ADL5375 noise floor

Hello,

I'm looking into the noise density of the ADL5375 IQ modulator and I noticed that it degrades when the LO frequency goes above approximately 4 GHz. Up to 4 GHz noise density is around -155 dBm/Hz as expected but with LO frequency of 5 GHz it degrades by 10 dB as it can be seen in the attached figure. Moreover, the noise floor degrades not only around the LO frequency but also at the low frequencies. 

Is this something I have to live with or is there a way to improve the performance? According to the datasheet noise density shouldn't degrade that much with the frequency so I wouldn't expect this to happen.

In the figure white shows noise floor with LO at 3 GHz and red shows noise floor with LO at 5 GHz. LO power is 0 dBm in both cases. 

I also nulled the LO leakage in the 5 GHz case but it doesn't make any difference.  Also. I checked the LO source and the noise density is the same at both 3 GHz and 5 GHz.

  • 0
    •  Analog Employees 
    on Sep 9, 2021 9:55 PM

    Hello Gec,

    I tried duplicating your results and unfortunately have not been successful. Specifically I zoomed into the 100MHz to 1GHz region that is shown elevated in the plot and did not see any appreciable difference with varied LO frequency. Although, given my spectrum analyzer floor is on the order of -110dBm, so not sure if that's making a difference or not.

    When you collect the two traces are you making any changes to the spectrum analyzer settings?  Can you zoom into the same frequency region for both sweeps, such as 100MHz to 1GHz?  How about measuring the source by itself to rule that out?

    Best Regards,

    David

  • Hello David,

    Thank you very much for your reply.

    - There was no change in the spectrum analyser settings between the two measurement.

    - There was also no baseband input signal so that wasn't the cause. Only LO signal was present its spectrum was fine.

    - Your spectrum analyser floor seems to be the same as mine. In the attached figure the left hand axis is in dB and right hand side axes is spectral noise density in dB/Hz so that's why is -155.

    Do you think it may be the problem with the device or the evaluation board?

    I will try to do some more testing as ASAP with different spectrum analyser jut to rule it's not coming from there for some reason.

    Kind regards,

    Dragan Gecan

  • 0
    •  Analog Employees 
    on Sep 10, 2021 8:13 PM in reply to Gec

    Hi Dragan,

    During my testing I applied 0.5V common mode voltage to all baseband inputs with slight offsets to cause the LO signal to pop up and pass through (opposite of nulling). Also, I had the on board amplifier bypassed, so looking at the ADL5375 output directly.

    Did you apply the common mode voltage? The mixer inputs will need this common mode voltage for proper biasing.

    I'm not sure if it's the device or board; haven't seen an issue like this before.

    Usually RF sources are the first place I look when seeing elevated noise levels as it can be quickly ruled out.

    If you do more testing and it appears to be from the device or board, the next steps would be: 1) get another board to see if it persists, 2) feedback your test setup and I'll try again to duplicate/identify the problem.

    Best Regards,

    David

  • Hi David,

    Thank you for your help.

    I think I found the source of the noise. It is the DAC I'm using for common mode voltage and LO nulling. By using different DC supply the noise at the high frequency end goes away but at the low end it's still there I'll try to use some inline filter or capacitor to ground to see if that's also coming from the DC source.

    Since I can't use DC supply in the final product, can you recommend me a circuit for the common mode voltage supply and LO nulling?

    At the moment I'm resistively feeding DAC ports to the modulator inputs. Baseband signal is fed by differential OPAMPs. I made rough drawing of the circuit.  

  • 0
    •  Analog Employees 
    on Sep 16, 2021 2:38 PM in reply to Gec

    Hi Dragan,

    You're welcome. Good to hear you were able to troubleshoot it to the baseband common mode voltages.

    In terms of baseband interface, the following are possibilities you might consider:

    1. Use differential output transmit DAC with common mode voltage as your primary driver (transmit signal and common mode).
    2. Use the single ended to differential amplifiers you have in the schematic above, with common mode coming from a DAC, such as the AD5624. You would need to be sure the AC signal isn't leaking through the DAC path, which could be done with a inductor or the resistors on each leg. It would be good to put placeholders for filtering capacitors to ground in parallel with the existing resistors to ground.
    3. Use the ADRF6521 DVGA with single-ended to differential input baluns. This would also require a DAC to apply offset voltages to do LO nulling. Although, it's two voltages instead of four. The single common mode pin could be provided as static voltage or you could use a DAC for flexibility; same goes for the gain control pin. If you go with this approach, pay close attention to the amplitude and phase imbalance of the baluns, so that you can achieve good unwanted sideband rejection.

    Best Regards,

    David