PWM Fan Controller - how to get a stable frequency?

[Didix]

Intel's specification for  4-Wire Pulse Width Modulation (PWM) Controlled Fans asks for a 25kHz modulation:
http://www.formfactors.org/developer/specs/4_Wire_PWM_Spec.pdf

Nidec proposes a NE555 based circuit:
http://www.nidec.net/apps.htm

This circuit works for my fan - so far that's fine.

However, the frequency strongly changes when changing the duty cycle (10kHz .. 20kHz).

How could I do better to get a much more stable frequency when changing the duty cycle (by using a NE555)?

-Didix

[void_error]

Move the 1k resistor from the output to the wiper of the 100k pot, that way any load at the 555's output won't affect the frequency that much. In other words, short the 1k resistor and put one 1k resistor between the 100k pot wiper and the TRIG-THRES node.

[NiHaoMike]

The inverter in the fan motor won't mind a big swing in PWM frequency. In fact, after the input level converter, it most likely either goes into a low pass filter or a gated counter, with maybe a few Hz of control bandwidth.

[Didix]

Meanwhile I have replaced the capacitance to 300pF.

To neglect the influence of the load I have removed it.
This does not heavily change the fact, that the frequency covers that wide range  (21..38kHz).

I have also moved the 1k resistor as proposed to the wiper.
This changes the effect when having the load connected a little (24..36).
However, when the load ist unconnected I still see the same wide range (21..38).

-Didix