PC817 Optocoupler Response Time and Load Resistance for 6kHz Motor RPM Signal

Hi forum,
For a particular application I am reading the motor rpm with reluctance type of sensor and interfacing it to the MCU circuit with a PC817 optocoupler( How Optocoupler Works and the Optocoupler Basics: here).

The drive motor is variable speed AC drive and you would have guessed what follows next - noise !!

The PC817 data sheet specifies the response time(Rise) as 4us with a load of 100 Ohm and Vcc of 5V. In my case the load is at about 1 kOhm and Vcc is 5V.

What I am keen to know is whether by properly dimensioning the load resistance, whether the response time can be altered up or down ?

(or)

Does it have anything to do with the noise rejection ? ( my valid input signal extends upto 6kHz and I want to cut down anything above it )

I have set the LED input current at about 15mA.

While the machine is running OK , I would like to improve upon the stability of the RPM values.

Any tips-n-tricks?

Many thanks,
Joshua

Hi Joshua-My first thought is- Where are you getting the power for the reluctance sensor and the LED side of the optocoupler? One mistake I see engineers make is that they will often use the same power source for both sides of the optocoupler.  This drastically reduces the isolation that it would normally provide. If this is the case, could you try temporarily powering the reluctance sensor and LED side of the optocoupler from a battery, and make sure there is no common connection between both sides of the coupler. If this helps, replace the battery with an isolated DC-DC converter module.-Rick