[sdiy] Ring Mod (was Re: Hadamard Transform Network)

[Tim Ressel]

Here is my pontificating take on the subject.

4QMs are mathematically pure, aside from distortions, etc. An AD633, 
when given 2 and 3 volts on the input, will give 0.6V on the output (the 
actual function is (X*Y)/10 for scaling).  One excellent use for an 
AD633 is in an analog computer.

Diode ring modulators use steering diodes to switch the polarity of the 
modulator input. The carrier is used to switch the diodes on and off, 
and must of a high enough level to turn on the diodes. In theory the 
diode ring mod is identical to a 4QM with one input being a square wave. 
The square wave has harmonics, of course, and these also interact with 
the modulation input.

In practice, it gets muddy. The diodes don't just turn on and off. They 
go through a curve as they transition. This adds stuff to the sound. One 
can turn down the carrier so the diodes are just conducting to get an 
effect. Also the ring mod uses transformers (mostly) and that adds 
another dimension to it. If the diodes are not matched (and lets face 
it, they ain't), another type of non-perfection is introduced. So the 
diode ring mod is charmingly imperfect, mathematically speaking.

There is a third circuit that makes that 'ring mod' sound: an exclusive 
OR gate. This produces a really dirty grungy raspy sound.

So take your choice. I have and use all three.


On 5/19/2017 3:34 PM, Neil Johnson wrote:
> Tom,
>
>>> David G Dixon wrote:
>>>> Also, if a "ring modulator" and a "4-quadrant balanced modulator" give
>>>> exactly the same transfer function, then why can't the terminology by used
>>>> interchangeably?
>>> They don't give the same transfer function, so you shouldn't
>>> interchange the words.
>> Further explanation please, Neil.
> In the general case, discussed above, they don't have the same
> transfer function.
>
>> Like David, it was my understanding that a diode ring modulator gave the same sum and difference frequencies when fed with clean square waves like you suggested. In which case, it would be doing 4 quadrant multiplication in that limited case.
>>
>> Is that not right? Why not?
> In the *specific* case of driving both with a perfect square wave
> carrier yes they do produce similar outputs.  But that's a specific
> case, and David's comment was a general case (no mention of
> specifics).
>
> The two main areas where they don't produce the same outputs are when
> either of the carrier or signal has DC offsets (transformers are very
> good at blocking DC), or when the carrier is not a square/pulse wave
> (the diodes of the ring modulator add colour that don't get in a 4QM).
> More on this topic in the MW thread (I appear on page 2):
>
> https://www.muffwiggler.com/forum/viewtopic.php?t=127474
>
> Neil

-- 
--Tim Ressel
Circuit Abbey
timr at circuitabbey.com