in the
    frequency range between 1MHz - 200MHz.
    Please, let me know what IC can be useful to this purpose.
    Thank you in advance.
  2. VCOs have usually only an octave band, or a bit less.
    If you need 1 to 200MHz, you have several possibilities :

    take a higher band which is 200MHz wide and mix it down,
    such as 1000 to 1200MHz.
    Or switch several VCOs,
    Or use a DDS.

    What is the application ?

  3. I guess you will never find such an extra wideband VCO. Best models have an
    octave bandwidth or so (ie 100 to 200MHz for example, see or similar suppliers). In order to have a full coverage
    from 1MHz to 200MHz you have I think two solutions :

    1/ Heterodyne generator : Use for example a fixed 300MHz oscillator and a
    300 to 500MHz VCO, send to two signals to a mixer, low-pass filter the
    output to 200MHz, and you have a DC to 200MHz signal (in theory...)

    2/ DDS generator : I guess it will be by far the easiest method. Look at
    solutions from analog devices, they offer DDS chips with clock rates far
    above 400Msps (up to 1Gsps I think), enough to generate a clean sine signal
    up to 200MHz with impressive resolution.

    If you need more help with this design, just ask...
    Friendly yours,
  4. colin

    colin Guest

    VCOs do often have wide ranges such as the ubiqutous 4046 wich has a VCO
    that goes up to 20mhz in 74hc type and wil cover a range of about 20:1 or
    more with less linearity. 200:1 would be posible but would be rather non
    linear at the low end.

    However similar devices that would woork upto 200mhz probably wldnt be able
    to handle the range. the old 74s124 dual vco is specd upto 85mhz but i doubt
    these are available at all.

    Maybe maxim have one. im sure ive seen a high frequency wide range ring
    oscilator VCO but it was buried in some comunication IC.

    Colin =^.^=
  5. Andrew Holme

    Andrew Holme Guest

    You won't get much more than 2:1 range without switching, or you could
    mix down the output of a higher frequency VCO.

    You don't say if you want sine or square wave output.
  6. Ken Smith

    Ken Smith Guest

    A VCO that covers that range is likely to be very-very noisy and very
    complex. Can you live with that? If not you may want to consider band

    Circuits can be made to cover this wide of a range, continuously, but they
    are not simple nor linear. You have to sort of "band fade" between the
    components that work at the bottom end and those that work at higher
    frequencies and then again to another set and so on until you get to the
    top. Unless you really really need to do it, this is a method you should
    avoid like the plague.
  7. Marco

    Marco Guest

    I need a sine wave output. I have found the TLC2934 from Texas
    Instruments, and it seems to have a good behaviour in a wide range,
    but not near 200MHz.
  8. Marco wrote...
    The TLC2934 = NRND? There are their slower '2932 and '2933 chips.

    For high-frequency VCOs, check out the offerings by ICS.

    For example the ICS1523 works to 250MHz, and should meet your needs.
    Or you might prefer the ICS331 or ICS307-02, both good to 200MHz.
  9. Joerg

    Joerg Guest

    Hi Marco,

    I'd consider a nice DDS chip. Since you need sine wave it should be
    followed by switchable low passes. The switching can be done with SD5400
    or with PIN diodes if you take those with a really long carrier
    lifetime. PIN diodes may be a challenge at the 1MHz limit but look at
    the Agilent HSMP series if interested in that approach. I have used
    those as low as 3MHz, maybe lower is possible but I never needed that.

    Regards, Joerg
  10. Marco

    Marco Guest

    Hi, thanks for the hints. For my application I have to swap all this
    frequency range with a sine wave continuously, in order to study some
    chemical properties of a material.
  11. Marco wrote...
    Why don't you purchase an RF sweep generator on eBay? If you want
    to have full computer control over your experiment, you can get a
    synthesized signal generator, with a GPIB interface. We just got
    two HP 8657A, which go to 1GHz, for a good price. Older models
    like the 8656A and 8656B are usually being offered, for example,

    They are big, heavy units.
  12. Joerg

    Joerg Guest

    Hi Marco,

    Ok, here is another trick: Generate a frequency range of, say
    800-1000MHz which should be easy to do with a PLL or whatever you
    prefer. Now mix with a fixed frequency of 800MHz. That results in
    0-200MHz and 1600-1800MHz. The latter you can easily filter away with a
    simple lowpass. The mixer can be had for a few Dollars at:

    Or go with Winfield's idea and buy a used generator. The larger ones are
    cheap and you can get incredible precision with some of the big HP
    boxes. Nicely HPIB controlled from a PC.

    Regards, Joerg
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