TRF372017: TRF372017 OIP3 at L band

Part Number: TRF372017

Hi,

I am using TRF372017 as a I/Q modulator to operate at 1.65GHz, According to datasheet, the OIP3 can be 25dBm at L band, and P1dB is about 11dBm. 

But when I tested the OIP3 with two tone (150KHz space) on EVM board, and I can not get 25dBm OIP3, only around 20dBm at 0dBm output on our EVM board .

I do not know if the RF output of TRF372017 was matched to 50 ohm cross a wide band. and there is no impedance information on datasheet. on the EVM board, there is only a 22pF capacitor at the RF output.

As to OIP3, normally the output matching will affect OIP3 very much, Could you please tell the optimized matching impedance for OIP3 at 1.65GHz?

And are there any configuration setting in the chip to optimise OIP3?

Thank you very much!

Jerry Chen

 

  • Hi Jerry,

    I am looking into this.

    Abdallah Obidat

  • Jerry,

    The best load matching you could do for this modulator is simply 50 ohms, which was determined empirically.  Judging by figure 52 of the datasheet, however, it seems that your result is somewhat reasonable. You could consider operating at a different common-mode voltage, or baseband voltage level. See Figures 55 and 56 of the datasheet.

    Regards,

    Abdallah 

  • In reply to Abdallah Obidat:

    Abdallah,

    Thank you for the quick response.

    Do you mean all the OIP3 data in datasheet were measured with 50ohm load cross the band from 700MHz to 2700MHz?

    On our EVM board(TI's EVM) without any matching, we can get about 25dBm OIP3 at 850MHz, and only 20dBm OIP3 at 1650MHz. Vocm was biased at 1.7V. and tried other voltages,1.7V~1.8V is the best range, and no big difference in the range.

    At 1650MHz,I tried a serial 3.0nH inductor at the modulator ouput as matching circuit, the OIP3 can be improved to about 24dBm at 0dBm output power(2-tone,space 150KHz,-3dBm each tone), but the bandwidth with 24dBm OIP3 is only about 40MHz.

    For the Figure 56 in the datasheet, normally, the input single ended baseband voltage 300mVrms(or-10.45dBV),

    differential voltage is -10.45dBV+6=-4.45dBV
    -4dB convertion gain,
    then output is -4.45-4=-8.45dBV
    or-8.45+13=4.55dBm(each tone)
    Total ouput power :4.55+3=7.55dBm
    And the P1dB is 11dBm
    So the test tones are very high.

    0.5Vrms single ended input basebande will drive the modulator close to saturation, so almost half of the data in the figure 56 in the datasheet are meanless.

    Could you please check your EVM board at L band? I still think the OIP3 should has an optimized impedance at different frequency.

    Thank you very much!

    Jerry
  • In reply to jerry chen89:

    I'm going to attempt to reproduce your setup in the lab. What are the frequencies of your baseband tones?

  • In reply to Abdallah Obidat:

    Abdallah,

    We feed a 75KHz sin wave tone on both I and Q channel, the TRF372017 output frequence is 1.65GHz+/-75KHz, the two tone space is 150KHz. and the output power is around 0dBm.

    Please also check the OIP3 at high and low frequency of 1.62GHz and 1.68GHz.

    Thank you very much.

    Jerry

  • In reply to jerry chen89:

    Hi Jerry,

    I measured OIP3 using 75kHz on I and Q, and found a result that is comparable to the datasheet. My only other suggestion would be to offset your input tones from DC, such that the IM2 product doesn’t fall directly on the second harmonic of your signal (which will be modulated as well).

  • In reply to Abdallah Obidat:

    Hi Abdallah,

    Thank you for the testing and suggestion!

    Regarding to the test plot,could you please let me know:

    1. What are the baseband signal on I and Q? Are they both sin wave of like cos(2*pi*75KHz*t)? or can you describe the I/Q in mathematical expression?

    Is there any frequency offset of base band signal in your test?

    2.For Lower 3rd IMD, It looks like the actual 3rd IMD frequency should be 1.64978GHz, not 1.64977GHz if the space of 2-tone is 150KHz, right?

    then what are the 2 spurs(noise like) very close (around 10KHz away) to the 3rd IMD?  Are they from LO's phase noise? or do they have any relation to baseband signal of 75KHz?

    Thanks again,

    Jerry

     

  • In reply to jerry chen89:

    1. Yes they are both cosine waves at 75kHz.

    2. There is no frequency offset in the test results shown above. The baseband signal is centered at DC.

    3. I believe that the 3rd IM is at the correct frequency, and the spacing is indeed 150 kHz.

    1.65 GHz – 75 kHz = 1.649925 GHz, and that minus 150 kHz gives you 1.649775 GHz. I’m assuming that my signal generator can display only so many significant digits, and it may have truncated the value, rather than round it. I think that this is the cause of the discrepancy. 

    4. The spurs seem to be caused by the device itself. I do not think it is related to the baseband signal, as I measured OIP3 on TRF3722 and they were absent. If they are the main issue, you could consider using the TRF3722 as it had similar OIP3 performance at this frequency.

    Regards,

    Abdallah

  • In reply to Abdallah Obidat:

    Abdallah,

    Thank you very much for the answers!
    1.Could you please send me a copy of the REGISTERs'setting, Maybe I missed some settings.
    2.Thank you for the suggestion of TRF3722, I notice that the Vcm is only 0.25V,which is very small, maybe it is good for wide bandwidth modulation with SSB. But our application is narrow bandwidth upto 150KHz and DC was include. Are there any good suggestions for design an active LPF on the baseband with Vcm=0.25V?

    Regards,
    Jerry
  • In reply to jerry chen89:

    Regrettably the file has been changed since I made the above measurement. However, if you follow the attached users guide, and start from the default settings, you should only have to change the RF Step Size, LO Frequency, and maybe Vref, which is used to adjust the Vcm. Hit the calc button on the GUI to have the program automatically change any other registers related to producing the desired LO frequency.

    For the TRF3722, I would suggest starting with the LPF filter that is on the TRF3722EVM schematic, which I’ve attached.

    slwu068a.pdf

    TRF3722RGZEVM-SCH_A.pdf

    Hopefully this helps.

    Regards,

    Abdallah