 # l297 question, about chopper rate

the circuits of the l297 always have a 33nf capacitor and a 22k Ohm resistor
connected to the OSC pin, like for example circuit below:

From the datasheet of the l297 i read:

pin OSC :
An RC network (R to VCC, C to ground) connected to this terminal
determines the chopper rate. This terminal is connected to ground
on all but one device in synchronized multi - L297 configurations. f
@ 1/0.69 RC

Well english isnt my first language
And i’m not that deeply into electronics to understand what it means,
and… i dont have a 33nf capacitor, i got a 10nf would that be a problem ?
i’m not sure what would happen from the text i think more values might be used, but i might be wrong to,
i dont understand the role of “chopper rate”.

i dont have a 33nf capacitor, i got a 10nf would that be a problem

Yes. The value of the capacitor determines the frequency. As you mentioned, f=1/0.69 RC. With a capacitor 3 times smaller, the frequency will be about 3 times higher.

Use 3 of them in parallel If you have the parts... That will get you close enough.

Doc

frequency i have read somewhere without explanation that its frequency is about 20khz but when i try

is this like f = 1 / 0,69 * 22000 * 33 = 1052173,9130434782608695652173913 about 1.052.173 Hz ? seams way to high or is it f = 1 / ( 0,69*22000 * 33 ) = seams way to small

perhaps if i know the formula correctly i could use different resistors and capacitors to get about the same end value.

Still i wonder what this freuqency exactly does, i think it generates a block-wave from which the signals are made. But it isnt the clock signal, the clock determines the motor step moments, it must be something else but i wonder for what purpose it is used?

You forgot the nano in nF f = 1 / ( 0,69 * 22000 * 33 * 10-9) = ~ 2 kHz
Is that realistic ?

I saw a 3,3 nF somewhere ( which would yield a SYNC frequency of 20 kHz ), if my calc and understanding is correct.

You could of course compensate the smaller C by a bigger R, but the data sheet schematics propose a fixed R=22k and a variable C.
Give it a try with a 68k resistor. If that does not work, what about a 100 nF / 6.8k pair ?
How critical is that SYNC frequency, in general ?