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TRF370417: Generating FM signals from 50 to 500MHz

Kristian Kruse
Prodigy 10 points
Part Number: TRF370417
Other Parts Discussed in Thread: LMX2571

Tool/software:

Hi all

This is very new for me. I have been given a task, that is not my usual area of expertise.

I need to generate a FM modulated signal, i need to be able to gate the following:

A sine carrier signal at a specific frequency. for simplicity 100 MHz.
Frequency modulate the signal by +/-5 MHz with a bandwidth of 100 KHz or more.
in some cases the center frequency will be 80 MHz, others 120MHz or more. 

Ideally the center frequency should be selectable from 50 MHz to 500 MHz.
But as a minimum 70 MHz to 120 MHz.

I have been looking at various RF parts, but it is not clear to me if they generate a sine wave or square wave on the output of the device.

Further more it is not clear to me if all parts support FM modulation via an analog signal.

I would appreciate some input on how to get started and figuring out if the output is indeed sine. 

Simply put: how would one go about generating a 100 MHz sine with +/-5 MHz modulation depth and a constant sine amplitude?

Best regards
Kristian K

  • 0
    TI__Guru 71959 points

    Kristian,

    The TRF370417 will not be able to generate FM signal by itself. You will need a DAC to go with it to generate the baseband signal to modulate up. I am routing you applications to another team for guidance.

  • 0 in reply to Kang Hsia
    Prodigy 10 points

    Hi Kang

    Could you point towards another IC that will enable me to do the FM.

    Also does the part in question generate square wave or sine wave output signals?

    Best regards

  • 0 in reply to Kristian Kruse
    TI__Guru 71959 points

    Hi

    The TRF370417 only output whatever the baseband input provided. Please refer to the datasheet. 
    Other TI groups will provide their recommendations

    Kang

  • 0 in reply to Kang Hsia
    Prodigy 10 points

    If the RF group is not the correct one, where should I then ask?

  • 0 in reply to Kristian Kruse
    TI__Intellectual 1875 points

    Kristan

    You can look the TI RF synthesizer products like the LMX2571. There is an app note  that includes Analog FM Modulation Example

  • 0 in reply to Carey Ritchey
    Prodigy 10 points

    Hi Carey

    I have previously looked at this part, but in 8.1.8 RF Output Buffer Type it mentions that the output is square wave.
     - Then I need to transform that in to a sine, and I would expect that the amplitude of this sine will vary quite a lot between 80 MHz and 120 MHz, let alone attempting 50 MHz to 500 MHz.

    It also seems as if there is a finite number of frequencies available, due to the "digital nature" of how this part works. This may just be how most PLLs work, is this correctly assumed?

    anyways thank you for the reference to the app note.

  • 0 in reply to Kristian Kruse
    TI__Intellectual 1875 points

    OK

    Typically FM modulator outputs are square wave as only the frequency of the carrier  is needed (no information is encoded on the amplitude of the RF signal), an output band pass filter is used to band limit the signal if needed. 

    If the LMX2571 does not serve your requirements you can either build the FM modulator utilizing:

    A  discrete analog design by  modulating a VCO/PLL loop with your input analog signal. Maybe a challange to find a VCO that has linear modulation bandwidth to support your +/-5 MHz modulation

    Or a digital design  with

    1. ADC to capture your input analog signal

    2. processor to convert the baseband digital data into frequency modulation data.

    3. utilise a high speed DAC to produce the FM modulated RF signal.

  • 0 in reply to Carey Ritchey
    Prodigy 10 points

    Thank you for the clarification on the output waveform, this saves me a lot of headaches with respect to finding a suitable IC.
    It sounds as I need to solve this task a bit differently than firs anticipated. 

    Do you have some literature which you could recommend?