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Could anyone please tell me how LO rejection is defined in a mixer? I've heard Image Rejection but not LO rejection.

Please help!

Thanks.

Missfresstyle
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    LO feedthru leakage – Tony Stewart EE75 Feb 01 '18 at 20:29
  • After the mixer you have a ceramic or crystal filter that only lets the i.f. frequency pass to be amplified. In todays radios of any type a ADC takes over to finish processing the signal. –  Feb 01 '18 at 20:39
  • @TonyStewart.EEsince'75 Thanks for your reply! It makes some sense to me now. But how would the mixer design relate to LO rejection? Let say if I choose a ring diode mixer, how would these didoes affect LO rejection? I am weak on this topic. Please help! thank! – Missfresstyle Feb 01 '18 at 20:59
  • What stage are you at? Choosing a mixer for a particular application, or designing a mixer? Normally we don't worry about the types of diode for the first one, we just pick a mixer based on specifications. If you're designing a mixer, thne you need to have a set of specifications to hit, what are they? – Neil_UK Feb 02 '18 at 07:53

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In an ideal balanced mixer, the LO is fed via balun in antiphase to a matched set of diodes, and so cancels completely at the other ports.

In practice, the balance of the balun and the didoes is not perfect, and some LO leakage emerges. This is more problematic at the RF port which has a similar frequency range to the LO, than at the IF where it's easy to filter.

LO rejection is defined as the ratio of leakage out to LO in. For a mixer without active controlled LO cancellation, -40dB might be an aspiration, -30dB is commonly seen, sometimes you have to put up with -20dB.

Neil_UK
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  • Thanks for the explanation! If LO signal leaks to IF output port due the imperfect diodes, the leakage should be less(better LO rejection) if I choose diodes that have low off-state parasitic capacitance. Am I right? – Missfresstyle Feb 01 '18 at 21:47
  • Not really. There are so many factors that affect it. If all other things are equal, then yes. But you're pretty much stuck with the best diodes that you want to afford for the frequency and power level. Assymetric component assembly and balun symmetry are every bit as important as diode matching. – Neil_UK Feb 01 '18 at 21:56
  • I see what you are saying :) I am still at a stage that trying to figure out what diodes work the best, therefor I am assuming all other things are equal for now. – Missfresstyle Feb 01 '18 at 22:08
  • Examine the NE602, or newer versions. Below 200MHz, this is excellent circuit. – analogsystemsrf Feb 02 '18 at 03:32
  • There is no such thing as 'a mixer'. You have a frequency range, and a dynamic range, some specifications, and then you optimise the inevitable compromise between them. I've worked with GHz mixers, and MHz mixers, and although the fundamentals may be similar, the components, details and achievable specifications are very different. 'What diodes work the best' Best for what? Schottky diodes for conventional ring mixers, but actively driven FET switches use less LO power and deliver far higher linearity. It will be interesting to see what they achieve. – Neil_UK Feb 02 '18 at 07:46
  • One part of this project is to investigate how different kinds of diode affect the mixer performances. For example, PN diodes and Schottky diodes. How do these diodes relate to mixer parameters? I am not designing a mixer with certain specifications at this point. Diode ring mixers is my choice for now. I've been reading a lot of paper and trying to make the connections. But it's not easy :( – Missfresstyle Feb 02 '18 at 14:50
  • Diodes act like switches right? Since each kind of diode has different switching performance, I am trying to make some connections between them. – Missfresstyle Feb 02 '18 at 14:59
  • Diodes, when driven by an LO that has sufficient power, with a signal that has much lower power than the LO, work like switches. The higher power the LO, the better the switch, but more LO leakage, and drive cost. FETs work as better switches, that's why it will interesting to see how mixers based on FETs fare in the next decade compared to schottky diode ring mixers, which will probably stay cheaper. Schottkies need less power to turn on than junction diodes, and are faster (read 'better switch') than PINs, which is why you only hear of schottky diodes being used in diode mixers. – Neil_UK Feb 02 '18 at 16:27
  • Schottky diodes have zero reverse recovery time in theory, which is one of the big advantages. But I am stuck... Besides switching speed, there should be other parameters that are important... right? (I couldn't get rid of this question in my mind.. Hmmm – Missfresstyle Feb 02 '18 at 18:58
  • The part about a large LO and small signal is important. The diodes need to be good switches, which also means constant on resistance independent of signal current. A small LO, or a large signal, makes that less true, and so gives rise to signal distortion. That is why the FETs I've been banging on about are interesting, they have more constant on resistance and so less distortion. High drive level mixers use multiple diodes to increase the usable LO power, to improve the LO/signal ratio. – Neil_UK Feb 02 '18 at 19:06