Frequency Estimator/ Resolution

Jay wrote:
> 
> Ok,
> 
> Perhaps I'm way off in suggesting this but what about a hardware
> solution?
> 
> Take your sine wave --> Schmitt trigger to make it into a square wave,
> time the rising edges with a fast clock(10MHz or faster) and a deep
> counter and you'll get a good idea of the period of the waveform in
> counter ticks.
> 
> Take counter value * System_Clock_Period and invert that to get a number
> in Hz...
> 
> The faster your system clock (which requires a deep counter) the better
> resolution you can get. Even with something as "coarse" as 10MHz, at
> 6kHz +/- 100 ns will result in a frequency error of ~3.6 Hz.
> 
> -- Jay.

Noise will affect the time of zero crossing. Certain waveforms will
count a multiple of the fundamental frequency.* Barring such waveforms,
the error due to noise can be reduced by measuring the time for more
than one cycle; precision is traded for time (sound familiar?).

Jerry
_____________________________________
* A square wave with inverted fundamental is a simple example of this. 
-- 
Engineering is the art of making what you want from things you can get.
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"L. Bawr" <lba@engineer.com> wrote in message news:<3F2D7F90.9050708@engineer.com>...
> Thank you very much eveyrone for the pointers. I really appreciate your 
> interest in this post. It really helps. Thanks for the link with MATLAB 
> scripts. I'll take some time to try a couple of them.
> Do these technics you guys have mentioned perform well on coherent 
> signals? I was told that technics similar to MUSIC (called parametric 
> methods in Fred's link) don't. My signal might contain several coherent 
> single-frequency components. Any feedback on this point.
> TIA.
> 
> L.B.

Well, I have never really understood what these "coherent signals" that 
are supposed to be so problematic, really are. From what I see in the 
literature, one property of coherent signals is that the covariance matrix 
somehow becomes rank deficient. I can't really see how this can happen with
real-life measured data. 

In the case of one microphone, I interpret two coherent signals as two 
sines of the same frequency. In that case, MUSIC will (according to my 
intuitive understanding) "see" only one component. As long as amplitude 
or phase is not to be estimated, I think MUSIC will find the frequency. 

In the case of Direction of Arrival (DoA) estimation, I don't see that 
coherence becomes a problem at all. If two signals of the same frequency 
impinges from two different directions, they would separate along the 
wavenumber dimension. The only problematic situation I can see, is when 
two signals impognes on the array from the same DoA. Again, both frequency 
and DoA (but not amplitude or phase) should be possible to estimate.

But then, MUSIC and thelikes were never designed to estimate amplitude 
and phase. Only frequency/DoA.

Where did I go wrong?

Rune