sub harmonic mixers vs. harmonic mixers

Hello,

can somebody explain why mixers using higher LO harmonics for mixing, are sometimes called sub-harmonic-mixers and sometimes just harmonic mixers.
It seems, mixers that expand spectrum analyzer operating frequency ranges are called harmonic mixers, while the same mixer type for different applications (up- and downcoversion...) are called sub-harmonic.
Just convetion ?

thanks
-e

I am not sure that there really IS a difference. People get pretty sloppy about their use of technical device "names". On engineer means one thing, another means something else.

In MY way of thinking, here is the difference:

Subharmonic mixer: lets say I need to receive a 10 GHz signal, and I want a 1 GHz IF frequency. I could use a standard mixer and use a 9 GHz LO. But that is hard to generate. I could also use a subharmonic mixer and either use a 4.5 GHz LO or a 3 GHz LO (depending on how I configure the diodes inside of the mixer for odd or even harmonics). My mixer now produces a 1 GHz LO, and I am a happy camper

Harmonic mixer: well this might mean the following to me. I am upconverting, want a 20 GHz signal. I put in a 1 GHz IF with a strong signal, and a weaker 9 GHz signal. They up convert nicely to 10 GHz. But also, there is a nice 2nd harmonic output, maybe 10 dB lower, at 20 GHz. So I am using the non-linearity of the mixer to act like it has a built-in frequency doubler.

Hi biff,


what you describe as harmonic mixing is using spurious products of a fundamental mixer. I dont think anybody would call this harmonic mixing.

To me, there is no difference between sub-harmonic-mixers and harmonic mixers. Both use antiparallel diodes to mix with LO harmonics.
I just wanted to hear some other engineer's opinion...

-e

you do know that antiparallel diodes only work for X2, X4, etc

Harmonic and "sub-harmonic" name in mixers relates to the input LO frequency if it is much lower than the "nominal LO frequency.
As the LO input power is used to drive the non-linear element(s) in the mixer to switch ON and OFF, harmonics are generated and all are mixed (linearly combined) with the input RF or IF frequency. Filters are then used to select the desired output from that mess.

If a sub-harmonic frequency is used for LO input (meaning an integer fraction of the nominal LO frequency), mixer action is possible but due to the loss in multiplying, the conversion loss of such mixer is higher than a typical value of 6...8 dB deemed good.
Harmonic mixers are typically used in spectrum analyzers (or before them). If properly calibrated, they allow to extend the input frequency range.
There are many ways how to design a harmonic mixer, but expect rather high conversion loss, typically ~10 dB more for each integer multiple of the input LO frequency. For LO/2 input, the CL would be >16 dB; for LO/3, 26 dB, etc.
SOme MMIC makers integrate LO multipliers in their chips, then the CL can be < 10 dB even for LO/3 input. Check Min Circuits and Hittite.

so a comb generator driving a mixer would be called "harmonic" mixing, because of the myriad of "LO tones"

It is possible to use a comb generator. Usually the harmonics are generated by mixer diode(s) only.

Comb generators are often used at low RF frequencies where it is easy to use e.g. TTL logic circuits for such function. I used a 74F04 to multiply 26 MHz quartz signal, to get ~780 MHz for a precision down converter. Quite successful experiment.

At microwaves, generating other than sine signals becomes more difficult, so it is easier to overdrive the mixer diode(s) and generate harmonics that way.

So, to finalize the discussion.
There is no real difference between harmonic mixers and sub-harmnonic mixers.
These use LO harmonics for mixing.

---

Using spurious mixing products of fundamental mixers within the application (as outlined above) is something special and does not have
a name.

Using comb generators at the LO port is interesting for some applications, but does also have nothing to do with the terms harmonic resp. sub-harmonic mixers.

--e