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TRF372017: Spurious problem on RFout

Other Parts Discussed in Thread: TRF372017, DAC3174

art Number: TRF372017

Hi all,
There is a problem that spurious occurs about the output of TRF372017 in customer's system.

If the output of the I / Q signal from the DAC3174 is modulated to the center frequency= 713.142857 MHz, strong spurious will occur around 715 MHz and around 722 MHz which seems to be from TRF372017.

I think spurious around 715 MHz is affected by 65.015873 MHz.

Please advise on how to find out the causes of these spurs and how to prevent them.

The PFD frequency of TRF372017 is REFIN / 4 and it is 16.25396825 MHz and the following constant of the filter for VCO is used.

Regards,
Toshi

  • Hey Toshi,

    Try using one of these suggested PFD frequencies and the corresponding loop filter values found on page 48 in the Datasheet.

  • Hi Yusuf-san,
    thanks for your response.

    Is there a possibility that spurious of about 722 MHz will occur due to insufficient adjustment of offset of I and Q output from DAC?

    Regards,
    Toshi
  • Hey Toshi, 

    Unfortunately i am having a hard time understanding what you are trying to do. The diagram says that the RFoutput from the modulator is 725MHz, yet you say that the signal is modulated to a center frequency of 713MHz at the Dac output. So can you give me a brief description of what exactly it is your trying to do?.What is your baseband signal frequency?: What is your desired LO frequency?etc... Also, is clocking the DAC at a higher sample rate an option? Try clocking the DAC at a higher frequency as an experiment and see if the location of the spurs change or if the spur performance improves overall. 

    Yusuf. 

  • Hi Yusuf-san,

    thanks for your response.

    Application is a CATV transmitter device which converts digital TV signal into UHF band signal and transmits it, and its transmission method is based on Rec. ITU-T J.83 Annex C specification.

    T-REC-J.83-200712-I!!PDF-E.pdf

    TRF372017 is used to convert to UHF frequency (UHF13, UHF14  - UHF64) and  LO output is not used.

    They have a plan of 130MHz of DAC sampling rate  as the next version, but current software is based on 65MHz sampling.

    Anyway, the 725 MHz RFoutput of the diagram is incorrect, 713.142857 MHz is the correct value. I'm sorry for the confusion.

    I hope this explanation is the answer to your question.

    Best regards,
    Toshi

  • Hi Yusuf-san,
    I received the additional information from my customer.

    The DAC and TRF372017 are AC coupled. They removed C from DAC and TRF372017 and operated in the open state of IQ input of TRF372017.
    BBI_P and BBI_N or BBQ_P and BBQ_N are only terminated with 100 ohm register.
    Since all spurious at 65 MHz rerated disappeared, it was judged that it was mixed from IQ input to TRF 372017.
    Since there is no contamination from the CLOCK input of the TRF372017 and the power supply GND, it is contaminated from the IQ input terminal, so they believe that improvement of S / N around the DAC is necessary.

    Even in this state, the spurious of the center frequency of the modulation frequency does not change.
    They think that there is no difference in IQ because the internal selection of the IQ input of TRF372017 is selected, so they think if there is something good advice on the setting of the TRF372017 side.

    Best regards,
    Toshi
  • Hey Toshi-san,

    Can you send me a bit more information? Can you reproduce the error like in your first message and take more pictures for me? Please set the span to 200M instead of 60M so that I can get a better view of the spectrum. Please use and label markers in the pictures on the spectrum analyzer to help me identify which signals are spurs, and which are desired.  Here are some debug tips you can try: Adjust the LO signal and monitor the behavior of the spurs. Do they move? If so, by how much relative to the LO adjustment and in what direction? Try monitoring the signal coming out of the LOP SMA terminal and verify that the LO is 713 MHz. Check to make sure that there are no spurs on the LO output. Is there a way for you to monitor the DAC output on a spectrum analyzer? Can you verify that baseband signal is centered at 0Hz? Also, please take pictures of your test setup as well and send them to me.

    Yusuf