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HMC8411 Instability

Hi everybody. I designed a custom down-conversion board comprising an HMC8411 for the amp stage and an ADL5830 for the I/Q demodulation. The board is realized on RO4350B substrate and correctly dimensioned to be matched at 5.8GHz. I tested the board without the HMC8411 and the demodulator is working just fine.

The problem comes when mounting the HMC8411. The interface circuit to the DC bias and the RF was designed following what is prescribed by the datasheet, also inspired by the schematic of the EVB. However, many spurious are generated, that seems to be without meaning.

When sectioning the circuit by "breaking" the RF branch before the ADL5380 (so having just the HMC on the RF branch), when I connect the INPUT to the Spectrum Analyzer I can see a pure clean tone at ~5.3GHz, with 10dBm power. It seems like a problem on the feedback loop, but I cannot figure out how to suppress this harmonic that is feedthrough towards the input.

How can I arrange some tests or a solution to suppress this spurious that I think is generating some issues on the demodulator, and so, to the whole circuit?

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  • Hi Jim, thanks for your kind reply.

    I tried to manage a possible "handmade" solution. By scratching the microstrips I arranged to put the L2 on the trace but still the harmonic is present (but now it has 9dBm power instead of 11-12dBm) on the input.

    With the demodulator connected, I can see the expected output but with an attenuation of more than 15dB! (and still A LOT of other frequency harmonics ranging from the expected one to 10-15GHz).

    Best regards,