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RF Mixer level - feedback path in GC5325SEK

Prem Krishnan57034
Prodigy 100 points


Hi Joe,

I had analyzed the path taken by LO_IN in the GC5325SEK. We observed that when LO_IN is varied from -6 dBm to +6 dBm at 800 MHz, in steps of 1 dB, the variation in the levels of TX1_LO and TX0_LO is within 0.6 dB. When LO_IN is varied from -6 to +6 dBm, TX1_LO varies from 0 dBm to 7.29 dBm and TX0_LO varies from 0.59 dBm to 7.80 dBm. The level of RX_MIX_LO under similar conditions varies from 3.05 dBm to 10.69 dBm.

Through further analysis, we could understand that in the case of RX_LO, ( in the pi attenuator) R147 and R196 are labelled as DNI and not installed. Moreover R250 is set to 0 ohms.In the case of TX0_LO and TX1_LO, pi attenuators have been provided.

We were advised by the TI Local support in Bangalore to use a level of  +6 dBm for LO_IN. It translates to a RX_MIX_LO level of + 10.69 dBm. We also got an RX_MIX_LO level of 10.3 dBm even for a LO_IN level of 2 dBm. Given the fact that the recommended LO drive for HMC483MS8GE(U64) is between -4 and +4 dBm and the Absolute Max rating is 10 dBm,  we are operating in the margin.

We have been facing issues with regard to the response of the aforementioned mixer over the frequency range of 700 MHz to 1400 MHz. Is it due to the higher level of RX_MIX_LO vis-a-vis TX_LO ? Would you recommend putting the pi atteuator in the path of RX_LO to solve the issue?

Regards,

Prem Krishnan

 

 

 

 

 

  • 0
    TI__Mastermind 37500 points

    Hello Prem,

    I have no special notes, other than to follow the jumper settings in the User Guide for the GC5325SEK.  If the feedback LO drive is too high, it will generate more distortion

    in the feedback path, which will lower the DPD performance.  I have forwarded your question to Ken Chan, so he can analyze your RF concern. 

    If you input a two tone sequence with varied frequency and amplitude, and get the capture buffer (not running DPD) you can observe the data capture SNR.  We normally want

    an observation path SNR at least 3 db better than the desired spectral mask.

    Regards,

    Joe Quintal

     

  • 0 in reply to Radio-Joe
    TI__Genius 12965 points

    Prem,

     

    The FB mixer performance will degrade a bit if it is driven with an excessively large LO power.  The larger level of LO suggested by the India team may have been appropriate for an earlier version of the GC5322 RF card, however for the existing GC5325 SEK we would recommend something closer to the -7dBm for the lower freqency mixer and around 2dBm for the higher frequency mixer as suggested by the users guide.  This range of gain is due to the gain slope of the wideband RF amp that is on the board and used to amplifier the LO signal.

     

    By design there is about a 3dB difference in power for the LO between the TX modulator and RX mixer.  I would back off your LO in power by about 10dB, to about -4dBm.  This should make the RX LO level about 0dBm and your TX LO around -3dBm which should fit within the operating levels of the modulator and mixer parts.

     

    Regards,

    Ken.