FFT reading for 10Hz - 200Hz with frequency resolution of 0.5Hz.

[Shivanand P]

Want to measure frequency with FFT,
Frequency range = 10Hz - 200Hz.
Frequency resolution = 0.5Hz


Which controller suits for this, and which FFT algorithm will give better results. I am new to this FFT concept. suggest your best...

 

[KlausST]

Hi,

Upper frequency = 200Hz ---> Sample frequency at least 400Hz
Frequency resolution 0.5Hz --> FFT window size = 1/0.5Hz = 2 seconds
FFT --> 2^n: 400:0.5 = 800. Next higher = 1024

My recommendaton:
Sample frequency = 0.5Hz x 1024 = 512Hz
Window size 2s = 1024 samples

What controller:
Generally every controller could do this. I´ve done similar calculations with 8 bit AVR.
But you should choose what controller you know best and choose about calculation speed. Maybe an ARM.

How often do you want to calculate?
What do you do with the FFT output?
What resolution (bit count) do you need?
What interfaces do you need?

Klaus

 

[Shivanand P]

Thanks for the information KlausSt,

Edited by Dora: from now on, could you put quotation marks when you quote somebody?

I am familiar with PIC18F series controllers.

How often do you want to calculate?

Every 1 Second i want to calculate frequency,

What do you do with the FFT output?

From the output of FFT, I want to measure RPM of an engine with battery ripples.

What resolution (bit count) do you need?

Resolution of what, I couldn't get you sorry.

What interfaces do you need?

One adc conversion, and 2Uart communications.

 

[KlausST]

Hi,

What resolution / bit count.
--> you say you are reading battery ripples. Voltage or current? Usually this is done with an ADC. What resolution (How many bits?) has the ADC?

What interface:
--> adc conversion is no interface. What interface do you use for the ADC?

****

FFT for a RPM measurement... a lot of effort, with relatively low resolution.
But for sure doable.
Isn´t there any other, simpler way?

Klaus

 

[Easyrider83]

Simplest way is to readout the values directly from CAN bus. But it requeres connection to OBD-II. Autor will not measure engine RPM, only generator x3, which is much higher than engine.

 

[Shivanand P]

What resolution / bit count.
I am reading battery voltage, and my ADC is with 12bit resolution.
What interface:
Built in ADC, i am using built in functions like read adc(); and all..

There is a way of measuring frequency with much bigger hardware with multiple filters. but its varying from vehicle to vehicle..
I got a built in library for android application, which is giving almost same result with many vehicles..

- - - Updated - - -

@Easyrider,,
Thanks for the information, Yes with OBD-II, it works fine but i need to develop my own hardware to read rpm of vehicle..

 

[KlausST]

Hi,

Resolution.
Good information. --> 12bit will be transferred to signed 16 bit data for the FFT input.

ADC:
Good information. Mind for FFT analysis you need a fixed (hardware driven) sample rate with low jitter to get good results. Don't use software to start each conversion.

Analog signal:
I recommend to use a high pass filter to get rid of DC voltage. Then amplify this signal to get good voltage resolution, but avoid clipping.

Klaus

 

[Shivanand P]

as off my required frequency is 10-200Hz, But the input for adc have frequency of inbetween 2Khz-5Khz because of noise,
Is there any way to control it via software or i have to go for hardware filter for attenuation of higher frequency signal?

 

[KlausST]

Hi,

you have to attenuate all signals with frequency >= F_sample/2 to a unproblematic level at the analog side.
Else you will see alias frequencies at the digital side.

Read about "nyquist"
Read about "anti aliasing filter"

Klaus