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ADL5385 Output Level and LO Drive

Hi, we're using the ADL5385 to produce a modulated waveform (1.2MHz BW) from 200MHz to 400MHz. To do this we drive the LO port with 400MHz to 800MHz from a synthesizer (ADF4112 + VCO). I have a couple of questions based on observations made so far.

1) The datasheet implies an output power at the modulator of around +5dBm. We are driving each of the the IQ inputs with 700mVpp biased at 500mV, our power is around -5dBm. Is this what you would expect? Can we increase the IQ drive to 1Vpp, how would that degrade performance?

2) When driving the LO input single-ended (using the eval board) from our synthesizer, the performance is pretty poor (the constellation wobbles and shows excessive magnitude imbalance (and phase to a lesser extent) yielding an EVM greater than 20%. If we add a low-pass filter to remove VCO harmonics the performance gets pretty good (EVM < 2%). I've read a bit on the issue and if I understand correctly what we are seeing maybe due to even-order harmonics of the VCO upsetting the divider timing inside the modulator. Two options are then offered to counter this: low-pass filter the VCO output to reduce the even-order harmonics (is 20dB sufficient? 30dB?), or use a balun (or transformer) to drive the LO differentially (do you have a recommendation?). Are the second-order VCO harmonics (or for that matter, any VCO harmonics) the root cause of what we're observing; and if so, what do you recommend to rectify this?

Thanks.

  • ADL5385 is characterized at -7dBm LO, I/Q inputs=1.4Vp_p biased at 500mV, 1MHz CW. At this condition, it outputs above +5dBm. So if 700mVp_p means 700mVp_p at I+ /I- /Q+ /Q-(so 1.4Vp_p at I and Q), then -5dBm is too low. What is LO driving level even I think it is not from LO amplitude? When I checked output power over LO amplitude(-10~6dBm), almost no change. Please make sure if there are 700mVp_p at each baseband inputs of ADL5385.

    We can increase to 1Vp_p at each of the I/Q inputs(so 2Vp_p at I and Q) and Figure 8 and 9 at datasheet show the performance over baseband amplitude.

    At divide-2 Modulator such as ADL5385, duty cycle is important at accurate quadrature signal generation. Even-order harmonics could effect on this duty cycle and result in performance degradation such as EVM. As mentioned,it will improve even-order harmonics performance by driving LO at differential. Best way would be driving LO at differential with Filter like below(we recommend below circuit to interface PLL to Modulator). 30dBc is the number discussed at x1 modulator as harmonic level but no data at x2 modulator now.

    Best Regards

  • Update: Our problem with the output level turned out to be operator error. After re-addressing the drive circuit, the measurements of output power were as expected.

    The EVM issue with the LO drive is still not quite right, but for now it is acceptable. Driving the LO input differentially (using a transformer/balun) had little if any effect. Filtering the LO input had a positive effect however. Second harmonic rejection on the LO input around -20dBc appears to be adequate.

  • It is good to hear that the output power of ADL5385 is now normal. In terms of EVM, please check LO Leakge level which is also related to EVM performance. If possible, you can get better EVM by nulling LO Leakage level. Refer attached file for correcting imperfections in modulator. And it might be also valuable to check it by adjusting LO driving amplitude.

    AN-1039IQModulatorErrorCorrection.pdf