[sdiy] passive ring modulator transformers

[Richie Burnett]

Hi Neil,

Thanks for the video, I drew out those exact same diagrams last year explaining the diode mixer to a colleague at work!  It's a good summary of mixing and amplitude-modulation, covering sum and difference frequencies, harmonics, etc.

My point was that his two cases of a gain of +1 and -1 are just the saturated gains. There are times when the diodes aren't fully conducting (operating somewhere on the curved part of their I-v characteristic) where they present significant resistance to the passage of the signal. Here the gain is somewhere between 0 and 1, in either the positive or negative direction. From about 8m onwards in the video he even says that the real diode mixer doesn't behave as if it switches abruptly when compared to the squarewave modulated perfectly mathematically generated version. He suggests that this is because sometimes the diodes are all off, which is true. But what he fails to discuss is that the diodes present a whole range of resistances between a few ohms and open-circuit depending on the instantaneous applied forward voltage. This is what causes the proportional behaviour.

The diode mixer or ring modulator is a proportional device, and in fact Minicircuits even recommend a sinusoidal local oscillator signal at +7dBm level for optimum operation. With harder LO drive the behaviour is less linear and tends more towards the saturated switching behaviour, with the addition of a whole load more distortion products in the output.

Yes, that dafx paper is the one I was thinking about. Thanks for the links! I only skimmed over it but it looked good.

-Richie,

Sent from my Xperia SP on O2

---- Neil Johnson wrote ----

>Hi Richie,
>
>> Neil, in practice the diode ring modulator is somewhere in between the "product generator" and the "switching modulator" that you described. There is some proportional multiplication behaviour going on there because of the incremental resistance of the diodes. (They don't switch abruptly from fully blocking to fullt conducting.)
>
>Any resistance in the diodes is an imperfection, which is balanced out
>to reduce carrier bleed.  As long as both directions see the same
>resistance then that should cancel out and no carrier seen in the
>output.
>
>> The diodes can be made to switch sharply by over driving one of the inputs, or using a square wave like you said, and this is often the case in RF mixing applications where you're just interested in a single IF frequency at the output, and filtering away all the other crap.
>
>Here's a rather useful video showing a ring diode mixer in action on the bench:
>
>https://www.youtube.com/watch?v=junuEwmQVQ8
>
>The original application for the ring modulator was in telephone
>circuits, where it was used for FDM'ing telephone channels together.
>See the original Cowan patent for background.
>
>> But it certainly doesn't have to be like that. It's not a strictly on/off modulator, it is *a bit* proportional too :-)
>
>Yes, it's quite funny that the world of electronic music has taken the
>ring modulator and kind of claimed it as its own - see the wikipedia
>page on "Ring Modulation" for example: almost the entire content, and
>list of references, are in the electronic music domain.
>
>> Sure you could introduce non-linearities into a mathematically perfect double balanced modulator though to make it sound like a vintage diode ring modulator. Someone wrote a very good paper on a digital algorithm for modelling a diode ring modulator, but I seem to have the link saved.
>
>Did you mean this one?  http://recherche.ircam.fr/pub/dafx11/Papers/66_e.pdf
>
>Cheers,
>Neil
>--