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AWR1243: Cascading, External Clocks, and Phase Noise

michael saxton
Prodigy 30 points
Part Number: AWR1243


The AWR1243 datasheet specifies the device uses a 40 MHz clock as the local oscillator.  It states, and responses to others' questions have confirmed, that that clock can be shared/cascaded to other devices.  Elsewhere, the datasheet also specifies –91 dBc/Hz (77 to 81 GHz) of phase noise at 1 MHz.

My questions are as follows:

(1) What does the phase noise look like at frequencies closer to the carrier?

(2) Are the 40 MHz LO or 20 GHz RF synthesizer the primary source of this phase noise?  I.e., if fed by an external device with better phase noise can the system's phase noise be improved from the -91 dBc/Hz value from above?

(3) Is there any meaningful limit to how many devices can be cascaded if an external clock or synthesizer are used?

  • 0
    TI__Guru 58035 points
    Hello Michael,
    You are right, incase of cascading it is possible to feed the 40Mhz reference clock from the master to the slave device.

    Incase you use an XTAL with our internal oscillator , phase noise is predominately governed by the internal synthesizer. So by improving the external 40Mhz clock phase noise the synth phase noise might not improve drastically.

    The Synth output phase noise at 1Mhz is typically ~ -91dBc/Hz for the 77-81Ghz band and ~-93.5dBc/Hz for 76-77Ghz band. At a 100Khz offset these numbers is typically ~-74dBc/Hz.

    We will get back to you on the number of devices that can be cascaded. But this would be mostly governed by PCB layout and clock drivers. You would need to phase match the LO clock to all the devices and the drivers must be able to provide the clock to all the devices with good amplitude. The more devices you have this part gets more challenging. We have tried 4 devices in cascade mode.

    Regards,
    Vivek
  • 0
    TI__Guru 50240 points
    We apologize that we were not able to provide you a timely answer.
    If this is still an issue we recommend that you start a new thread.