Need LFO (ramp) for sweep generator

[Sooner Boomer]

I always wanted to buy/build a function generator based on the Exar XR2206 chip.  In playing with effects pedals over the past year or so, I finally decided I needed one and bought the Jameco kit.  What I need to add on is an LFO to do FM so that I can get the chip to sweep through a frequency range to look at the response curve of pedals. 

Chip datasheet: https://www.sparkfun.com/datasheets/Kits/XR2206_104_020808.pdf

kit catalog page: https://www.jameco.com/z/JE2206-Function-Generator-Kit-Assembly-Required_20685.html

Any ideas, circuits, or references?

[duck_arse]

Code Select Expand

7/AO/44a  Low Cost Audio Sweeper - 1    Aug 92
electronics australia published an audio sweeper using the xr2206, which I still have - much modified so it doesn't sweep.

[merlinb]

The resistor used to set the sinewave frequency is acting as a current sink in much the same way a PT2399 delay pin works, so you can apply the same principle. A simple transistor current sink to take the place of the resistor, and an LFO (sawtooth?) to drive the transistor. Presumably you'll need a trigger output so you can view the envelope on your o'scope.
R.A. Penfold published a sweep generator using the LM13700 which did the same thing, I'll see if I can dig it out. (These days you would use an MCU and a DAC to do all this stuff, but old skool circuits still have a place in my heart!)

EDIT: Do you need the sweep to be logarithmic so it displayes more like a conventional frequency response graph on the scope? That will need a bit of extra circuitry.

[BetterOffShred]

Not a whole lot of help again, but the echo dream 2 uses a 2206 to drive the modulation, schematic is around.  It has a square and some wave setting, plus depth and rate control. 

Merlin's solution sounds pretty good though

[merlinb]

R.A. Penfold published a sweep generator using the LM13700 which did the same thing, I'll see if I can dig it out.

Here it is:
https://www.dropbox.com/s/qbs0563rakwedcf/1983_Sweep_Generator_Penfold.pdf?dl=0

[Sooner Boomer]

Thanks, I'll take a look at it.