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Problem of switching output channel on the ADF5901

When switching TX channel of ADF5901, We find that the output frequency will jump and is not a stable value,and only happens when you switch TX channel.

Test chirp signals as shown in the figure below.When switching TX, we only configured DB6 and DB7 of registers R0


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  • Hi, I am thinking this is a measurement quirk being picked up when the Tx is enabled. The Tx does not get enabled right away, it takes around 100ns to each full power and the analyzer could picking this period as a blip. Do you know what signal power the frequency blips are at? For example the time domain plot you have sent is reporting there is a signal at 24GHz-40MHz, but what power is this signal it has detected and plotted?

  • I don't think that is the reason of the test, you can use the maxhold spectrum, you will see on spectrum anomaly, and the anomaly has something to do with the timing of the antenna switch, if you are at the end of the positive slope is switching antenna, you will see in spectrum remote exceptions, if you are at the end of the negative slope switching antenna, you will see in the proximal exceptions, if not switch antenna is normal.

    1)Switch antenna when positive slope ends

    2)Switch antenna when negative slope ends

    3)not switch antenna 

  • Hi,

    Yes you are correct.

    These glitches are a result of some expected VCO pulling that occurs when switching the Tx off/on, we give a spec for it in the VCO section of the DS spec table. 

    VCO pulling can also be exacerbated by any supply ripple that may be occurring when switching the Tx. So it could be worth monitoring the voltage on ADF5901's VCO supply when you switch Tx, and also looking at the decoupling you have on the ADF5901's supply pins.

    You could try switching Txs during the down ramp to help mitigate some of the pulling.