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I have a mixer that is behaving erratically. It has low conversion loss, my system is showing low output, what is the cause?

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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/handling damage and DC current application.

Thermal and handling damage usually occur in concert to surface mount components during the assembly process. The application of excessive heat combined with improper care in handling can cause obvious mechanical damage to the part.

The other major failure mode is the application of excessive current. While we have never seen a mixer damaged from excessive RF input power, damage due to excessive DC is quite common. This is particularly common in IQ mixer structures where a DC offset is used to improve sideband suppression. The addition of DC connected circuitry can expose the mixer to increased risk of damage due to ESD discharge as well, since the mixer is generally protected by RF shielded cables and connectors from any discharges.

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 IF to the grounded case. Next reverse the leads, with the positive lead on the ground and the negative lead on the IF 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 the mixer. The conversion loss of each side should be within the datasheet spec for conversion loss. Ensure that you are supplying sufficient LO power. If you are driving it below the datasheet spec, then it will have high conversion loss.

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.

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