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MMIC layout

Discussion in 'Electronic Design' started by Jamie Morken, Oct 26, 2005.

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  1. Jamie Morken

    Jamie Morken Guest


    For this MMIC amplifier:

    Why does the evalboard layout have small rectangular copper
    areas near C1 and near the amplified RF output? These copper
    pads are not hooked up to anything so I am not sure what they
    are for.

    (pics of the evalboard copied from the datasheet:)

    Why is the input RF signal narrowed down after C1?

    The datasheet specifies an input VSWR of 2:1, not sure what effect
    that has on the layout.

    Also this chip requires a gate voltage input of about -1.2Volts
    (-0.6V to -2.0V min/max specified). Is this voltage ok to just be in
    the min/max range or does it require precision? I was thinking of
    generating this signal with a small cap switcher IC.

    I figured I might as well do a layout for this MMIC, before trying a
    transistor based amplifier.

  2. Chris Jones

    Chris Jones Guest

    Probably so that you can put some copper foil on them and a blob of solder
    to join them up to the track with the signal on it, as a sort of stub, to
    alter the impedance match. It tends to indicate that they weren't really
    confident that it would be matched up properly when they built it for the
    first time. That is understandable, it isn't easy to get right first time.
    This narrow section is the two transmission lines T1 and T2, which have a
    higher impedance than the wider section. Wherever you put C2 is the place
    which defines where T1 ends and where T2 starts. That's what I reckon
    No idea. You might want to try different voltages to see if it affects
    gain, linearity, efficiency, etc., then you will know if it is important or
    not. If you're lucky, the manufacturer will provide you with curves
    showing these effects so you don't have to do the experiment.

    If you use a switching converter to generate supplies for this IC, be aware
    that any ripple on the supplies might end up modulating the output signal
    (e.g. AM) which will produce sidebands at an offset from the carrier equal
    to the ripple frequency (and perhaps harmonics thereof), and this might
    interfere with other things, though in the ISM band you are relatively
    safe. E.g. if you use a 1MHz switched-mode converter, and you transmit at
    2450MHz, you could expect to see something at 2451MHz and 2449MHz.
    I expect that it would be hard to do better than a MMIC at these
    frequencies. If they will give/sell you some parts then this sounds great.
    Don't cook yourself!

  3. Jamie Morken

    Jamie Morken Guest

    Chris Jones wrote:

    Sounds reasonable to me! :) Also does C2 act as part of an LCL lowpass
    filter? I was wondering why you need C2 at all? I am tempted just to
    run a straight 50ohm trace from the input connector to the chip with
    just the AC coupling cap..
    Ok, I will make sure to filter that signal down to a couple uV noise
    I don't think I need to worry about that unless I use a high gain
    antenna pointed at my head. :)

  4. Chris Jones

    Chris Jones Guest

    probably one of the following in order of decreasing respectability
    to make the input look more like 50 Ohms resistive,
    to make the gain or output power greater in order to boast about it
    to stop it from oscillating (if it is a shonky design)
  5. Joerg

    Joerg Guest

    Hello Chris,

    How could it make the gain greater?

    ROFL! I have seen those designs where someone hung a cap somewhere in
    hopes that it would now please, please not oscillate anymore.

    Regards, Joerg
  6. Jamie Morken

    Jamie Morken Guest

    They must not of had an ARRL UHF/microwave experimenters manual :)
    I got mine in the mail a couple days ago.

    Figure 16 on Page 6-15 looks very interesting, it shows 8 different
    impedance matching circuits. I still don't know how to pick which one
    of the 8 to use (yet) but it looks cool. :)


  7. Chris Jones

    Chris Jones Guest

    If the input impedance is badly mismatched, the power gain may appear low
    because the input power is being reflected back to the source. If the
    matching is improved, this can make the measured gain increase (when all
    measurements are made with the same source impedance).
  8. Joerg

    Joerg Guest

    Hello Chris,
    That would be unlikely with a non-resonant circuit like an RC. It
    increases the dissipation in the resistor before the C and drops the
    gain. If you had LC that would be another story.

    OT: I have seen you on a German NG. How did you learn that language?
    From the quality of your posts there it can't just be from a few years
    in school. Did you live there?

    Regards, Joerg
  9. Joerg

    Joerg Guest

    Hello Chris,
    Well, in that case it would probably fall under the category that you
    had called "shonky design" ;-)
    Yeah, they are now Infineon and I am not so fond of them anymore.
    Excellent products but their customer support, well, let's not go there.

    Your German does look good and is easy to read. Don't worry about
    adjective endings and stuff like that, even Germans get those wrong at
    times. Also, they all understand English pretty well. Except when they
    travel to the US and then hear Archie Bunker on TV "I ain't gonna have
    none of this no more at this here table ...".

    Regards, Joerg
  10. Chris Jones

    Chris Jones Guest

    I think at 2.4GHz, pretty much everything looks like a combination of R, L
    and C, in proportions which are not easy to determine, let alone influence
    in the desired direction.
    That's very kind of you to say so, but I know my German isn't that good. I
    studied it in school for quite a few years but I was quite lazy and didn't
    learn nearly as much as I should have. In '97 I spent 3 months doing work
    experience in Munich (with Siemens semiconductor group as it was then). In
    spite of these opportunities I never learnt which adjective ending to use
    when, and my grammar remained very bad. Since then, I have been slowly
    forgetting what little I knew and now I resort to the dictionary for
    several words in most sentences.

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