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AD8345 I/Q Modulator Instability

Question asked by Kapati on Feb 22, 2013
Latest reply on Mar 22, 2013 by Kapati

We have three manual phase shifter modules as lab equipment. These were an in-house development using I/Q modulator AD8345.  The I/Q inputs are DC levels from dual TxDAC AD9761 with 0.1% resistors; the programming is from a sin/cosine table and is fully static.  The board is 1.6mm FR4, nothing special and the RF  layout is reasonable but could be improved. 

 

The units function OK, but are troubled with an instability.  This appears as a ~130MHz oscillation but only at particular angle settings, maybe with full gain on the Q channel and zero gain on the I channel.  Monitoring is with a scope RF probe connected to a resistor attenuator directly after the AD8345.The oscillation remains if the RF LO is replaced with an RF short.  I have experimented with changing the supply decoupling capacitors, adding series R, changing the RF capacitors, grounding and so on, but the oscillation frequency doesn't seem affected.  The frequency can be tuned by dropping the supply voltage or sometimes by changing the I/Q angle a few degrees.  In conclusion, it seems the oscillation frequency is set by the IC, not so much by the external circuit.

 

In the worse case, we could make a new PCB using the ADL5370 device, but if we are doing something wrong, maybe this doesn't help.

 

Any suggestions?

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