In hardware or software ?

This is a amplitude modulated wave where 0.33 Hz modulates a higher frequency signal of  3 to 6 Hz.. The rc circuits are the most appropriate to generate such waveform. You can use wien bridge oscillator or phase shift oscillators to generate the sine waves. Then you can use a multiplier circuit build the required modulated eave. You have to make the amplitude of the higher frequency signal higher then that tof the low frequency signal. You can use a trans-conductance amplifier to implement the the multiplication where the the bias of the amplifier is controlled by the lower frequency signal and so the gain of the amplifier will be changed according to the controlling signal which is the lower frequency signal. Also all mixer circuits can be used for this function.

wish you success

Many different options here.  How much noise is there?  Is the noise in-band or out-of-band?  How fast does the frequency of the 3-6 Hz vibration change?  How fast does the 0.33 Hz envelope change - both in amplitude and in frequency.

The following should work with either analog or digital signal processing:

Band pass filter the vibration signal to reject DC and low frequencies below 3 Hz and frequencies higher than 6 Hz.

Extract the amplitude of the vibration envelope.  Options include precision rectification followed by a low pass filter or peak-follower circuit with appropriate (e.g. ~0.5 s) decay time.

Multiply the envelope signal by a 0.33 Hz cosine wave and by a 0.33 Hz sine wave, and low pass filter each product to derive separate in-phase and quadrature signals.  The time constant of the filter determines the detection bandwidth.  Square and add the in-phase and quadrature signals and take the square root of the result to extract the amplitude of the 0.33 Hz envelope modulation.  For analog processing, the multiplication, square and square root operations can be implemented using 4-quadrant multiplier integrated circuits.

Alternatively, if the frequency and phase of the 0.33 Hz changes reasonably slowly, apply a phase-locked loop to extract a stable 0.33 Hz reference signal, and use this for synchronous (lock-in) detection of the 0.33 Hz envelope signal.  This can offer lower noise than the RMS detection scheme outlined in the preceding paragraph.

Thanks for suggestions..

Please let me know, the vibration signal is of 3 to 6 hz, (excluding noise, assuming 0 level noise), suppose I need envelope of 0.33 hz then what is my LPF cuttoff??

What basis I need to chose cuttoff of LPF for envelope?? Is there any theory please share me..

If there is no noise, then you don't need to band-pass filter the vibration signal.  Just DC block (AC couple).

How stable is the vibration frequency?  If there is appreciable phase noise, then a band-pass filter will result in phase to amplitude conversion, and a spurious amplitude signal after envelope detection.

You have not given enough information to select an optimum low pass filter bandwidth after 3-6 Hz envelope detection.  A value around 1 Hz would be a reasonable starting point.  This is low enough to attenuate the 6 to 12 Hz and higher frequency components remaining after rectification or square law detection, without significant attenuation of the 0.33 Hz signal you are trying to detect.

A high order filter with a rapid roll-off will improve rejection of high frequency components, but will also produce transient oscillations in response to impulsive or step changes in the the envelope amplitude.  A linear phase or Bessel filter will minimise such transients and should be a safe choice, but will not necessarily be optimum.

I understand that you are processing sampled data in Matlab.  What is the sample rate?  Both the sampling process and envelope detection are non-linear processes, and can introduce spurious frequency components through inter-modulation and harmonic distortion.  Is the sample rate high enough to avoid aliasing?

Sampling rate is 100Hz.. Max frequency in the signal is 6Hz.

23Hz signal will be there very less, its a noise so BPF is used.

Dear Venatesh,

May be the problem needs more clarification. I understood that you want to generate the signals so my first comment was about that. But i hardly now understand tat you have a composite signal from 6 Hz enveloped by .33Hz.  According to your description  if the signal is frequency analysed then it contains the carrier at 6Hz and two side bands at .33Hz apart from the carrier.

To extract he 6 Hz from the signal there are many techniques but the the two coming techniques may be suitable:

- You can use the PLL such as 565 to recover a clean 6 Hz signal with loop filter having a bandwidth of about 0.1Hz much less than that of e envelope.

- The other solution is to amplify and limit the signal to clip out amplitude variations.

Then if you are interested in a sine wave you may filter out the harmonics in the output by a low pass filter with a cut off frequency at the signal frequency at 6Hz.

Hope this help this time. Your sampling frequency is Okay.

wish you success

Hello Venkatesh,

I suggest cross-correlation with a 0.33Hz reference internal digital signal after sampling. A complex form of the reference (and therefore a complex correlator) may help you to identify the 0.33Hz regardless of it's phase in regards to your reference.

Best

Hello Venkatesh,

I would like to suggest a different approach altogether. There are two ways that you can address this problem - i) Use a DDS chip to generate a highly stable sine wave of say the carrier frequency 3-6Hz that you are looking for. Next for the modulating wave, you can either use conventional function generator ICs such as XR22XX series or ICL7038 or its equivalent.

ii) The second approach I would suggest and strongly recommend to you is to generate these sine waves using a micro(controller) and a suitable DAC which in opinion will be the easiest way to implement the solution for your requirement.

You can refer one of my article published in this regard in EFY, October 2000. It is also available on RG.

Please feel free if you need any further help.

Thanks and best wishes!

If you have sampled data already and you can process data offline, you can get envelope info by Hilbert transform.  Just pay attention to model implied with the technique.

Dear friend

it is very simple

use Diode IN 4007 and RC combination (Low Pass) with

R= 100 K Ohm

C= 10 mic Farad

Let me guess: you are working on breathing-heart beating separation!

However, I deeply suggest to base your design on digital elaboration. There you will find the widest capability in terms of filtering. In addition, if you are going to design phase demodulation in order to extract the information I suggest to adopt pre-ADC low-pass filters designed adopting analog linear-phase response filters. Let me know your opinion. Fabrizio

Hello,

Since the frequence is very less, it looks the application required is Biomedical signal processing, hence you can apply reduction techniques to operate in lower frquency