Laptop Metal Detector utilising Digital Lock-in Amplifier
Bookmark this page:
 Yahoo!
 Google
 Windows Live
 del.icio.us
 digg
 Netscape
|
|
|||||||||||||
|
Posted by Don Bowey on August 17, 2008, 10:17 am
Please log in for more thread options On 8/17/08 5:47 AM, in article 48a81818.1430283@news.sysmatrix.net, "Bob >> Hi Bob,
>> >> indeed, this is really an interesting and fascinating project. The >> digital lock-in amplifier is a pure software implementation which is >> applied on the input signal (A/D converted receive signal). The lock- >> in amplifier is a very sensitive phase detector even the signal is >> buried in high noise. Any small changes can be detected with it (=B5V >> measuring). Using a 24 bit sound-card at 96 kHz sample rate increases >> the signal-to-noise ratio and dynamic range. It will also work on 16 >> bit and 44.1/48 kHz with reduced SNR and dynamic range. >> >> It is quite time-critical application. You must not loose the >> synchronisation of the transmitted to received signal. The continious >> wave form is buffered to the sound-card to avoid signal gaps due to >> operating system task switches. But this is easy to handle and only >> the DMA is busy and relieves the CPU. Laptop has enough CPU power for >> doing this and much more in real-time (FFT, digital filter, lock-in >> amp, detection, signal generation, synch generation, graphical >> output ..). >> >> The operating frequency for the sensor is between 5 to 24 kHz (VLF >> range). It depends only on the resonant frequency of the search head >> (L,C resonant defining elements) and sampling rate (fmax=3DSR/4). The >> higher the operating frequency, the better the sensitivity of the >> sensor (Faraday's law). So it is mostly defined by the sensor >> specification. >> >> The sensors are typically D shaped coils with same inductivity L for >> transmit and receive coil. This will allow a simple matching of the >> capacitors (same for transmitter and receiver). The coils are in >> overlapped co-planar position and forming a circle (two D's). The >> receive coil should have a minimum of signal level (10-50 mV rms). >> This position must be found by moving one of the coils. >> >> >> What about digital lock-in amplifier for your application? This would >> be a quite useful feature. >> >> Regards, >> Aziz >> >
> Aziz: > > Thanks for the explanation. Yes, I am quite familiar with real-time > issues. Daqarta needs perfect sync to do synchronous waveform > averaging for noise reduction, so I've been there and done that! > > A digital lock-in would be a definite possibility for Daqarta. I'll > put that on my "Wish List" for future enhancements. I probably won't > offer the "lock-in" (PLL) part that hardware lock-ins have, since I've > always thought that was pretty silly unless you really do need to sync > to an external signal. > > For those who are following this thread and aren't familiar with > lock-in amplifiers, they are essentially a single-frequency Fourier > Transform. You separately multiply the incoming signal by the sine > and cosine of the reference signal, and low-pass filter the results. > From the old high-school formula for the product of sinusoids, you get > only terms at sum and difference frequencies. It's the difference > term we want here. The low-pass removes the sum and produces an > output only if the input is exactly the same frequency as the > reference (difference = 0), or very near. > > (See <www.daqarta.com/eex01.htm> for an FFT explanation that > goes into more detail about this.) > > The only real difference between a lock-in and the output of an FFT > is that the FFT has a very crude low-pass filter (one for each > spectral line) and the lock-in usually has a better filter (longer > time constant in lock-in terms). That statement assumes that the > FFT has a spectral line just where you wanted the lock-in reference > frequency. This is no big deal if you are generating the output > frequency yourself... just make sure it lands squarely on a spectral > line. > > Daqarta can already do this. And it can get the noise reduction by > synchronous waveform averaging before the FFT. The only thing > is that it doesn't display the data in lock-in format, with separate > sine and cosine or magnitude and phase readouts. That would be a good > addition! > > Best regards, > > > Bob Masta > > DAQARTA v4.00 > Data AcQuisition And Real-Time Analysis > www.daqarta.com > Scope, Spectrum, Spectrogram, Sound Level Meter > FREE Signal Generator > Science with your sound card! Great discussion, excellent commercial. Thanks guys. | |||||||||||||
|
Posted by Jasen Betts on August 18, 2008, 6:47 am
Please log in for more thread options > On Sat, 16 Aug 2008 11:06:52 -0700 (PDT), oeguet@gmx.de wrote:
> Thanks for the explanation. Yes, I am quite familiar with real-time
> issues. Daqarta needs perfect sync to do synchronous waveform > averaging for noise reduction, so I've been there and done that! > > A digital lock-in would be a definite possibility for Daqarta. I'll > put that on my "Wish List" for future enhancements. I probably won't > offer the "lock-in" (PLL) part that hardware lock-ins have, since I've > always thought that was pretty silly unless you really do need to sync > to an external signal. > > For those who are following this thread and aren't familiar with > lock-in amplifiers, they are essentially a single-frequency Fourier > Transform. You separately multiply the incoming signal by the sine > and cosine of the reference signal, and low-pass filter the results. > From the old high-school formula for the product of sinusoids, you get > only terms at sum and difference frequencies. It's the difference > term we want here. The low-pass removes the sum and produces an > output only if the input is exactly the same frequency as the > reference (difference = 0), or very near. you make the lock-in amplifier sound very much like a synchronous detector. in-fact if you're not implementing the PLL it seems more like a synchronous detector than it is a lock-in amplifier. Bye. Jasen | |||||||||||||
|
Posted by Bob Masta on August 19, 2008, 8:31 am
Please log in for more thread options
>you make the lock-in amplifier sound very much like a synchronous
>detector. in-fact if you're not implementing the PLL it seems more >like a synchronous detector than it is a lock-in amplifier. > Exactly so! Hardware lock-in amplifiers are just sine and cosine synchronous detectors with PLLs to sync to an external reference. I don't know what the current situation is, but for years the lock-in designers were off in the ozone somewhere. The typical design had a built-in reference oscillator that appeared to be an afterthought... it was completely separate, and even in models that allowed you to change the reference frequency remotely (via control voltage or serial bus), it still had to go through the PLL. Which meant you had to wait for the PLL to settle ("acquire lock") instead of instant response like they would have gotten had they just allowed the oscillator to *be* the reference. It seems there are not that many applications that really need to sync to an independent external reference these days. The original lock-ins were used with optical choppers, where the reference depended on the rotation of a wheel. (And they used simple switches instead of true multipliers, so they were effectively multiplying by square waves and thus were sensitive to all the odd harmonics.) Ahh, the good old days... <g>
Best regards, Bob Masta DAQARTA v4.00 Data AcQuisition And Real-Time Analysis www.daqarta.com Scope, Spectrum, Spectrogram, Sound Level Meter FREE Signal Generator Science with your sound card! | |||||||||||||
|
Posted by YD on August 17, 2008, 11:46 pm
Please log in for more thread options
Late at night, by candle light, oeguet@gmx.de penned this immortal opus: >Hi Bob,
> >my experimental measurement system has already a scope, spectrum >analyzer, digital lock-in amplifier, signal generator and much more. > >So this experiment is a typical proof of concept which is saying: it >is working! > >Basic operation of the laptop metal detector: >Harmonic sine wave is sent through the earphone output. The impedance >of the earphone output is round about 20 Ohms which delivers enough >current for the transmitter coil. The transmitter coil has a capacitor >connected defining a LC resonant tank. The transmitted frequency is >the resonant frequency of the LC tank. >The receive coil is in inductively balanced position (less coupled to >transmitter coil) and has also a capacitor which also defines a LC >resonant tank (same of transmitt frequency). The receive coil is >inducing a small signal. The signal amplitude and phase will change >upon a metal target nearby the search coil appears. The sound-card is >used in full-duplex mode (while transmitting a signal, the receive >signal is acquired). >The digital lock-in amplifier (a two channel I & Q lock-in amplifier) >detects the signal magnitude and phase of the receive coil. While the >laptop knows the reference frequency (internally generated), it >detects the changes by the receive coil. > >This is the most simple and sentive metal detector ever designed. You >don't need any active electronics between the laptop and search coil. >Search coil has only some capacitors and the inductors (transmit & >receive coil). All the rest is done by the software using a high >definition sound-card operated at 96 kHz sampling rate and 24 bit >resolution. > >The experiment is showing amazing sensitivity results. It can compete >with professional VLF detectors. >Aziz Why lug along a lap-top if the needed circuitry fits in a box the size of a cigarette pack? All you need is a LED display and possibly a beeper. - YD. -- Remove HAT if replying by mail. | |||||||||||||
|
Posted by Jasen Betts on August 18, 2008, 6:54 am
Please log in for more thread options
> Why lug along a lap-top if the needed circuitry fits in a box the size
> of a cigarette pack? well. the size of a mobile phone anyway :) > All you need is a LED display and possibly a
> beeper. you'd probably want to be able to tune it to discriminate between scrap iron and lost coins. Bye. Jasen | |||||||||||||
>