Single ended operation in ADL5380 for DC output

We have the eval board of ADL5380. We desire to operate it in a single-ended mode. As suggested in the datasheet the RF and LO input already has a 100pf capacitor connected to one port and ground on the eval board for single-ended mode.  According to the datasheet per pin impedance is 25ohm so in the eval board with the onboard baluns connected do we need to match it at 50ohm since according to the datasheet all the pins are at 25ohm impedance or all are at 50ohm differential. Please correct if any confusion in understanding. 

We have given 5.81Ghz input to RF port at -10dBm and 5.81Ghz input to LO at -5dBm. The board I guess is by default configured to single-ended operation since the SMA connecter not connected to RFIN & LOIN and 100pf already connected. ILO & QLO we have terminated with 50ohm. Is the experiment setup correct or we have to make changes to work in a single-ended configuration? 

We expect a DC signal at the output I and Q single-ended but while observing on the scope but we find the noise at 10mV. We want to observe a DC signal flat line and only the phase difference between the RF and LO. Please correct us if any mistakes and suggest a way to achieve the same. Attaching images of the test setup and observations on the scope for 5.81Ghz at the RF and LO. Using this setup we have also done a trial with 4.7Ghz and 4.8Ghz at the RF and LO respectively to and we could observe 100Mhz signal at around 160mV as shown in the attached image.

    

  • 0
    •  Analog Employees 
    on Sep 3, 2020 9:15 PM 27 days ago

    Hi akshay92,

    • Attached is the latest version of the schematic for the board which you have in the photo.
    • To help troubleshoot, please markup the schematic and send back with any changes you have made.
    • Be sure the RF and LO sources are synchronized or there could be residual frequency error due to the internal references of each source. The residual error could cause an AC signal at the baseband outputs.
    • To measure true DC you will need to bypass the transformers on the baseband outputs, since the transformers will block DC.
    • The baseband differential load impedance should be kept above 200 ohms.
    • When connected with the baseband transformers and terminated into 50 ohms single ended, the differential load impedance presented to the device is 450 ohms.

    PDF

    Best Regards,

    David

  • Thank you for the timely response. We have gone through the updated schematic and I think it is the same what we have and we have not connected or removed any component.

    We are giving trigger to RF and LO at the same time, if there is any other way to make sure if signals are in sync then please guide for the same.

    From datasheet page number 34, table 5 it is shown that for the single-ended output, the resistors R2x-R5x needs to be populated for bypassing the output transformer and R13x-R18x needs to be populated for a single-ended mode. Please correct us if anything is written is incorrect. According to our understanding we have to populate registers at R2x-R5x.

    Another point which is not clear is about the 200ohm impedance, is the eval board configured for 50ohm per pin? please elaborate a little on this. And I guess since the output being DC we might not require any matching at the output ports, please correct if any mistake in understanding.

  • 0
    •  Analog Employees 
    on Sep 4, 2020 1:06 PM 26 days ago in reply to akshay92

    Hi akshay92,

    You're welcome. Please see below for some more guidance.

    • Instrument Synchronization:
      • You should make sure the 10MHz references on the rear panel of the instruments are connected.
      • You can use one instrument as the master by connecting to the 10MHz REF OUT port.
      • Be sure to check on a spectrum analyzer or scope that there is in fact a reference signal coming out.
      • Sometimes you need to enable reference signal in the instrument utility configuration.
      • Then connect this 10MHz REF OUT port to the other instruments 10MHz REF IN port.
      • This will ensure that all instrument internal references are synchronized, eliminating any residual error.

    • Bill of Material:
      • Please disregard datasheet figure 102, figure 103, and table 5, as these are out of date.
      • Attached is the latest bill of materials for the board you have.

    • Load Impedance Information:
      • From datasheet table 3 pin descriptions:
        • I Channel and Q Channel Mixer Baseband Outputs. These outputs have a 50 Ω differential output impedance (25 Ω per pin). Each output pair can swing 2 V p-p (differential) into a load of 200 Ω. The output 3 dB bandwidth is ~400 MHz.
      • From datasheet page 25:
        • The baseband outputs QHI, QLO, IHI, and ILO are fixed impedance ports. Each baseband pair has a 50 Ω differential output impedance. The outputs can be presented with differential loads as low as 200 Ω (with some degradation in gain) or high impedance differential loads (500 Ω or greater impedance yields the same excellent linearity) that is typical of an ADC. The TCM9-1 9:1 balun converts the differential IF output to a single-ended output. When loaded with 50 Ω, this balun presents a 450 Ω load to the device. The typical maximum linear voltage swing for these outputs is 2 V p-p differential. The output 3 dB bandwidth is 390 MHz. Figure 85 shows the baseband output configuration.
      • Once you bypass the transformers, please follow this guidance and use a differential load impedance of greater than 200 ohms.

    PDF

    Best Regards,

    David

  • Thank you for such an elaborate explanation. Will do this and get back as soon as possible.

  • I am making the register changes to bypass transformer balun as written in my previous message and use the differential load impedance as suggested by you. While testing for DC may I get some reference data or input Vs expected output if any observations are recorded at your end to verify proper configuration of the board. Or else please guide on verification of correct configuration.

    And if I bypass the balun, according to my understanding please correct me if wrong I will get differential output but I want single ended output so how to achieve that for DC output at 5.81ghz RF and LO.

    We also tried to see the inputs of the balun at the I and Q side after giving 5.81Ghz at RF and LO. We observed 2.4 V DC at both I and Q. But this value did not change even after changing the cable length at RF. We were expecting to see a voltage change with respect to the phase change due to cable length change at the RF but at both times we observed same 2.4V at the input of the Balun. Here we have not yet made any change to the eval board. We did testing this way to check the desired DC signal since the balun will block our desired DC signal. Actually we dont think this voltage is due to the signal but it was also present when the RF and LO were OFF and only board was powered on. Please correct us if there is any confusion in our understanding.