How to Measure Spurs (with actual measurements)

In a previous post we gave a block diagram for how to measure spurs from a microwave mixer. This post is to show some actual data for it. Experimental Setup: I measured an M1-0620NP using two Anritsu synthesized sweepers and an Agilent spectrum analyzer. The power into the mixer was measured using an HP power meter (this is important, as the power will change with the added filters). An IF of 2.2 GHz was input at a power level of -10 dBm, and an LO of 7.4 GHz was input with a power level of 13 dBm. Here are the spurs shown with no additional filtering: Here is the output in the X band (8-12 GHz). You can see the high side output product at 9.6 GHz with a power of -16.52 dBm (6.52 dB conversion loss). There are two spurs, one at the 2*LO – 2*IF at 10.4 GHz with a value of -57.55 dBm (41 dBc) and one at the 1*LO x 2*IF at 11.8 GHz with a value of -68.73 dBm (52.2 dBc). Next we add a filter to the IF. Since this a low frequency we used our DPX-3, which will pass the 2.2 GHz but suppress the 2*IF of 4.4 GHz. Here are the results from that: Fundamental: -15.95 dBm (input power increased .5 dB) 2 LO x 2 IF: -63.8 dBm (improvement of 6 dBc) 1 LO x 2 IF: -62 dBm (improvement of 6 dBc) This is the most important filter, as the IF harmonics tend to be the most prominent. Next we add an LO filter: Fundamental: -15.91 dBm (same) 2 LO x 2 IF: -58.8 dBm (degradation of 5 dBc) 1 LO x 2 IF: -60.4 dBm (degradation of 1.5 dBc) The spurs got worse with an LO filter? This illustrates how touchy spur measurements can be with mixers. The filter at the LO will change the impedance seen by the mixer across many frequencies, which will cause the spur powers to change unpredictably. Ideally we would put an attenuator on each port, which would ensure a good 50 ohm match at all ports. We didn’t do this in this experiment because we didn’t have the LO power to spare. Also things as small as slightly tightening connectors can affect spur measurements. Every little thing counts. Fundamental: -26.37 dBm (power decreased by 10 dB, same as the pad) 2 LO x 2 IF: -70.72 dBm (44.35 dBc, 3 dB better than originally measured, but not the lowest we’ve measured) 1 LO x 2 IF: -71.4 dBm (44.63 dBc, not as good as we originally measured) Here again the spur levels change unpredictably. This is because the ESA front end consists of a mixer as well, and adding the pad changes the way the intermodulation products reflect back and forth in the system. Spur levels are very low; they can be unpredictable in actual systems.

Now we add an attenuator after the IF filter but before the mixer:Fundamental: -26.63 dBm (same) 2 LO x 2 IF: -73.99 dBm (47.36 dBc) 1 LO x 2 IF: -79.12 dBm (52.49 dBc).

This is our final answer. The good 50 ohm match at the IF caused a significant improvement. Note that the filter may be matched at the IF frequency, but for our purposes it has to be matched at the spurious frequencies, which for our purposes means all frequencies.

With the RF and IF ports matched to 50 ohms and the filters in place for the synthesizers, these are the spurs. Note that these aren’t the spurs that you’ll see in your system, unless your system provides filtering at the synthesizers and a good 50 ohm match at all ports.