Re: FG 504 function generator odd behaviour

Chuck Harris <cfharris@...>

Traditionally, the "function generator" had a voltage source
that fed into a polarity switcher that fed into an integrator.

The output of the integrator was a linear ramp that changed
direction whenever the polarity switcher changed polarity.

To make the function generator oscillate, the output of the
integrator was fed into a Schmidt trigger that in turn fed the
polarity switcher, and made it switch whenever the ramp got
above or below the Schmidt trigger's threshold voltages.

Draw yourself a picture using blocks.

The output of the integrator is a triangle wave, and the output
of the Schmidt trigger is a square wave.

The sine wave was made by feeding the triangle wave into a soft
clipper that clipped off the triangle wave's pointy peaks giving
an approximation to a sine wave. A good bit of art went into
that soft clipper.

The manual will describe the FG504's specific implementation, but
I would concentrate on the soft clipper stage. It typically
has a lot of trimmers, and diodes.

-Chuck Harris

Colin Herbert via wrote:

I have an FG 504 which is showing some odd behaviour. The triangle-wave shows asymmetry in that the negative peaks of the wave are rounded-off at any frequency and the positive ones have a similar but lesser problem at higher frequencies. This also seems to manifest itself as what looks like cut-off on the negative peaks of the sine-wave and lesser cut-off on positive peaks. While the triangle-waves are more-or-less symmetrical, the sine-waves are definitely not so, showing a nearly three-to-one voltage ratio in favour of positive peaks. There is also noticeably overshoot on the square-waves at anything above 1kHz and only a DC offset at frequencies below 80Hz.
I am totally ignorant of how this function generator produces triangle-waves, but I do understand that the square-waves and sine-waves are derived from the triangle-wave. Does anyone have any help that they can offer? I would be grateful to get this useful function-generator working properly.

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