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I’m the first person to admit that mixers are confusing. Even drawing a mixer schematic can be challenging—what with all the crossing over of lines and the 4 and sometimes 8 diodes configurations. For better or worse, the complexity of mixers means (a) most companies don’t want to design mixers, and (b) Marki Microwave’s customers sometimes need a lot of coaching and advice. To put things in perspective, there are literally hundreds of texts relating to RF and microwave amplifier design, and about 3 relating to mixer design. It is not your fault you are confused!
To help you, I have come up with a list of “The Mixer 10 Commandments”. If you can follow these simple rules, I promise they’ll help to make your life a little easier when it comes to using mixers.
1. Thou shalt not starve the mixer of LO drive.
2. Thou shalt not blame conversion loss ripple on VSWR problems when reflective filtering is present at the IF and/or RF ports.
3. Thou shall carefully follow the recommended solder reflow temperature profile when mounting surface mount mixers.
4. Thou shalt not measure phase noise using mixers made using GaAs devices (FETs, diodes, or otherwise). Silicon schottky diodes are preferred for phase noise measurements.
5. Thou shall phase lock your synthesizers to a reference oscillator when making mixer measurements.
6. Thou shall test mixer performance in a broadband 50 Ohm system.
7. Thou shall place the LO driver amplifier as close to the mixer as possible when laying out a PCB.
8. Thou shalt not provide attenuation on the LO port unless it is absolutely necessary.
9. Thou shall assume mixer simulations are always wrong, unless corroborated with measured data. This is especially true when simulating mixer nonlinear performance (i.e. single tone and multi-tone IMD). (This is subject to change as nonlinear modeling matures in the coming years).
10. Thou shalt not starve the mixer of LO drive.
Here’s a hint, #1 and #10 are by far the most important. Too often engineers believe they are saving board space, power and money by under-driving the mixer, only to find later in development that they have unintentionally caused catastrophic penalties in terms of conversion loss, isolation and IMD. If you add up all the time and money wasted in order to correct this relatively straightforward mistake, you quickly find that the safest, fastest, most economical approach is to drive the mixer at the recommended LO level. Think of it this way: you wouldn’t expect an amplifier to work properly if you don’t bias it to the manufacturer’s recommended DC levels, so why should you expect the mixer to “turn on” with too little LO drive?
To learn more about how mixers work and how to use them, check out our Mixer Tutorial.
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