## Personal web pages of## Tim Stinchcombe |

## Triangle wave to sine wave conversionI find the idea of fashioning a sine wave from a triangle wave by passing it through some sort of nonlinear waveshaping device a fascinating one, especially since many of these devices, superficially at least, seem to bear no relation whatsoever to a sine function. Some years ago I started documenting references whenever I came across a new one, and the list has now grown pretty large. I thus thought I would make a page dedicated to this subject: I'm not quite sure just what format it will eventually take, as I'm a little hesitant about copying lots of schematics from many books, and thus how useful it will be, as without doing so, the page may seem a little I have split the page into three: I will also point out that whilst I have all the papers and books mentioned, I have only practical experience of a very few of these circuits, and so it should be clear that my interest Please note that not all links on book or journal paper titles are going to take you to a [The main focus of this page is to provide information on sine wave approximation using ## Piecewise linear approximationThis is probably the oldest method—here resistors with diodes and/or transistors are arranged to approximate a sine curve with a series of straight-line segments. Many different types of arrangements are possible, but an obvious disadvantage is that to accurately model the sine wave, many linear segments are required, hence requiring many components, and soon we encounter difficulties matching all the component values etc. An example of the complexity such a scheme can reach is the (now obsolete) ICL8038 function generator chip, of which, here is the equivalent schematic of the sine converter: Being fabricated in a monolithic chip, matching between the transistors is probably quite good, but I do not know how accurately they needed the resistors—THD is quoted as "close to 0.5%" when external pots are used for trimming. ## Journal refs
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## Overdriven differential pair approachprobably most popular; makes use of the 'bow-tie' curve that arises as the differential voltage across pair increases. does anti-parallel diodes (a la Doepfer) fall into this category? ## Journal refs
## Synth-related book refs(Several of the Thomas Henry books have been made available again through Magic Smoke Electronics.)
## Other book refs
## Middlebrook & Richer JFET-based methodsomewhere need Buchla links, and how this is a hybrid of MR + cusp cancelling - no knowledge of whether this actually helps, as clearly as it does for the BJT case... The Middlebrook & Richer technique uses the nonlinear properties of a JFET to produce the sine-shaping, using the JFETs symmetry in an elegant and very distinctive configuration: It was popular in many older synth circuits; to maintain accuracy, the amplitude of the input triangle wave needs to be well controlled, and this needs to be in the correct relation to the chosen JFET's threshold voltage in order to ensure that the JFET does indeed cut-off at the top of the triangle; the original paper also says one of two resistors can be trimmed, but implementations of the circuit are rarely seen with this. ## Journal refs
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## Electronotes refsNot surprisingly there are a lot of references in
[Page last updated: 15 Dec 2010] |