Question

I’m currently working on a design that will incorporate your T3-20 mixer.  In the specification it is recommended to utilize your ADM1-0026PA amplifier module to drive the LOs (will be using single LOs to different T3-20s in the 11 to 13 GHz range).  What I’m curious about, and this is obviously based on the spec for the ADM1, is whether I should include a filter between the amplifier and the mixer and if so, what design would you recommend for the filter?  So basically I’m looking for your recommendations for the best way to drive your T3 mixers.

Answer

Thanks for the question and the interest in our T3 mixers. In general we recommend against filtering the LO for two reasons:

  1. Placing a filter between the T3 and the LO amplifier will create a non-50 ohm environment for the mixer, which can cause increased spur levels and decrease the predictability of the spurs.
  2. Filtering the LO will eliminate the desirable odd harmonics (third, fifth, etc) and reduce the linearity of the T3, which will reduce the dynamic range of the system.

The one caveat to this is if you are concerned about the noise contribution from your LO source degrading your system noise figure. This can occur when you have an LO source that is not very clean. In general the mixer will reject LO noise according in a down-conversion to its LO-IF isolation and the LO-RF common mode conversion loss (typically around the LO-RF isolation plus the conversion loss). This means that if your LO noise is more than 30 dB below your signal path noise you probably don’t have to worry about it. Otherwise you can mitigate the impact of the LO noise by using a high pass filter on the LO up to the IF frequency that you care about. This will allow you to keep the odd harmonics that are beneficial, although it may still impact your spur performance.

Good luck!


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