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How to detect signal from microwave sensor?

Started by gpezzella June 20, 2010
Dear Friends

My application should detect signal that come from MicroWave cavity (the
one used in house alarm).
When people walk in front a MW, a little signal of few mV and frequency
between 10-50Hz is generated (beat signal). I should detect this!

I'm developing on ATTiny micro family. Now: 
Since AtTiny have 20x amplifier buit in, which is better:

a) add 3 new bit by Oversampling and Decimation 
b) amplify the signal and use the basic 10 bit ADC

The scope is understand if someone walked if front sensor. Now how I SHOULD
PROCEED??

c) Filter the signal with FIR like sen(x)/x with cut frequency = 50 Hz? If
yes, which are the minimum point I must acquire (M coefficient
http://www.dspguide.com/ch16/2.htm) 

d) Calculate a FFT? 

Please try to be coincise in reply because in previous thread the reply 
have only add confusion....  
I need to understand which are the step to follow and few other detail
suggested by experience that I not have
Thanks

On 06/20/2010 02:53 PM, gpezzella wrote:
> Dear Friends > > My application should detect signal that come from MicroWave cavity (the > one used in house alarm). > When people walk in front a MW, a little signal of few mV and frequency > between 10-50Hz is generated (beat signal). I should detect this! > > I'm developing on ATTiny micro family. Now: > Since AtTiny have 20x amplifier buit in, which is better: > > a) add 3 new bit by Oversampling and Decimation > b) amplify the signal and use the basic 10 bit ADC > > The scope is understand if someone walked if front sensor. Now how I SHOULD > PROCEED?? > > c) Filter the signal with FIR like sen(x)/x with cut frequency = 50 Hz? If > yes, which are the minimum point I must acquire (M coefficient > http://www.dspguide.com/ch16/2.htm) > > d) Calculate a FFT? > > Please try to be coincise in reply because in previous thread the reply > have only add confusion.... > I need to understand which are the step to follow and few other detail > suggested by experience that I not have
Perhaps from your perspective the replies have only added to the confusion, but from the perspective 'out here' a whole bunch of smart people have worked hard to help you out, and you have more or less refused to acknowledge that you have some real, live, actual _work_ that you need to undertake. There _aren't_ any easy -- or at least no work-free -- answers. How should you proceed? for (a) vs. (b)? Well, take a real live oscilloscope, spend some real live time, and take a real live look at the real live signal. Then ask yourself if, when that signal is amplified by 20x, it will range outside the voltage bounds of the ADC. If the answer is "no", then of course a 20x preamplifier will help. (c) No. This has been explained, both in words and with links to articles that go into far more depth than one normally wants to devote to a single reply on a newsgroup. (d) No. An FFT isn't magic, it's just a means of filtering. In this case it's probably the wrong filter -- much like your sinc filter, but for different reasons. Perhaps instead of asking the same question over, hoping that the answer will change if only you're stubborn, you could review that whole thread, and ask for clarification of the parts that you don't understand. Expect it to be obscure, and possibly even hard -- if DSP were easy, they'd teach it to kids in grade school. -- Tim Wescott Control system and signal processing consulting www.wescottdesign.com

gpezzella wrote:
> Dear Friends > > My application should detect signal that come from MicroWave cavity (the > one used in house alarm). > When people walk in front a MW, a little signal of few mV and frequency > between 10-50Hz is generated (beat signal). I should detect this! > I'm developing on ATTiny micro family.
All you have to do is a basic bandpass filter, rectifier and adaptive threshold. ~20 lines of C code. Or you can drop ATTiny and do the work with LM358 and few analog parts.
> Now: > Since AtTiny have 20x amplifier buit in, which is better: > > a) add 3 new bit by Oversampling and Decimation > b) amplify the signal and use the basic 10 bit ADC > > The scope is understand if someone walked if front sensor. Now how I SHOULD > PROCEED??
There are two ways to proceed: do it yourself or hire a consultant. The whole job should take a couple of hours.
> c) Filter the signal with FIR like sen(x)/x with cut frequency = 50 Hz? If > yes, which are the minimum point I must acquire (M coefficient > http://www.dspguide.com/ch16/2.htm) > d) Calculate a FFT?
General rule: don't mess with the methods that you don't understand. It is not going to help.
> Please try to be coincise in reply because in previous thread the reply > have only add confusion....
> I need to understand which are the step to follow and few other detail > suggested by experience that I not have
You can contact Tim, Randy, or me, but you have to realize that nobody is going to do your work for free. Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com
On 06/20/2010 06:54 PM, Vladimir Vassilevsky wrote:
> > > gpezzella wrote: >> Dear Friends >> >> My application should detect signal that come from MicroWave cavity (the >> one used in house alarm). >> When people walk in front a MW, a little signal of few mV and frequency >> between 10-50Hz is generated (beat signal). I should detect this! >> I'm developing on ATTiny micro family. > > All you have to do is a basic bandpass filter, rectifier and adaptive > threshold. ~20 lines of C code. Or you can drop ATTiny and do the work > with LM358 and few analog parts. > >> Now: Since AtTiny have 20x amplifier buit in, which is better: >> >> a) add 3 new bit by Oversampling and Decimation b) amplify the signal >> and use the basic 10 bit ADC >> >> The scope is understand if someone walked if front sensor. Now how I >> SHOULD >> PROCEED?? > > There are two ways to proceed: do it yourself or hire a consultant. The > whole job should take a couple of hours. > >> c) Filter the signal with FIR like sen(x)/x with cut frequency = 50 >> Hz? If >> yes, which are the minimum point I must acquire (M coefficient >> http://www.dspguide.com/ch16/2.htm) d) Calculate a FFT? > > General rule: don't mess with the methods that you don't understand. It > is not going to help. > >> Please try to be coincise in reply because in previous thread the >> reply have only add confusion.... > >> I need to understand which are the step to follow and few other detail >> suggested by experience that I not have > > You can contact Tim, Randy, or me, but you have to realize that nobody > is going to do your work for free.
The ATTiny is a lot smaller than the cap you'd need to implement a decent low-pass filter in an analog circuit. I suppose you could use a Tantalum if the exact cut-off frequency wasn't too much of an issue, but you'd still be constantly fiddling with the circuit. There is a lot for the OP to learn to make this work -- he either needs to hire someone, or dig in and start learning. -- Tim Wescott Control system and signal processing consulting www.wescottdesign.com
On 6/20/2010 5:53 PM, gpezzella wrote:
> Dear Friends > > My application should detect signal that come from MicroWave cavity (the > one used in house alarm). > When people walk in front a MW, a little signal of few mV and frequency > between 10-50Hz is generated (beat signal). I should detect this! > > I'm developing on ATTiny micro family. Now: > Since AtTiny have 20x amplifier buit in, which is better: > > a) add 3 new bit by Oversampling and Decimation > b) amplify the signal and use the basic 10 bit ADC > > The scope is understand if someone walked if front sensor. Now how I SHOULD > PROCEED?? > > c) Filter the signal with FIR like sen(x)/x with cut frequency = 50 Hz? If > yes, which are the minimum point I must acquire (M coefficient > http://www.dspguide.com/ch16/2.htm) > > d) Calculate a FFT? > > Please try to be coincise in reply because in previous thread the reply > have only add confusion.... > I need to understand which are the step to follow and few other detail > suggested by experience that I not have
MAX is the strongest signal your ADC can accommodate without overload. LITTLE is the strength of the the strongest beat signal you will get. MAX/LITTLE is the gain of the amplifier you want, but a quarter of that will let you digitize with 8 bits. I understand that you want to use no additional components: tough. Most people know how to cook fried eggs. Only a master chef can make fried eggs without using eggs. What in the world would an FFT do for you? Grease the pan? Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������