What happens when you underdrive a mixer?

The oldest question in mixer tech support is probably “what happens when I drive the mixer with X dBm LO?”, where X is some number lower than what we recommend. In general, and particularly in the past, we have avoided this question. Our recommendation was and is to never underdrive a mixer. The reason for this is that a mixer with insufficient LO drive does not act as a switching device, but as a square law device. If the LO does not turn on the diodes then the physics of mixer operation change completely, and all of our carefully laid design work is thrown out the window.

Indeed, when you underdrive a mixer the conversion loss is not the only thing that changes. All of the specs change, and in unpredictable ways. The LO side in particular responds weirdly, because a lot of deficiencies and inefficiencies on the LO side are exposed when it is underdriven and concealed under normal operation. In this post we will show all the bad things that can happen when you don’t supply an adequate LO drive to the mixer, and then leave it to you as the user to decide what LO drive to design with.

The mixer we will use as our guinea pig is the ML1-0110LSM. This is a strategic choice, since it is one of the few mixers that our PNA-X can actually produce enough LO drive to measure. This brings me to another important point: datasheet mixer performance is specified only for the recommended LO drive levels. This seems obvious, but is can be nefarious when you are trying to measure return losses in your system, and you use a VNA that cannot produce enough power to turn the diodes on.

First let us look at conversion loss:

The conversion loss actually suffers less than you might expect at the beginning. The rule that is sometimes repeated that ‘1 dB less LO drive leads to a 1 dB increase in conversion loss’ obviously has no basis in fact. In truth the conversion loss near the peak efficiency points for the LO balun (1 high frequency and 1 low frequency) is barely affected, but the ripple across the band and the conversion loss at the band edges suffers dramatically.

Next lets look at the isolations:

Here the results are a little subtle and unpredictable. The isolation generally improves with increasing LO drive, but there are resonance points where it improves more at one point than at another, and there are resonance points where it gets worse rather than better. This is also less repeatable from mixer to mixer. The RF to IF isolation barely changes, since it is largely determined by the passive structures.

Next we test the return losses:

Here we see dramatic changes. When the diodes are not turned on the power cannot penetrate the structure of the mixer, and so much of it is reflected. As the diodes start to turn on some resonance points disappear and the return loss becomes much lower and flatter for both the LO and RF.

Finally a quick, and incomplete, look at a spur, the 2×2 downconverted:

Here is where we see more unpredictability in the underdriven mixer. Spurs depend much more than other specs on resonances, making them the most difficult to predict in software and engineer out. In general the spur increases with LO drive until you hit the nominal drive level. After that, it increases at some frequencies and decreases at other frequencies. This can affect you by almost 10 dB at some frequencies. Note that this is only a spur in a single mixer with a single frequency plan. The spur power will vary more with LO power from mixer to mixer, and is a major reason to not underdrive the mixer. I have not even addressed IP3, which is its own can of worms.