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Discussion Groups | Comp.DSP | Basics of Frequency Shifting


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Basics of Frequency Shifting - Mason - 2008-11-05 19:40:00

Can someone refer me to a source of basic information on frequency shifting? 
I have an audio frequency input tone consisting of a single relatively 
stable signal that is in the range of 5 kHz +/- 5 Hz.  I want the circuit to 
shift the tone to precisely 5 kHz.

I have limited DSP knowledge so if someone could point me at some basic 
reference info that would be a great help.


Re: Basics of Frequency Shifting - DigitalSignal - 2008-11-05 19:54:00

Question: After the frequency shift, what do you want to do? Do you
want to look at the time signal after the frequency shift, or the
frequency spectrum after the frequency shift?

James
www.go-ci.com


Re: Basics of Frequency Shifting - Mason - 2008-11-05 19:59:00

At the time spectrum.  Specifically, I'm interested in the phase of the 
shifted signal.

Mason


"DigitalSignal" <d...@yahoo.com> wrote in message 
news:0...@s9g2000prg.googlegroups.com...
> Question: After the frequency shift, what do you want to do? Do you
> want to look at the time signal after the frequency shift, or the
> frequency spectrum after the frequency shift?
>
> James
> www.go-ci.com
>


Re: Basics of Frequency Shifting - DigitalSignal - 2008-11-05 20:34:00

Here is what I would do:

take the original signal, x(t), multiply it by: cos(2 PI dF t), sin(2
PI dF t)

R(t) = x(t) cos(2 PI dF t)
I(t) = x(t) sin(2 PI dF t)

where dF is the delta frequency to be shifted.

Now you have two time series signals: R(t) and I(t).

Conduct complex Fourier transform using R(t) as real and I(t) as
imaginary parts. The results will be frequency shifted spectrum.

James
www.go-ci.com


Re: Basics of Frequency Shifting - emre - 2008-11-05 20:54:00

>Can someone refer me to a source of basic information on frequency
shifting? 
>I have an audio frequency input tone consisting of a single relatively 
>stable signal that is in the range of 5 kHz +/- 5 Hz.  I want the circuit
to 
>shift the tone to precisely 5 kHz.

If you have no other signal than this tone, you can find its amplitude as
a function of time, and multiply (mix) that by a "pure sine".   You can
achieve this by filtering the signal first.  In simplest terms, you could
design an RC-circuit to get the envelope of the signal and mix that with
the purer tone.

What is your application?  What is the tone?

Emre


Re: Basics of Frequency Shifting - Mason - 2008-11-05 20:59:00

OK, that's possible if I first determine the amount of shift I need by 
measurement of the input frequency.

What I need is simpler though.  All that I need to do is to AFC the signal - 
that is, shift it to a constant frequency (5 kHz in my example).

Can you suggest a simple way to do that?

Thank you.

Mason



"DigitalSignal" <d...@yahoo.com> wrote in message 
news:f...@u29g2000pro.googlegroups.com...
> Here is what I would do:
>
> take the original signal, x(t), multiply it by: cos(2 PI dF t), sin(2
> PI dF t)
>
> R(t) = x(t) cos(2 PI dF t)
> I(t) = x(t) sin(2 PI dF t)
>
> where dF is the delta frequency to be shifted.
>
> Now you have two time series signals: R(t) and I(t).
>
> Conduct complex Fourier transform using R(t) as real and I(t) as
> imaginary parts. The results will be frequency shifted spectrum.
>
> James
> www.go-ci.com


Re: Basics of Frequency Shifting - Mason - 2008-11-05 21:06:00

The input signal is essentially a pure tone as it is received, at approx but 
not exactly 5 kHz.  I don't follow what you mean by mixing it with a pure 
sine.  My goal is to shift (mix) it to precisely 5 kHz, always 5 kHz, like 
an AFC circuit.

The application is to determine the difference in phase shift through two 
circuits driven by separate oscillators.  Since the oscillators have 
slightly different frequencies I first need to normalize the frequencies 
then measure the phase difference between the signals.

Mason


"emre" <e...@ece.neu.edu> wrote in message 
news:8...@giganews.com...
> >Can someone refer me to a source of basic information on frequency
> shifting?
>>I have an audio frequency input tone consisting of a single relatively
>>stable signal that is in the range of 5 kHz +/- 5 Hz.  I want the circuit
> to
>>shift the tone to precisely 5 kHz.
>
> If you have no other signal than this tone, you can find its amplitude as
> a function of time, and multiply (mix) that by a "pure sine".   You can
> achieve this by filtering the signal first.  In simplest terms, you could
> design an RC-circuit to get the envelope of the signal and mix that with
> the purer tone.
>
> What is your application?  What is the tone?
>
> Emre


Re: Basics of Frequency Shifting - Jerry Avins - 2008-11-05 21:25:00

Jason wrote:
> OK, that's possible if I first determine the amount of shift I need by 
> measurement of the input frequency.
> 
> What I need is simpler though.  All that I need to do is to AFC the signal - 
> that is, shift it to a constant frequency (5 kHz in my example).
> 
> Can you suggest a simple way to do that?

You might find what you need at http://www.dspdimension.com/

You mentioned wanting to compare phase. Compare t5o what?

Jerry
-- 
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯


Re: Basics of Frequency Shifting - Jerry Avins - 2008-11-05 21:28:00

Mason wrote:

   ...

> The application is to determine the difference in phase shift through two 
> circuits driven by separate oscillators.  Since the oscillators have 
> slightly different frequencies I first need to normalize the frequencies 
> then measure the phase difference between the signals.

Give up. Frequency can be looked at as rate of change of phase. You 
can't measure phase shift of signals at different frequencies.Arrange to 
use a single oscillator, even if it seems too complicated.

Jerry
-- 
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯


Re: Basics of Frequency Shifting - DigitalSignal - 2008-11-05 22:55:00

On Nov 5, 6:06 pm, "Mason" <plsnos...@plsnospam.net> wrote:
> The input signal is essentially a pure tone as it is received, at approx but
> not exactly 5 kHz.  I don't follow what you mean by mixing it with a pure
> sine.  My goal is to shift (mix) it to precisely 5 kHz, always 5 kHz, like
> an AFC circuit.
>
> The application is to determine the difference in phase shift through two
> circuits driven by separate oscillators.  Since the oscillators have
> slightly different frequencies I first need to normalize the frequencies
> then measure the phase difference between the signals.
>
> Mason
>
> "emre" <egu...@ece.neu.edu> wrote in message
>
> news:8...@giganews.com...
>
>
>
> > >Can someone refer me to a source of basic information on frequency
> > shifting?
> >>I have an audio frequency input tone consisting of a single relatively
> >>stable signal that is in the range of 5 kHz +/- 5 Hz.  I want the circuit
> > to
> >>shift the tone to precisely 5 kHz.
>
> > If you have no other signal than this tone, you can find its amplitude as
> > a function of time, and multiply (mix) that by a "pure sine".   You can
> > achieve this by filtering the signal first.  In simplest terms, you could
> > design an RC-circuit to get the envelope of the signal and mix that with
> > the purer tone.
>
> > What is your application?  What is the tone?
>
> > Emre- Hide quoted text -
>
> - Show quoted text -

Jerry is right. You can't compare the phase between the two sines from
two oscillators unless they are somehow correlated. Maybe we did not
understand your question correctly?

James
www.go-ci.com


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