In Mixers

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 ourMixer Tutorial.

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Showing 5 comments
  • K. Giannopoulos
    Reply

    This is a nice article I have read before. Difficult to find specific information about mixers indeed. The only valuable information I had before I read your tutorial was from Charles Wenzel, very helpfull.
    A few months ago, I decided that this information would be valuable to the Greek Microwave Group members, so I have included the tutorial link to the GMG website (under the permission of your company).
    If you have any other helpfull documents you wish to share pease notify us.
    I am really considered about phase noise of mixers especially in the X-band region, but lacking the equipment to measure it, I have been based only on articles and hints to judge their phase performance. What are the hints you can give us for an ultra low phase noise mixer at these frequencies?
    Generally are these so called “brick block mixers” (usually SMA I/O ports) low noise diode ones or gilbert cell ones?

  • Denis Jaisson
    Reply

    Hello Christopher
    Regarding your 10 commandments…
    “6.Thou shall test mixer performance in a broadband 50 Ohm system”
    => this is ather accademic; such a test is a good indication of the future performance after integration into a system, only if the latter involves broadband 50 Ohm souces and loads
    “9.Thou shall assume mixer simulations are always wrong, unless corroborated with measured data”
    => Is it not fair to trust simulations in the case of a diode mixer – and only such a mixer?
    “This is subject to change as nonlinear modeling matures in the coming years”
    => …and manufacturing stability! I know of accademics who spend years and years modelling devices whose manufacturing tolerances are in the -50% to +200%(!) Most of them are German; I belkieve it is cultural…
    Regards,
    Dr. D.J.

  • Christopher Marki
    Reply

    Comments about your comments:
    1. Thanks for you link to our tutorial, we are very happy that people find it useful. We are always writing new mixer application notes, make sure to check markimicrowave.com to stay up to date.
    2. Phase noise is an important topic, and poorly understand by most people. I will be addressing this in the future. A very common mixer type people buy for phase noise measurements is the Marki M8 mixer.
    3. The “measure in a 50Ohm system” commandment is important for the same reason commandment #2 is important; failure to resistively terminate all IMD tones can result in very surprising behavior because reactive (i.e. reflective) loads can cause spurs to remix.
    4. People will always attempt to simulate mixers, it is inevitable in this era. In my experience, mixer diodes are not necessarily easier to simulate than, say, FET mixers. For example, I have yet to successfully simulate a Marki T3 mixer. The high two-tone performance of the T3 (> +35 dBm), was not predicted accurately using the harmonic balance engine. After lots of debugging, I eventually decided that it was a waste of time to tweek the simulation and chose, instead, to simply build the mixer and measure it. Sometimes old fashioned is a GOOD thing.
    Cheers
    Christopher

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