We’ve put a 13dBm ~3GHz input at the LO port and 0-0.5V DC at the IQ ports.

At the beginning, the RF port did give an output, but the ‘I’ input did not affect it (only the ‘Q’).

After about an hour, we stopped getting any output from the RF port.
Note that we did not exceed any of the input power limitations…

We tried replacing the IQ mixer by a “Marki IQ-0307″ we got from another lab and everything worked, so our testing setup is OK.

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Unfortunately, it sounds like the mixer may have been damaged by the application of DC current. Marki mixers are remarkably robust, with many of our mixers lasting in harsh conditions in the field for decades without failure or degradation. There are very few known degradation mechanisms for our mixers, and the two most common are thermal damage and DC current application.

To test if your mixer is still good perform the following tests:

– Take a handheld multimeter, and set it to the ‘diode’ setting. The diode setting will measure how much voltage is required to drive 10 mA of current from the positive to the negative lead. For a standard silicon Schottky diode this value would be 0.7 Volts. For most Marki low barrier diodes it will be around 200-300 millivolts. Measure from the center pin of the I to the grounded case then from the center pin of the Q port to ground. Next reverse the leads, with the positive lead on the ground and the negative lead on the I and Q center pins. Now compare the measurements. If they are roughly the same, then the diodes are ‘alive’, and if one is significantly different, the diodes are likely ‘dead’. If one voltage is higher than the others it indicates that one of the diodes in the ring is probably burned out.

– If possible, measure the conversion loss of each side of the IQ mixer. The conversion loss of each side should be within the datasheet spec for conversion loss. If one side creates a significantly lower output signal than the other, then it is likely damaged.

After you have these results you can send an email including your findings to support at markimicrowave. Unfortunately when diodes are burned out due to the application of DC current, this is considered field damage and the parts are not returnable under the warranty, so please be extremely careful when using this technique for IQ compensation, and consider one of these alternative methods. Application of the means to apply DC voltage, even, can expose the unit to potential power surges and ESD discharge that it is normally protected from by the shielded connectors.

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