When I first entered the world of mixers, customer concerns about spurs were very cryptic. When one spur mattered against another seemed totally arbitrary. Over time I learned that certain spurs matter in certain situations. Here are those situations:
1 LO x 0 IF/0 RF (LO-IF/RF Isolation): As we discuss in the mixer basics primer, this matters all the time, but particularly in an upconversion with a low frequency IF. In this case the LO will need to be filtered from the nearby RF. This is actually a good reason to use a lower frequency IF before final transmission, since that makes the final stage filtering easier. The LO – IF can also be a problem in conversions with a high IF.
1 RF x 0 LO (RF-IF isolation): This isolation, like the LO-IF isolation, is important in conversions with a high IF frequency. For example a DC-6 GHz up/downconversion to 7-13 GHz will need the RF/IF filtered out of the output.
1 LO x n IF: This spur is important in the same situation: upconversion from a low frequency IF. In this case the LO will be on the inside of the LO + IF and LO – IF, and the other side will be the LO + 2 IF or LO – 2 IF, as below:
This is an upconversion of a 20 MHz IF with a 6 GHz LO using an ML1-0220L. You can see that the LO is suppressed due to the RF-IF isolation, and the 1 LO x 2 IF is suppressed due to the balance of the mixer.
2 RF x 2 LO (also m LO x m RF): This is the brother of the 1 LO x n IF, but for downconversions. In a downconversion to a low IF, the 2×2 will show up at double the IF frequency, and requires a low pass filter between the IF and 2IF.
m IF x 0 LO (Harmonic IF Isolations): This is important for medium to high level IF frequencies translated to low to medium level RF frequencies. For example, a 1 GHz IF translated to a 3 GHz RF will have to contend with the 3xIF harmonic, which is probably fairly strong. Odd harmonics, in general, are stronger than even harmonics.
n LO x 0 IF (Harmonic LO Isolations): These are generally only a problem for band conversions, usually for satellite work. If the LO is lower than the IF or RF, then it becomes a problem similar to the harmonic IF isolations, but worse because the LO is stronger. The 2LO can be a problem for high IFs as well.
2LO x n IF: These are an issue when doing a low side upconversion. The LO – 2,3,4 IF will cross the fundamental when the IF increases high enough, as shown below (from our spur calculator):
n RF – (n-1) LO: These will show up in a typical downconversion with a high side LO:
n LO – (n-1) RF: These, similarly, will show up in a low side LO downconversion:
Other spurs will show up in many disparate situations, with differing levels, particularly in unusual conversions. One conversion that you haven’t seen mentioned much in this blog entry? The high side upconversion. This is because the high side upconversion is the frequency translation with the fewest spurs:
In this case the only interfering spur is the 3rd harmonic of the IF, and that is relatively low level. The high side upconversion causes a reversal of the frequencies, so we recommend a heterodyne system with a high side upconversion to a high IF, filtering, andd then a downcoversion to a lower IF for final processing. The other downside? You need a wideband mixer to use the high frequency LO. A wideband mixer much like the ones sold by Marki….
