Dear Members

I want to measure frequency of power line. I am using 16-bit ADC (sampling
frequency is 12.8KHz). My question is how to measure frequency with high
accuracy (+/ - 8mHz) in presence of Harmonics ( I think it can be done by
Appling FFT) . Can any one through some light on this?

Thanks in Advance.

Regards,

SVS

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# Frequency measurement

Started by ●June 30, 2006

Reply by ●July 1, 20062006-07-01

>Dear Members

> I want to measure frequency of power line. I am using 16-bit ADC

>(sampling frequency is 12.8KHz). My question is how to measure frequency

>with high accuracy (+/ - 8mHz) in presence of Harmonics ( I think it can

>be done by Appling FFT) . Can any one through some light on this?

The possibel approaches are

1. Use hardware zero cross detection and take the inverse of timer gap between two interrupts and calibrate.

2. Implement the same in software

3. Use a digital PLL, means keep estimating the phase angle for every sample using a PI control algorithm and use phase incremental between two samples to calculate frequency

BHASKAR REDDY . N

Mysore

Mobile: +91 9880 177365

> I want to measure frequency of power line. I am using 16-bit ADC

>(sampling frequency is 12.8KHz). My question is how to measure frequency

>with high accuracy (+/ - 8mHz) in presence of Harmonics ( I think it can

>be done by Appling FFT) . Can any one through some light on this?

The possibel approaches are

1. Use hardware zero cross detection and take the inverse of timer gap between two interrupts and calibrate.

2. Implement the same in software

3. Use a digital PLL, means keep estimating the phase angle for every sample using a PI control algorithm and use phase incremental between two samples to calculate frequency

BHASKAR REDDY . N

Mysore

Mobile: +91 9880 177365

Reply by ●July 3, 20062006-07-03

Sita-

> I want to measure frequency of power line. I am using 16-bit ADC

> (sampling frequency is 12.8KHz). My question is how to measure frequency

> with high accuracy (+/ - 8mHz) in presence of Harmonics ( I think it can

> be done by Appling FFT) . Can any one through some light on this?

To exceed +/- 8 mHz accuracy using an FFT approach, then you would need 4

mHz magnitude bin spacing. This would require a 262,144 pt FFT size at

12.8 kHz sampling rate.

So you're going to need a substantial amount of SDRAM in your C6x design,

at least 2 Mbyte -- maybe 4 Mbyte to be safe.

-Jeff

> I want to measure frequency of power line. I am using 16-bit ADC

> (sampling frequency is 12.8KHz). My question is how to measure frequency

> with high accuracy (+/ - 8mHz) in presence of Harmonics ( I think it can

> be done by Appling FFT) . Can any one through some light on this?

To exceed +/- 8 mHz accuracy using an FFT approach, then you would need 4

mHz magnitude bin spacing. This would require a 262,144 pt FFT size at

12.8 kHz sampling rate.

So you're going to need a substantial amount of SDRAM in your C6x design,

at least 2 Mbyte -- maybe 4 Mbyte to be safe.

-Jeff

Reply by ●July 3, 20062006-07-03

First you should remove the harmonics and superposed noise on the signal by

applying some kind of low- or band-pass filter. After that, you can measure

the frequency by counting the time between two subsequent zero crosses of

the filtered signal. If you find out that your measurements aren't accurate

enough, try measuring the time between a larger number of zero crosses.

Besides the FFT, another useful tool for spectrum analysis is the Chirp-Z

transform (see http://www.embedded.com/showArticle.jhtml?articleID301593

).

Regards,

Guy Eschemann

Vienna, Austria.

On 7/1/06, Bhaskar Reddy N wrote:

>

> >

> >

> >Dear Members

> > I want to measure frequency of power line. I am using 16-bit ADC

> >(sampling frequency is 12.8KHz). My question is how to measure frequency

> >with high accuracy (+/ - 8mHz) in presence of Harmonics ( I think it can

> >be done by Appling FFT) . Can any one through some light on this?

>

> The possibel approaches are

> 1. Use hardware zero cross detection and take the inverse of timer gap

> between two interrupts and calibrate.

> 2. Implement the same in software

> 3. Use a digital PLL, means keep estimating the phase angle for every

> sample using a PI control algorithm and use phase incremental between two

> samples to calculate frequency

>

applying some kind of low- or band-pass filter. After that, you can measure

the frequency by counting the time between two subsequent zero crosses of

the filtered signal. If you find out that your measurements aren't accurate

enough, try measuring the time between a larger number of zero crosses.

Besides the FFT, another useful tool for spectrum analysis is the Chirp-Z

transform (see http://www.embedded.com/showArticle.jhtml?articleID301593

).

Regards,

Guy Eschemann

Vienna, Austria.

On 7/1/06, Bhaskar Reddy N wrote:

>

> >

> >

> >Dear Members

> > I want to measure frequency of power line. I am using 16-bit ADC

> >(sampling frequency is 12.8KHz). My question is how to measure frequency

> >with high accuracy (+/ - 8mHz) in presence of Harmonics ( I think it can

> >be done by Appling FFT) . Can any one through some light on this?

>

> The possibel approaches are

> 1. Use hardware zero cross detection and take the inverse of timer gap

> between two interrupts and calibrate.

> 2. Implement the same in software

> 3. Use a digital PLL, means keep estimating the phase angle for every

> sample using a PI control algorithm and use phase incremental between two

> samples to calculate frequency

>