As detailed in our Mixer Basics Primer, the isolations of a mixer refer to how well a balanced mixer cancels out each of the arms inputs from the outputs of the other arms. There are three isolations of different importance:
1) LO-RF Isolation: This is most important because the LO tends to be very close to the RF (usually the IF is low frequency). Therefore the LO will be in the band of the following components and contaminate the RF circuitry, whether in an upconversion or a downconversion.
2) LO-IF Isolation: This is also important as it is the worst of the isolations. The LO can contaminate the IF circuitry, especially if it is wideband, or cause conversion loss problems if it is reflected from the IF circuitry.
3) RF-IF Isolation: This is less of a problem because the LO is always the strongest signal in the system.
So how do you determine what the isolations are? For a given frequency conversion it is simple: measure the output of the RF or IF on a spectrum analyzer and measure the leaking component. LO power input divided by output power at the LO frequency at the RF or IF port output (or minus, in dBm units) gives the isolation. This is what matters ultimately for the system.
For making a mixer datasheet, however, things are more complicated. We want to know what the isolation is across the entire operating band of the mixer. The obvious way to do it is to hook it up to a network analyzer and measure the leakage. However, since the mixer is by definition a non-linear element, we have to pay attention to the parameters we choose. The power levels usually used to measure linear devices are much too low to measure a mixer, because the mixer diodes don’t turn on until the rated drive level of the mixer (around +10-+13 dBm for a typical double balanced L diode mixer). Some VNAs cannot produce that much power across the whole band. Fortunately ours generally can. Below are two graphs of the isolations of an M1-0412LA mixer, one with measurement power of -5 dBm, and one with measurement power of more than 10 dBm.
As you can see the isolations can vary dramatically with input power. Increasing the input power by 20 dB in this case can lead to an isolation improvement of 20 dB or better!
The last conundrum is how to measure RF-IF isolation, since the normal operating mode of a mixer is with a strong LO present. The truth of this is that generally the LO and RF ports of a mixer are interchangeable. This is actually a trick that we use sometimes to improve the spurious performance of a mixer (sometimes it works better backwards!). So if you measure the RF-IF isolation the same way as the LO-IF isolation (with a high power RF signal input), then it should replicate closely the actual performance you would see under operating circumstances. In any case, this is the least important isolation for most applications anyways.
Nice document. I have two questions.
First, to measure LO to RF isolation, I can measure the transmission(S21) with a network analyzer. In this case, is the IF port left open or terminated by 50Ohm?(They give different results.)
Second, in the plot shown in this post, both LtoR and LtoI isolation curves have the same color. Can you tell me which one is which? Or can you use different color for clarity?
Excellent questions, and very important. All ports of a mixer need to be terminated in 50 ohms to get the datasheet performance. Otherwise the energy that would be delivered to the IF will be reflected to the RF, degrading the isolation generally, and increasing the ripple. Ripples in any measurement of the mixer are usually an indication of a non-50 ohm load on one of the ports.
On these plots, at the lowest frequency, the bottom one is the L-R isolation (blue), the middle is the L-I isolation (purple), and the top is the R-I isolation (red).
Thanks for quick reply.
About the color, I thought that L-R and L-I are in the same color(they looked so on my screen), but they look slightly different now.
Hi,
I have some questions on the mixer measurements.
1) I would like to measure the isolation from LO to IF using a network analyser. Let’s say the mixer is driven at 0dBm. Do I terminate the RF port to 50 ohm and measure the isolation between LO and IF with source power set to 0dBm?
2) I like to measure the mixer matrix spuriuos table e.g. 2×2 (2RF, 2LO). What test setup do you recommend? Are filters at RF and LO (in this case 2RF, 2LO) recommended to filter out harmonics from signal generator.
These are good questions, thank you for submitting.
1) To measure the LO-IF isolation of the mixer, simply terminate the RF in a 50 ohm load and measure the insertion loss from LO to IF. However, you must measure at the appropriate LO drive power for the mixer. None of our mixers work at 0 dBm, so if you are using our mixer you would see worse LO-IF isolation than the datasheet: http://www.markimicrowave.com/blog/what-happens-when-you-underdrive-a-mixer/
2) To measure spurious suppression, filters are HIGHLY recommended when your input LO/RF harmonic suppression is worse than 40 dBc (as a rule of thumb). See http://www.markimicrowave.com/blog/how-to-measure-spurs-with-actual-measurements/
Hello Doug
Thank you for replying. Basically drive the LO at the power what is meant to be designed for e.g. if drive level is 5dBm, set power on analyser as 5 dBm.
I read the post about the test setup. It mentions 1.2-1.8xfc. Is this for a low pass filter? Could a bandpaas filter be ok?
Correct, the mixer must be driven at the system LO drive level to get an accurate isolation measurement. If the mixer is underdriven then the isolation will degrade.
Either a lowpass or bandpass filter is suitable for rejecting higher order harmonics, as long as it does not have re-entrant modes at the harmonics.
Hello,
I connected Port 1 of the network analyser to LO and Port 2 to one of the differential IF port. The other IF port and RF port are terminated to 50 ohm.
Is it S21 that I need to measure as isolation from LO to IF or is it S12?
S21 is typ. 28dB and S12 ás 46dB?
The S parameter you need to measure to determine LO-IF isolation is S21, if the LO is connected to port 1 and the IF is connected to port 2 (S parameters are TO the first number FROM the second number). I would not, however, expect to see such a big difference between the forward and reverse results.
Any one can explain to me what is the difference between leakage and isolation. My mixer is downconverting mixer and has LO-IF leakage about >-21 , is that means that my LO to IF isolation is about 80 dB? My LO input level is 0dBm so I expect that my LO level power would be attenuated by 80 dB? is that right?
LO to IF isolation is defined as the attenuation of the input LO power to the IF port. LO to IF leakage is defined as the output power of the LO from the IF port.
In general the LO to IF isolation is weakly dependent on the LO input power, so as long as the LO input power is within the specified operating range of the mixer the Leakage will be the same as the input LO power minus the LO to IF isolation. If your LO input level is 0 dBm and the LO to IF isolation is 21 dB, then your leakage from your IF port will be -21 dBm. Of course this assumes that 0 dBm is within the operating range of the mixer, which would not be the case for almost all of our mixers as it would be too low.