A Brief Guide to Mixer Spurs
Spurious products can quickly overcrowd your output spectrum and destroy the dynamic range of your system if you aren’t careful. The threat of these spurs can be minimized if a designer can predict their exact frequency and power early in the design phase. In this short (45 minute) webinar, we will discuss:
• Where single tone intermodulation products come from within a mixer
• How to model a mixer to get close approximations of spurious levels
• How to get the most accurate prediction possible without sitting in front of a VNA
Q & A
How do you get a square wave at microwave frequencies? Aren’t you limited by BW of the system?
Yes they are limited by bandwidth, since square waves are comprised of odd harmonics, if you have a 30GHz square wave, that already puts the 3rd harmonic at 90GHz! How we like to generate square waves is by saturating distributed amplifiers. These can generate higher order tones and different topologies of amplifiers are good at generating odd tones. Another method would be to use a limiter circuit to clip the top of your signal. For more information on this, you should sign up for our webinar “Amplifiers for Synthesizer and Local Oscillator Generation: The Unsung Application” on May 28.
Conceptually, how does increasing diode voltage reduce spurs?
Increasing the diode’s Vf essentially makes it even harder for the weak RF signal to modulate the diode. Since spurs are generated in the finite time that the LO and RF signal intermodulate on the diode, if we can make it harder for the RF signal to modulate the diode, we reduce the spur level.
Are mixer parameters frequency dependent?
Yes, nonlinear mixer parameters are highly dependent on operating frequency and the power levels of incoming signals as well.
How do Marki Microwave’s LO driver amps make perfect square waves and generate all odd harmonics? What is the typical performance for LO square wave generation?
Our square wave driver amps do not generate “perfect” square waves. They are capable of generating strong third and fifth harmonics. For an idea of hat that looks like in the time domain, here is a plot of the output from an ADM1-0026PA with a 1GHz +15dBm input sine wave.
How will multitone modulated signals at the input of a mixer affect performance?
This is an interesting question that can be explored and simulated using our nonlinear device models!
Can the spur web model dual conversions?
Our spur calculator does not currently offer this capability.
Where can I learn more about the Spur Web?
You can learn more about the spur web here or by emailing [email protected] with questions
Do we need attenuators at RF & IF outputs of mixers to avoid multiple reflections into mixers from output filters? How does this affect mixer spurious performance?
You do not always need attenuators, however there are instances where a small amount of attenuation on the mixer’s ports can improve performance. This is commonly seen when you take a swept measurement and the plot is jagged. This is usually caused by a standing wave that can be nullified with attenuation.
All of your mixer spur tables are given for typical 100MHz IF frequency. How can I get an idea of spurious performance with my frequency plan?
There are a few tools you have at your disposal to verify spurs in your custom frequency plan. You could use our spur calculator, you could simulate with our nonlinear device models, or you could email [email protected] with the RF/LO/IF frequencies and power levels, which spurs you want measured data for, and on which mixer. Someone will run the test for you and send you the data.
What is reasonable design margin for the spur level with respect to the spur data provided in the datasheets?
With respect to the values placed on the spur tables, we have seen variation from this number by as much as 30dB across band. This is because the spur tables offer a median value of suppression across the mixers entire operating band. For a more realistic suppression value you can use the swept spur plots in our datasheets, you can simulate, or you can email [email protected] with your frequency plan and someone will reach out to you with measured data on a unit.
Are the mixer models available for download?
Yes. Our nonlinear device models are available to download at this link: https://www.markimicrowave.com/engineering/resources/non-linear-device-models/
Do you have Genesys nonlinear device model?
Unfortunately, we do not have nonlinear device models for Genesys
If we increase LO power above +20dBm to +25dBm what will be the typical IIP3 improvements? Will increasing LO power around 25dBm will reduce IMD spurs?
The IIP3 improvements will be dependent on the mixer. In theory, as you drive a mixer harder with more LO power, the IIP3 should improve as should spurious performance. This is especially true of T3-family mixers which uses feedback circuitry to sense the LO power and change the operating point of the diodes.
What are Configuration A and Config B in your spurs matrix?
Configuration A vs configuration B simply describes which port the large signal enters. If you inject the large signal into the port that we label LO then the mixer is in configuration A. If the large signal enters what we label as the RF port, then the mixer is in configuration B. In reality, any port on our mixers can be used as input or output ports.
Can you explain a little about IQ mixers
In short, IQ mixers are two mixers in parallel, driven in quadrature. For information on IQ, IR, and SSB mixers read our primer here: https://www.markimicrowave.com/assets/appnotes/IQ_IR_SSB_Mixer_Primer.pdf
Spur data is given for an RF power level of -10dbm. How much will the spur reduce if we reduce the RF power to -15 or -20dbm
Spurious suppression is scaled for different RF power levels by (n-1), where “n” is the RF spur order. For example, if the 2RF x 2LO spur is 69 dBc for a -10 dBm input, then for a -20 dBm RF input, the 2RF x 2LO spur will be approximately (2-1) x (-10 dB) lower, or 79 dBc
You have ADM & APM series Square wave amplifiers? Do both will serve same purpose for T3 mixers?
Both amplifier families are able to drive T3 mixers, however for a more in depth discussion of this, you should sign up for our webinar “Amplifiers for Synthesizer and Local Oscillator Generation: The Unsung Application” on May 28.
Why are there small variations between Config A and Config B suppression values?
In configuration A vs configuration B, the signals take a slightly different path and as such there are different parasitics that come into play in configuration A vs configuration B. Typically configuration A is better for loss and configuration B is better for IP3 and spurs, but this is not always the case. This is the reason we characterize both configurations, since one may be better optimized for a given frequency plan than the other. Take the example below. At a 5GHz RF, you may want to use configuration B, but if you are operating at a 13GHz RF then configuration A is superior.
For wideband RF frequency plans, to avoid spurs we typically choose our first IF as our up-converter stage and then in the second stage we down-convert. For first IF stage, Can we use down-conversion configuration if mixer RF, LO, IF within the mixer bandwidth?
Any port can be used as an input or an output so long as the signal falls within the bandwidth of the port. In fact, choosing different configurations will result in different spur levels, so playing with the configuration of the mixer may yield higher linearity.
Do you see PIM (passive intermodulation) in balun becoming an issue in spurs. I heard that the PIM is caused by hysteresis in the core of the baluns.
If you use a magnetic core, PIM becomes a little more noticeable, but we do not have the ability to measure it at Marki. The diode intermodulation terms are always orders of magnitude higher than those from the balun itself. In a standard transmission line balun, the PIM is essentially infinitely small.
Doesn’t the LO input look like a weird, time-varying, level-varying impedance? What concerns does this present to an LO driver?
While yes, that is true, we typically find that the LO port match is actually quite good. It is typically the RF and IF ports, especially RF port, that does not have as good a match.
How much advantage does a triple balanced mixer gives over double balance?
A triple balanced mixer offers very wideband operation on all ports.