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ADRF6821 output

Hello,

I have two questions about ADRF6821.

First, what is the use of DACs that are internally attached?

Second, how to determine the output power level provided from ADRF6821?

If I wanna use the ADRF6821 in my receiver system that should provide a sensitivity of -120dBm. How to determine the amplification required in my system?

thanks in advance.

  • HI Ahmed,

    On Page 21 of the DS, it shows that the 2 DACs provide the DC compensation needed for the I and Q outputs.

    The ADRF6821 uses dc compensation digital-to-analog converters (DACs) for both I and Q outputs. DC compensation covers a
    range of ±40 mV. Control the dc compensation value via the CODE_DC_IDAC_RF0 bits (Register 0x0051) for the I output and the CODE_DC_QDAC_RF0 bits (Register 0x0052) via the Q output for Channel 0. For Channel 1, use the CODE_DC_IDAC_RF1 bits (Register 0x0053) for the I output and the CODE_DC_QDAC_RF1 bits (Register 0x0054) for the Q output.

    These DACs provide the DC compensation for the I and Q outputs. It Controls the dc correction applied to the IF path for the I and Q outputs.

    From the LPF, the IF or baseband signal passes to a linear output amplifier to drive the baseband output pins (IF_IOUT+, IF_IOUT−, IF_QOUT−, and IF_QOUT+). The IF amplifier provides the overall gain for the ADRF6821 and, with the required 25 Ω series resistors, allows the ADRF6821 to drive
    a 100 Ω load directly. The ADRF6821 can be interfaced to a variety of analog-to-digital converters (ADCs), and the common-mode output can be adjusted with the external VCM pin.

    With regard to your second question,

    The sensitivity is dependent on the noise figure of the 6821, which doesn’t look like it varies with mixer gain settings. 

    Looking at the specs on the ADRF6821 datasheet, I see noise figure numbers around 15dB, depending on operating conditions. 

    Noise figure is spec’d based on noise density (dBm/Hz) relative to -174 dBm/Hz. This is equal to KT at 25 deg C. 

    If the noise figure is 15dB, the sensitivity will then be -159 dB, assuming we are working with a signal bandwidth of 1Hz. 

    I think you want to know  what bandwidth can we increase the signal and still get -120dB sensitivity.   

    The amount of noise in a signal increases by 3dB every time we double the bandwidth. Based on that, if we have a sensitivity of -159dB @ 1Hz bandwidth, and want to go to -120dB, the difference is 39dB. Because 39/3 is 13, the bandwidth we can achieve and still have -120dB sensitivity is 2^13, or about 8kHz BW.

    Hope this helps.

    Warm Regards,

    Rachana

     

    Based on that, the ADRF6821 should be able to achieve -120dB sensitivity with a signal bandwidth of 8kHz or less.

     

  • Thank you for your detailed reply!

    I have another question. According to this part
    "The ADRF6821 can be interfaced to a variety of analog-to-digital converters (ADCs), and the common-mode output can be adjusted with the external VCM pin."

    Does this mean that the output voltage of I/Q can be controlled through the Vcm pin? and the I/Q output does not depend on the RF input power?
    The datasheet does not provide anything about the relation between I/Q output and RF input.

  • HI Ahmed,

    The ADRF6821 can be interfaced to a variety of analog-to-digital converters (ADCs), and the common-mode output can be adjusted with the external VCM pin.

    The VCM I/Q inputs can be adjusted to null out carrier and sideband rejection as well as optimize for output P1dB as shown in figure 11.  The output of the IQ voltage is not designed to control the through the VCM pin, only to optimize RF performance.  The RF input, combined with the DSA is directly proportional to the IF / IQ outputs. 

    Hope this helps.

    Warm Regards,

    Rachana