Variable frequency ramp generator

[SneaKSz]

Hello Guys,

I'm having problems with designing this "Variable frequency ramp generator". It has to be somewhere around 10 Hz - 200 kHz. I tried with the 555 as ramp generator but then got stuck with adapting the voltage to vary the frequency :

.

Someone whos knows a decent circuitry/IC to retrieve this goal?

EDIT : So with variable I mean controlled from a µC

Kind regards !

 

[Nigel Goodwin]

You're not going to get anywhere near that in a single range, the value of R sets the charging current, and thus the frequency - make that a variable pot (with a fixed resistor in series) and make C switchable for different ranges.

 

[SneaKSz]

Hi Nigel, thanks for the reply. ( I've edited my first post !)

Well it is there some kind of IC (digitally controlled) to set parallel to the R, in order to change the load current.

Kind regards

 

[alec_t]

If the frequency is to be digitally controlled by a micro, why not just get the micro to apply a train of current pulses to the capacitor to generate the ramp?

 

[SneaKSz]

Well are you able to supply some sort of schematic ? Maybe adapt R1 en R2 to a µC output and regulate the ON time?

Thx

 

[Nigel Goodwin]

Why not just generate the ramp via a DAC from a micro?.

What EXACTLY are you trying to do?.

 

[SneaKSz]

Some kind of function generator and it needs sine, square, sawtooth and traingle. the " sine, square, sawtooth" are done by a DDS and now I need the sawtooth.
Indeed I also thought that, generate it from a SPI DAC, but lets take a look.

fosc µC = 40MHz, spi clk = fosc/4 = 10 MHz ( SPI DAC = 20 MHz), 2 bytes need to be sent to set the output voltage => update at = 10MHz/16 =625 kHz. Lets say a sawtooth 0 to 5V, Deltva V? 100 steps? f sawtooth = 6,25kHz . I can use some kind of recovery filter but the cut off frequency has to be variable...