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Using I/Q modulator in AM modulation application?

Alex Marcoux
Intellectual 515 points
Other Parts Discussed in Thread: TRF370417, DAC3484, DAC3482, DAC34H84

Hi,

Got a question from a customer about using and I/Q modulator like the TRF370417 in an AM modulation type of setup. I'm not an RF guru, so your inputs are welcome. Here is the statement from my client:

I might have an application requiring I/Q vector modulator for my next project starting from 110MHz and to 340MHz (this application uses AM modulation for Air Traffic Guidance equipment), I think that I can use I/Q vector mixer to control phase of side band of AM and its amplitude (side bands are at 90Hz and 150Hz modulates RF carrier in between 110MHz to 340MHz under Amplitude Modulation mode, or you can see it as AM carrier that has side bands at +/-90Hz and +/-150Hz offset from carrier);  AM spectrum should as per plot below if you ignore intermodulation caused by a non-linearity of high power amplifier at 60Hz, 240Hz, 300Hz, and 180Hz; Design architect and approach is not designed yet and nothing has been started yet. I used I/Q vector modulator for QPSK type for direct conversion but not quite sure how well it works under AM.

Any feedback would be appreciated if this is a valuable option.

Thanks,

Alex 

  • 0
    TI__Intellectual 1650 points

    Hello Alex,

    I believe the TRF370417 would probably work for an application like this.  Without knowing a lot of the specs or seeing the plot the customer discusses its a little difficult to say for sure.

    If the signals are in the 110M to 340M range than maybe this could be done with direct conversion, just using one of the following DACs -  DAC3482, DAC3484, or DAC34H84 w/o a I/Q modulator.

    EVMs are available online for these parts if the customer wants to experiment.

    Chris

  • 0 in reply to Chris Pearson
    Intellectual 515 points

    Chris,

    I have new information on this.

    First, the DAC348x would work, but it would prove a more expensive solution than a DAC + TRF solution.

    New, questions are: 

    • Can the TRF I/Q input support inputs down to 90Hz
    • Do we have better noise plot at low frequency? Datasheet plot stops at 800MHz (figure 20 & 21)
    • Can we support input with a DC component?
    Thanks,
    Alex
  • 0 in reply to Alex Marcoux
    TI__Intellectual 1650 points

    Alex,

    Interesting. 

    The I/Q inputs do work down to DC. 

    At DC there is the carrier suppresion  (LO) spur at RF out that is present in all modulators.  Depending on what the customer is trying to do they will most likely have to calibrate the carrier suppresion spur out before running DC there.  I'm not sure I understand the DC component question exactly, so if this doesn't answer the question let me know.

    For noise, in general the noise is better are equivalent at lower LO frequencies when compared to higher LO frequencies. 

    Chris