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TRF372017: Image Rejection

Expert 1460 points

Replies: 7

Views: 130

Part Number: TRF372017

Hello,

    We are using the TRF372017 in conjunction with a DAC3484 to generate a modulated carrier. We are generating single tone cosine and sine and are feeding them to the I and Q of the modulator. We are getting equal sidebands at the moment (when there in concept should only be one). (f_LO +/- f_IF, both are present). 

Is there some setting that we are missing? 

Thanks for your help in advance,

  • Hi SM,

    you can first check the waveform at the DAC3484 output are truly 90 degree phase shifted. After that, please also check the interface network is matched correctly for the common mode level requirement for both the DAC and the TRF part. Below app note will help:

    -Kang

  • Expert 1460 points

    In reply to Kang Hsia:

    Hello Kang,

             We checked the signals on an oscilloscope and they appeared in quadrature (the scope measured a phase difference of 97 degrees). We also checked the dc levels on them at R62 and R54 with respect to ground. One of them was at 1.37V and the other at 2.11V. Could this be affecting it?

    We checked the differential outputs (BBI_P and BBI_N and they are two sinusoids out of phase and the same for BBQ_P and BBQ_N)

    The register settings in use are:

    Reg 1: 60100409
    Reg 3: 0000000B
    Reg 5: 0D03A28D
    Reg 6: 9090100E
    Reg 7: D041100F
    Reg 4: 4A01000C
    Reg 2: 8880620A

    The traces on the PCB are length matched. 

    Lets say they are not in perfect quadrature for some reason, we should still see some level of image suppression. At the moment they are within 1-2dB of each other. 

    Following is our schematic for the same: the series inductors are set to zero Ohms. 

    Regards,
    SM.

  • In reply to SM:

    Hi SM,

    SM
    We also checked the dc levels on them at R62 and R54 with respect to ground. One of them was at 1.37V and the other at 2.11V. Could this be affecting it?

    This is quite extreme. Given the DAC to TRF is AC coupled, there must be some mis-configuration or soldering issues at the TRF372017 side. The DC offset adjustment capability on the TRF372017 is in the mV range. Therefore, I suspect this is a soldering issue.

    SM
    Lets say they are not in perfect quadrature for some reason, we should still see some level of image suppression. At the moment they are within 1-2dB of each other. 

    I agree.

    I have also asked our colleague to look into this issue as well to brainstorm.

    -Kang

  • In reply to Kang Hsia:

    When you see both carriers at the same level then it means one of two things: 1) the BB signals are not in quadrature or 2) one channel's signal is not getting to the modulator input.  You are reporting that the signals are indeed in quadrature which implies there is signal on both I/Q; hence, one channel must not be getting through.  As mentioned previously, the DC imbalance is excessive and problematic.  The voltages on all legs (if the internal Vcm is selected) should be hovering around 1.7V.

    As an experiment, disable one of the DAC outputs and monitor the modulator output.  This can help to deduce which leg is not working. If for example you disable DACA and there is no change, then that channel's signal is not getting through.

    As a side note, the DAC loading has an inductor to ground.  This will set the DAC Vcm to 0V.  That is a bit strange configuration and will create a HPF response and I am not sure if the DAC will "like" being DC shorted.

    --RJH

  • Expert 1460 points

    In reply to RJ Hopper:

     @RJ Hopper

    The idea of switching OFF one of the DAC outputs to find the non-working channel was good. We turned OFF both the channels and found that the I port of the modulator is not working and this is the port which has the common mode voltage around 2.13V. We checked the soldering and also reheated the device but there is no difference. 

    What I mean by I port not working is that when a signal was fed on the I port and no signal on the Q port there was no output from the modulator, while vice versa (no signal on I and signal on Q produced an output). 

    Regarding the shunt inductor to ground on the DAC output side, we will get remove it (don't remember how we got it there)

    Thanks again for helping me out and looking forward to your inputs,

  • Expert 1460 points

    In reply to SM:

    Hello,

        Looking forward to your inputs,

    Thank you

  • In reply to SM:

    Ok, so you determined the I port was not working.  That explains why the Vcm is off and why there was no sideband suppression.  Something still must not be configured properly in that path.  Sometimes it is difficult to debug.  Perhaps you can break the connection to the modulator and use a probe to see if the AC and DC signals are expected.  My experience in cases like these is that there is a wrong resistor value or bad solder joint that is the root of the problem.

    --RJH